Metal Cutting

THEORY OF METAL MACHINING

1. Overview of Machining Technology2. Theory of Chip Formation in Metal Machining3. Force Relationships and the Merchant

Equation4. Power and Energy Relationships in Machining5. Cutting Temperature

Material Removal Processes

A family of shaping operations, the common feature of which is removal of material from a starting workpart so the remaining part has the desired geometry

• Machining – material removal by a sharp cutting tool, e.g., turning, milling, drilling

• Abrasive processes – material removal by hard, abrasive particles, e.g., grinding

• Nontraditional processes - various energy forms other than sharp cutting tool to remove material

Cutting action involves shear deformation of work material to form a chip • As chip is removed, new surface is exposed

Figure 21.2 (a) A cross‑sectional view of the machining process, (b) tool with negative rake angle; compare with positive rake angle in (a).

Machining

Basic Mechanics of Metal Cutting

Special aspects of Metal cutting

• Thickness of chip is greater than actual depth of cut??? Chip is shortened

• No Flow of metal at right angles to the direction chip flow

• Flow lines are evident on the side and back of chip – shearing mechanism

• Considerable thermal energy is associated with the cutting process

Role of friction in cutting

Friction can be reduced by• Improved tool finish and sharpness of the

cutting edge• Use of low friction work or tool material• Increased sliding speed• Improved tool geometry• Use of cutting fluids

Metal Cutting thermal effects

• Engineering Mechanics• Material Testing• Engineering Plasticity• Fundamentals of lubrication, friction and wear• Basic concepts of chemistry and physics• Principles of metallurgy• Thermodynamics and Heat Transfer

Fundamentals of cutting

• Fig 20.3 Schematic illustration of a two-dimensional cutting process,also called orthogonal cutting.Note that the tool shape and its angles,depth of cut,to,and the cutting speed are all independent variables.

Fig 20.1 Examples of cutting process

Fig 20.2 Basic principle of turning operation

Why Machining is Important

• Variety of work materials can be machined– Most frequently used to cut metals

• Variety of part shapes and special geometric features possible, such as:– Screw threads– Accurate round holes– Very straight edges and surfaces

• Good dimensional accuracy and surface finish

Disadvantages with Machining

• Wasteful of material – Chips generated in machining are wasted material, at least

in the unit operation

• Time consuming – A machining operation generally takes more time to shape

a given part than alternative shaping processes, such as casting, powder metallurgy, or forming

Machining in Manufacturing Sequence

• Generally performed after other manufacturing processes, such as casting, forging, and bar drawing – Other processes create the general shape of the

starting work part– Machining provides the final shape, dimensions,

finish, and special geometric details that other processes cannot create

Objectives During Machining

High Material Removal Rate

(MRR)

Good accuracy and Surface

finish

Long tool life

Contradicting

Cost

Cutter RelatedMaterial

Geometry Mounting

Workpiece RelatedMaterial (composition, homogeneity)

Geometry (bar, block, casting etc.)Depth of cut

Spindle speed Feed rate

Machine RelatedCutting fluid type andapplication method

Depth and Width of cutSpindle speed

Feed rate

Others – Cutting fluid type and application

method Depth and Width of cut

Spindle speedFeed rate

Processing Parameters in Machining

Cutting forces and

Torques and

power

Tool temperature

Frictional effects

on tool face

Built up edge

Formation

Chatter, noise and

Vibrations

Effects of Processing Parameters

Work hardening

Thermal softening

Hot spots on the

machined surface

Deflection and

diameter variations

Tool life

Surface finish

Theories of Chip Formation

Chip formation studies helps in understanding mechanics of metal

cutting or physics of machining

They lead to equations that describe the interdependence of the

process parameters such as depth of cut, relative velocity, tool

geometry etc. These relations help us in selecting optimal process

parameters.

Theories of Chip Formation – Theory of Tear

A crack propagates ahead of the tool tip causing tearing similar

to splitting wood [Reuleaux in 1900]

Theories of Chip Formation – Theory of Tear

Against the traditional wisdom, the tool was observed to wear, not at the

tip, but a little distance away from it. Therefore, this theory was subscribed

by many researchers for a long time.

Theories of Chip Formation – Theory of Tear

Further studies attributed the wear away from the tip to the

following:

Chip velocity w.r.t. the tool is zero at the tip.

The tip is protected by BUE.

Temp is also high a little away from the tip due to the

frictional heat.

Subsequent studies proved the chip formation as shear and

not tear. Thus the theory of tear was rejected.

Theories of Chip Formation – Theory of Compression

The tool compresses the material during machining.

This was based on the observation that the chip length was shorter

than the uncut chip length.

Later it was established that this shortage in length corresponds to the

increase chip thickness.

Thus this theory too was wrong

Theories of Chip Formation – Theory of Shear

The excessive compressive stress causes shear of the chip at an angle to

the cutting direction [Mallock in 1881].

Theories of Chip Formation – Theory of Shear

Emphasis on the influence of friction at chip-tool interface

Studied the effect of cutting fluids

Studied the influence of tool sharpness

Studied chatter

His observations on the above studies still hold good although

he could not explain all of them at that time.

Mallock’s other contributions

Difficulties in Machining Mechanics studies

Several physical phenomenon such as plastic flow, fracture, friction,

heat, molecular diffusion and chatter are involved. Some of them occur

in extrême conditions

Friction – sticking; deformation – high strain and strain rate; nascent

surface exposed after deformation is very active causing diffusion

The cutting zone is covered by chips and coolant.

Typical machining is oblique, i.e., forces, torques and deflections exist

in all 3 directions.

Simplified 2-D model of machining that describes the mechanics of machining fairly accurately

Figure 21.6 Orthogonal cutting: (a) as a three‑dimensional process.

Orthogonal Cutting Model

Facing of thin pipe on a lathe with the cutting edge radial to the pipe.

Orthogonal Cutting

Mechanism of Oblique Cutting• The cutting edge is at an angle i, called inclination angle.• The chip movement is in lateral direction

Fig: a)right hand cutting tool.Although these tools have traditionally been produced from solids tool-steel bars,they have been largely replaced by carbide or other inserts of various shapes and sizes,as shownin b).The vcarious angles on these tools and their effects on machining are described

Difficulties in Machining Mechanics studies

The typical machining operations are too short and the stock (depth

and width of cut) keeps changing.

Furthermore, velocity also may change along the cutting edge as well

as over time. These changes further compound the difficulties to

observe the process carefully.

Orthogonal cutting experiments were developed to overcome

these difficulties.

A wedge shaped tool is used

Cutting edge is perpendicular to the direction of cut. In other words,

cutting edge angle and cutting edge inclination angle

Uncut chip thickness is constant along the cutting edge and w.r.t.

time.

Cutting edge is longer than the width of the blank and it extends on its

both sides.

Cutting velocity v is constant along the cutting edge and w.r.t. time

Characteristics of Orthogonal Cutting

Quick stopping devices to freeze the chip formation

Cutting wax manually slowly so as to observe it

Marking grids on the side of the work piece and study their deformation.

Microscopic studies

Photoelastic studies (tools made of transparent material such as persbex or

resin (araldite); work piece is wax. Resulting fringe patterns are observed

under polarized glasses.

Observation using high speed cameras

Force, torque and power measurements using dynamometers.

Temp measurements

Orthogonal Cutting - Experiments

Machining Operations

• Most important machining operations:– Turning– Drilling– Milling

• Other machining operations:– Shaping and planing– Broaching– Sawing

Single point cutting tool removes material from a rotating workpiece to form a cylindrical shape

Figure 21.3 Three most common machining processes: turning,

Turning

Used to create a round hole, usually by means of a rotating tool (drill bit) with two cutting edges

Drilling

Rotating multiple-cutting-edge tool is moved across work to cut a plane or straight surface

• Two forms: peripheral milling and face milling

peripheral milling, face milling.

Milling

Cutting Tool Classification

1. Single-Point Tools– One dominant cutting edge– Point is usually rounded to form a nose radius– Turning uses single point tools

2. Multiple Cutting Edge Tools– More than one cutting edge– Motion relative to work achieved by rotating – Drilling and milling use rotating multiple cutting edge

tools

Figure 21.4 (a) A single‑point tool showing rake face, flank, and tool point; and (b) a helical milling cutter, representative of tools with multiple cutting edges.

Cutting Tools

Cutting Conditions in Machining

• Three dimensions of a machining process: – Cutting speed v – primary motion– Feed f – secondary motion– Depth of cut d – penetration of tool below

original work surface

• For certain operations, material removal rate can be computed as

RMR = v f d where v = cutting speed; f = feed; d = depth of cut

Figure 21.7 Shear strain during chip formation: (a) chip formation depicted as a series of parallel plates sliding relative to each other, (b) one of the plates isolated to show shear strain, and (c) shear strain triangle used to derive strain equation.

Shear Strain in Chip Formation

Shear Strain

Shear strain in machining can be computed from the following equation, based on the preceding parallel plate model:

= tan( - ) + cot

where = shear strain, = shear plane angle, and = rake angle of cutting tool

Figure 21.8 More realistic view of chip formation, showing shear zone rather than shear plane. Also shown is the secondary shear zone resulting from tool‑chip friction.

Chip Formation

Four Basic Types of Chip in Machining

1. Discontinuous chip2. Continuous chip3. Continuous chip with Built-up Edge (BUE)4. Serrated chip

Factors influencing cutting process

Parameter Influence and interrelationship

Cutting speed depth of cut,feed,cutting fluids.

Tool angles

Continuous chip

Built-up-edge chip

Discontinuous chip

Temperature rise.

Tool wear

Machinability

Forces power,temperature rise,tool life,type of chips,surface finish.

As above;influence on chip flow direction;resistance to tool chipping.

Good surface finish;steady cutting forces;undesirable in automated machinery.

Poor surface finish,thin stable edge can product tool surface.

Desirable for ease of chip disposal;fluctuating cutting forces;can affect surface finish and cause vibration and chatters.

Influences surface finish,dimensional accuracy,temperature rise,forces and power.

Influences surface finish,dimensional accuracy,temperature rise,forces and power.

Related to tool life,surface finish,forces and power

Types of chips

• Continuous• Built up edge• Serrated or segmented • Discontinuous

Fig20.5 Basic types of chips and their photomicrographs produced in metal cutting (a) continuous ship with a narrow,straight primary shear zone; (b) secondary shear zone at the chip tool interface;(c) continuous chip with large primary shear zone; (d) continuous chip with built-up-edge;(e) segmented or nonhomogeneous chip and (f) discontinuous chips

• Brittle work materials

• Low cutting speeds• Large feed and

depth of cut• High tool chip ‑

friction

Figure 21.9 Four types of chip formation in metal cutting: (a) discontinuous

Discontinuous Chip

Discontinuous

• Typically associated with brittle metals like –Cast Iron

• As tool contacts work, some compression takes place

• As the chip starts up the chip-tool interference zone, increased stress occurs until the metal reaches a saturation point and fractures off the workpiece.

Discontinuous• Conditions which favor

this type of chip – Brittle work material– Small rake angles on

cutting tools– Coarse machining feeds– Low cutting speeds– Major disadvantage—

could result in poor surface finish

Continuous

• Continuous “ribbon” of metal that flows up the chip/tool zone.

• Usually considered the ideal condition for efficient cutting action.

Continuous

• Conditions which favor this type of chip: – Ductile work– Fine feeds– Sharp cutting tools– Larger rake angles– High cutting speeds– Proper coolants

Surface finish on 1018 steel in face milling

Surface finish in turning 5130 steel with a built-up edge

• Ductile work materials

• High cutting speeds• Small feeds and

depths• Sharp cutting edge• Low tool chip ‑

friction

Figure 21.9 (b) continuous

Continuous Chip

• Ductile materials• Low to medium ‑ ‑

cutting speeds• Tool-chip friction

causes portions of chip to adhere to rake face

• BUE forms, then breaks off, cyclically

Figure 21.9 (c) continuous with built‑up edge

Continuous with BUE

Continuous with a built-up edge(BUE)

• Same process as continuous, but as the metal begins to flow up the chip-tool zone, small particles of the metal begin to adhere or weld themselves to the edge of the cutting tool. As the particles continue to weld to the tool it effects the cutting action of the tool.

Continuous with a built-up edge(BUE)

• This type of chip is common in softer non-ferrous metals and low carbon steels.

• Problems– Welded edges break off and

can become embedded in workpiece

– Decreases tool life– Can result in poor surface

finishes

• Semicontinuous - saw-tooth appearance

• Cyclical chip forms with alternating high shear strain then low shear strain

• Associated with difficult-to-machine metals at high cutting speeds

Serrated Chip

Figure 21.9 (d) serrated.

Chip Breakers• Long continuous chip

are undesirable• Chip breaker is a piece

of metal clamped to the rake surface of the tool which bends the chip and breaks it

• Chips can also be broken by changing the tool geometry,thereby controlling the chip flow Fig 20.7 (a) Schematic illustration of the action of

a chip breaker .(b) Chip breaker clamped on the rake of a cutting tool. (c) Grooves in cutting tools acting as chip breakers

Chip Breakers

Fig:Various chips produced in turning: a)tightly curled chip b)chip hits workpiece and breaks c)continuous chip moving away from workpiece;and d)chip hits tool shank and breaks off

Cutting Conditions for Turning

Figure 21.5 Speed, feed, and depth of cut in turning.

Roughing vs. Finishing

In production, several roughing cuts are usually taken on the part, followed by one or two finishing cuts

• Roughing - removes large amounts of material from starting workpart– Creates shape close to desired geometry, but leaves some

material for finish cutting– High feeds and depths, low speeds

• Finishing - completes part geometry– Final dimensions, tolerances, and finish– Low feeds and depths, high cutting speeds

Machine Tools

A power driven machine that performs a ‑machining operation, including grinding

• Functions in machining:– Holds workpart– Positions tool relative to work– Provides power at speed, feed, and depth that have been

set

• The term is also applied to machines that perform metal forming operations

Forces in Metal Cutting

• Equations can be derived to relate the forces that cannot be measured to the forces that can be measured:

F = Fc sin + Ft cos

N = Fc cos F‑ t sin

Fs = Fc cos F‑ t sin

Fn = Fc sin + Ft cos

• Based on these calculated force, shear stress and coefficient of friction can be determined

• Friction force F and Normal force to friction N • Shear force Fs and Normal force to shear Fn

Figure 21.10 Forces in metal cutting: (a) forces acting on the chip in orthogonal cutting

Forces Acting on Chip

Resultant Forces

• Vector addition of F and N = resultant R• Vector addition of Fs and Fn = resultant R'

• Forces acting on the chip must be in balance:– R' must be equal in magnitude to R – R’ must be opposite in direction to R– R’ must be collinear with R

Coefficient of FrictionCoefficient of friction between tool and chip:

Friction angle related to coefficient of friction as follows:

NF

tan

Shear StressShear stress acting along the shear plane:

sinwt

A os

where As = area of the shear plane

Shear stress = shear strength of work material during cutting

s

s

AF

S

Chip Thickness Ratio

where r = chip thickness ratio; to = thickness of the chip prior to chip formation; and tc = chip thickness after separation

• Chip thickness after cut always greater than before, so chip ratio always less than 1.0

c

o

tt

r

Determining Shear Plane Angle• Based on the geometric parameters of

the orthogonal model, the shear plane angle can be determined as:

where r = chip ratio, and = rake angle

sincos

tanr

r

1

• F, N, Fs, and Fn cannot be directly measured• Forces acting on the tool that can be measured:

– Cutting force Fc and Thrust force Ft

Figure 21.10 Forces in metal cutting: (b) forces acting on the tool that can be measured

Cutting Force and Thrust Force

The Merchant Equation • Of all the possible angles at which shear

deformation can occur, the work material will select a shear plane angle that minimizes energy, given by

• Derived by Eugene Merchant• Based on orthogonal cutting, but validity

extends to 3-D machining

2245

What the Merchant Equation Tells Us

• To increase shear plane angle – Increase the rake angle – Reduce the friction angle (or coefficient of

friction)

2245

• Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperature

Figure 21.12 Effect of shear plane angle : (a) higher with a resulting lower shear plane area; (b) smaller with a corresponding larger shear plane area. Note that the rake angle is larger in (a), which tends to increase shear angle according to the Merchant equation

Effect of Higher Shear Plane Angle

Power and Energy Relationships

• A machining operation requires power• The power to perform machining can be

computed from: Pc = Fc v

where Pc = cutting power; Fc = cutting force; and v = cutting speed

Power and Energy Relationships • In U.S. customary units, power is

traditional expressed as horsepower (dividing ft lb/min by 33,000) ‑

where HPc = cutting horsepower, hp

00033,vF

HP cc

Power and Energy Relationships • Gross power to operate the machine

tool Pg or HPg is given by

or

where E = mechanical efficiency of machine tool Typical E for machine tools 90%

EP

P cg

EHP

HP cg

Unit Power in Machining • Useful to convert power into power per

unit volume rate of metal cut• Called unit power, Pu or unit horsepower,

HPu

or

where RMR = material removal rate

MR

cU R

PP =

MR

cu R

HPHP =

Specific Energy in MachiningUnit power is also known as the specific

energy U

Units for specific energy are typically N‑m/mm3 or J/mm3 (in‑lb/in3)

wvt

vF

R

PPU

o

c

MR

cu ===

Cutting Temperature

• Approximately 98% of the energy in machining is converted into heat

• This can cause temperatures to be very high at the tool chip ‑

• The remaining energy (about 2%) is retained as elastic energy in the chip

Cutting Temperatures are Important

High cutting temperatures 1. Reduce tool life2. Produce hot chips that pose safety hazards to

the machine operator3. Can cause inaccuracies in part dimensions

due to thermal expansion of work material

Cutting Temperature• Analytical method derived by Nathan Cook

from dimensional analysis using experimental data for various work materials

where T = temperature rise at tool‑chip interface; U = specific energy; v = cutting speed; to = chip thickness before cut; C =

volumetric specific heat of work material; K = thermal diffusivity of work material

333040 ..

Kvt

CU

T o

Cutting Temperature

• Experimental methods can be used to measure temperatures in machining – Most frequently used technique is the tool chip thermocouple‑

• Using this method, Ken Trigger determined the speed temperature relationship to be of the ‑form:

T = K vm where T = measured tool chip interface ‑temperature, and v = cutting speed

Tool Selection Factors

• Inputs• Work material• Type of cut• Part geometry and size• lot size• Machinability data• Quality needed• Past experience of the decision maker

Constraints

• Manufacturing practice• Machine condition• Finish part requirements• Workholding devices• Required process time

Tool Selection Process

Elements of an Effective Tool

• High hardness• Resistance to abrasion and wear• Strength to resist bulk deformation• Adequate thermal properties• Consistent tool life• Correct geometry

Tool Materials

• Wide variety of materials and compositions are available to choose from when selecting a cutting tool

Tool Materials

• They include:– Tool steels - low end of scale. Used to make some

drills, taps, reamers, etc. Low cost equals low tool life.

– High speed steel(HSS) - can withstand cutting temperatures up to 1100F. Have improved hardness and wear resistance, used to manufacture drills, reamers, single point tool bits, milling cutters, etc. HSS cutting tools can be purchased with additional coatings such as TiN which add additional protection against wear.

Tool Materials

– Cobalt - one step above HSS, cutting speeds are generally 25% higher.

– Carbides - Most widely used cutting tool today. Cutting speeds are three to five times faster than HSS. Basic composition is tungsten carbide with a cobalt binder. Today a wide variety of chemical compositions are available to meet different applications. In addition to tool composition, coatings are added to tool materials to incerase resistance to wear.

Tool Materials

– Ceramics - Contain pure aluminum oxide and can cut at two to three times faster than carbides. Ceramic tools have poor thermal and shock resistance and are not recommended for interrupted cuts. Caution should be taken when selecting these tools for cutting aluminum, titanium, or other materials that may react with aluminum oxide.

Tool Materials– Cubic Boron Nitride(CBN) - This tool material maintains

its hardness and resistance to wear at elevated temperatures and has a low chemical reactivity to the chip/tool interface. Typically used to machine hard aerospace materials. Cutting speeds and metal removal rates are up to five times faster than carbide.

– Industrial Diamonds - diamonds are used to produce smooth surface finishes such as mirrored surfaces. Can also be used in “hard turning” operations to eliminate finish grinding processes. Diamond machining is performed at high speeds and generally fine feeds. Is used to machine a variety of metals.

Tool Geometry

• The geometry of a cutting tool is determined by (3) factors:– Properties of the tool material– Properties of the workpiece– Type of cut

Tool Geometry

• The most important geometry’s to consider on a cutting tool are – Back Rake Angles– End Relief Angles– Side Relief Angles

Tool Geometry

Rake Angles

• Back-Allows the tool to shear the work and form the chip. It can be positive or negative– Positive = reduced cutting forces, limited

deflection of work, tool holder, and machine– Negative = typically used to machine harder

metals-heavy cuts

• The side and back rake angle combine to from the “true rake angle”

Rake Angles

• Small to medium rake angles cause: – high compression– high tool forces– high friction– result = Thick—highly deformed—hot chips

Rake Angles

• Larger positive rake angles – Reduce compression

and less chance of a discontinuous chip

– Reduce forces– Reduce friction– Result = A thinner, less

deformed, and cooler chip.

Rake Angles

• Problems….as we increase the angle:– Reduce strength of tool– Reduce the capacity of the tool to conduct heat

away from the cutting edge.– To increase the strength of the tool and allow it to

conduct heat better, in some tools, zero to negative rake angles are used.

Negative Rake Tools• Typical tool materials which utilize negative

rakes are: • Carbide• Diamonds• Ceramics

• These materials tend to be much more brittle than HSS but they hold superior hardness at high temperatures. The negative rake angles transfer the cutting forces to the tool which help to provide added support to the cutting edge.

Negative Rake Tools

Summary Positive vs. Negative Rake Angles

• Positive rake angles– Reduced cutting forces– Smaller deflection of work, tool holder, and machine– Considered by some to be the most efficient way to

cut metal– Creates large shear angle, reduced friction and heat – Allows chip to move freely up the chip-tool zone– Generally used for continuous cuts on ductile

materials which are not to hard or brittle

Summary Positive vs. Negative Rake Angles

• Negative rake angles– Initial shock of work to tool is on the face of the

tool and not on the point or edge. This prolongs the life of the tool.

– Higher cutting speeds/feeds can be employed

Tool Angle Application

• Factors to consider for tool angles– The hardness of the metal– Type of cutting operation– Material and shape of the cutting tool– The strength of the cutting edge

Carbide Inset Selection

Carbide Inset Selection

A.N.S.I. Insert Identification System ANSI - B212.4-1986

M1-FineM2-Medium

M3-S.SM4-Cast ironM5-General

Purpose

M1-FineM2-Medium

M3-S.SM4-Cast ironM5-General

Purpose

Carbide Inset Selection

Tool Life: Wear and Failure

1. Flank wear :It occurs on the relief face of the tool and the side relief angle.

2. Crater wear:It occurs on the rake face of the tool.

3. Chipping :Breaking away of a small piece from the cutting edge of the tool .

Fig (a) Flank and crater wear in a cutting tool.tool moves to the left. (b) View of the rake of a turning tool,showing nose radius R and crater wear pattern on the rake face of the tool c)View of the flank face of a turning tool,sowing the average flank wear land VB and the depth-of-cut line (wear notch)

Wear and Tool Failures: Crater wear

Fig (a) Schematic illustrations of types of wear observed on various types of cutting tools .(b) Schematic illustrations of catastrophic tool failures.A study of the types and mechanism of tool wear and failure is essential to the development of better tool materials

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Tool Wear• Productivity and economy of manufacturing by machining are significantly affected by life of

the cutting tools. Cutting tools may fail by brittle fracture, plastic deformation or gradual wear. Turning carbide inserts having enough strength, toughness and hot hardness generally fail by gradual wears. With the progress of machining the tools attain crater wear at the rake surface and flank wear at the clearance surfaces, as schematically shown in following Figure (next slide) due to continuous interaction and rubbing with the chips and the work surfaces respectively. Among the aforesaid wears, the principal flank wear is the most important because it raises the cutting forces and the related problems.

Flank Wear

Crater Wear

Principal Cutting Edge

Shank

Rake or Face

FlankAuxiliary

Cutting Edge

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K

BKM

A A

V

N

V

B

V

M

VS

VS

M

K

T

Notch

Grooving wear

Section A-A

Auxiliary Flank

Principal Flank

Rake Surface

Crater wear

Major Features of Wear of Turning Tool

VB = Average flank wear

VN = Flank notch wear

VM = Maximum flank wear

VS = Average auxiliary flank wear

VSM = Maximum auxiliary flank wear

KT = Crater depth

KM = Distance from center of crater

KB = Crater width

22/120

• The life of the tools, which ultimately fail by systematic gradual wear, is generally assessed at least for R&D work, by the average value of the principal flank wear (VB), which aggravates cutting forces and temperature and may induce vibration with progress of machining. The pattern and extent of wear of the auxiliary flank (VS) affects surface finish and dimensional accuracy of the machined parts.

• However, tool rejection criteria for finishing operation were employed in this investigation. The values established in accordance with ISO Standard 3685 for tool life testing. A cutting tool was rejected and further machining stopped based on one or a combination of rejection criteria:

i. Average Flank Wear ≥ 0.3 mm

ii. Maximum Flank Wear ≥ 0.4 mm

iii. Nose Wear ≥ 0.3 mm

iv. Notching at the depth of cut line ≥ 0.6 mm

v. Average surface roughness value ≥ 1.6 µm

vi. Excessive chipping (flanking) or catastrophic fracture of cutting edge.

22/121

Effects of Tool WearThe wear on a tool causes the following effects.

– The cutting force increases– The dimensional accuracy of the work decreases– The surface roughness of the work increases– The tool-work system may start vibrating– The work piece may get damaged or tool may break ultimately.

22/122

Mechanism of Tool WearTo know the right mechanism of tool wear and its reasons, the researchers all over the world conducted lots of experiments. Due to the inabilities of the researchers to observe the wear actually taking place on different places of a tool, the bulk of the knowledge is based primarily on theory supported by limited investigations. In general there are seven basic types of wear that affect a cutting tool:

Abrasion: Mechanical wearing, hard particles in workpiece removes small portions of the tool, that cause flank and crater wear. This is the dominant cause of flank wear.Adhesion:Two metals contact under high pressure and temperature that cause welding between the materials. Diffusion:Atoms on the boundry of workpiece and tool changes place. This is the principle cause for crater wear. Chemical Reactions: The high temperatures and clean surfaces at the chip-tool interface in machining at high speeds can result in chemical reactions, in particular, oxidation, on the rake surface of the tool. The oxidized layer, being softer than the parent tool material, is sheared away, exposing new material to sustain the reaction process. Plastic Deformation: Cutting forces acting on the cutting edge at high temperature cause the edge to deform plastically. This cause flank wear.

22/123

Tool Life • Defined as the cutting time required for complete failure of the tool,• The time necessary to produce a given amount of flank wear on the

tool. • Tool life is a measure of the length of time a tool will cut satisfactorily • Tool life is an important factor in production work since considerable

time is lost wherever a tool is ground and reset. • The tool life is affected by several variables, the important ones being:

– Cutting speed (Vc)– Feed rate (So) – Depth of cut (t)– Work material hardness– Tool material– Shape and angles of cutting tool– Types of cutting fluid and its method of application

22/124

Tailor Tool Life Equation• As cutting proceeds, various wear mechanisms result in increasing levels of wear

on the cutting tool. The general relationship of tool wear versus cutting time is shown in following Figure. Although the relationship shown is for flank wear, a similar relationship occurs for crater wear. Three regions can usually be identified in the typical wear growth curve.

Break-in period

Machining Time (min)

Tool

Fla

nk W

ear (

VB)

Steady-state wear regionFailureregion

Rapid initial wear

Uniform wear rate

Acceleratingwear rate

Final failure

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• The first is the break-in Period, in which the sharp cutting edge wears rapidly at the beginning of its use. This first region occurs within the first few minutes of cutting.

• The break-in period is followed by wear that occurs at a fairly uniform rate. This is called the steady state wear region.

• In this figure, this region is pictured as a linear function of time, although there are deviations from the straight line in actual machining. Finally, wear reaches a level at which the wear rate begins to accelerate.

• This marks the beginning of the failure region, in which cutting temperatures are higher and the general efficiency of the machining process is reduced. If allowed to continue, the tool finally fails by temperature failure.

• Frederick W. Taylor did pioneering work in the field of metal cutting. He conducted numerous experiments and in 1907 gave the following relationship between tool life and cutting speed.

CTV nc

Constant C

t.environmen andn combinatio work and on tool dependsIt index. life Tool n

life Tool T , velocity Cuttingc

V

Where,

22/126

Tool-life curves for a variety of cutting-tool materials as shown in the following Figure. The negative inverse of the slope of these curves is the exponent n in the Taylor tool-life equations and C is the cutting speed at T = 1 min.

CTV nc

The following values may be taken for nn = 0.10 to 0.15 for HSS tools

n = 0.20 to 0.40 for carbide toolsn = 0.40 to 0.60 for ceramic tools

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Cutting Tool Materials for Machining• A wide variety of tool materials have been developed to fulfill the severe

demand of present-day production. No one of' these materials is superior in all respects, but rather each has certain characteristics which limits its field of application. Depending upon the type of service, the proper tool material should, therefore, be selected. The best material to use for a certain job is the one that will produce the machined part at the lowest cost. A good type of tool material should possess certain desired properties such as – The material must remain harder than the work material at

elevated operating temperature.– The material must withstand excessive wear even though the

relative hardness of the tool-work materials changes.– The frictional coefficient at the chip-tool interface must remain

low for minimum wear and reasonable surface finish.– The material must be sufficiently tough to withstand the shocks

of intermittent cutting; if not reinforcement must be provided.– The tool material should also possess high thermal conductivity

for quickly removing heat from the chip-tool interface, have a low coefficient of thermal expansion, not be distorted after heat treatment, be easy to regrind and also easy to weld to the tool holder

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Types of Cutting Tool Materials• Carbon Tool Steels

– medium alloy steels– poor properties above 200OC– Inexpensive– Uses: Taps and core drills for machining soft materials and wood working tools

• High Speed Steels (HSS)– Hot hardness is quite high, so the HSS cutting tools retain the cutting ability upto 600OC– Wear resistance is high– The hardenability is good– Uses: Drills, reamers, broaches, milling cutters, taps, lathe cutting tool, gear hobs etc. are made of

HSS.• Carbides

– “A hard material made of compacted binary compounds of carbon and heavy metals, used to make tools that cut metal.”

– made using powder metallurgy– usually as an insert

• Ceramics– high abrasion and high hot hardness– not good for interrupted cutting– requires dry, or constant profuse cutting fluids

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• All carbides, when finished, are extremely brittle and weak in their resistance to it impact and shock loading. Due to this, vibrations are very harmful for carbide tools. The machine tools should be rigid, faster and more powerful. Light feeds, low speeds and chatter are harmful. Due to the high cost of carbide tool materials and other factors, cemented carbides are used in the form of inserts or tips which are brazed or clamped to a steel shank as shown in the following Figure.

Methods of attaching inserts to tool shanks

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Cutting Fluid • Machining is inherently characterized by generation of heat and high

cutting temperature. At such elevated temperature the cutting tool if not enough hot hard may lose their form stability quickly or wear out rapidly resulting in increased cutting forces, dimensional inaccuracy of the product and shorter tool life. The magnitude of this cutting temperature increases, though in different degree, with the increase of cutting velocity, feed and depth of cut, as a result, high production machining is constrained by rise in temperature. This problem increases further with the increase in strength and hardness of the work material. So, the use of a cutting fluid during a machining operation is very essential. Its application at the workpiece-tool interface produces the following effects:

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Properties of Good Cutting Fluid • Good cooling capacity and lubricating qualities• Rust resistance and stability- for long life• Resistance to rancidity and foaming• Non-toxic• Transparent-to allow the operator to see the work clearly during

machining• Relatively low viscosity-to permit the chips and dirt to settle quickly• Nonflammable-to avoid burning easily and should be non-combustible • Ability to disposed of in an environmentally responsible way.• In addition, it should not smoke excessively, form gummy deposit which

may cause machine slide to become sticky, or clog the circulating system.

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Types of Cutting Fluids• Cutting fluids are used in metal machining for a variety of reasons such as improving

tool life, reducing workpiece thermal deformation, improving surface finish and flushing away chips from the cutting zone. Practically all cutting fluids presently in use fall into one of four categories:

– Straight oils – Soluble oils – Semi-synthetic fluids – Synthetic fluids

• Straight oils are non-emulsifiable and are used in machining operations in an undiluted form. They are composed of a base mineral or petroleum oil and often contain polar lubricants such as fats, vegetable oils and esters as well as extreme pressure additives such as Chlorine, Sulphur and Phosphorus. Straight oils provide the best lubrication and the poorest cooling characteristics among cutting fluids.

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• Soluble oil fluids form an emulsion when mixed with water. The concentrate consists of a base mineral oil and emulsifiers to help produce a stable emulsion. They are used in a diluted form (usual concentration = 3 to 10%) and provide good lubrication and heat transfer performance. They are widely used in industry and are the least expensive among all cutting fluids.

• Semi-synthetic fluids are essentially combination of synthetic and soluble oil fluids and have characteristics common to both types. The cost and heat transfer performance of semi-synthetic fluids lie between those of soluble oil fluids and synthetic fluid.

• Synthetic fluids contain no petroleum or mineral oil base and instead are formulated from alkaline inorganic and organic compounds along with additives for corrosion inhibition. They are generally used in a diluted form (usual concentration = 3 to 10%). Synthetic fluids often provide the best cooling performance among all cutting fluids.

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Machining Economics• Optimizing cutting speed is formulated by W. Gilbert with respect to Taylor’s tool life formula. There

are two objectives in this optimization

– Maximizing production rate– Minimizing unit cost

• Both objectives seek a balanced MRR and tool life.

Maximizing Production Rate

Choose cutting speed to minimize machining time per production unit.In turning 3 elements contribute to the total production cycle time for one part

Part handling time (loading+ unloading+ starting the machining)=Th

Machining time (actual machining)=Tm

Tool change time (at the end of tool life, the tool must be changed)=T t .

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Therefore total time per unit product for the operation cycle Tc = Th +Tm +Tt /np

Where np =integer number of parts we can produce within the tool life.

Our objective is to minimize Tc, which is the function of the cutting speed.

Remember in Turning operation, Tm = π .D. L/ V .So

Taylor’s tool life formula, V.Tn =C T=(C/ V)1/n

np=T/ Tm np =(C/ V)1/n . V .So / π .D. L = C1/n. So

/ π .D. L . V(1/n) -1

So, Tc becomes, Tc = Th +π .D. L/ V .So +(Tt . π .D. L . V(1/n) -1 )/ C1/n. So

To minimize we need to take derivative of Tc w.r.t V, and equate it to 0.

Therefore the maximum V= Vmax =C/[{(1/n)-1}Tt] n

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We have maximum production for this value of V. The corresponding tool life is Tmax =[(1/n )– 1]. Tt

Minimizing Cost per Unit

Choose cutting speed to minimize production cost per unit product.In turning 4 elements contribute to the total production cost for one part (cost rate is $/min)

Cost of part handling time(cost of the time that operator spends loading and unloading the part)=Co

.Th

Cost of machining time= Co . Tm

Cost of tool change time= Co . Tt /np

Tooling cost= Ct /np,

where, Ct =Cost for cutting edge=Pt/ne

Pt =Price of the toolne =Number of cutting edges

Co=Cost rate ($/min) for the operator and machine

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If the tool is regrindable, Ct =Pt/ng+Tg . Cg

Where,ng =number of tool lifesTg =time to grindCg =grinding labor costTherefore total cost per unit product for the operation cycle,

n1

Co

S

1n1

πDLV t

Ct

To

C

oS V

πDLo

C

hT

oC

cC

pn

tC

pn

tT

oC

mT

oC

hT

oC

cC

To minimize the cost we need to take derivative of Cc w.r.t υ, and equate it to 0.

Therefore the minimum V, Vmin =C.[{n/ (1-n)}.(Co / (Co. Tt +Ct)] n

Means that it is the cost minimizing speed, and the corresponding tool life isTmin=[(1/n)-1].(Co. Tt +Ct)/ Co