Lecturer:-Zeradam Y.
March, 2016
Ambo University Institute of Technology
Department of Mechanical Engineering
Manufacturing Engineering II (MEng 3182)
Outline
• Sheet metal forming operation• Applications of sheet metal• Cutting operation
ShearingBlankingPunching
• Engineering analysis of sheet metal cutting
Part ISheet metal forming
Sheet metal working includes cutting and forming operations performed on relatively thin sheets of metal.
Shearing Blanking Punching
Bending Drawing Spinning Stretching
Cutting Forming
Con..
Typical sheet metal thicknesses are below 6mm (1/4in). When the thickness exceeds about 6mm,the stock is
usually referred to as plate rather than sheet. The sheet or plate stock used in sheet metalworking is
produced by rolling.
Applications
Construction equipments
Automobile and truck bodies
Ship building furniture Railway Cars
Airplane
Con..
space industries;
Chemical industry;Drink & food industry.
Nuclear
Con..
Shower cabinets and others Washing machines
Cookers Refrigerator bodies
Domestic use
Five basic sheet metal operations
Cutting Shearing Blanking Punching
Con..
Forming Bending, Spinning, Stretching, Drawing
Cutting operation
• Cutting of sheet metal is accomplished by a shearing action between two sharp cutting edges.
• The upper cutting edge (the punch) sweeps down past a stationary lower cutting edge(the die).
Con..
c
(1) Just before the punch contact the work
Con..
• c
(2) Punch begins to push into the work, causing plastic deformation
Con..
• c(3) Punch compresses and penetrates into the work and causing a smooth cut surface
Con..
• c(4) Fracture is initiated at the opposing cutting edges that separate the sheet.
Symbols V and F represents motion and applied force , respectively, t = stock thickness , c = clearance.
Shearing
• Is a sheet metal cutting operation along a straight line between two cutting edges.
• Shearing is typically used to cut large sheets into smaller sections for subsequent press working operations.
Con..
• It is performed on a machine called a power shear, or squaring shear.
• The upper blade of the power shears is often inclined (to reduce
cutting force).
• The shearing operation constitutes the first stage of any forming
process by producing either
The starting material (cutting out of a sheet) OR
Preparing an existing work piece e.g by punching a hole or a series
of holes before forming.
Blanking
• Involves cutting of sheet metal along a closed outline in a single step to separate the piece from the surrounding stock, as shown in figure a the part that is cut out is the desired product in the operation and is called the blank.
Punching
• Is similar to blanking except that the separated piece is scrap, called the slug. the remaining stock is desired part. the distinction is illustrated in figure b.
Engineering analysis of sheetmetal cutting
• One of the important parameters in sheet metal cutting is clearance between the punch and die
• The clearance c in a shearing operation is the distance between the punch and die.
• Typical clearance in conventional press working range between 4% and 8% of the sheet metal thickness t.
Con..
• Measured perpendicular to the direction of the blade movement.
• It affects the finish of the cut(burr) and machine’s power consumption.
Con..
• If the clearance is too small, then the fracture line tend to pass each other ,causing a double burnishing and large cutting force.
con..
• If the clearance is too large ,the metal becomes pinched between the cutting edges and an excessive burr results.
Con..
• The correct clearance depends on sheetmetal type and thickness.
• The recommended clearance can be calculated by the following formula:
C = Act
Where:C=Clearance , mm;
Ac = Clearance allowance (percentage of the material thickness)
t=Stock thickness, mm.
Con..
• The clearance allowance is determined according to the type of metal 4.5%, 6% or 7.5% of the material thickness.
• For convenience, metals are classified into three groups with an associated allowance value for each group.
Clearance allowance for three sheet metal groups
Con..
• The calculated clearance values can be applied to conventional blanking and hole-punching operations to determine the proper punch and die sizes.
• The die opening must always be larger than the punch size(obviously).
• Because of the geometry of the sheared edge, the outer dimension of the part cut out of sheet will be larger than the hole size.
Con..
• Punch and die sizes for a round blank of diameter Db are determined as;
Blanking punch diameter =Db-2c
Blanking die diameter =Db
• Punch and die sizes for a round hole of diameter Dh are determined as:
Hole punch diameter = Dh
Hole die diameter = Dh+2c
Con..
Angular Clearance Purpose: allows slug
or blank to drop through die
Typical values: 0.25 to 1.5 on each side
Cutting force
• Estimates of cutting force are important because this force determines the size (tonnage) of the press needed. Cutting force F in sheetmetal working can be determined by
Where S= Shear strength of the metal, Mpa t = Sthock thickness, mm
L= length of the cut edge,mm
• In blanking ,punching ,slotting ,and similar operations , L is the perimeter length of the blank or hole being cut.
•
Con..
Example
A round disk of 150mm diameter is to be blanked from a
strip of 3.2mm cold rolled steel whose shear
strength=310MPa. Determine (a) the appropriate punch
and die diameters, and (b) blanking force.
Lesson II
Outline
Other sheet-metal cutting operations
Sheet Metal Bending Operation
Con..
Other sheet-metal cutting operations
Cutoff
Is a shearing operation in which blanks are separated from a sheet-metal strip by cutting the opposite sides of the part in sequence.
Con..
With each cut, anew part is produced.
Distinguish it from a conventional shearing operation.
• The cut edges are not necessarily straight
Parting
Involves cutting a sheet-metal strip by a punch with two cutting edges that match the opposite sides of the bank.
Con..
This might be required because the part outline has an irregular shape.
Parting is less efficient than cutoff in the sense that it results in some wasted materials.
Slotting
Is the term sometimes used for a punching operation that cuts out an elongated or rectangular hole.
Perforating
Involves the simultaneous punching of a pattern of holes in sheet metal.
The hole pattern is usually for decorative purposes, or allow passage of light, gas, of fluid.
Notching and Seminotching
• Notching involves cutting out a portion of metal From the side of the sheet or strip.
• Seminotching removes a portion of metal from the interior of the sheet.
Con..
Seminotching might seem the same as punching and slotting operation.
The difference is that the metal removed by seminotching creats part of the blank outline, while punching and slotting creates holes in the blank.
Shaving
Shaving is a shearing operation performed with very small clearance to obtain accurate dimensions and cut edges that are smooth and straight, as pictured in figure (a).
Shaving is typically performed as a secondary or finishing Operation on apart that have been previously cut.
Fine blanking
Is a shearing operation used to blank sheet metal parts with close tolerances and smooth, straight edges in one step, as illustrated in figure (b).
Con..
Pressure pad applies holding force(Fh) in order to prevent distortion.
The punch then descends with slower than the normal velocity and smaller clearance to provide the desired dimension and cut edge.
The process in usually for small stock thicknesses. v
Part II
Bending operation Bending in sheet metal work is defined as the straining of the
metal around a straight (bend) axis. (figure a). During bending the metal on the inside of the neutral plane is
compressed ,while the metal on the outside of the neutral plane is stretched (figure b).
Con..
• The metal is plastically deformed so the bend takes a permanent set upon removal of stresses that caused it.
• Bending produces little or no change in the thickness of the sheet metal.
V-Bending and Edge bending
Bending operations are performed by using punch and die tooling .
The common bending methods are 1. V-Bending ,performed with a V-die.
Con..
2. Edge bending , performed with wiping die.
Con..
Edge bending is limited to bend of 90° or less. More complicated
wiping dies can be designed for bend angles greater than 90°.
Because of the pressure pad, wiping dies are more complicated
than V-dies and generally used for high production work.
Engineering analysis of bending
The metal of thickness t is bend through an angle called the bend angle α.
This results in a sheet metal part with an included angle α’, where α + α’= 180°
Con.. R=Inside Bend radius
W= The bend is made over the width of the work piece W.
Kba= k factor ( the location of the neutral axis in the
material.
• Since neutral axis is the theoretical location at which the
material is neither compressed nor stretched .we use the
following recommended values of Kba.
Con..
Recommended value of Kba
R<2t,Kba=0.33R ≥2t,Kba=0.50
t=stock thickness
Bend allowance
The bend allowance (Ab) is the length of the neutral axis between the bend lines, or the arc length of the bend.
Where: α = bend angle
Spring back When the bending pressure removed at the end of deformation ,elastic
energy remains in the bent part ,causing it to recover particularly to its original shape. This elastic recovery is called springback.
Figure: (1) during the operation ,the work is forced to take the radius Rt and included angle α’t determined by the bending tool (punch in v-bending) (2) after the punch is removed ,the work springs back to radius R and included angle α’. Symbol: F=applied bending force.
Con..
• Spring back in bending show itself as a decrease in bend angle and an increase in bend radius.
Sb= spring backα’= Included angle of the sheet metal (after spring
back)α’t =Included angle of the bending tool
Compensation for springback
Can accomplished by several methods one of these methods is over bending.
Due to this elastic recovery, it is necessary to over-bend the sheet a precise amount to achieve the desired bend radius and bend angle . This is called over bending.
Bending force
Bending Force: The force required to perform bending depends on
the Geometry of the punch and die and the strength, thickness, and
length of Sheetmtal. The maximum bending force can be
estimated by means of the following equation.
Con..
Where F = Bending forceTS = Tensile strength of the sheet metalW = Width of part in the direction of the bend axis.t= stock thicknessD=die opening dimension
Kbf= Is a constant for differences encountered in actual bending process. Its value depends on type of bending: For V-bending, Kbf=1.33; and for edge bending, Kbf=0.33
Con..
• Die opening dimension a) V-die b) Wiping Die
Con..
Example:A sheet metal blank is to be bent as shown in figure below. The metal has a modulus of elasticity=205(103) MPa, yield strength=275MPa, and tensile strength=450MPa. Determine (a) the starting blank size and (b) the bending force if a V-die is used with a die opening dimension = 25mm.
v
Other bending operations
Flanging
Con..
Con..
Miscellaneous bending operations
• Figure: miscellaneous bending operations: (a) Channel bending (b) U-bending(c) Air bending (d) Offset bending (e) Corrugating symbol : F=applied force.
Part III
Drawing
Drawing is a sheet metal forming operation used to make cup shaped ,box-shaped ,or other complex curved, hollow shaped parts.
It is performed by placing a piece of sheet metal over a die cavity and pushing the metal into the opening with a punch.
The blank must be held down flat against the die by blank holder.
Con..
( a ) Drawing of cup shaped part :(1) start of operation before punch contacts work, and (2) near end of stroke
Con..
(b) Workpart: (1) starting blank, and (2) drawn part .
Symbols: C=clearance, Db=blank diameter, Dp= punch diameter, Rp=punch corner radius, F=drawing force,Fh=holding force.
Parts produced
Ammunition shellsBeverage
Sinksand
Other parts 12
Mechanics of drawing
A blank diameter Db is drawn into a die by means of a punch of diameter Dp.
The punch and die must have a corner radii, given by Rp and Rd .
Con..
If the punch and die were to have sharp corners (Rp and Rd=0) a hole punching process would-be accomplished rather than drawing operation.
The sides of the punch and die are separated by a clearance c.
The clearance in drawing is about 10% grater than the stock thickness.
C= 0.1t
Con..
The punch applies a downward force F to accomplish the deformation of the metal, and a downward holding force Fh is applied by the blankholder, as shown in the sketch.
Stages in deformation of the work
1) Punch makes initial contact with work
2) bending
Con..
3) straightening 4) Friction and compression
5) Final cup shape
Con..
Engineering analysis of drawing
Checking for feasibility of the operationFirst Drawing ration: The ratio of blank diameter Db to
punch diameter Dp.
Un upper limit on the drawing ration is a value of 2.0 (DR≤2.0)
Con..
Second Reduction r
The value of reduction r should be less than 0.50
Con..
Third Thickness to diameter ration
It is desirable the ratio to be greater than 1%.
Con..
Example 1
A drawing operation is used to form a cylindrical cup with inside diameter=75mm and height = 50mm. The starting blank size=138mm and the stock thickness=2.4mm.based on these data, is the operation feasible?
Drawing force
F = Drawing force, Nt = Original blank thicknessTS= Tensile strengthDb and Dp = Starting blank and punch diameter.
Constant 0.7 is correction factor to account for friction.
Forces: The drawing force required to perform a given operation can be estimated by the formula:
Con..
Where Fh=holding force in drawing ,NY=yield strength of the sheet metalt =starting stock thickness,mmRd=die corner radius, mm
The holding force (Fh) required can be calculated by using the following formula.
Con..
Example 2
For the drawing operation of Example 1 determine(a) drawing force and (b) holding force, given that the tensile strength of the sheet metal(low-carbon steel)=300MPa and yield strength=175MPa.The die corner radius=6mm.
Part IVStretch Forming
Con..• Stretch forming: is a sheet metal deformation process in which
the sheet is internationally stretched and simultaneously bent in order to achieve shape change.
• Figure below shows stretch forming for relatively simple gradual bend.
Con..
The workpart is gripped by one or more jaws on each end and
then stretched and bent over a die containing the desired form.
The metal is stressed in tension to a level above its yield point.
When the tension loading is released, the metal has been
plastically deformed.
The combination of stretching and bending results in relatively
little springback in the part
Con..
Where; F=Stretching Force, N L=length of the sheet in the direction perpendicular to stretching(mm) t =Instantaneous stock thickness, mm Yf=flow stress of the metal, Mpa
Con..
Yf=flow stress, Mpa; K=Strength coefficient , n =strain hardening exponent ε = True strain
Con..Example
• A 508 mm long sheet-metal workpiece is stretched in a stretch forming operation to the dimensions shown in Figure next. The thickness of the beginning stock is 4.76mm and the width is 216mm. The metal has a flow curve defined by a strength coefficient of 517 N/mm2 and a strain hardening exponent of 0.20. The yield strength of the material is 207 N/mm2. (a) Find the stretching force F required near the beginning of the operation when yielding first occurs. Determine (b) true strain experienced by the metal, (c) stretching force F, and (d) die force Fdie at the very end when the part is formed as indicated in Figure next.
Use ε = 0.002 as start of yielding.
Part VSheet metal spinning
Con..
Spinning is a metal forming process in which an axially symmetric part is gradually shaped over a mandrel or form by means of a rounded roller.
• The tool or roller applies a very localized pressure(almost a point contact) to deform the work by axial and radial motions over the surface of the part.
Con..
• Basically, Cups, Cones, Hemispheres, Tubes, and cylinders are produced.
• 3 types 1) Conventional spinning2) Shear spinning 3) Tube spinning
Con..
Conventional Spinning:
A sheet metal disk is held against the end of a rotating Mandrel of the desired
inside shape of the final part, while the tool or roller deforms the metal against the
mandrel.
In some cases, the starting workpart is other than a flat Disk.
The process requires a series of steps, as indicated in the fig, to complete the
shaping Of the part.
•The tool position is controlled either by human operator, by an automatic
method such as numerical control.
Con..
These alternatives are manual spinning and power
spinning.
Power spinning has the capacity to apply higher forces
to the operation, resulting In fast cycle times and
greater work size capacity.
It also achieves better process control Than manual
spinning.
Con..
Figure: conventional spinning: (1) setup at start of process ;(2) during spinning; and (3) completion of process
Conventional spinning bends the metal around a moving circular axis
to conform to the outside surface of the axisymmetric mandrel.
Con.. The stock thickness of the metal therefore remains unchanged(more or
less) relative to the starting disk thickness.
The diameter of the disk must therefore be somewhat larger than the
diameter of the resulting part.
The required starting diameter can be figured by assuming constant
volume, before and after spinning.
Con..
Also known as flow turning ,shear forming, spin forging.
Figure: Shear spinning (1) setup and (2) completion of process
Shear spinning: in shear spinning, the part is formed over the mandrel by a shear deformation process in which the outside diameter remains constant and the wall thickness is therefore reduced, as in figure next.
Con..• For the simple conical shape in our figure, the resulting
thickness of the spun wall can readily be determined by the sine law relationship.
• Where tf=the final thickness of the wall after spinning=the starting thickness of the disk, and α=the mandrel angle. Thinning is sometimes quantified by the spinning reduction r.
Con..
Figure :Tube spinning (a) external;(b) internal; and (c) profiling
Tube spinning: is used to reduce the wall thickness and increase the length of a tube by means of a roller applied to the work over a cylindrical mandrel, as in figure next. Tube spinning is similar to shear spinning except that the starting workpiece is a tube rather than a flat disk. The operation can be performed by applying the roller against the work externally(using a cylindrical mandrel on the inside of the tube ) or internally (using a die to surround the tube).it is also possible to form a profile in the walls of cylinder ,as in figure next.
The End
