metal cutting ip430

7/31/2019 Metal Cutting Ip430

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THEORY OF METAL MACHINING

1. Overview of Machining Technology

2. Theory of Chip Formation in Metal Machining

3. Force Relationships and the MerchantEquation

4. Power and Energy Relationships in Machining

5. Cutting Temperature


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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


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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


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Material Removal Processes

A family of shaping operations, the common featureof which is removal of material from a startingworkpart 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


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tool

work


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Machining


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Chip Formation


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Basic Mechanics of Metal Cutting


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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


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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


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Machining in Manufacturing

Sequence

Generally performed after othermanufacturing processes, such ascasting, 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


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Objectives During Machining

High Material Removal Rate

(MRR)

Good accuracy and Surface

finish

Long tool life

Cost


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Cutter RelatedMaterial

Geometry

Mounting

Workpiece RelatedMaterial (composition, homogeneity)

Geometry (bar, block, casting etc.)

Depth of cutSpindle speed

Feed rate

Machine RelatedCutting fluid type andapplication method

Depth and Width of cut

Spindle speed

Feed rate

Others

Cutting fluid type and applicationmethod

Depth and Width of cutSpindle speed

Feed rate

Processing Parameters in Machining


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Cutting forces andTorques 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


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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

geometryetc. These relations help us in selecting optimal process

parameters.


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Theories of Chip FormationTheory of Tear

A crack propagates ahead of the tool tip causing tearing similar

to splitting wood [Reuleaux in 1900]


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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.


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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.


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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 theincrease chip thickness.

Thus this theory too was wrong


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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].


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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.

Mallocks other contributions


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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 extrme conditions

Friction sticking; deformation high strain and strain rate; nascentsurface 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.


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Simplified 2-D model of machining that describesthe mechanics of machining fairly accurately

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

Orthogonal Cutting Model


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

Orthogonal Cutting


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Oblique Cutting tool


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Oblique Cutting tool

nomenclature


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h f h l


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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

h l i i


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Quick stopping devices to freeze the chip formation

Cutting wax manually slowly so as to observe itMarking 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


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Machining Operations

Most important machining operations:

Turning

Drilling

Milling

Other machining operations:

Shaping and planing

Broaching

Sawing


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

Figure 21.3 Three most common machining processes: turning,

Turning


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

Drilling


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

Two forms: peripheral milling and face milling

peripheral milling, face milling.

Milling


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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


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Cutting Tools


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Cutting Conditions in Machining

Three dimensions of a machining process: Cutting speed v primary motion m/min

Feedf secondary motion mm/rev or mm/min

Depth of cut d penetration of tool below original work surface -

mm

For certain operations, material removal rate

can be computed as

RMR = v f d

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


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Four Basic Types of Chip in Machining

Discontinuous chip

Continuous chip

Continuous chip with Built-up Edge (BUE)

Serrated chip

Factors influencing cutting process


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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 toolsurface.

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


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Brittle work materialsLow cutting speeds

Large feed and depth of

cut

High tool - chip friction

Discontinuous Chip

Discontinuous


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Discontinuous Typically associated with brittle metals likeCast 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.

Conditions which favor this type of chip

Brittle work material

Small rake angles on cutting tools

Coarse machining feeds

Low cutting speeds

Major disadvantagecould result in poor surface

finish


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Continuous

Continuous ribbon of metal that flows up

the chip/tool zone.

Usually considered the ideal condition for

efficient cutting action.

Contin o s


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Continuous Conditions which favor this type of chip:

Ductile work

Fine feeds

Sharp cutting tools

Larger rake angles

High cutting speeds

Proper coolants

Low tool - chip friction

Surface finish on 1018 steel in face milling


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Continuous with a built-up edge(BUE)

Same process as continuous, but as the metalbegins 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.


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Ductile materials Low - to- medium cutting

speeds

Tool-chip friction causes

portions of chip to adhere torake face

BUE forms, then breaks off,cyclically

Continuous with BUE

Built up Edge


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Continuous with a built-up edge(BUE)

This type of chip is commonin softer non-ferrous metalsand low carbon steels.

Problems

Welded edges break off andcan become embedded inworkpiece

Decreases tool life Can result in poor surface

finishes


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Semicontinuous -saw-toothappearance

Cyclical chip formswith alternating highshear strain thenlow shear strain

Associated withdifficult-to-machinemetals at highcutting speeds

Serrated Chip

Figure 21.9 (d) serrated.


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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 toolgeometry,thereby

controlling the chip flowFig 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

Chi B k


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Chip Breakers

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

breaks off


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Cutting Conditions for Turning

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


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Roughing vs. Finishing

In production, several roughing cuts are usuallytaken on the part, followed by one or twofinishing cuts

Roughing - removes large amounts of materialfrom 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


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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


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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 + Ftcos

N = Fc

cos- Ftsin

Fs

= Fc

cos- Ftsin

Fn

= Fc

sin+ Ft

cos

Based on these calculated force, shear stress

and coefficient of friction can be determined


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Friction force Fand 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 inorthogonal cutting

Forces Acting on Chip


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Resultant Forces

Vector addition ofFand N = resultant R

Vector addition ofFs

and Fn

= resultant R'

Forces acting on the chip must be in balance:

R' must be equal in magnitude to R

Rmust be opposite in direction to R

Rmust be collinear with R


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Coefficient of Friction

Coefficient of friction between tool and chip:

Friction angle related to coefficient of frictionas follows:

N

F

tan


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Shear Stress

Shear stress acting along the shear plane:

sin

wtA os

where As= area of the shear plane

Shear stress = shear strength of work materialduring cutting

s

s

A

FS


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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

cot

tr


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Determining Shear Plane Angle

Based on the geometric parameters ofthe orthogonal model, the shear planeangle can be determined as:

where r= chip ratio, and = rake angle

sin

costan

r

r

1

C tti F d Th t F


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F, N, Fs

, and Fn

cannot be directly measured

Forces acting on the tool that can be measured: Cutting force F

cand Thrust force F

t

Figure 21.10 Forces

in metal cutting: (b)

forces acting on the

tool that can bemeasured

Cutting Force and Thrust Force


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The Merchant Equation

Of all the possible angles at which sheardeformation can occur, the work materialwill select a shear plane angle thatminimizes energy, given by

Derived by Eugene Merchant

Based on orthogonal cutting, but validity

extends to 3-D machining

2245


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What the Merchant Equation Tells Us

To increase shear plane angle

Increase the rake angle

Reduce the friction angle (or coefficient of

friction)

2245


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Higher shear plane angle means smaller shear

plane which means lower shear force, cuttingforces, power, and temperature

Figure 21.12 Effect of shear plane angle : (a) higherwith aresulting lower shear plane area; (b) smallerwith a correspondinglarger shear plane area. Note that the rake angle is larger in (a), whichtends to increase shear angle according to the Merchant equation

Effect of Higher Shear Plane Angle

d l h


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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


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P d E R l i hi


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Power and Energy Relationships

Gross power to operate the machinetool P

gor HP

gis given by

or

where E= mechanical efficiency of machine tool

Typical Efor machine tools 90%

E

PP cg

E

HPHP cg

U i P i M hi i


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Unit Power in Machining

Useful to convert power into power perunit volume rate of metal cut

Called unit power, Pu or unit horsepower,HP

u

or

where RMR= material removal rate

MR

c

UR

P

P =MR

c

uR

HP

HP =

S ifi E i M hi i


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Specific Energy in Machining

Unit power is also known as the specificenergyU

Units for specific energy are typically

N-m/mm3 or J/mm3 (in-lb/in3)

wvt

vF

R

PPU

o

c

MR

c

u===

Shear Strain in Chip Formation


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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

M t l C tti th l ff t


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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

C tti T t


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Cutting Temperature

Approximately 98% of the energy in machiningis 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

C tti T t I t t


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Cutting Temperatures are Important

High cutting temperatures

1. Reduce tool life

2. Produce hot chips that pose safety hazards to

the machine operator

3. Can cause inaccuracies in part dimensions

due to thermal expansion of work material

Cutting Temperature


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Cutting Temperature

Analytical method derived by Nathan Cook

from dimensional analysis using experimentaldata 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

..

K

vt

C

U

To

C tti T t


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Cutting Temperature

Experimental methods can be used to measuretemperatures in machining

Most frequently used technique is the tool-chip thermocouple

Using this method, Ken Trigger determined thespeed-temperature relationship to be of the

form:

T= K vm

where T= measured tool-chip interface

temperature, and v= cutting speed

T l S l ti F t


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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


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Constraints

Manufacturing practice

Machine condition

Finish part requirements

Workholding devices

Required process time

Tool Selection Process


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Elements of an Effective Tool


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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


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Tool Materials

Wide variety of materials and compositionsare available to choose from when selecting a

cutting tool

Tool Materials


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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 speedsteel(HSS) - can withstand cuttingtemperatures 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 additionalcoatings such as TiN which add additional protection

against wear.

Tool Materials


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Tool Materials

Cobalt - one step above HSS, cutting speeds aregenerally 25% higher.

Carbides - Most widely used cutting tool today.

Cutting speeds are three to five times faster thanHSS. 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


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Tool Materials

Ceramics - Contain pure aluminum oxide and cancut 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


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

its hardness and resistance to wear at elevatedtemperatures and has a low chemical reactivity to thechip/tool interface. Typically used to machine hardaerospace materials. Cutting speeds and metalremoval rates are up to five times faster than carbide.

Industrial Diamonds - diamonds are used to producesmooth surface finishes such as mirrored surfaces. Canalso be used in hard turning operations to eliminatefinish grinding processes. Diamond machining isperformed at high speeds and generally fine feeds. Isused to machine a variety of metals.

Tool Geometry


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Tool Geometry

The geometry of a cutting tool is determinedby (3) factors:

Properties of the tool material

Properties of the workpiece

Type of cut

Tool Geometry


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Tool Geometry

The most important geometrys to consider ona cutting tool are

Back Rake Angles

End Relief Angles Side Relief Angles

Tool Geometry


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Tool Geometry

Rake Angles


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Rake Angles

Back-Allows the tool to shear the work andform 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 fromthe true rake angle

Rake Angles


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Rake Angles

Small to medium rake angles cause: high compression

high tool forces

high friction

result = Thickhighly deformedhot chips

Rake Angles


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Rake Angles

Larger positive rakeangles

Reduce compressionand less chance of a

discontinuous chip Reduce forces

Reduce friction

Result = A thinner, less

deformed, and coolerchip.

Rake Angles


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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


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Negative Rake Tools

Typical tool materials which utilize negativerakes are:

Carbide

Diamonds

Ceramics

These materials tend to be much more brittlethan HSS but they hold superior hardness athigh temperatures. The negative rake anglestransfer the cutting forces to the tool which

help to provide added support to the cuttingedge.

Negative Rake Tools


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Negative Rake Tools

Summary Positive vs. Negative Rake


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Angles

Positive rake angles Reduced cutting forces

Smaller deflection of work, tool holder, and machine

Considered by some to be the most efficient way tocut 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


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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


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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


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A.N.S.I. Insert Identification SystemANSI - B212.4-1986

M1-FineM2-Medium

M3-S.S

M4-Cast iron

M5-General

Purpose



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Tool Life: Wear and Failure


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1. Flank wear :It occurson the relief face ofthe tool and the siderelief angle.

2. Crater wear:It occurson the rake face ofthe tool.

3. Chipping :Breaking

away of a small piecefrom the cutting edgeof the tool .

Fig (a) Flank and crater wear in a cutting tool.toolmoves to the left. (b) View of the rake of aturning tool,showing nose radius R and crater

wear pattern on the rake face of the toolc)View of the flank face of a turningtool,sowing the average flank wear land VBand the depth-of-cut line (wear notch)

Wear and Tool Failures: Crater wear


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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

Tool Wear


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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

Major Features of Wear of Turning Tool


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K

B

KM

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

VB = Average flank wear

VN = Flank notch wear

VM = Maximum flank wearVS = Average auxiliary flank

wear

VS

M

= Maximum auxiliary flank

wear

KT = Crater depth

KM = Distance from center of

crater

KB = Crater width


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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 ofwear 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 m

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

Effects of Tool Wear


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Effects of Tool Wear

The 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.

Mechanism of Tool Wear


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To 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 theknowledge 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 thetool, 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 weldingbetween the materials.

Diffusion:Atoms on the boundry of workpiece and tool changes place. This is the principlecause 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 therake surface of the tool. The oxidized layer, being softer than the parent tool material, issheared away, exposing new material to sustain the reaction process.

Plastic Deformation:Cutting forces acting on the cutting edge at high temperature causethe edge to deform plastically. This cause flank wear.

Tool Life


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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 considerabletime 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

Tailor Tool Life Equation


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q 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, asimilar relationship occurs for crater wear. Three regions can usually be identified

in the typical wear growth curve.

Break-in period

Machining Time (min)

To

olFlankWear(VB) Steady-state wear region

Failureregion

Rapid initial wear

Uniformwear rate

Acceleratingwear rate

Finalfailure

The first is the break-in Period, in which the sharp cutting edge wearsrapidly at the beginning of its use. This first region occurs within the first

few minutes of cutting.


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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 machiningprocess 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.

CTVn

c

ConstantC

t.environmenandncombinatioworkandon tooldependsItindex.lifeTooln

lifeToolT,velocityCuttingc

V

Where,


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

CTVn

c

The following values may be taken for nn = 0.10 to 0.15 for HSS toolsn = 0.20 to 0.40 for carbide toolsn = 0.40 to 0.60 for ceramic tools

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


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in all respects, but rather each has certain characteristics which limits itsfield of application. Depending upon the type of service, the proper toolmaterial should, therefore, be selected. The best material to use for a

certain job is the one that will produce the machined part at the lowestcost. A good type of tool material should possess certain desiredproperties such as

The material must remain harder than the work material atelevated operating temperature.

The material must withstand excessive wear even though therelative hardness of the tool-work materials changes.

The frictional coefficient at the chip-tool interface must remainlow for minimum wear and reasonable surface finish.

The material must be sufficiently tough to withstand the shocksof intermittent cutting; if not reinforcement must be provided.

The tool material should also possess high thermal conductivityfor quickly removing heat from the chip-tool interface, have alow coefficient of thermal expansion, not be distorted afterheat treatment, be easy to regrind and also easy to weld to thetool holder

Types of Cutting Tool Materials


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yp g 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 ofHSS.

Carbides

A hard material made of compacted binary compounds of carbon and heavy metals, used to maketools 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

Cutting Fluid


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g 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 rapidlyresulting 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 withthe 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:

Properties of Good Cutting Fluid


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p g 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 duringmachining

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

Types of Cutting Fluids


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yp g 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 flushingaway 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 undilutedform. They are composed of a base mineral or petroleum oil and often contain polarlubricants 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 andthe 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 goodlubrication 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 (usualconcentration = 3 to 10%). Synthetic fluids often provide the best cooling

performance among all cutting fluids.

Machining Economics


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g Optimizing cutting speed is formulated by W. Gilbert with respect to Taylors 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 RateChoose cutting speed to minimize machining time per productionunit.

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)=Tt .


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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 = .D.L/ V .So

Taylors tool life formula, V.Tn =C T=(C/ V)1/n

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

=C1/n.So/ .D.L.V(1/n) -1

So, Tc becomes, Tc = Th + .D.L/ V .So +(Tt . .D.L

.

V(1/n) -1

)/ C1/n

.

So

To minimize we need to take derivative of Tcw.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.Inturning 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 tool

ne=Number of cutting edges

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


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metal cutting ip430

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