influence of high cutting speeds
TRANSCRIPT
Report No. ERC/NSM - S-96-19
INFLUENCE OF HIGH CUTTING SPEEDS ON THE QUALITY OF
BLANKED PARTS
by
Martin Grünbaum, Visiting Scholar
University of Stuttgart, Germany
and
Jochen Breitling, Staff Engineer, ERC/NSM
Taylan Altan, Professor and Director, ERC/NSM
NSF Engineering Research Center for
Net Shape Manufacturing
The Ohio State University
1971 Neil Avenue
Columbus, Ohio 43210
May, 1996
Advanced Copy
For Limited Distribution Only
(This report is an advance copy subject to modification and is distributed only to
members of the ERC for Net Shape Manufacturing. Approval must be requested
from the ERC prior to distribution to other organizations or individuals.)
i
FOREWORD
This document has been prepared for the Engineering Research Center for Net
Shape Manufacturing (ERC/NSM). The Center was established on May 1, 1986
and is funded by the National Science Foundation and the member companies.
The focus of the Center is net shape manufacturing with emphasis on cost-
effective manufacturing of discrete parts. The research concentrates on
manufacturing from engineering materials to finish or near finish dimensions via
processes that use dies and molds. In addition, to conduct industrially relevant
engineering research, the Center has the objectives to
a) establish close cooperation between industry and the university,
b) train students and
c) transfer the research results to interested companies.
This report summarizes experimental and simulation work, investigating the
effects of very high cutting speeds. The goal of the first part of the project was to
monitor and analyze the velocity profile obtained with a Lourdes
electromagnetic impact press. For that purpose the press was equipped with a
velocity and proximity sensor in order to monitor the velocity-stroke curve. In
addition, the influence of material properties, cutting velocity and punch-die
clearance on the quality of the part edge was investigated.
Information about the ERC for Net Shape Manufacturing can be obtained from
the office of the Director, Taylan Altan, located at the Baker Systems Engineering
Building, 1971 Neil Avenue, Columbus, Ohio 43210-1271, phone 614/292-5063.
ii
INFLUENCE OF HIGH CUTTING SPEEDS ON THE QUALITY OF
BLANKED PARTS
Martin Grünbaum, Visiting Scholar
Report No. ERC/NSM - S-96-19
EXECUTIVE SUMMARY
This report summarizes the influences of different parameters on the part edge
quality of blanked parts. Experiments have been conducted using different
materials, punch-die clearances and cutting speeds. In order to determine the
reachable cutting speeds and to calculate the energy required for blanking,
velocity-stroke curves were obtained. In addition, blanking simulations with
DEFORM 2D have been performed. The results of these simulations have then
been compared with the results obtained by the experiments.
The evaluation of the part edges shows that higher cutting speeds can improve
the part edge quality, resulting in smaller burr height and rollover, and a larger
shear zone. Furthermore, it could be observed that the part quality improvement
when blanking with high cutting speeds (up to 12 ft/sec) is much more distinct
for steel than for copper or aluminum. According to theory, this improvement
was expected because copper and aluminum have much higher heat conduction
coefficients. Therefore, the heat dissipates faster and the desired stress relief
effect does not take place to the same degree as for steel.
ERC/Net shape Manufacturing
339 Baker Systems / 1971 Neil Avenue
Columbus, OH 43210 ph: 614-292-9267 fax: 614-292-7219
iii
TABLE OF CONTENTS
FOREWORD.................................................................................................................. i
EXECUTIVE SUMMARY............................................................................................. ii
TABLE OF CONTENTS.............................................................................................. iii
LIST OF TABLES.......................................................................................................... v
LIST OF FIGURES....................................................................................................... vi
CHAPTERS PAGE
1. INTRODUCTION.....................................................................................................................................1
2. THE BLANKING PROCESS ..................................................................................................................2
2.1 DEFINITION OF SHEARING AND BLANKING .................................................................................................2
2.2 INFLUENCES ON THE BLANKING PROCESS ...................................................................................................2
2.3 PHASES OF THE BLANKING PROCESS ..........................................................................................................3
2.4 STRESS CONDITIONS IN SHEARING ..............................................................................................................6
2.5 FORMATION OF THE PART EDGE..................................................................................................................8
2.6 HIGH SPEED BLANKING.............................................................................................................................10
3. EQUIPMENT USED...............................................................................................................................13
3.1 THE LOURDES PRESS 100-OH ..................................................................................................................13
3.2 EXPERIMENTAL SETUP .............................................................................................................................17
3.2.1 Punches and Dies ............................................................................................................................17
3.2.2 Stock materials ................................................................................................................................19
3.2.3 Sensors.............................................................................................................................................20
3.2.3.1 Analog Proximity Sensor ..........................................................................................................................20
3.2.3.2 Linear velocity transducer .........................................................................................................................20
3.2.4 Data Acquisition and Signal Analysis .............................................................................................21
3.3 TECHNIQUES FOR EVALUATING THE PART EDGE .......................................................................................22
3.3.1 Measuring the penetration depth, the shear zone and the rollover .................................................22
3.3.2 Technique for burr height measuring..............................................................................................23
3.3.2.1 Definition of the burr height......................................................................................................................23
3.3.2.2 Requirements and design of the device .....................................................................................................24
iii
4. EXPERIMENTAL PROCEDURE ........................................................................................................27
4.1 INVESTIGATION OF PRESS CHARACTERISTICS............................................................................................28
4.2 VELOCITY INVESTIGATIONS......................................................................................................................29
4.3 PART QUALITY INVESTIGATIONS...............................................................................................................30
5. EXPERIMENTAL RESULTS ...............................................................................................................31
5.1 PRESS CHARACTERISTICS AND PRELIMINARY RESULTS .............................................................................31
5.2 VELOCITY INVESTIGATIONS......................................................................................................................33
5.3 PART QUALITY INVESTIGATIONS...............................................................................................................41
5.3.1 Experiments with low carbon steel ..................................................................................................42
5.3.2 Experiments with high strength steel ...............................................................................................50
5.3.3 Experiments with aluminum ............................................................................................................57
5.3.4 Experiments with copper .................................................................................................................64
5.3.5 Material comparison .......................................................................................................................71
6. FEM SIMULATIONS WITH DEFORM 2D........................................................................................78
6.1 THE FINITE ELEMENT CODE DEFORM 2D ..............................................................................................78
6.1.1 Pre-processor ..................................................................................................................................78
6.1.2 Simulation engine ............................................................................................................................78
6.1.3 Post-processor .................................................................................................................................79
6.2 SIMULATIONS OF THE HIGH SPEED BLANKING PROCESS ............................................................................79
6.2.1 Simulation settings...........................................................................................................................79
6.2.2 Simulation results ............................................................................................................................81
6.2.3 Comparison of the experimental and simulated results...................................................................82
7. SUMMARY AND CONCLUSIONS......................................................................................................84
8. LIST OF REFERENCES .......................................................................................................................87
APPENDIX A
APPENDIX B
APPENDIX C
APPENDIX D
v
LIST OF TABLES
CHAPTERS PAGE
TABLE 1: INFLUENCES ON THE FORMATION OF THE CUTTING EDGE /10/, /4/, /8/, /11/, /12/................................10
TABLE 2: HEAT CONDUCTION COEFFICIENTS FOR DIFFERENT MATERIALS /19/ ..................................................11
TABLE 3: LOURDES 100-OH PRESS SPECIFICATIONS (ACCORDING TO THE MANUFACTURER) .............................14
TABLE 4: CLEARANCES IN PERCENT FOR DIFFERENT PUNCH DIAMETERS AND MATERIAL THICKNESSES (DIE
BUTTON DIAMETER: 0.500")...................................................................................................................19
TABLE 5: CLEARANCES IN PERCENT FOR DIFFERENT PUNCH DIAMETERS AND MATERIAL THICKNESSES (DIE
BUTTON DIAMETER: 0.514")...................................................................................................................19
TABLE 6: CUTTING SPEEDS AT DIFFERENT POWER LEVELS OF THE PRESS FOR DIFFERENT MATERIALS.................32
TABLE 7: CUTTING FORCE FOR DIFFERENT MATERIALS.....................................................................................38
TABLE 8: ENERGY LEVEL AT DIFFERENT CUTTING SPEEDS.................................................................................40
TABLE 9: CUTTING ENERGY FOR THE STOCK MATERIALS ...................................................................................41
TABLE 10: CROSS SECTION AND SIDE VIEW FOR LOW CARBON STEEL. CLEARANCE: 3/4%. ..................................47
TABLE 11: CROSS-SECTION AND SIDE VIEW FOR LOW CARBON STEEL. CLEARANCE: 15%. ..................................48
TABLE 12: CROSS-SECTION AND SIDE VIEW FOR LOW CARBON STEEL. CLEARANCE: 21/24%. .............................49
TABLE 13: CROSS-SECTION AND SIDE VIEW FOR HIGH STRENGTH STEEL. CLEARANCE: 3%. ................................54
TABLE 14: CROSS-SECTION AND SIDE VIEW FOR HIGH STRENGTH STEEL. CLEARANCE: 14%. ..............................55
TABLE 15: CROSS-SECTION AND SIDE VIEW FOR HIGH STRENGTH STEEL. CLEARANCE: 20/24%..........................56
TABLE 16: CROSS-SECTION AND SIDE VIEW FOR ALUMINUM. CLEARANCE: 5%. .................................................61
TABLE 17: CROSS-SECTION AND SIDE VIEW FOR ALUMINUM. CLEARANCE: 15%. ...............................................62
TABLE 18: CROSS-SECTION AND SIDE VIEW FOR ALUMINUM. CLEARANCE: 21%. ...............................................63
TABLE 19: CROSS-SECTION AND SIDE VIEW FOR COPPER. CLEARANCE: 6%. ......................................................68
TABLE 20: CROSS-SECTION AND SIDE VIEW FOR COPPER. CLEARANCE: 13%. ....................................................69
TABLE 21: CROSS-SECTION AND SIDE VIEW FOR COPPER. CLEARANCE: 19%. ....................................................70
TABLE 22: INPUT DATA OF THE LOW CARBON STEEL SHEET AND THE PUNCH......................................................80
TABLE 23: INPUT DATA OF THE DIE BUTTON AND THE BLANK HOLDER...............................................................80
TABLE 24: COMPARISON OF THE EXPERIMENTAL AND SIMULATED RESULTS (LOW CARBON STEEL) ......................83
vi
LIST OF FIGURES
CHAPTERS PAGE
FIGURE 1: SCHEMATIC ILLUSTRATION OF THE SHEARING PROCESS.......................................................................3
FIGURE 2: PHASES OF THE BLANKING PROCESS /6/ ............................................................................................6
FIGURE 3: SHEARING FORCES ............................................................................................................................7
FIGURE 4: STRESS CONDITIONS IN THE SHEARING ZONE ......................................................................................8
FIGURE 5: DIFFERENT ZONES OF THE PART EDGE ...............................................................................................9
FIGURE 6: THE LOURDES PRESS 100-OH ........................................................................................................13
FIGURE 7: POSITION OF MATERIAL STRIP IN THE PRESS .....................................................................................14
FIGURE 8: SCHEMATIC PICTURE OF THE LOURDES PRESS 100 - OH ..................................................................15
FIGURE 9: PUNCH VELOCITY VERSUS THE POWER LEVELS OF THE PRESS DEPENDING ON THE STROKE LENGTH. NO
CUTTING CONDITION. .............................................................................................................................17
FIGURE 10: STRIPPER, DIE BUTTON AND PUNCH ...............................................................................................18
FIGURE 11: SENSOR WIRING (SCHEMATICALLY).................................................................................................22
FIGURE 12: MICROSCOPE PICTURE SHOWING THE CROSS SECTION OF A SLUG /4/...............................................23
FIGURE 13: BURR HEIGHT MEASUREMENT DEVICE............................................................................................24
FIGURE 14: METHOD OF WORKING OF THE BURR HEIGHT MEASURING DEVICE...................................................25
FIGURE 15: VELOCITY/DISPLACEMENT-TIME CURVE FOR NO CUTTING CONDITION AT POWER LEVEL 5 OF THE
PRESS ....................................................................................................................................................28
FIGURE 16: VELOCITY-STROKE CURVE FOR NO CUTTING CONDITION. POWER LEVEL 5. ......................................29
FIGURE 17:MAXIMUM CUTTING SPEED FOR DIFFERENT PUNCH-DIE CLEARANCES AND STOCK MATERIALS...........31
FIGURE 18: UNIFORMITY OF PART EDGE (LOW CARBON STEEL, 14% CLEARANCE, 4 FT/SEC)...............................33
FIGURE 19: VELOCITY/DISPLACEMENT-TIME CURVE. MATERIAL: LOW CARBON STEEL. POWER LEVEL 3, STROKE
LENGTH 0.5"..........................................................................................................................................34
FIGURE 20: VELOCITY/DISPLACEMENT-TIME CURVE. MATERIAL: LOW CARBON STEEL. POWER LEVEL 9, STROKE
LENGTH 1.5"..........................................................................................................................................35
FIGURE 21: PUNCH VELOCITY VERSUS DISPLACEMENT. MATERIAL: LOW CARBON STEEL. POWER LEVEL 3, STROKE
LENGTH 0.5"..........................................................................................................................................36
FIGURE 22: PUNCH VELOCITY VERSUS DISPLACEMENT. MATERIAL: LOW CARBON STEEL. POWER LEVEL 9, STROKE
LENGTH 1.5"..........................................................................................................................................37
FIGURE 23: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE (LOW CARBON STEEL)..............................................42
FIGURE 24: % SHEAR VERSUS PUNCH-DIE CLEARANCE (LOW CARBON STEEL) .....................................................43
FIGURE 25: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE (LOW CARBON STEEL)...............................................45
FIGURE 26: % PENETRATION VERSUS PUNCH-DIE CLEARANCE (LOW CARBON STEEL)..........................................46
FIGURE 27: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE (HIGH STRENGTH STEEL) .........................................50
vi
FIGURE 28: % SHEAR VERSUS PUNCH-DIE CLEARANCE (HIGH STRENGTH STEEL) ................................................51
FIGURE 29: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE (HIGH STRENGTH STEEL) ..........................................52
FIGURE 30: % PENETRATION VERSUS PUNCH-DIE CLEARANCE (HIGH STRENGTH STEEL) .....................................53
FIGURE 31: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE (ALUMINUM) ..........................................................57
FIGURE 32: % SHEAR VERSUS PUNCH-DIE CLEARANCE (ALUMINUM)..................................................................58
FIGURE 33: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE (ALUMINUM)............................................................59
FIGURE 34: % PENETRATION VERSUS PUNCH-DIE CLEARANCE (ALUMINUM).......................................................60
FIGURE 35: % SHEAR VERSUS PUNCH-DIE CLEARANCE (COPPER) ......................................................................65
FIGURE 36: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE (COPPER) ................................................................66
FIGURE 37: % PENETRATION VERSUS PUNCH-DIE CLEARANCE (COPPER) ...........................................................67
FIGURE 38: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE, 0.5 FT/SEC.............................................................71
FIGURE 39: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE, 12 FT/SEC..............................................................71
FIGURE 40: % SHEAR VERSUS PUNCH-DIE CLEARANCE, 0.5 FT/SEC....................................................................73
FIGURE 41: % SHEAR VERSUS PUNCH-DIE CLEARANCE, 12 FT/SEC.....................................................................73
FIGURE 42: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE, 0.5 FT/SEC..............................................................75
FIGURE 43: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE, 12 FT/SEC...............................................................75
FIGURE 44: % PENETRATION VERSUS PUNCH-DIE CLEARANCE, 0.5 FT/SEC.........................................................76
FIGURE 45: % PENETRATION VERSUS PUNCH-DIE CLEARANCE, 12 FT/SEC..........................................................76
1
1. Introduction
Since raw-material as well as energy will become scarce and hence more
expensive in the future, the costs in the production industry will increase.
Despite the pressure of rising costs, it is very important to stay competitive in the
market. Therefore it is essential to think about economic production and high
productivity during the early part of the design phase.
Netshape or near netshape manufacturing is becoming more important to
decrease the costs of production. The final part has to be produced with fewer
manufacturing processes and natural resources. Many sheet metal parts are
produced by a blanking operation or include blanking within their
manufacturing process. In particular, when blanking with high punch velocities
it is possible to manufacture near netshape parts by improving the part edge
quality. The burr height, the rollover and the penetration depth are decreased
and the shear zone is increased when blanking at high speeds /1/. The overall
goal is to improve the blanking process in such a way that the produced part
does not need to be reworked. Therefore, the process parameters have to be
optimized carefully.
In order to determine the influence of different parameters on the part edge
quality, investigations have been made by varying process parameters, like the
punch-die clearance and the cutting speed for different materials. The punch
velocity and displacement was continously monitored by means of a velocity and
proximity sensor. Based on these two values it was possible to calculate the
required energy for blanking a part.
2
2. The blanking process
The following sections provide general information about the blanking process,
the different phases of the cutting operation and the various parameters that
influence the process. In addition, the formation of the resulting part edge will be
discussed.
2.1 Definition of Shearing and Blanking
Shearing is defined as the cutting of a workpiece between two die components. The
material is stressed between two cutting edges to the point of fracture or beyond
its ultimate strength. During this process, the material is subjected to tensile as
well as compressive stresses /2/.
Various metal forming operations are based on the shearing process. Blanking is
cutting of parts out of sheet material to a predetermined contour. The contour is
defined by the punch and the die (Figure 1). The ejected slug is the part and the
remaining skeleton is considered scrap, in contrast with punching where the
sheared slug is discarded and the rest is the part /2/.
2.2 Influences on the blanking process
The cutting process is influenced by many parameters. The primary variables
affecting the cutting process are listed below /3/, /4/:
• punch - die clearance
• punch velocity
• stock material (thickness, mechanical properties, chemical
composition, microstructure and grain size)
• cutting tools (materials, cutting edge, tool wear)
• lubrication
3
• alignment of the tools and
• strain rate.
punch motion
punchstripper
d ie
workpiece
Figure 1: Schematic illustration of the shearing process
Figure 1 shows the shearing process in principle. In industrial applications a
stripper or blankholder is used to strip or remove the material from the punches.
The simpliest type of stripper is the fixed one, in which the material is guided in
a gap between the die and stripper plate (fixed channel stripper) /5/.
2.3 Phases of the Blanking Process
The different phases of the cutting process are shown schematically in Figure 2.
Schüssler explains the process in terms of the following steps /6/, /7/:
Phase1: The punch moves downwards in the direction of the sheet with a
certain velocity. There is no contact between punch and die.
4
Phase 2: The punch reaches the stock. Due to elastic and plastic deformation,
the contact area increases until enough force is applied for material
deformation.
Phase 3: The applied forces deform the stock elastically. The amount of the
bending moment and the elastic bending of the sheet depends on the
clearance between punch and die. The bending force consists of
tension and compression.
Phase 4: As the cutting elements penetrate further into the material, the
stresses within the deformation zone reach the shearing strength of
the material. The material starts yielding into the die. In this phase,
the draw in zone/rounded edge of the final part is created.
Phase 5: The material is sheared along the cutting edge. During this phase the
material which flows into the cutting gap is strain hardened within
the deformation area. The maximum cutting force is reached in this
step.
Phase 6: As soon as the shear stress reaches the tensile strength of the
material, material rupture starts. Due to high radial and tangential
stresses rupture starts behind the cutting edge on the surface area of
the die and spreads out to the surface area of the punch. This phase is
completed when the maximum rupture force of the material is
reached.
Phase 7: The blanked part is separated from the stock. If the incipient cracks
which start at the cutting edge of punch and die are not aligned and
5
do not meet, the material is still not completely separated. In this case
the complete material separation occurs after the cutting elements
move further together.
Phase 8: The two parts are now separated and the shape of the shear area is
fully developed. Due to the elastic springback effects, the diameter of
the slug increases and the diameter of the skeleton decreases. The
result is a pressure on the surface of the cutting elements.
Phase 9: The punch moves down to the bottom dead center and ejects the
blanked part. During this phase, the surface pressure is still effective.
Phase 10: At the bottom dead center, the direction of motion is inverted. Due to
the friction between the stock and the surface of the punch, the
surface pressure is intensified. A stripper or blank holder has to strip
the blank from the punch.
6
Figure 2: Phases of the Blanking Process /6/
2.4 Stress conditions in shearing
The cutting forces do not act linearly at the cutting edge. Instead, the vertical
force FV and horizontal force FH act in a small area near the cutting edge (Figure
3). The distribution of those compressive forces is nonuniform. The distance l
between the forces causes a moment which either bends or tilts the workpiece.
This moment has to be compensated by a counterbending moment which results
in bending stresses and horizontal normal stresses on the workpiece and tool
/4/, /8/. In addition to the above forces, frictional forces also act on the tooling.
The horizontal forces result in the frictional forces µFH and µF'H and the shearing
forces in µFV and µF'V .
7
Figure 3: Shearing forces
The stress condition in the shearing zone during crack formation is triaxial.
According to Tresca, the flow criterion is given by (Figure 4) /9/:
τσ σ σ
max =−
=1 3
2 2f (1)
σ1 principal tensile stress
σ3 principal compressive stress
σf flow stress
τ max maximum shear stress
During the process, the shear yield stress increases because of the strain
hardening effect. As shown in Figure 4, the principal stress circle enlarges until
the shearing strength is reached /8/.
8
As described in chapter 2.3, the stress condition changes throughout the
deformation process. The cracks propagate in the direction of the maximum
shear stresses /9/.
tensioncompression normal stress
shear stress
shearing strength
shear yield stress
fractureshearing
σ σ3 1
τmax
Figure 4: Stress conditions in the shearing zone
2.5 Formation of the part edge
The part edge is characterized by distinct regions as shown below (Figure 5) /4/.
The smooth and shiny area created by shearing the material is called shear zone.
The rough surface of the rupture zone is caused by material fracture/3/. The
penetration depth of the cracks depends on the material and the clearance between
punch and die. If the cracks do not run towards each other, secondary shear
formation may occur /8/. Stresses between the tool and the workpiece, and
between the two sheared surfaces, generate friction that stretches the metal into a
thin, ragged protrusion called a burr (see also 2.4). The side opposite the burr
9
develops a rounded edge called rollover, as material is drawn away from the
surface /1/. The rollover is caused by plastic deformation, which is mainly
affected by material ductility, tool wear and clearance.
The quality of the part edge (ratio of the different zones) is mainly influenced by
the punch-die clearance, material properties, material thickness, cutting speed
and tool wear (see also chapter 3.2.1). The burr height, for instance, increases
with increasing clearance and increasing ductility of the material. Previous
investigations have shown that the cutting speed can have a remarkable
influence on the formation of the part edge (see also chapter 2.6) /6/. The
influences on the formation of the different zones of the part edge are
summarized in Table 1.
rollover
shear zone
rupture zone
burr
secondary shear
depth of the crackpenetration
Figure 5: Different zones of the part edge
10
zone mainly influenced by:
burr ductility of the material, clearance, tool
wear, cutting speed
rollover material properties, clearance, tool
wear,
penetration depth tool wear, material properties,
clearance
shear zone material properties, tool wear, cutting
speed, clearance
secondary shear ductility of the material, clearance,
sheet thickness
Table 1: Influences on the formation of the cutting edge /4/, /8/, /10/, /11/, /12/
2.6 High speed blanking
Blanking at high speeds could mean that the process speed/stroke rate is high or
that the punch speed is high, or both. In this study, high speed blanking is
refered to as blanking with high punch velocities. The result are high strain rates
within the material. The strain rate within a deforming material describes the
variation of strain over time. The strain rate (dimension: [ s−1]) affects the
temperature of the workpiece as well as of the tool. Huml found out that as
much as 95% of all the work performed when forming and cutting materials is
converted into heat /13/.
This means that an increase in cutting velocity results in a temperature increase
in the forming zone and the tool surface. Since the heat is generated faster than it
can dissipate into the material (depending on the heat conduction coefficient,
Table 2), the result is a very high temperature concentration in a narrow shearing
zone.
11
This effect produces three main benefits:
• High temperatures of up to 1800°F in the shearing zone create a stress release
effect within the material /14/, /15/. The higher the speed, the more the
effect of stress release is apparent. The strain hardening effect caused by the
material deformation may even be neutralized. Because of the stress release
the material can withstand more shearing until the shearing strength is
exceeded and the material fractures.
• Due to the small deformation area, the springback of the parts is negligible
compared to blanking at lower shearing speeds. This phenomena results in
less return stroke load and therefore less tool wear /16/.
• Temperature related internal stresses and fractures are dramatically reduced
/17/.
These effects result in an improvement of the part edge quality by means of
decreasing the burr height and the rollover and increasing the shear zone. In
addition, less distortion is created than in blanks produced at low speeds /18/.
The heat transfer is characterized by the heat conduction within a material. A
characteristic value for the heat conduction is the heat conduction coefficient. The
following table provides a comparison for different materials:
Material Heat conduction coefficient [W/Km]
Steel (0.2 % C) 50
Steel (0.6 % C) 46
Copper 350...370
Aluminum (99.5%) 221
Brass 80...120
Table 2: Heat conduction coefficients for different materials /19/
12
Table 2 shows that the heat conduction coefficient of copper and aluminum is
much higher than the one for steel. Therefore, the heat which is created by high
cutting velocities is also dissipating faster when cutting copper or aluminum.
This results in lower quality improvement for copper and aluminum when
blanking with high punch speeds.
Contrary to high speed blanking, the stress relief effects are negligible while
blanking at lower speeds. The edge quality of the blanked part is mainly affected
by the material properties and the tool geometry. Due to the stress profile and
the movement of the tools, the material strain hardens in the area close to the
cutting tool surface. Since this area can withstand higher stresses than the
material next to it, the rupture takes place in the direction of the maximum shear
stress within the unhardened material. The resulting stress profile created by the
force couple has two separate shear zones which grow towards each other from
opposite sides of the workpiece. In the zone between the two shear planes, the
stress builds up to the material’s tensile strength, causing it to rupture. This
results in an S-shaped edge on the blanked part /1/.
All these effects, seen at low speeds, result in blanked parts with a larger
deformation zone, more part deformation, a higher burr and a rough rupture
zone compared to high speed blanking /20/.
Ideal process conditions in high speed blanking are only achieved when the
strain hardening effect is reduced as much as possible by the stress relief effects.
This means that the optimum shearing speed has to be high enough that stress
relief occurs, but not too high in order to allow enough time for stress relieving,
which is a time dependent process.
13
3. Equipment used
In the following chapters the press, tooling, instrumentation and data acquisition
system are described. Furthermore, a technique for measuring the burr height
and the materials used for the experiments will be introduced.
3.1 The Lourdes Press 100-OH
Figure 6: The Lourdes Press 100-OH
The experiments were conducted with a Lourdes Electro Activated Die Set with
the following specifications:
14
Force (0.030” above bottom) 10 tons
Overall dimensions 10”x10”x19”
Maximum work area 4.5”x10”
Stroke length 0.5" to 1.5”
Open height 5.0”
Shut height 3.5”
Approx. weight 100 lbs
Approx. weight of untooled top plate 35 lbs
Table 3: Lourdes 100-OH press specifications (according to the manufacturer)
The Lourdes High Speed Press 100-OH uses high tool speed, rather than force or
pressure to perform work. It accelerates the tooling to speeds up to
approximately 12 ft/sec /17/.
Figure 7: Position of material strip in the press
The Lourdes Electromagnetic Press uses tractive Solenoids, comprised of a coil
structure, a ferro-magnetic flux path, and an Armature, to accelerate the motion
plate. The magnetic accelerator mounts directly to a precision die set (as shown
in Figure 8) /21/.
15
The microprocessor control precisely energizes the accelerator causing the punch
to be rapidly accelerated towards the die. The control regulates the tool speed
and disconnects the driving forces just before the tool impacts the material. The
kinetic energy or momentum of the moving tool holder is converted to work as
the tooling impacts the material. Finally, any unused energy is absorbed by the
urethane stops and the tool holder plate is returned, aided by spring force, to the
initial position /17/.
AIR IN COOLING FAN
ARMATURE SPACERS
ARMATURE
GUARD (TRANSPARENTFOR CLARITY ONLY)
COIL ENCLOSURE
AIROUT
RETURN RATEADJUSTING
RETURNSPRING
POWER CABLE
TO CONTROL
AIROUT
URETHANERETURN STOP
STROKEADJUSTING
ARMATURESUPPORT
BALL BEARINGBUSHING
GUIDEPINBALL RETAINERADJUSTORURETHANESTOP
Figure 8: Schematic picture of the Lourdes Press 100 - OH
In order to minimize wear on the die set and tooling and to get the best blanking
results, three different press adjustments have to be made:
16
1) Return Rate Adjustment
The return rate adjustment is made by two nuts, located on the top of each
Return Rate Spring Rod (Figure 8). This spring force only serves to return the
punch plate to the top of its stroke after the Die Set has been fired. It is not
intended for the use of material stripping. This return force has to be adjusted
depending on the upper die weight. In order to leave the maximum amount of
energy available for the actual application, the return rate has to be set to a
minimum.
2) Stroke Adjustment
This adjustment is located between the motion plate and top plate, and plays an
important part in developing the maximum force of the unit. The stroke length
has to be adjusted depending on the selected power level.
3) Armature Spacer Adjustment
The armature spacers (washers) located on top of the armature have to be
adjusted only if the tooling shut height differs from the press shut height. The
armature should sit flush with the top of the spring loaded T-bar that enters the
coil from the bottom side when the tooling is closed.
As soon as the adjustments described above have been made, the cutting speed
which is nesessary for each specific application has to be determined. For the
Lourdes 100 - OH press there are 9 power levels available. The cutting speed
varies between 2.5 (power level 2) and 12 ft/sec (power level 9), depending on
the blanked material.
The following graph shows the dependence of the cutting speed on the stroke
length for each power level. Shown are the settings for 0.5" (minimum stroke
length) and 1.5" (maximum stroke length). However, it is possible to set the
stroke length at any number in between these two extremes.
17
0
3
6
9
12
15
1 2 3 4 5 6 7 8 9power level
velo
city
[ft/s
ec]
stroke length 1.5"stroke length 0.5"
Figure 9: Punch velocity versus the power levels of the press depending on the stroke
length. No cutting condition.
Figure 9 shows that up to power level 4 it is more efficient to set the stroke length
to 0.5" in order to reach the maximum punch velocity, whereas for power level 5
and higher, the maximum punch velocity can only be reached by setting the
stroke length to 1.5 ".
3.2 Experimental Setup
3.2.1 Punches and Dies
Usually the punch-die clearance is defined as a relative clearance per side in
percent of the material thickness (equation (2)) /17/.
cd d
td p=−
⋅2
100%
(2) c radial clearance [%]
dd diameter of the die
dp diameter of the punch
t material thickness
18
The radial clearance is important, since it will affect the part edge quality,
distortion of the blanks, tool wear and production costs in general /18/.
According to previous investigations, increasing the punch-die clearance has the
following effects /3/, /6/, /10/, /18/, /22/:
• more rollover
• higher burr
• less shear and more rupture.
In general, small clearances (< 8%) create high strains on punch and die /6/.
Therefore, proper alignment of the punch and the die is necessary in order to
minimize tool wear. On the other hand, the part quality increases with
decreasing the punch-die clearance. To reduce strain, clearances higher than 8%
are preferred when using high strength steels /6/.
The tooling of the Lourdes Press consists of a punch, a die button and a polymer
stripper, which are shown in the following figure. All punches and die buttons
which were used for the experiments are made of M-2 high speed steel and are
mounted to the retainers by means of a ball lock (for fast punch and die change).
The polymer stripper is mounted directly to the punch and is used for stripping
the skeleton off the punch after blanking /23/.
Figure 10: Stripper, die button and punch
19
In order to obtain different punch-die clearances a whole set of round punches
and die buttons was available. The following two tables show the resulting
clearances depending on the material thickness.
punch
diameter
material thickness [in]
[in] 0.016” 0.033” 0.041" 0.054” 0.488 38 18 14.5 11 0.491 28 14 11 8.5 0.494 19 9 7 5.5 0.496 13 6. 5 3.5 0.497 9 4.5 3.5 3 0.498 6 3 2.5 2 0.499 3 1.5 1.2 1
Table 4: Clearances in percent for different punch diameters and material thicknesses
(die button diameter: 0.500")
punch
diameter
material thickness [in]
[in] 0.033” 0.041" 0.054” 0.488 / / 24 0.491 / 28 21 0.494 30 24 19 0.496 27 22 17 0.497 26 21 16 0.498 24 20 15 0.499 23 18 14
Table 5: Clearances in percent for different punch diameters and material thicknesses
(die button diameter: 0.514")
3.2.2 Stock materials
Four different stock materials were used for conducting the experiments:
• Low carbon steel (0.0033" thickness),
• high strength steel (0.054" thickness),
• copper 110, annealed temper (0.016" thickness),
20
• aluminum 2008 (0.041" thickness) and
• brass (0.031" thickness). Brass was only used for preliminary experiments.
More detailed information about these materials and their properties can be
found in the appendix A.
3.2.3 Sensors
In order to monitor the velocity profil over the stroke, the press was equipped
with a velocity transducer and a proximity sensor.
3.2.3.1 Analog Proximity Sensor
For monitoring the relative position of the punch an inductive proximity sensor
was used. The sensor (resolution: 0.0002", linearity: ± 4%, range: 1") was
mounted to the base plate and was detecting the motion plate (Figure 6). An
inductive proximity sensor consists of a coil and a ferrite core arrangement. The
oscillator creates a high frequency field, radiating from the coil in front of the
sensor, centered around the axis of the coil. The ferrite core bundles and directs
the electro-magnetic field to the front.
When a metal object (target) enters the high-frequency field, eddy currents are
induced in the surface of the target. This results in a decrease of energy in the
oscillator circuit and, consequently, a smaller amplitude of oscillation /24/. The
signal was further processed through an oscillator/demodulator, a signal
conditioner and an operational amplifier. For monitoring the relative position of
the slide, the probes were connected to a data acquisition board, which will be
described in chapter 3.2.4.
3.2.3.2 Linear velocity transducer
In addition to the proximity sensor, a linear velocity transducer was used in
order to monitor the velocity of the punch at different stages during the cutting
process. The velocity transducer consists of high coercive force permanent
21
magnet cores which induce sizable DC voltage while moving concentrically
within shielded coils.
As shown in Figure 6, the shielded coil is mounted to the stationary chassis and
the permanent magnet core via a brass rod to the motion plate of the press. The
induced output voltage of the coil is directly proportional to the magnet's relative
velocity and field strength. For reducing the noise of the signal, an aluminum foil
shielding around the probes was used to improve the results. However, for
getting the most accurate results, one should operate in a magnetically shielded
enclosure /25/. The signal was also converted from high to low impedance by
means of an operational amplifier in order to stabilize the signal and avoid
compatibility problems with the data acquisition board.
3.2.4 Data Acquisition and Signal Analysis
The data acquisition PC was equipped with a National Instruments AT-MIO-
16F-5 data acquisition board in conjunction with the National Instruments
LabVIEW software used for real time monitoring of the sensor signals up to 200
kHz. A Labview program (the program code is shown in the appendix B) was
written for our specific application.
The program has the following features:
• It displays the velocity as well as the penetration of the punch in respect to
the time. One can read the correlating punch velocity for a given punch
position.
• It has an option to save the acquired data for further investigations/analysis
of the cutting speed.
Figure 11 shows the scheme of the discussed sensor wiring.
22
linear velocitytransducer
voltagedivider
operationalamplifier
proximitysensor
oscillator/demodulator
signal conditioner
DAQ
BoardPC
Figure 11: Sensor wiring (schematically)
3.3 Techniques for evaluating the part edge
According to Lange there are basically four values, that should be considered
when evaluating the part edge of a blanked part (Figure 12) /4/:
• the burr height hb ,
• the percentage of shear with respect to the material thickness (called % shear),
• the percentage of rollover with respect to the material thickness (called
% rollover),
• the penetration depth tc .
Depending on the final use of the part, an ideal part edge has a minimum of
rollover and burr and at least 75 % shear.
3.3.1 Measuring the penetration depth, the shear zone and the rollover
The values for the penetration depth and the % rollover have been obtained from
the cross section of the slugs by using a microscope. To achieve the cross
sections, the slug first has to be sheared close to the centerline, then be mounted
in polymer, and finally be ground and polished in order to get a smooth surface.
For investigating the amount and the constancy of shear and rupture, the slug
has been microscoped from the side. Since the % shear value is not as constant as
the penetration depth or the % rollover for a certain clearance and cutting speed,
each slug has been measured at 4 different locations around the circumference.
23
Pictures for different materials showing cross sections as well as side-views can
be found in chapter 5.3.
Figure 12: Microscope picture showing the cross section of a slug /4/
3.3.2 Technique for burr height measuring
A device for fast and reliable burr height measurements had to be developed.
Requirements for such a measuring device, a short overview about existing
methods and the final design are discussed in this chapter.
3.3.2.1 Definition of the burr height
Cross sections of the cutting edge show that the burrs may be either sharp-edged
or rounded. Due to different material properties the part may remain flat or
becomes domed during the blanking process. For that reason, the burr height is
defined as the difference between the highest point of the burr and the surface of the part
immediately adjacent to the burr.
tc
hb
se
cross section
with:
s material thickness
se edge draw-in/rollover
ss shear zone
sr rupture zone
hb burr height
tc penetration depth
% rollover = sse
% shear = sss
sr
ss s
24
3.3.2.2 Requirements and design of the device
In order to measure the burr height in a precise and repeatable way, a new
device had to be developed. Since the burr is very small and soft, it is very
important that the burr is not damaged by the device while measuring. High
accuracy of the measurement tool guarantees repeatable results.
A literature review showed that there have been basically 3 different principles
of measuring the burr height:
• A device based on the principle of a caliper /26/,
• measuring the surface profile of the skeleton /22/,
• optical solutions /27/.
The advantages and disadvantages of these principles have already been
discussed in a previous report /28/. A technique based on the "caliper-method"
was chosen.
Figure 13: Burr height measurement device
1 2
1 micrometer head
2 reference tip
25
Figure 13 shows a picture of the designed burr height measurement device,
which works as follows (Figure 14):
The head has to be moved down until the slug is clamped by the reference pin.
The pin clamps the part 0.1 mm (ca. 0.0039 in) beside the burr. After that, the
user turns the micrometer dial until the tip of the micrometer touches the burr.
The height of the burr can be read from the micrometer dial. The metal tip of the
micrometer and the plane table (with the part) are electrically isolated from each
other. A device which senses conductivity is connected across the micrometer
head and the table. As soon as the tip of the micrometer touches the burr, the
instrument indicates that the electrical circuit is closed. The main advantage of
this principle is a very small load on the burr during the measurement.
Figure 14: Method of working of the burr height measuring device
0.0039 in
reference pin
tip of micrometer
head
slug
flat surface
26
Calibration of the device:
The device is calibrated by putting the reference pin directly on the plane table.
The micrometer is then lowered until it touches the table. The calibration value,
which has to be deducted from the measured burr height, is the value read on
the scale of the micrometer.
This device reduces the load on the burr to a minimum and has a resolution of
approximately 0.0003 inches. In contrast to other solutions, the device can also be
used to measure the skeleton.
27
4. Experimental Procedure
Different experiments have been conducted in order to achieve the following
objectives:
1. Investigating the performance characteristics of the press.
2. Investigations of characteristics of the punch velocity during one stroke
(velocity investigations).
3. Determining the influence of high punch velocities in conjunction with
different punch-die clearances on the part edge quality depending on
different materials (part quality investigations).
Before performing any experiments with the press, preliminary work had to be
completed:
• Four different stock materials had to be chosen (chapter 3.2.2).
• The tooling had to be selected. Chapter 3.2.1 contains more detailed
information on the punches and dies. With the chosen punches and dies it is
possible to conduct experiments in a clearance range from 4% up to 24%.
• Punch-die alignment. The retainers had to be adjusted precisely.
In order to obtain repeatable results, the following points had to be taken into
account:
• Once the retainers were adjusted well, the adjustment was kept for all the
experiments. Only the punches and die buttons had to be changed for the
experiments with different punch-die clearances.
• During each measurement both channels (displacement and velocity) were
monitored continously through the stroke with a maximum sampling rate of
10,000 samples per second. This sampling rate was necessary so as not to lose
small signal peaks.
• All the experiments were conducted with sharp punches and die buttons.
28
4.1 Investigation of press characteristics
First of all it is important to know in which velocity range the press is
performing. Therefore experiments using different power levels of the press have
been conducted under no cutting condition. The displacement as well as the
velocity of the punch have been monitored by means of a proximity sensor
(chapter 3.2.3.1) and a velocity transducer (chapter 3.2.3.2). Thus, a displacement-
time- and velocity-time-curve could be obtained (Figure 15). In addition, a
velocity-stroke-curve could be obtained by combining the information of these
two curves (Figure 16).
-9
-6
-3
0
3
6
9
12
0 0.005 0.01 0.015 0.02 0.025
time [sec]
punc
h ve
loci
ty [f
t\sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
stop blocks
BDC
Figure 15: Velocity/displacement-time curve for no cutting condition at power level 5 of
the press
These first experiments under no cutting condition gave a rough idea, which
punch velocity could theoretically be reached at the 9 power levels of the press.
In addition, several influences on the performance of the press (like the
29
adjustment of the return springs or the stroke length) have been examined. For
determining the minimum and maximum cutting velocity the different materials
were blanked. For each material five measurements were made, then the average
value was taken.
4.2 Velocity investigations
The main goal of these investigations was to obtain characteristic displacement-
time, velocity-time and the resulting velocity-stroke curves, as shown in Figure
16. The two curves in respect to the time (Figure 15) are important for roughly
calculating the cutting force (chapter 5.2) later on. They are also used to show
when blanking starts, when blanking is completed and when the polymer stop
blocks are reached.
0
2
4
6
8
10
-1 -0.5 0 0.5displacement [in]
punc
h ve
loci
ty [f
t/sec
]
stop blocks
Figure 16: Velocity-stroke curve for no cutting condition. Power level 5.
30
In order to check the accuracy of the velocity transducer a monitored
displacement-time-curve was derived at particular points and the results were
compared. The measured and calculated velocity were corresponding with a
variation of ± 5%.
4.3 Part quality investigations
The following four characteristic values for evaluating the part edge quality were
measured and are shown in 4 different graphs for each material (chapter 5.3):
• burr height (measured with the burr height measurement device, chapter
3.3.2),
• % shear (expressed as a percentage of the material thickness, measured by
using the side view, that can be seen under a microscope),
• % rollover (expressed as a percentage of the material thickness, measured at
the cross-sections of the parts which were mounted in epoxy),
• % penetration (expressed as a percentage of the material thickness, measured
at the cross-sections of the parts which were mounted in epoxy),
The values mentioned above are shown on the y-axis whereas the x-axis shows
the punch-die clearance. Separate curves are shown for different cutting
velocities.
Before conducting the experiments to get the actual graphs that are shown in
chapter 5.3, preliminary experiments (using 3 different cutting speeds, 2 different
clearances and 6 different materials) have been conducted in order to find out
which materials, cutting speeds and clearances are the most promising to
investigate further.
31
5. Experimental Results
5.1 Press characteristics and preliminary results
Figure 17 shows the highest cutting speed that could be reached for the different
stock materials. It can be seen that there is almost no influence of the clearance on
the cutting speed. The shown values represent the average of five measurements.
For each of the four materials the highest cutting speed is approximately 12
ft/sec.
10
10.5
11
11.5
12
12.5
0 5 10 15 20 25clearance[%]
cutti
ng s
peed
[ft/s
ec]
low carbon steelAl 2008110 Copperhigh strength steel
Figure 17:Maximum cutting speed for different punch-die clearances and stock materials
The following table shows the average value of the cutting speed that could be
reached at different power levels for each material. The measurements were
made with ideal stroke length.
32
materials
low carbon
steel
high strength
steel
aluminum copper
power level 2 4.5 ft/sec 6 ft/sec (PL 3) 4.5 ft/sec 3 ft/sec
power level 5 9 ft/sec 8.5 ft/sec 9 ft/sec 9 ft/sec
power level 9 12 ft/sec 12 ft/sec 12 ft/sec 12 ft/sec
Table 6: Cutting speeds at different power levels of the press for different materials
The lowest cutting speed that could be reached with this press is about 4 ft/sec.
This is already regarded as high speed. In order to obtain curves at low cutting
speeds, additional experiments were conducted manually. This means that the
tooled plate of the press was moved down by means of an extension arm,
resulting in approximately 0.5 ft/sec.
As mentioned in chapter 4, the punch-die alignment was a very important issue
before conducting the experiments. The retainers for the punch and for the die-
button had to be adjusted in order to minimize wear on these tools and to
achieve a uniform part edge around the circumference. The retainer adjustment
was then checked by punching low carbon steel (14 % clearance, punch velocity:
4 ft/sec) and measuring the burr height and percentage of shear at four different
locations around the circumference of the slug. The results of these experiments
are shown in Figure 18. The burr height varied within a range of 3/10,000 of an
inch, the percentage of shear within a range of 5 %. Both values are regarded as
acceptable.
33
0
0.0005
0.001
0.0015
0.002
0.0025
1 2 3 4
locations around the part
burr
hei
ght (
in)
0
10
20
30
40
50
% s
hear
burr height% shear
1
2
3
4
slug feeding direction
Figure 18: Uniformity of part edge (low carbon steel, 14% clearance, 4 ft/sec)
5.2 Velocity investigations
Since the velocity-time, displacement-time and velocity-stroke curves look
similar for the different materials, only the curves for low carbon steel are shown
and discussed in this chapter. The according graphs for the other stock materials
can be found in appendix C.
34
-9
-6
-3
0
3
6
9
12
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
time [sec]
punc
h ve
loci
ty [f
t\sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
stripper
start blanking
blanking completed
Figure 19: Velocity/displacement-time curve. Material: low carbon steel. Power level 3,
stroke length 0.5"
Figure 19 and Figure 20 show the velocity and displacement of the punch versus
time for low carbon steel. The following points could be observed:
• The polymer stripper touches the material.
• The punch hits the sheet and blanking starts. This also causes vibrations due
to which the curve is oscillating afterwards.
• Blanking is completed, the punch "enters" the die button.
• The polymer stop blocks are reached. They are compressed and absorb the
remaining energy.
The bottom dead center (BDC) is reached. The moving direction of the punch is
inverted which results in zero velocity.
35
-9
-6
-3
0
3
6
9
12
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
time [sec]
punc
h ve
loci
ty [f
t\sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
stripper
start blanking
blanking completed
BDC
stop blocks
Figure 20: Velocity/displacement-time curve. Material: low carbon steel. Power level 9,
stroke length 1.5"
Until blanking starts the punch velocity is constantly increasing. Compressing
the urethane stripper does not result in a velocity decrease. However , if the
maximum velocity is only 4 ft/sec, the blanking force is high enough to result in
a velocity decrease. As soon as the material fractures the velocity curve starts
oscillating due to the energy release of all structural components of the press
(also known as "snap-through" effect).
The following graphs show the punch velocity over the stroke. These curves
were used for calculating the forces that are shown in Table 7. Whereas Figure 21
shows a velocity decrease, Figure 22 shows an almost constant velocity during
blanking.
36
0
1
2
3
4
5
6
-0.05 -0.025 0 0.025 0.05displacement [in]
punc
h ve
loci
ty [f
t/sec
]
vst vm
vd
Figure 21: Punch velocity versus displacement. Material: low carbon steel. Power
level 3, stroke length 0.5"
In Figure 21 and Figure 22 the following abbreviations are used:
vSt = punch velocity when the stripper touches the material.
vm = punch touches the material (blanking starts).
vd = blanking completed.
vs = polymer stop blocks are reached.
37
0
2
4
6
8
10
12
14
-0.05 0 0.05 0.1 0.15 0.2displacement [in]
punc
h ve
loci
ty [f
t\sec
] vmvst vd
vs
Figure 22: Punch velocity versus displacement. Material: low carbon steel. Power
level 9, stroke length 1.5"
The cutting force when blanking with low strain rates and cutting velocities can
be calculated by using the shear resistance of the blanked material and the tool
geometries /4/, /29/. A table including the shear resistance of the different
materials is shown in the appendix A.
F d tc s= ⋅ ⋅ ⋅π σ ( 3 ) with:
Fc = blanking force
σs = shear resistance
d = disk diameter
t = sheet thickness
38
On the other hand, it is possible to calculate approximately the blanking force by
using the change of speed during the cutting operation. The force is calculated by
using the following equation:
F m a m dvdt
= ⋅ = ⋅ ( 4 ) with:
F = cutting force
a = acceleration
m = weight of the moving mass
If the time range of the deceleration is very short, the equation can be simplified
to:
F m vt
m v vt t
= ⋅ = ⋅−−
∆∆
1 2
1 2
( 5 )
The following table shows cutting forces calculated with the two different
equations described above for the four stock materials:
material thickness
[ in ]
cutting force, calculated
with equation (5) at
4 ft/sec
cutting force, calculated
according to equation (3)
low carbon steel 0.032 10.7 kN 334 kN/in 8.1 kN 253 kN/in
high strength steel 0.054 30.1 kN 557 kN/in 17.5 kN 324 kN/in
Al 2008 0.041 5.1 kN 124 kN/in 5.0 kN 122 kN/in
110 Copper 0.016 1.8 111 kN/in 3.2 kN 290 kN/in
Table 7: Cutting force for different materials
39
In order to compare the forces of the different material thickness' which were
blanked, the actual forces have been divided by the material thickness.
A comparison of the numbers shown in Table 7 shows that there is no reliable
accordance between the forces calculated in different ways. Therefore, it is not
advisable to calculate the force derived from a measured velocity. If one wants to
know the cutting force, the experimental setup should be equipped with a force
measurement device.
In addition to the force, the required energy for blanking was investigated. There
are two different ways to approach this problem. Since the cutting force
calculated with equation (3) and the entry depth of the punch before fracturing
begins are known, it is possible to calculate the energy as follows:
E F lc c f= ⋅ ( 6 ) with:
Fc=cutting force
lf=entry depth until fracture
The entry depth before fracturing begins is approximately the experimentally
observed length of the shear zone (see also % shear curves). On the other hand,
the cutting velocity and the energy used for blanking are related through the
following equation:
E mv=12
2 ( 7 ) with:
E = energy [J]
m = weight of the moving mass [kg]
v = velocity of the moving mass [m/s]
40
Since the two distinct points, start of blanking and blanking completed, are
known, the cutting energy can be calculated as the energy difference between
these two points:
E E E m v vc = − = −1 2 12
221
2( ) ( 8 )
The weight of the tooled plate of the press is 40 lbs. This leads to the following
calculation for the two curves shown:
Ec represents the energy used for blanking with a speed of 4 ft/sec (using
equation (8)):
E ft lb ft Jc 4 150 6 32
2sec sec.
= ≈
In contrast to this equation, it is not possible to calculate the energy needed for
blanking with 12 ft/sec. The velocity is staying almost constant during the
blanking operation, Figure 22. This means that the energy required for cutting is
very small compared to the available energy of the moving mass (compare with
Table 8).
Energy available for cutting at 4 ft/sec in [ J ]: 14
Energy available for cutting at 12 ft/sec in [ J ]: 121
Energy used for cutting (at 4 ft/sec) in [ J ]: 6
Table 8: Energy level at different cutting speeds
41
material thickness
[ in ]
entry
depth until
fracture
[in]
cutting
velocity
[ ft/sec ]
cutting
energy
according to
(6) in [ J ]
cutting
energy
according to
(8) in [ J ]
low carbon steel 0.032 0.0128 4 2.64 6.3
high strength steel 0.054 0.0162 5 7.21 19.5
Al 2008 0.041 0.0144 4 1.82 8.2
110 Copper 0.016 0.0112 2.5 0.92 1
Table 9: Cutting energy for the stock materials
Table 9 shows approximate energy numbers for cutting the four different stock
materials. It shows that the thinner and softer materials require less energy for
cutting. But also in this case it could be observed that the energy derived from
the cutting velocity does not correlate very well with the calculated numbers.
This shows that the fastest and most reliable way to get information about
blanking force and energy is by implementing a load sensor in the experimental
setup. This will be done in the next phase of the project.
5.3 Part quality investigations
It should be mentioned that the discussion of the results is divided based on the
blanked materials. The part edge quality is evaluated for the different parameter
settings and the four stock materials (see also appendix A). After drawing
conclusions regarding the process conditions for each material, the results of the
different materials are compared.
42
5.3.1 Experiments with low carbon steel
The results of the experiments conducted with low carbon steel are shown in the
next four graphs. In addition, seven cross-sections and side views are shown.
0
1
2
3
4
5
6
0 5 10 15 20 25punch-die clearance [%]
burr
hei
ght [
0.00
1 in
]
12 ft/sec9 ft/sec4 ft/sec0.5 ft/sec
Figure 23: Burr height versus punch-die clearance (low carbon steel)
Figure 23 shows the burr height versus the punch-die clearance for different
cutting speeds. The following characteristics were observed:
• The burr of the slugs blanked with 0.5 ft/sec is constantly increasing with an
increasing clearance. Table 10, pictures 1, 7 and 11 confirm that.
• For velocities above 4 ft/sec the burr height is almost constant at 0.0013" up to
14 % clearance. Increasing the clearance further results in a linear burr height
growth.
• When blanking with low cutting speeds of 0.5 ft/sec and a small clearance the
burr is about 0.0035", which is about three times as much as when blanking
with high speed.
43
This graph shows that in the commonly used clearance range between 5 and 10%
the burr height is less than half as large when blanking at cutting speeds above 4
ft/sec compared to low speed blanking.
0
20
40
60
80
100
0 5 10 15 20 25punch-die clearance [%]
% s
hear
12 ft/sec9 ft/sec4 ft/sec0.5 ft/sec
Figure 24: % shear versus punch-die clearance (low carbon steel)
Figure 24 shows the influence of the cutting speed on the formation of the shear
zone. The following characteristics were observed:
• Increasing the cutting speed above 4 ft/sec has no influence on the amount of
shear.
• The punch-die clearance has a major influence on the percentage of shear. For
all cutting speeds there is a large difference in the percentage of shear
between 4% clearance and 14 % (compare Table 10, Table 11: pictures 2 and 8
or pictures 6 and 10). Increasing the clearance further has only a minor
influence on the amount of shear.
44
• The influence of high velocities on the percentage of shear is not as distinct as
the clearance influence. However, the percentage of shear is about 5-20 %
higher for parts blanked with high speeds compared to parts blanked with
low speeds (see also Table 11, pictures 8 and 10).
Although there is an increase in the percentage of shear when using high speeds,
it is not enough to compensate the decrease of % shear which is caused by an
increasing clearance. This means that for getting a part edge of 75% shear, one
still has to go with a relatively small clearance of about 5 to 8%.
The next graph (Figure 25) shows the % rollover in respect to the punch-die
clearance for cutting speeds of 0.5 ft/sec (low speed) and 12 ft/sec (high speed).
The following characteristics can be seen:
• Up to 15% clearance, the % rollover of the slugs is constantly increasing for all
cutting velocities (see Table 10, Table 11: picture 1 and 7 and pictures 5 and 9).
For a further clearance increase the rollover stays constant.
• At a clearance of 15%, the % rollover is about 3 times as big as at 4%
clearance.
• Using high cutting speeds results in a decrease of % rollover (compare Table
10: pictures 1, 3 and 5). For 4 % clearance, the % rollover is more than twice as
big at low speeds than at high speeds.
45
0
5
10
15
20
25
30
0 5 10 15 20 25punch-die clearance [%]
% ro
llove
r
0.5 ft/sec12 ft/sec
Figure 25: % rollover versus punch-die clearance (low carbon steel)
It is obvious that the selected clearance has the main influences on the plastic
deformation of the part which results in the rollover zone. However, for a given
clearance the % rollover can be always reduced by blanking with high speeds.
Figure 26 shows the % penetration in respect to the punch-die clearance for high
and low cutting speeds:
• For both velocities the curves show a constant increase of % penetration with
an increasing punch-die clearance (Table 10, Table 11, Table 12: pictures 5, 9
and 13).
• High cutting speeds result in less penetration depth, whatever clearance is
selected (Table 10: pictures 1, 3, 5). At small clearances, around 4% to 50%
reduction could be seen.
46
Since the edge of the slugs should be as straight as possible for most applications,
it is important that the penetration depth is as small as possible. With a low
cutting speed it is not possible to produce parts with a % penetration smaller
than 8%, even if the clearance is very small. By using high cutting speeds,
however, it is possible to reach a % penetration as low as 4% (Table 10, picture 5).
0
5
10
15
20
0 5 10 15 20 25punch-die clearance [%]
% p
enet
ratio
n
0.5 ft/sec
12 ft/sec
Figure 26: % penetration versus punch-die clearance (low carbon steel)
47
Cross-section side view
Clearance: 4 %.
Characteristic part
edge for 0.5 ft/sec
punch velocity.
Clearance: 3%.
Characteristic part
edge for 4 ft/sec
and 9 ft/sec punch
velocities.
Clearance: 3 %.
Characteristic part
edge for 12 ft/sec
punch velocity
Table 10: Cross section and side view for low carbon steel. Clearance: 3/4%.
1 2
3 4
5 6
48
Cross-section side view
Clearance: 15 %
Characteristic
part edge for 0.5
ft/sec punch
velocity.
clearance: 14 %
Characteristic
part edge for 4
ft/sec, 9 ft/sec
and 12 ft/sec
punch velocites.
Table 11: Cross-section and side view for low carbon steel. Clearance: 15%.
7 8
9 10
49
Cross-section side view
Clearance: 21 %
Characteristic
part edge for 0.5
ft/sec punch
velocity
clearance: 24 %
Characteristic
part edge for 4
ft/sec, 9 ft/sec
and 12 ft/sec
punch velocities.
Table 12: Cross-section and side view for low carbon steel. Clearance: 21/24%.
11 12
13 14
50
5.3.2 Experiments with high strength steel
In this chapter the results of the experiments conducted with high strength steel
are discussed. In particular, the influences of clearance and cutting speed on the
formation of the different zones of the part edge are discussed.
0
1
2
3
4
5
6
7
0 5 10 15 20 25punch-die clearance [%]
burr
hei
ght [
0.00
1 in
]
12 ft/sec8.5 ft/sec6 ft/sec0.5 ft/sec
Figure 27: Burr height versus punch-die clearance (high strength steel)
Figure 27 shows the burr height versus the punch-die clearance for different
cutting speeds. The following characteristics were observed:
• The major changes in the burr height are taking place between 3% and 6%
clearance. The smallest burr is obtained at 6% clearance. A further increase of
the clearance has only a minor effect on the formation of the burr.
• The cutting speed has an influence on the formation of the burr. The largest
burr height decrease is seen between 0.5 and 6 ft/sec cutting speed.
Increasing the cutting speed further than 8.5 ft/sec does not give any
improvement concerning the burr.
51
The previous graph shows that at the commonly used clearance of 6% the burr
height is reduced by 70% when blanking at high speeds.
0
20
40
60
80
100
0 5 10 15 20 25punch-die clearance [%]
% s
hear
12 ft/sec8.5 ft/sec6 ft/sec0.5 ft/sec
Figure 28: % shear versus punch-die clearance (high strength steel)
The effect of the punch-die clearance and the cutting speed on the % shear is
shown in Figure 28:
• Increasing the cutting speed above 6 ft/sec has no remarkable effect on the
amount of shear (see also Table 14: pictures 8 and 10).
• There is a large influence of the punch-die clearance on the % shear when
using clearances smaller than 14%. Larger clearances have only a minor
influence on the amount of shear (compare Table 13: picture 4 and Table 14:
picture 10).
• An increase of the percentage of shear when using higher cutting speeds can
only be noted at clearances smaller than 14%.
52
0
10
20
30
40
50
0 5 10 15 20 25punch-die clearance [%]
% ro
llove
r
0.5 ft/sec12 ft/sec
Figure 29: % rollover versus punch-die clearance (high strength steel)
Figure 29 shows the % rollover in dependance of the punch-die clearance and the
cutting speed. The following characteristics were observed:
• In general, the % rollover of the slugs is linearly increasing with increasing
clearance for all cutting speeds (see also Table 13: pictures 1, 7 and 11).
• For small clearances the % rollover can be reduced by 50% when blanking
with high speeds (compare Table 13: pictures 1 and 3).
• For getting less than 10% rollover one has to blank with high speeds and
small clearances.
The next graph shows the % penetration in respect to the punch-die clearance for
cutting speeds of 0.5 ft/sec and 12 ft/sec. The following characteristics can be
seen:
53
• Independent of the cutting speed, the % penetration is constantly increasing
with an increasing punch-die clearance (compare Table 13: pictures 1, 7 and
11).
• High cutting speeds result in less penetration. To get a straight part edge with
less than 10% penetration one has to go with high speeds and small
clearances (see also Table 13: pictures 1 and 3).
0
5
10
15
20
25
30
0 5 10 15 20 25punch-die clearance [%]
% p
enet
ratio
n
0.5 ft/sec
12 ft/sec
Figure 30: % penetration versus punch-die clearance (high strength steel)
54
Cross-section side view
Clearance: 4 %.
Characteristic part
edge for 0.5 ft/sec
punch velocity.
Clearance: 3%.
Characteristic part
edge for 6 ft/sec,
8.5 ft/sec and 12
ft/sec punch
velocities.
Clearance: 3 %
Characteristic part
edge for 12 ft/sec
punch velocity.
Ca. 100% shear!
Table 13: Cross-section and side view for high strength steel. Clearance: 3%.
1 2
3 4
5 6
55
Cross-section side view
Clearance: 14 %
Characteristic
part edge for 0.5
ft/sec and 6
ft/sec punch
velocity.
clearance: 14 %
Characteristic
part edge for 8.5
ft/sec and 12
ft/sec punch
velocities.
Table 14: Cross-section and side view for high strength steel. Clearance: 14%.
7 8
9 10
56
Cross-section side view
Clearance: 20 %
Characteristic
part edge for 0.5
ft/sec punch
velocity
clearance: 24 %
Characteristic
part edge for 6
ft/sec, 8.5 ft/sec
and 12 ft/sec
punch velocities.
Table 15: Cross-section and side view for high strength steel. Clearance: 20/24%.
11 12
13 14
57
5.3.3 Experiments with aluminum
The results of the experiments conducted with aluminum are shown in the next
four graphs. As in the previous chapters, cross-sections and side views are
discussed as well.
Figure 31 shows the burr height versus the punch-die clearance for different
cutting velocities. The following characteristics were observed:
• The major influence on the formation of the burr is the punch-die clearance.
High speed has no effect on the formation of the burr.
• For all cutting speeds there is an optimum clearance of about 7%, where the
burr is smallest.
• For clearances smaller or bigger than 7% the burr is constantly increasing
regardless of the cutting speed.
0
0.5
1
1.5
0 5 10 15 20 25punch-die-clearance [%]
burr
hei
ght [
0.00
1 in
]
12 ft/sec9 ft/sec4 ft/sec0.5 ft/sec
Figure 31: Burr height versus punch-die clearance (aluminum)
58
The next figure shows the percentage of shear versus the punch-die clearance.
The following characteristics can be seen:
0
20
40
60
80
100
0 5 10 15 20 25punch-die clearance [%]
% s
hear
12 ft/sec9 ft/sec4 ft/sec0.5 ft/sec
Figure 32: % shear versus punch-die clearance (aluminum)
• The bigger the punch-die clearance gets, the less % shear occurs. The
percentage of shear doubles by decreasing the clearance from 21% to 5% (see
also Table 16, Table 17, Table 18: pictures 2, 6 and 10).
• The cutting speed has almost no influence on the percentage of shear
(compare Table 10: pictures 2 and 4).
59
0
10
20
30
40
0 5 10 15 20 25punch-die clearance [%]
% ro
llove
r
0.5 ft/sec12 ft/sec
Figure 33: % rollover versus punch-die clearance (aluminum)
Figure 33 shows the % rollover versus the punch-die clearance for high and low
speed. It could be observed that:
• Up to clearances of 15% neither the clearance nor the cutting speed is
influencing the % rollover (see also Table 16, Table 17: pictures 1, 3, 5 and 7).
• Only at clearances around 20% can the plastic deformation be decreased by
choosing high cutting speed.
The next figure shows the influence of the punch-die clearance and the cutting
speed on the % penetration. The following characteristics can be seen:
• Since the material fractures earlier when blanking with large clearances also
more penetration is observed.
• Like with the rollover, high cutting speeds show the biggest improvement in
combination with large clearances (Table 18: pictures 9 and 11).
60
0
5
10
15
20
0 5 10 15 20 25punch-die clearance [%]
% p
enet
ratio
n
0.5 ft/sec
12 ft/sec
Figure 34: % penetration versus punch-die clearance (aluminum)
61
Cross-section side view
Clearance: 5 %.
Characteristic part
edge for 0.5 ft/sec
punch velocity.
Clearance: 5%.
Characteristic part
edge for 4 ft/sec,
9 ft/sec and 12
ft/sec punch
velocities.
Table 16: Cross-section and side view for aluminum. Clearance: 5%.
1 2
3 4
62
Cross-section side view
Clearance: 15 %
Characteristic
part edge for 0.5
ft/sec punch
velocity.
clearance: 15 %
Characteristic
part edge for 4
ft/sec, 9 ft/sec
and 12 ft/sec
punch velocities.
Table 17: Cross-section and side view for aluminum. Clearance: 15%.
7 6
7 8
5
63
Cross-section side view
Clearance: 21 %
Characteristic
part edge for 0.5
ft/sec punch
velocity
clearance: 21 %
Characteristic
part edge for 4
ft/sec, 9 ft/sec
and 12 ft/sec
punch velocities.
Table 18: Cross-section and side view for aluminum. Clearance: 21%.
9 10
11 12
64
5.3.4 Experiments with copper
The softest material used in the experiments was copper. It also has the highest
heat conduction coefficient in comparison to the other investigated materials.
The results of the experiments conducted with copper are shown in the following
graphs, side views and cross-sections.
There is no graph showing the burr height versus the punch-die clearance,
because the burrs of the blanked parts were too small to be measured with the
measurement device described in chapter 3.3.2.2. Since the resolution of that
device is 0.0003", it can be noted that the burr of the copper slugs (no matter
which clearance or cutting velocity) was always less than 0.0003".
In comparison to the other materials, the copper slugs were domed. This made
the mounting in epoxy as well as all other measurements more complicated, and
is also the reason for the inclined shear zone (see part cross sections).
Figure 35 shows the percentage of shear versus the punch-die clearance. The
following characteristics were observed:
• Increasing the punch-die clearance results in a decrease of % shear. Between 6
and 19 % clearance this decrease is as much as 25% (see also Table 19 and
Table 21: pictures 4 and 12).
• Increasing the cutting speed up to 12 ft/sec results in an increase of %shear of
approximately 15% (Table 19: pictures 2 and 4).
65
0
20
40
60
80
100
0 5 10 15 20 25punch-die clearance [%]
% s
hear
12 ft/sec9 ft/sec4 ft/sec0.5 ft/sec
Figure 35: % shear versus punch-die clearance (copper)
The influence of the cutting speed and the punch-die clearance on the % rollover
is shown in the next figure. In particular, it can be observed that
• the clearance has the main effect on the % rollover. Like with most other
materials, the larger the clearance the larger the % rollover (compare Table 19
and Table 21: pictures 3 and 11).
• Also, with this material it was observed that the high cutting speeds only
show improvement when combined with large clearances.
66
0
10
20
30
0 5 10 15 20 25punch-die clearance [%]
% ro
llove
r
0.5 ft/sec12 ft/sec
Figure 36: % rollover versus punch-die clearance (copper)
Figure 37 shows the % penetration versus the punch-die clearance depending on
the cutting speed. The following characteristics can be observed:
• There is no obvious influence of high cutting speeds on the formation of the
% penetration.
• The penetration of the material fracture is only influenced by the clearance up
to 13%.
67
0
5
10
15
20
0 5 10 15 20 25punch-die clearance [%]
% p
enet
ratio
n
0.5 ft/sec
12 ft/sec
Figure 37: % penetration versus punch-die clearance (copper)
68
Cross-section side view
Clearance: 6 %.
Characteristic part
edge for 0.5 ft/sec
punch velocity.
Clearance: 6%.
Characteristic part
edge for 4 ft/sec,
9 ft/sec and 12
ft/sec punch
velocities.
Table 19: Cross-section and side view for copper. Clearance: 6%.
1 2
3 4
69
Cross-section side view
Clearance: 13 %
Characteristic
part edge for 0.5
ft/sec punch
velocity.
clearance: 13 %
Characteristic
part edge for 12
ft/sec punch
velocity.
Table 20: Cross-section and side view for copper. Clearance: 13%.
7 6
7 8
5
70
Cross-section side view
Clearance: 19 %
Characteristic
part edge for 0.5
ft/sec punch
velocity
clearance: 19 %
Characteristic
part edge for 4
ft/sec, 9 ft/sec
and 12 ft/sec
punch velocities.
Table 21: Cross-section and side view for copper. Clearance: 19%.
9 10
11 12
71
5.3.5 Material comparison
After discussing the influence of different parameters on the formation of the
part edge for every material, the different materials will be compared between
each other in this chapter.
0
1
2
3
4
0 5 10 15 20 25punch-die clearance [%]
burr
hei
ght [
0.00
1 in
]
low carbon steelhigh strength steelAl 2008Copper 110
Figure 38: Burr height versus punch-die clearance, 0.5 ft/sec
0
1
2
3
4
0 5 10 15 20 25punch-die clearance [%]
burr
hei
ght [
0.00
1 in
]
low carbon steelhigh strength steelAl 2008Copper 110
Figure 39: Burr height versus punch-die clearance, 12 ft/sec
72
Figure 38 and Figure 39 show the burr height versus the punch-die clearance for
low and high cutting speeds. The following conclusions could be drawn:
• At low cutting speeds, low carbon steel shows by far the highest burr.
• For copper, aluminum, and high strength steel the punch-die clearance has
the main influence on the formation of the burr. Only for low carbon steel is
the burr height decreased when blanking with high speeds.
• The two softest materials used in the experiments, copper and aluminum,
show the smallest burr, regardless of speed.
Another important value for the quality of the blanked part is the percentage of
shear, Figure 40 and Figure 41. The following characteristics were observed:
• For all materials the major influence on the percentage of shear is the
clearance.
• The only material which shows a shear zone increase due to high cutting
velocities is copper. When blanking with small clearances and high speeds a
shear zone of up to 95% could be reached.
• Regardless of speed, low carbon steel can be made to fracture after 80% of
shearing only when blanking with small clearances.
73
0
20
40
60
80
100
0 5 10 15 20 25punch-die clearance [%]
% s
hear
low carbon steelhigh strength steelAl 2008Copper 110
Figure 40: % shear versus punch-die clearance, 0.5 ft/sec
0
20
40
60
80
100
0 5 10 15 20 25punch-die clearance [%]
% s
hear
low carbon steelhigh strength steelAl 2008Copper 110
Figure 41: % shear versus punch-die clearance, 12 ft/sec
74
The next two figures show the percentage of rollover versus the punch-die
clearance for low and high cutting speeds, Figure 42 and Figure 43. The
following characteristics can be seen:
• In the commonly used clearance range of between 5 and 15%, there is an
obvious decrease of the % rollover by increasing the cutting velocity only for
high strength steel. The other materials show no velocity influence.
• When blanking low carbon steel with small clearances (< 5%) and high
cutting velocities, the plastic deformation could be decreased.
• The influence of high cutting speeds is more distinct when blanking with
more than 15% clearance.
Figure 44 and Figure 45 show the percentage of penetration versus the punch-die
clearance. It could be observed that:
• High strength steel shows the highest percentage of penetration.
• As expected, for all materials the % penetration is increasing with an
increasing punch-die clearance. That means that the clearance has a major
influence on the % penetration.
• For aluminum and high strength steel, increasing the cutting speed results in
a decrease of % penetration for all clearances.
• For low carbon steel and copper, a higher cutting speed results in less %
penetration for clearances smaller than 6%.
75
0
10
20
30
40
50
0 5 10 15 20 25punch-die clearance [%]
% ro
llove
r
low carbon steelhigh strength steelAl 2008Copper 110
Figure 42: % rollover versus punch-die clearance, 0.5 ft/sec
0
10
20
30
40
50
0 5 10 15 20 25punch-die clearance [%]
% ro
llove
r
low carbon steelhigh strength steelAl 2008Copper 110
Figure 43: % rollover versus punch-die clearance, 12 ft/sec
76
0
5
10
15
20
25
0 5 10 15 20 25punch-die clearance [%]
% p
enet
ratio
n
low carbon steelhigh strength steelAl 2008Copper 110
Figure 44: % penetration versus punch-die clearance, 0.5 ft/sec
0
5
10
15
20
25
0 5 10 15 20 25punch-die clearance [%]
% p
enet
ratio
n
low carbon steelhigh strength steelAl 2008Copper 110
Figure 45: % penetration versus punch-die clearance, 12 ft/sec
77
Overall, it can be noted that a positive influence of high cutting speeds on the
quality of the part edge is more obvious (especially concerning the burr height
and the percentage of rollover) for low carbon and high strength steels. Benefits
for aluminum and copper could be only observed for a few parameter
combinations. This agrees with the theory described in chapter 2.6: most of the
positive effects of high cutting speeds are temperature related. Copper and
aluminum have much higher heat conduction coefficients than steel. That means
that the heat produced by the increased cutting velocity is dissipating relatively
quick compared to steel. Thus, the benefits of high cutting speeds are not as large
for copper and aluminum as for steel. However, the punch-die clearance has the
major influence on the formation of the part edge, independent of which material
is blanked and which cutting speed is used.
78
6. FEM simulations with DEFORM 2D
6.1 The Finite Element Code DEFORM 2D
For simulating the blanking process a modified version of the FEM-code
DEFORM 2D (Version 4.1.5) was used. DEFORM includes three parts:
• The pre-processor,
• the simulation engine and
• the post-processor.
These parts will be described in the following /30/.
6.1.1 Pre-processor
The pre-processor consists of:
• An input module for introducing the model geometry and the process
conditions,
• an automatic mesh generation program, which creates a mesh taking various
process parameters into consideration (die and workpiece geometry, strain,
strain-rate and temperature),
• an interpolation module for interpolating the deformation history of the old
distorted mesh into the newly generated mesh.
These three tasks, called automatic remeshing, make it possible to perform a
continous simulation without any intervention by the user, even if several
remeshing steps are required. The automatic remeshing capability reduces the
total calculation time of the FE analysis. All the data generated in the pre-
processor is saved in a data base.
6.1.2 Simulation engine
This program provides different choices in the analysis mode:
79
• Isothermal, non-isothermal or heat transfer,
• rigid, plastic, elastic, elasto-plastic, porous object type.
As mentioned before, the simulation results are stored in a binary format and are
accessed by the post-processor.
6.1.3 Post-processor
The post-processor displays the simulation results in graphical or
alphanumerical form. The graphic presentation includes the mesh, contour plots
(line or continous tone in colors) of strain, strain rate, temperature, velocity
vectors and load-stroke curves. Two other important capabilities are 'point
tracking' (provides deformation histories of selected points in the workpiece
throughout the deformation) and 'flownet' (allows the user to observe the
deformation of the circles or rectangles defined on the underdeformed
workpiece).
6.2 Simulations of the high speed blanking process
6.2.1 Simulation settings
Four simulations have been performed for low carbon steel. The geometry of the
tooling was designed and meshed on CAEDS and then imported into DEFORM
2D via an universal file. The simulations were performed using standard units.
The input parameters of the different components of the simulation are shown in
the following tables:
80
Component
Sheet Punch
Object type rigid-plastic rigid
Number of elements ~4500 ~200
Material properties AISI 1015 none
Temperature at beginning 68 F 68 F
Thermal conductivity [Btu/sec/in/F] 5.082 E-4 3.74 E-4
Heat capacity [Btu/in3/F] 0.03106 0.03106
Emissivity 0.25 0.45
Table 22: Input data of the low carbon steel sheet and the punch
Component
Die button Blank Holder
Object type rigid rigid
Number of elements ~230 ~40
Material properties none none
Temperature at beginning 68 F 68 F
Thermal conductivity [Btu/sec/in/F] 3.74 E-4 3.74 E-4
Heat capacity [Btu/in3/F] 0.03106 0.03106
Emissivity 0.45 0.45
Table 23: Input data of the die button and the blank holder
The simulations were performed with the following settings:
• Material: low carbon steel. Properties from AISI 1015 were taken, because
they are very close to the properties of the low carbon steel that was actually
used. The sheet thickness was 0.033".
81
• Clearances: 5% and 18% (punch diameters: 0.488 in and 0.497 in; die button:
0.500 in),
• punch velocities: 0.5 and 12 ft/sec,
• non isothermal status,
• axisymmetric problem: though only one half of the geometry was simulated.
The inter object relationship was defined with a constant friction coefficient of
µ=0.05 for all contacts between tool and sheet. This value is typically used for
cold forming. The interface heat transfer coefficient was 0.3397 E-2
Btu/sec/in2/F.
Furthermore, the following restrictions apply:
• One has to be careful when interpreting the results of the simulations: high
speed blanking deals with high strains and high strain rates (ε ≈ 500% and
& /ε ≈ 10000 s ). Since there are only data for low strains (~70%) and low strain
rates (90/s) available, DEFORM extrapolates the values for higher strains and
strain rates. This is a very critical point concerning the simulation results,
because the accuracy of the simulation is influenced by the material flow-
stress curve.
• The non isothermal simulation mode was used. In this mode the program
was not able (softwarewise) to fracture the material. Therefore conclusions
can only be drawn about the plastic deformation at the beginning of blanking
resulting in rollover. This shows that we are still in the beginning stage of
simulating the high speed blanking process.
6.2.2 Simulation results
The pictures shown in the appendix D show results of the simulations for
different stages of the process. Each picture contains of three figures:
82
• A load versus time curve,
• the workpiece, the die button and the punch,
• a magnification of the area where the actual shearing takes place.
Simulations with a punch-die clearance of 18%:
Figure D-1 through D-5 show the simulated shearing process for low speed (0.5
ft/sec). Two intermediate steps are shown in addition to step1 (starting position,
figure D-1) and step 102 (shearing completed, figure D-5). Like in the
experiments, only the slug (left part of the sheared sheet in the pictures) was of
interest. As mentioned earlier, in this stage of the simulation software, only the
formation of the rollover could be observed. For 18% clearance and low as well
as high speed it is around 20 %. That means the simulations do not show an
influence of the high speeds on the formation of the rollover.
Simulations with a punch-die clearance of 5%:
The same characteristics as with 18% clearance were also seen with a punch-die
clearance of 5%: Increasing the cutting speed does not result in a decrease of the
rollover.
According to the simulations, the punch-die clearance is the main influence on
the formation of the rollover.
6.2.3 Comparison of the experimental and simulated results
Experimental and simulated results exist only for low carbon steel at clearances
of 5 and 18% and for low and high cutting speeds. Table 24 shows the percentage
of rollover for the different cases.
83
5% clearance 18% clearance
low speed high speed low speed high speed
experiment 12% 7% 26% 22%
simulation 12% 13% 21% 21%
Table 24: Comparison of the experimental and simulated results (low carbon steel)
Although the simulation results do not show the influence of high cutting speed,
the experimental and simulation results show a good correlation. Only for small
clearance and high speeds do the results not correlate.
It will be important in the future to be able to simulate the fracture as well and
accrue information about the burr height , % shear and penetration depth. These
values are necessary to predict the quality of the part edge by means of
simulations.
A more in depth comparison between simulations and experiments will be
possible as soon as the FEM program is able to handle high strains and strain
rates for the fracture simulation. This study will provide a large database for
fine-tuning the FEM simulation.
84
7. Summary and conclusions
This report discusses the influences of several parameters on the part edge
quality of blanked parts. Different experiments as well as blanking simulations
have been conducted in order to investigate the characteristics of the velocity-
stroke curve and to determine the influence of high punch velocities in
conjunction with different punch-die clearances on the part edge quality. Four
different materials: low carbon steel, high strength steel, aluminum and copper
have been blanked with punch-die clearances between 4% and 24%. All the
experiments were conducted with a Lourdes impact press which reaches cutting
speeds of up to12 ft/sec.
The displacement-time, velocity-time, and velocity-stroke curves were monitored
by means of a velocity and proximity sensor. This setup provided the velocity
when blanking starts and the velocity decrease during blanking. It was shown
that a velocity decrease due to blanking is only measurable for low cutting
speeds of around 4 ft/sec. The kinetic energy at high speeds is much higher than
the energy required for blanking. That results in an almost constant punch speed
while blanking. Blanking force and energy were determined based on the
velocity changes measured at low cutting speeds. The numbers matched only in
part with the numbers calculated from the shear strength of the different
materials. If one wants to know the exact cutting force the experimental setup
should be equipped with a force measurement device.
When evaluating the part edge quality of the blanked parts, all the different
zones were taken into consideration (burr height, shear, rupture/penetration
depth, and rollover). The results of the part quality investigations show that the
positive influence of high cutting speeds on the quality of the part edge is more
obvious for low carbon and high strength steels. Benefits for aluminum and
85
copper could only be observed for a few parameter combinations. This agrees
with the theory that most of the positive effects of high cutting speeds are
temperature related. Since copper and aluminum have much higher heat
conduction coefficients than steel, the heat generated by the increased cutting
velocity dissipates relatively fast. The temperatures in the shear band are not as
high as when blanking steel materials. Therefore, the benefits of high cutting
speeds are not as large for copper and aluminum as for steel. However, the
punch-die clearance has the major influence on the formation of the part edge,
regardless which material is blanked or which cutting speed is used.
Blanking simulations were performed for low carbon steel at different cutting
velocities and punch-die clearances. At the current stage of the development of
the simulation program, only the plastic deformation and shearing of the
material could be simulated. The experimental and simulation results show a
good correlation concerning the investigated zones of the part edge. However,
simulating small clearances when blanking with high velocities will need
additional investigations. For predicting the part quality by means of simulations
it will be necessary to simulate the whole process. Thus, a more in depth
comparison between simulations and experiments will be possible as soon as the
FEM program is able to handle high strains and strain rates for the fracture
simulation.
In the future it will be of particular interest to obtain more knowledge about the
effects of the high cutting velocities on tool wear. In conjunction, it will be
interesting to investigate the influence of lubricants and the running mode of the
press (either single or continous operation) on the tool wear. Also, it would be
desirable to obtain additional information about the high speed effects on the
part edge. Therefore, investigations concerning the microstructure of the part
edge as well as temperature measurements during blanking could be helpful. As
86
shown in previous investigations, further part edge quality improvements can be
achieved by increasing the cutting speed to 40 ft/sec /1/.
87
8. List of references
/1/ Svahn, O. Superfast blanking prevents defects
Pressworking
Industry Quaterly, Volume 8, No.4, 1993
/2/ Smith, D.A. Die Design Handbook,
Society of Manufacturing Engineers,
Dearborn, Michigan, 1990
/3/ Lascoe, O.D., Handbook of Fabrication Processes
ASM International, 1988
/4/ Lange, K. Blanking and Piercing
Handbook of Metal Forming
The McGraw-Hill Book Company, 1985
/5/ Smith, D. A. Fundamentals of Pressworking
Society of Manufacturing Engineers
Dearborn, Michigan, 1994
/6/ Schüssler, M. Hochgeschwindigkeitsscherschneiden im
geschlossenem Schnitt zur Verbesserung der
Teilequalität
Dissertation, University of Darmstadt, 1990
/7/ Breitling, J. Investigations of different loading conditions
Wallace, D. in a high speed mechanical press
88
Journal of Materials Processing Technology,
Volume PRO 059/1-2, pp. 18-23
/8/ Dannenmann Umdruck zur Vorlesung Schneiden
University of Stuttgart, Germany, 1992
/9/ Lange, K. Umformtechnik
Band3: Blechbearbeitung
Berlin, Heidelberg, New York, 1990
/10/ Neumann, C.-P. Die Schneidbarkeit von Elektroblech und ihre
Prüfung unter besonderer Berücksichtigung von
Blechwerkstoff und Schneidspalt
Dissertation, University of Hannover, Germany, 1979
/11/ Schenk, H. Schneidspaltoptimierung fuer Elektrobleche
Prölss, E. Fertigung 4/77, Germany, 1977
/12/ Kühne, H.-J. Der Schneidvorgang selbst und die Stempel-
geometrie als Ursache für
Maßungenauigkeiten und Spannungen beim
Scherschneiden von Elektroblechen
Magazin Trennen Rohre Profile, 1991
/13/ Huml, P. Der Einfluß der hohen Geschwindigkeit auf das
Schneiden von Metallen
Institute of metal forming, Stockholm
Annals of the CIRP, Volume 23/1/1974
89
/14/ Davies, R. Developments in High Speed Metalforming
Austin, E.R. The Machine Publishing Company Ltd
BN1 4 NH, Brighton, Sussex, 1970
/15/ Turkovich, B.F. On a class of Thermo-Mechanical Process during
rapid Plastic Deformation (with special reference to
metal cutting)
Annals of the CIRP, Volume 21/1/1972
/16/ Tobias, S.A. Hochgeschwindigkeitsumformen
Das Petro-Forge-Umformsysytem
Fertigung 1/1971
/17/ N.N. Brute Force vs. High Tool Speed
Bulletin, Lourdes Systems, Inc.
/18/ Jana, S. Effect of punch clearance in the High-Speed Blanking
Ong, N.S. of thick metals using an accelerator designed for a
mechanical press
Journal of Mechanical Working Technology
/19/ Beitz, W. Dubbel, Taschenbuch für den Maschinenbau
Küttner, K.-H. 17. Auflage, Springer Verlag, Heidelberg, 1990
/20/ Hippenstiel, H.-R. Elektrodynamische Hochgeschwindigkeitspresse
Röttger, R. Werkstatt und Betrieb, S. 683-687, Nr. 110, Germany
1977
90
/21/ N.N. Operating Instructions for Lourdes Electro Activated
die sets overhead units
Lourdes Systems Inc.
/22/ Cammann, J. H. Untersuchungen zur Verschleißminderung an
Scherschneidwerkzeugen der Blechbearbeitung
durch Einsatz geeigneter Werkstoffe und
Beschichtungen, Dissertation
University of Darmstadt, Germany, 1986
/23/ N.N. Pivot Basic Series, catalog 1000,
Pivot Punch Corporation, New York 1993
/24/ N.N. Sensors catalog
Turck Inc., 1992
/25/ N.N. Linear velocity transducer, Series 100
Transtek Inc., Bulletin S012-0028
/26/ Borchert, P. Einflüsse der Werkzeuggeometrie und der Maschine
beim Schneiden von kaltgewalztem Elektroblech
Dissertation, University of Hannover, Germany, 1976
/27/ Seidenberg, H. Presseneinwirkungen auf Werkzeugverschleiß und
Grathöhe beim Schneiden von Feinblech
im geschlossenen Schnitt
Dissertation, University Hannover, Germany, 1965
91
/28/ Pfeiffer, B. Investigations of the performance of in-die sensors for
high speed blanking
ERC Report, Columbus, 1996
/29/ N.N. Aida Press Handbook
Third Edition, Aida Engineering, Ltd., 1992
/30/ Wolff, Christian Metal flow simulations for flashless-forging of a cross
grooved inner-race
ERC Report NSM-B-95-25, Columbus, 1995
1
APPENDIX A Material low carbon
steel
high strength
steel
Al 2011 110 Copper
Thickness 0.033” 0.054” 0.041" 0.016"
Grade/heat treat. EDDQ 50-XF T3 annealed temper
Chemical composition (wt %)
C 0.003 0.07 Mn 0.1 0.39 P 0.006 0.006 S 0.007 0.004 Si 0.01 0.063 Cu 0.01 0.02 5.5 99.90 Ni 0.02 0.01 Cr 0.02 0.02 Mo 0.01 0.01 V 0.002 0.004 Sn 0.003 0.005 Al 0.049 0.058 Ti 0.056 Cb 0.001 0.01 N 0.004 0.005 B 0 0.0001
Ca 0.0004 0.004 Bi 0 0 0.4 Pb 0 0 0.4 O 0 0 0 0.04 Sb 0.0052 0.001
Tensile Properties
Yield [ksi] 20.55 n/a 43 10-53
UTS [ksi] 40.98 n/a 55 32-66
Shear resistance σs
[kgf/mm2]
~25 ~32 ~12 ~20
Table A-25: Material properties of the stock materials used
A - 1
1
APPENDIX B
Figure B - 1: Labview program code
B - 1
1
APPENDIX C
-9
-6
-3
0
3
6
9
12
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014
time [sec]
punc
h ve
loci
ty [f
t\sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
stripper
start blanking blanking
completed
Figure C - 1: Velocity/displacement-time curve. Material: high strength steel. Power
level 3, stroke length 0.5"
0
1
2
3
4
5
6
-0.05 -0.025 0 0.025 0.05 0.075displacement [in]
punc
h ve
loci
ty [f
t/sec
]
vmvst
vd
Figure C - 2: Punch velocity versus displacement. Material: high strength steel. Power
level 3, stroke length 0.5"
C - 1
2
-9
-6
-3
0
3
6
9
12
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014
time [sec]
punc
h ve
loci
ty [f
t/sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
start blanking
blanking completed
stop blocks
BDC
Figure C - 3: Velocity/displacement-time curve. Material: high strength steel. Power
level 9, stroke length 1.5"
0
2
4
6
8
10
12
14
-0.05 0 0.05 0.1 0.15 0.2displacement [in]
punc
h ve
loci
ty [f
t/sec
]
vst vmvd vs
Figure C - 4: Punch velocity versus displacement. Material: high strength steel. Power
level 9, stroke length 1.5"
C - 2
3
-9
-6
-3
0
3
6
9
12
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
time [sec]
punc
h ve
loci
ty [f
t/sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
stripper
start blankingblanking completed
Figure C - 5: Velocity/displacement-time curve. Material: aluminum 2008. Power
level 2, stroke length 0.5"
0
1
2
3
4
-0.05 -0.025 0 0.025 0.05 0.075displacement [in]
punc
h ve
loci
ty [f
t/sec
]
vst vm
vd
Figure C - 6: Punch velocity versus displacement. Material: aluminum 2008. Power
level 2, stroke length 0.5"
C - 3
4
-9
-6
-3
0
3
6
9
12
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
time [sec]
punc
h ve
loci
ty [f
t/sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
stripper
start blanking
blanking completed
stop blocks
BDC
Figure C - 7: Velocity/displacement-time curve. Material: aluminum 2008. Power
level 9, stroke length 1.5"
0
2
4
6
8
10
12
-0.05 0 0.05 0.1 0.15 0.2displacement [in]
punc
h ve
loci
ty [f
t/sec
]
vst vdvm
vs
Figure C - 8: Punch velocity versus displacement. Material: aluminum 2008. Power
level 9, stroke length 1.5"
C - 4
5
-9
-6
-3
0
3
6
9
12
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035
time [sec]
punc
h ve
loci
ty [f
t/sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
blanking completed
stripper
start blanking
BDC
Figure C - 9: Velocity/displacement-time curve. Material: copper 110. Power level 2,
stroke length 0.5"
0
1
2
3
4
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15displacement [in]
punc
h ve
loci
ty [f
t/sec
]
vst vmvd
Figure C - 10: Punch velocity versus displacement. Material: copper 110. Power level 2,
stroke length 0.5"
C - 5
6
-9
-6
-3
0
3
6
9
12
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035
time [sec]
punc
h ve
loci
ty [f
t/sec
]
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
disp
lace
men
t [in
]
velocitydisplacement
start blanking
blanking completed
BDC
stop blocks
Figure C - 11: Velocity/displacement-time curve. Material: copper 110. Power level 9,
stroke length 1.5"
0
3
6
9
12
-0.5 0 0.5 1 1.5displacement [in]
punc
h ve
loci
ty [f
t/sec
]
vmvst vd vs
Figure C - 12: Punch velocity versus displacement. Material: copper 110. Power level 9,
stroke length 1.5"
C - 6
1
APPENDIX D
Figure D - 1: Simulation model, Step 1 of 102, 18% clearance, 0.5 ft/sec
D - 1
2
Figure D - 2: Simulation model, Step 40 of 102, 18% clearance, 0.5 ft/sec
D - 2
3
Figure D - 3: Simulation model, Step 80 of 102, 18% clearance, 0.5 ft/sec
D - 3
4
Figure D - 4: Simulation model, Step 102 of 102, 18% clearance, 0.5 ft/sec
D - 4
5
Figure D - 5: Simulation model, Step 105 of 105, 5% clearance, 0.5 ft/sec
D - 5
6
Figure D - 6: Simulation model, Step 110 of 183, 5% clearance, 12 ft/sec
D - 6
1
Figure D - 7: Simulation model, Step 56 of 56, 18% clearance, 12 ft/sec
D - 7