Metal Cutting

Material removal processes • Characteristics- Closer dimensional accuracy.- Most of the heat treated parts may need

additional finishing operations such as grinding.

- Machining with a diamond cutting tool e.g. makes copper mirrors with very high reflectivity.

- Economical for job & batch production.- Size & shape do not add constraints.

Machine Tools• Specialization: GPM-Lathe, Shaper etc.

SPM-Gear cutting m/c.

• Surface Produced: Cylindrical & Flat

• Motion: Reciprocatory type m/c tools.

Rotary type of m/c tools.

• Automation: Manual Control, Semi-Automatic,

Automatic.

• Duty Cycle: Light, Medium, Heavy

• Energy Used: Conventional, Non-conventional

Surface roughness by machining

Machined Surface

Cutting processes

• Why do we study cutting physics?Product quality: surface, toleranceProductivity: MRR , Tool wear

• Physics of cuttingMechanicsForce, power

• Tool materials• Design for manufacturing

Cutting Tools

• Cutting tools may be classified according to the number of major cutting edges (points) involved as follows:

• Single point: e.g., turning tools, shaping, planning and slotting tools and boring tools

• Double (two) point: e.g., drills• Multipoint (more than two): e.g., milling

cutters, broaching tools, hobs,• gear shaping cutters etc.

Cutting Tools

Rake and clearance angles

Rake angles

Clearance angles

Mechanism of Metal CuttingMachining is a semi-finishing or finishing process essentially done to impart required or stipulated dimensional and form accuracy and surface finish to enable the product to

• Fulfill its basic functional requirements

• Provide better or improved performance

• Render long service life.

Mechanism of Metal CuttingMachining is a process or gradual removalor excess material from the preformedblanks in the form of chips.The form of the chips directly or indirectlyindicates :• Nature and behavior of the work material

under machining condition.• Specific Energy Requirement.• Nature & degree of interaction at the chip tool interface

Mechanism of Metal Cutting

Chip Formation in Ductile Material• During continuous machining the uncut layer of the work

material just ahead of the cutting tool (edge) is subjected to almost all sided compression as indicated in Fig.

• The force exerted by the tool on the chip arises out of the

normal force, N and frictional force, F as indicated in Fig.

Chip Formation in Ductile Material• Due to such compression, shear stress develops, within

that compressed region, in different magnitude, in different directions and rapidly increases in magnitude.

• Wherever the value of the shear stress reaches or exceeds the shear strength of that work material in the deformation region, yielding or slip takes place resulting shear deformation in that region.

• But the forces causing the shear stresses in the region of the chip quickly diminishes and finally disappears while that region moves along the tool rake surface towards and then goes beyond the point of chip-tool engagement.

• This phenomenon repeats rapidly resulting in formation and removal of chips in thin layer by layer.

Chip Formation in Ductile Material

• Metal Cutting phenomenon has been explained in a simple way by Piispannen using a card analogy as shown in Fig.

Chip Formation in Ductile Material• The pattern and extent of total deformation of the chips due

to the primary and the secondary shear deformations of the chips ahead and along the tool face, as indicated depends upon

• work material• tool; material and geometry

• the machining speed (VC) and feed (so)• cutting fluid application

Chip Formation in Brittle MaterialThe basic two mechanisms involved in chip formation are• Yielding – generally for ductile materials• Brittle fracture – generally for brittle materials• During machining, first a small crack develops at the tool tip

as shown in Fig.• Due to wedging action of the cutting edge. At the sharp

crack-tip stress concentration takes place.

Chip Formation in Brittle Material• In case of brittle materials the initiated crack quickly

propagates, under stressing action, and total separation takes place from the parent workpiece through the minimum resistance path as indicated in Fig.

• Machining of brittle material produces discontinuous chips and mostly of irregular size and shape. The process of

forming such chips is schematically shown in Fig.

Orthogonal & Oblique Cutting• While turning ductile material by a sharp tool, the

continuous chip would flow over the tool’s rake surface and in the direction apparently perpendicular to the principal cutting edge, i.e., along orthogonal plane which is normal to the cutting plane containing the principal cutting edge, as shown in fig

Orthogonal & Oblique Cutting• Practically, the chip may not flow along the

orthogonal plane for several factors like presence of inclination angle, λ, etc.

Orthogonal & Oblique Cutting• Effects of oblique cutting• Chip does not flow along the orthogonal plane; Positive λ causes, chip flow deviation away from the finished surface,

which may result lesser further damage to the finished surface but more inconvenience to the operator reduction of mechanical strength of the tool tip increase in temperature at the tool tip more vibration in turning slender rods due to increase in PY (transverse

force) Negative λ may enhance tool life by increasing mechanical strength

and reducing temperature at the tool tip but may impair the finished surface. The chip cross-section may change from rectangle (ideal) to skewed

trapezium The ductile metals( materials) will produce more compact helical chips if

not broken by chip breaker Analysis of cutting forces, chip-tool friction etc. becomes more

complex.

Types of Chips

• Continuous chips– Ductile material

• Ex: Mild steel, Al

– High speed– Low feed– Small depth of cut

• Discontinuous chips

– Brittle material• Ex: Cast iron

– Low speed– High feed– Large depth of cut

Types of Chips• Built Up Edge (BUE)

– Some of the cut material will attach to the cutting point.

– This tends to cause the cut to be deeper than the tip of the cutting tool and degrades surface finish.

– Also, periodically the built up edge will break off and remove some of the cutting tool. Thus, tool life is reduced.

Types of Chips• Built Up Edge (BUE)

– built up edge can be reduced by:

• Increasing cutting speed

• Decreasing feed rate • Increasing rake angle • Reducing friction (by

applying cutting fluid)

Cutting Forces

Fc

Ft

Fc – Cutting force, Ft – Thrust force

Externally applied forces

F

N

F – Friction force, N– Normal force

P

Cutting Forces

Fco

Fn

Ft

Fs

Fn

R

(λ –α)

α

αλ Fc

Merchant’s theory

λ – friction angle

t2

t1

Cutting Forces• As we know tool has two

elements ( rake angle) and clearance or relief angle.

Lets define • t1= chip thickness prior to chip

formation (depth of cut).• t2= increased chip thickness

along shear plane.• r = cutting ratio (which is

important when calculating cutting conditions).

= shear plane angle in orthogonal cutting model.

t1

t2

Cutting Forces• r = t1/t2

Chip thickness after cutalways greater than before,so chip ratio r always lessthan 1.0Determining Shear PlaneAngleBased on the geometricparameters of the orthogonalmodel, the shear plane angleφ can be determined as:

tanφ =r cosα/1-r sinα

t1

t2

Cutting Forces• 1/r = chip compression ratio (Measure of how thick the chip has

become compared to the depth of cut)• t1 is defined as depth of cut, but in turning operation it corresponds

to feed.• Practically we measure chip thickness by using micrometer or dial

caliper. From those values we calculate r, since we already know the rake angle we can easily calculate , and also shear strain ().

• We know = G. (shear stress) whereG = modulus of rigidity = shear strain (angle in radians)Remember=P/A (axial stress)P=axial force, A=area= E., E=modulus of elasticity=L/L , L=original length, L=deformation

Cutting Forces• Force relationship• F=Friction force• N=Normal force =coefficient of friction =F/N

R=Resultant forceFs=Shear forceFn=Normal to shear force =Shear stress= Fs /As

As =Area of the shear planeAs = to . w/Sin

=Average normal stress=Fn/As

R’=Resultant forceFor equilibrium R=R’ and angR= -ang R’

• These forces can not be measured, but they can be calculated

Cutting ForcesForce and energy dissipated (continue...) Measured forces

• Fc= Cutting force (measured by dynometers or force transducers, or by calculating power consumption that occurs during cutting)Fc supplies the energy required for the cutting.

• Ft= Thrust force

Ft important to know to keep the work part and the machine stiff.

• F= Fc . Sin +Ft . Cos

• N= Fc .Cos - Ft . Sin

• Fs = Fc . Cos - Ft . Sin

• Fn = Fc . Sin + Ft . Cos

Cutting ForcesMerchant EquationFrom = Fs /As = (Fc . Cos - Ft . Sin )/ ( to . w/Sin)

Work material will select a shear plane angle that minimizes the theenergy. Therefore, take the derivative of w.r.t , equate it to 0, theresult is Merchant’s euation.= 45+(/2)-(λ/2)Results:• As increases, increases.• As λ decreases, increases.

• If increases, then the area decreases so Fs, therefore machiningbecomes easier to perform.

Tool Wear

• Definition:

The change of shape of the tool from its original shape, during cutting, resulting from the gradual loss of tool material .

• Objectives: Study the general characteristics of tool wear. Understand the causes of tool wear and their

consequences. Set up the tool failure criteria and understand

the meaning of tool-life.

Tool Wear• Introduction: Cutting tools are subjected to an extremely severe

rubbing process. They are in metal-to-metal contact between the

chip and work piece, under conditions of very high stress at high temperature.

The situation is further aggravated (worsened) due to the existence of extreme stress and temperature gradients near the surface of the tool.

However, wear occurs during the cutting action, and it will ultimately result in the failure of the cutting tool.

Tool Wear• Tool wear phenomenon: The high contact stress between

the tool rake-face and the chip causes severe friction at the rake face, as well, there is friction between the flank and the machined surface.

The result is a variety of wear patterns and scars which can be observed at the rake face and the flank face Crater wear ,Flank wear, Notch wear, Chipping ,Ultimate wear

Tool Wear• Rake Face Wear

Crater wear:  The chip flows across the rake

face, resulting in severe friction between the chip and rake face, and leaves a scar on the rake face which usually parallels to the major cutting edge.

The crater wear can increase the working rake angle and reduce the cutting force, but it will also weaken the strength of the cutting edge.

The crater depth KT is the most commonly used parameter in evaluating the rake face wear.

Tool Wear

Effects of cutting speed V and cutting time T on crater wear depth KT

Tool Wear• Flank Wear (Clearance Surface) Wear on the flank (relief) face is

called Flank wear and results in the formation of a wear land.

Wear land formation is not always uniform along the major and minor cutting edges of the tool.

Results from abrasive wear of the cutting edge against the machined surface.

Monitored in production by examining the tool or by tracking the change in size of the tool or machined part.

Measured by using the average and maximum wear land size VB and VB max.

Tool WearTypical stages of tool wear in normalcutting1. Initial (or Preliminary) wear

region: Caused by micro-cracking, surface

oxidation and carbon loss layer. Small contact area and high

contact pressure will result in high wear rate.

The initial wear size is VB=0.05-0.1mm normally.

2. Steady wear region The micro-roughness is improved,

in this region the wear size is proportional to the cutting time.

The wear rate is relatively constant.

Tool WearTypical stages of tool wear in normalcutting (Continued….)3. Severe (or Ultimate or catastrophic)

wear: When the wear size increases to a

critical value, the surface roughness of the machined surface decreases, cutting force and temperature increase rapidly, and the wear rate increases.

Flank wear and chipping will increase the friction, so that the total cutting force will increase.

The component surface roughness will be increased, especially when chipping occurs. 

Flank wear will also affect the component dimensional accuracy.

Tool Wear• Notch Wear: Special type of combined

flank and rake face wear which occurs adjacent to the point where the major cutting edge intersects the work surface.

The gashing (or grooving, gouging) at the outer edge of the wear land is an indication of a hard or abrasive skin on the work material

Tool Wear• Chipping:

Involves removal of relatively large discrete particles of tool material.

• Ultimate Failure: The final result of tool wear is the complete removal

of the cutting point - ultimate failure of the tool. This may come about by temperature rise. An alternative mechanism of ultimate failure is the

mechanical failure (usually a brittle fracture) of a relatively large portion of the cutting tip.

Ultimate failure by melting and plastic flow is most common in carbon and high-speed-steel tools.

Fracture failures are most common in sintered carbide or ceramic tools.

Tool Wear• Causes of Tool Wear• Hard Particle Wear( Abrasive Wear) Caused by the impurities within the work piece material, such as

carbon, nitride and oxide compounds, as well as the built-up fragments.

This is a mechanical wear, and it is the main cause of the tool wear at low cutting speeds.

• Adhesive Wear Due to the high pressure and temperature, welding occurs between

the fresh surface of the chip and rake face (chip rubbing on the rake face results in a chemically clean surface).

• Diffusion Wear Diffusion results in changes of the tool and work piece chemical

composition.• Chemical Wear Corrosive wear (due to chemical attack of a surface) • Fracture Wear Fracture can be the catastrophic end of the cutting edge. Chipping of brittle surfaces

Tool Wear• EFFECTS OF THE TOOL WEAR ON

TECHNOLOGICAL PERFORMANCE MEASURES.

Increase the cutting force; Increase the surface roughness; Decrease the dimensional accuracy; Increase the temperature; Vibration; Lower the production efficiency, component

quality; Increase the cost.

Tool Life• Tool life is the time a tool can be reliably be used for

cutting before it must be discarded/repaired .• Some tools, such as lathe bits are regularly reground

after use .• Assessment of tool life• For R & D purposes, tool life is always assessed or

expressed by span of machining time in minutes, whereas, in industries besides machining time in minutes some other means are also used to assess tool life, depending upon the situation, such as

no. of pieces of work machined total volume of material removed total length of cut.

Tool Life• Measurement of tool wear• The various methods are :

i) By loss of tool material in volume or weight, in one life time – this method is generally applicable for critical tools like grinding wheels.ii) By grooving and indentation method – in this approximate method wear depth is measured indirectly by the difference in length of the groove or the indentation outside and inside the worn area.iii) Using optical microscope fitted with micrometer – very common and effective method.iv) Using scanning electron microscope (SEM) – used generally, for detailed study; both qualitative and quantitative.v) Talysurf, specially for shallow crater wear.

Tool Life• Taylor’s tool life equation

– tool life of any tool for any work material is governed mainly by cutting velocity, (VC), feed, (so) and depth of cut (t).

– Cutting velocity affects maximum and depth of cut minimum.

– The usual pattern of growth of cutting tool wear (mainly VB=0.3mm), principle of assessing tool life and its dependence on cutting velocity are schematically shown in Fig .

Tool Life

Tool Life

• If the tool lives, T1, T2, T3, T4 etc are plotted against the corresponding cutting velocities, V1, V2, V3, V4 etc as shown in Fig., a smooth curve like a rectangular hyperbola is found to appear.

Tool Life• When F. W. Taylor plotted the same

figure taking both V and T in log-scale, a more distinct linear relationship appeared as schematically shown in Fig.

• With the slope n and intercept c, Taylor derived the simple equation as

VTn = C

where, n is called, Taylor’s tool life exponent.

The values of both ‘n’ and ‘c’ depend mainly upon the tool-work materials and the cutting environment (cutting fluid application).

The value of C depends also on the limiting value of VB undertaken ( i.e., 0.3 mm, 0.4 mm, 0.6 mm etc.)

Tool Life• Example of use of Taylor’s tool life equation

Problem :• If in turning of a steel rod by a given cutting tool (material

and geometry) at a given machining condition (so and t) under a given environment (cutting fluid application), the tool life decreases from 80 min to 20 min. due to increase in cutting velocity, VC from 60 m/min to 120 m/min., then at what cutting velocity the life of that tool under the same condition and environment will be 40 min.?

• Solution :Assuming Taylor’s tool life equation, VT n = CV1T1

n = V2T2 n= V3T3 n= ................. = CHere, V1 = 60 m/min; T1 = 80 min.,V2 = 120 m/min; T2 = 20 min.V3 = ? (to be determined); T3 = 40 min.Taking, V1T1

n = V2T2 n

Tool Life

Use of Chip Breaker• Need and purpose of chip-breaking

The sharp edged hot continuous chip that comes out at very high speed

becomes dangerous to the operator and the other people working in the vicinity

may impair the finished surface by entangling with the rotating job

creates difficulties in chip disposal.Therefore it is essentially needed to break such continuous chips into small regular pieces for

safety of the working people prevention of damage of the product easy collection and disposal of chips.

Chip breaking is done for improving machinability by reducing the chip-tool contact area, cutting forces and crater wear of the cutting tool..

Use of Chip Breaker• Principles of chip-breaking(a) Self breaking of chips: depends upon

work & tool material, tool signature,vc &so

Use of Chip Breaker• Forced Chip Breaking: Built in Chip Breaker

Use of Chip Breaker• Forced Chip Breaking: Clamped type Chip

Breaker

Machinability

• ‘Machinability’ has been introduced for gradation of work materials w.r.t. machining characteristics.

• “Machinability” can be described in several ways such as:

It is generally applied to the machining properties of work material.

It refers to material (work) response to machining. It is the ability of the work material to be

machined. It indicates how easily and fast a material can be

machined.

Machinability• Attempts were made to measure or quantify

machinability and it was done mostly in terms of

tool life which substantially influences productivity and economy in machining

magnitude of cutting forces which affects power consumption and dimensional accuracy

surface finish which plays role on performance and service life of the product.

Machinability

• Machinability rating (MR):

The relative machining response of the work materials compared to that of a standard metal was tried to be evaluated quantitatively only based on tool life (VB* = 0.33 mm)

Machinability

Machinability

• Variables and factors involved in machining such as,(a) properties of the work material.(b) cutting tool; material and geometry.(c) levels of the process parameters.(d) machining environments (cutting fluid application etc)Machinability characteristics of any work – tool pair mayalso be further affected by, strength, rigidity and stability of the machine. kind of machining operations done in a given machine

tool. functional aspects of the special techniques, if employed.

Cutting Tool MaterialsNeed for development of cutting tool materials• to meet the growing demands for high

productivity, quality and economy of machining• to enable effective and efficient machining of the

exotic materials that are coming up with the rapid and vast progress of science and technology

• for precision and ultra-precision machining• for micro and even nanomachining

demanded by the day and future.

Cutting Tool Materials• Relative contribution of the cutting tool

materials on productivity, can be roughly assessed from Fig.

Cutting Tool MaterialsHistory of cutting tool materials

• Cutting tool used during the industrial revolution in 1800 A.D

• First cutting tool was cast using crucible method (1740) and slight hardened by H.T.

• 1868: R. Mushet found by adding Tungsten we can increase hardness and tool life ( Air Quenching)

Cutting Tool MaterialsHistory of cutting tool materials• F.W.Taylor in Pennsylvania did the most basic

research in metal cutting between 1880-1905– Invented high speed steel (better H.T. process)– Better alloy

• Tungsten Carbide was first synthesized in 1890.• Took 3 decades before we got Cemented

carbide• First used in Germany • Sintering technology was invented

Cutting Tool Materials

• Selection of cutting tool materials is very important

• What properties should cutting tools have– Hardness at elevated temperatures– Toughness so that impact forces on the tool

can be taken– Wear resistance– Chemical stability

Cutting Tool MaterialsTool Materials • 1.Carbon and medium-alloy steels (1880): Oldest tool

material, rarely used today. Poor hot hardness, need to be used in low cutting speed.

• 2. High speed steels (1900): Have good wear resistance, Tough (high resistance to failure), Low hot hardness (cutting speed cannot be high). Two basic types; T-type: Tungsten(tungsten and as alloying elements,

chromium, vanadium, cobalt), M-type: Molybdenum(tungsten and molybdenum,

chromium, vanadium, cobalt)

Cutting Tool Materials• 3. Cast cobalt alloys: (1915), Good wear resistance because of its hardness at high

temperatures Not as tough as HSS, not used in interrupted cutting

operations Capable heavy roughing cuts at speeds higher then HSS

• 4. Carbides(1930): (Cemented or sintered carbides); High hot hardness, allows high cutting speeds High strength -Tungsten Carbide (WC) (cutting steel, cast iron, non

ferrous materials) (replaced HSS cutting tools) Titanium Carbide (TiC) (higher wear resistance than WC,

but is not is tough)(machining of hard materials)

Cutting Tool Materials• 5. Alumina based ceramics: Ceramic tools. • Very high wear resistance, and hot hardness • Chemically more stable than HSS and carbides• Used in high speed turning of cast iron and steel.• Toughness is not good enough. (uninterrupted

operations)• 6. Coated tools (1960):• Coatings have unique properties • Lower friction• Higher resistance to wear• Act as a diffusion barries• Higher hot hardness

Cutting Tool Materials7. Cubic boron nitride: Hardest next to diamond, but very

expensive. It has good shock resistance and wear resistance. Good cutting edge strength.

8. Silicon nitride (SiN) Ceramics: Good thermal shock resistance. Not suitable for steel machining.

9. Diamond: Hardest tool material, used in high speed machining of non-ferrous metals and abrasive non-metals. Other metals are not practical, because of chemical affinity between metals and carbon. It has low friction, high wear resistance and used for good surface and accurate dimensions. Prefered in finishing cuts. It is brittle.

Cutting Tool Materials10.Whisker-reinforced and nanocrystalline

tool materials (new):

Tool material that supplies high fracture toughness, resistance to thermal shock, good cutting edge strength and good hot hardness developed.

Whisker: reinforcing fibers in composite cutting tool materials

Cutting Tool Materials

• Hot hardnessThe hardness of various cutting-tool materials as a function of temperature (hot hardness). The wide range in each group of materials is due to the variety of tool compositions and treatments available for that group.

General Characteristics of cutting tool material

Relative Time Required to Machine with Various Cutting-Tool Materials

Figure: Relative time required to machine with various cutting-tool materials, indicating the year the tool materials were first introduced. Note that machining time has been reduced by two orders of magnitude with a hundred years. Source: Courtesy of Sandvik.

Ranges of Mechanical Properties for Groups of Tool Materials

Figure : Ranges of mechanical properties for various groups of tool materials.

Heat Treatment of Steel

• Treatments induce phase transformations that influence mechanical properties.

• Depend on:- alloy composition and microstructure- degree of prior cold work- rates of heating and cooling during heat treatment

Heat Treatment of Tool Steel

Martensite• When austenite is cooled rapidly and fcc structure

transformed to bcc structure.• Does not have many slip systems, thus extremely hard

and brittle, low toughness and limited usage.

Tempered martensite• Tempering reduces martensite’s hardness and improves

toughness.• The bcc is heated to an intermediate temperature which

consists of bcc alpha ferrite and small cementite.

Heat Treatment of Steel• Annealing is the restoration of a cold/heat-

treated part to its original properties• Increase ductility and reduce hardness and

strength.• Also applies to thermal treatment of glasses and

weldments.• Annealing process:

1.Heating

2.Holding it at that temperature

3.Cooling it slowly

Heat Treatment of Steel

Process annealing• Restore workpiece ductility.• If the temperature is high, grain growth may

result adverse effects on the formability of annealed parts.

Stress-relief annealing• Residual stresses may have been induced

during phase transformations.• Stress relieving is allowing of slow cooling, such

as in still air.

Heat Treatment of Steel

• The steel is heated to a specific temperature and cooled at a prescribed rate.

• Use to reduce brittleness and residual stresses, increase ductility and toughness.

• In austempering, heated steel is quenched rapidly to avoid formation of ferrite/pearlite.

• Austempering is used to:

a)reduce cracking and distortion during quenching

b)improve ductility and toughness

Heat Treatment of Tool Steel

• Annealing– Full annealing– Normalising (faster rate of cooling)– Recovery annealing (longer holding time, slower rate

of cooling,)– Stress relieving (lower temperature)

• Martensite formation in steel– Austenitizing (conversion to austenite)– Quenching (control cooling rate– Tempering (reduce brittleness)

Heat Treatment of Tool Steel

• Precipitation hardening– Solution treatment (-phase conversion)– quenching– precipitation treatment (aging)

• Surface hardening– Carburizing– Nitriding– Carbonitriding– Chromizing and Boronizing

Heat Treatment of Tool Steel