thermal aspects in metal cutting

8/20/2019 Thermal Aspects in Metal Cutting

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THEORY OF METALCUTTING

Thermal aspects of Machining

Tool materials Tool !ear

C"tting fl"i#s an# Machina$ilit%&

PRODUCTION ENGINEERING

Module II


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C"tting Temperat"res

Of the total energy consumed in machining, nearly all of it is

converted into heat. The heat generated can cause

temperatures to be as high as 6000C at tool chip interface.

Cutting temperature has a controlling influence on the rate oftool wear and friction between tool and chip.

Elastic #eformation' Energy required for the operation is stored inthe material as strain energy and no heat is generated.

(lastic #eformation !ost of the energy used is converted as heat.


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Effect of c"tting temperat"re

The effect of cutting temperature, particularly when it is

high is mostly detrimental to both the tool and the *ob.

The ma*or portion of the heat is ta(en away by the chips.

+ut it does not matter because chips are thrown out.

o attempts should be made such that the chips ta(e awaymore and more amount of heat leaving small amount of

heat to harm the tool and the *ob.


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Effect of c"tting temperat"re on

tool The possible detrimental effects of the high cutting

temperature on cutting tool -edge are

#apid tool wear which reduces tool life

plastic deformation of the cutting edges if the tool

material is not enough hot/hard and hot/strong

thermal fla(ing and fracturing of the cutting edges

due to thermal shoc(s.

+uilt up Edge formation.


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Effect of c"tting temperat"re on )o$

The possible detrimental effects of the high cutting

temperature on machined *ob are

1imensional inaccuracy of the *ob due to thermal

distortion and e)pansion/contraction during and aftermachining

surface damage by o)idation, rapid corrosion,

burning etc.

induction of tensile residual stresses and micro crac(s

at the surface 2 subsurface.


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Effect of C"tting Temperat"re

3owever, often the high cutting temperature helps inreducing the magnitude of the cutting forces and cutting

power consumption to some e)tent by softening or

reducing the shear strength, * s of the wor( material ahead

the cutting edge.

To attain or enhance such benefit the wor( material

ahead the cutting &one is often additionally heated

e)ternally. This technique is (nown as 3ot !achining

and is beneficially applicable for the wor( materials

which are very hard and hardenable li(e high manganese

steel, 3adfield steel, 4i/hard, 4imonic etc.


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Factors Affecting Temperat"re


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+o"rces an# Ca"ses of heat generation

in Machining 1uring machining, heat is generated at the cutting point from

three sources, as indicated in 5ig.

Those sources and causes of development of cutting

temperature are (rimar% shear ,one -" where the ma*or part of the energy

is converted into heat.

+econ#ar% #eformation ,one -$ at the chip tool

interface where further heat is generated due to rubbingand 2 or shear.

t the !orn o"t flan-s -' due to rubbing between the tool

and the finished surfaces.


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+o"rces of heat generation in Machining


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Thermal Aspects of Machining The heat generated is shared by the chip, cutting tool and the blan(.

The apportionment of sharing the heat depends upon the

configuration, si&e and thermal conductivity of the tool wor(

material and the cutting condition.

The following figure visuali&es that ma)imum amount of heat is

carried away by the flowing chip.

5rom "0 to $07 of the total heat goes into the tool and some heat is

absorbed in the blan(.

8ith the increase in cutting velocity, the chip shares heat

increasingly.


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Thermal Aspects of Machining


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Temperat"re #istri$"tion in Metal

C"tting 5ig. shows temperature distribution in wor( piece and chip duringorthogonal cutting -obtained from an infrared photograph, for free/

cutting mild steel where cutting speed is 0.'9m2s, the width of cut is

6.':mm, the normal ra(e is '00 , and wor( piece temperature is 6""0C


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Temperat"re #istri$"tion in Metal

C"tting


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Anal%tical metho#s to comp"te C"tting

Temperat"res

Coo-.s Metho#

8here,

;T < !ean temperature rise at tool chip interface, C 0

= < pecific Energy in the operation, 4/m2mm'

> < Cutting peed, m2st 0 < Chip thic(ness before the cut, m

?C < >olumetric pecific heat of wor( material, @2mm'/C 0

A < Thermal diffusivity of the wor( material, m$ 2s

0.333

04.0

=

A

>t

C

= T

ρ

δ


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Meas"rement of tool'chip interface

temperat"re


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Tool !or- Thermoco"ple Techni/"e

Bn a thermocouple two dissimilar but electrically conductive

metals are connected at two *unctions.

8henever one of the *unctions is heated, the difference in

temperature at the hot and cold *unctions produce a

proportional current which is detected and measured by amilli/voltmeter.

Bn machining li(e turning, the tool and the *ob constitute the

two dissimilar metals and the cutting &one functions as the hot

*unction.

Then the average cutting temperature is evaluated from the m>

after thorough calibration for establishing the e)act relation

between m> and the cutting temperature.


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Tool !or- thermoco"ple techni/"e


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Em$e##e# thermoco"ple techni/"e Bn operations li(e milling, grinding etc. where the previous methods are

not applicable, embedded thermocouple can serve the purpose. 5ig. shows the principle.

The standard thermocouple monitors the *ob temperature at a certain

depth, hi from the cutting &one. The temperature recorded in oscilloscope

or strip chart recorder becomes ma)imum when the thermocouple beadcomes nearest -slightly offset to the grinding &one.

8ith the progress of grinding the depth, hi gradually decreases after each

grinding pass and the value of temperature, m also rises as has been

indicated in 5ig.

5or getting the temperature e)actly at the surface i.e., grinding &one, hihas to be &ero, which is not possible. o the m

vs hi curve has to be

e)trapolated up to hi < 0 to get the actual grinding &one temperature. Dog

log plot helps such e)trapolation more easily and accurately.


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Em$e##e# thermoco"ple techni/"e


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Infra're# photographic techni/"e

This modern and powerful method is based on ta(inginfra/red photograph of the hot surfaces of the tool, chip,

and2or *ob and get temperature distribution at those

surfaces.

%roper calibration is to be done before that. This way the

temperature profiles can be recorded as indicated in 5ig.

The fringe pattern readily changes with the change in any

machining parameter which affect cutting temperature.


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Tool !ear an# fail"re

The usefulness of tool cutting edge is lost through

0ear

1rea-age

Chipping

2eformation

Tool failure implies that the tool has reached a point

beyond which it will not function satisfactorily until it is

re/sharpened.


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Three Mo#es of Tool Fail"re

Fract"re fail"re8hen the Cutting force at tool point becomes e)cessive, it

leads to failure by brittle fracture.

Temperat"re fail"reCutting temperature is too high for the tool material, which

ma(es the tool point to soften, and leads to plastic

deformation along with a loss of sharp edge.

Gra#"al !ear

radual wearing of the cutting edge causes loss of tool

shape, reduction in cutting efficiency and finally tool failure.


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(referre# Mo#e of Tool Fail"re3

Gra#"al 0ear 5racture and temperature failures are premature failures

radual wear is preferred because it leads to the longest

possible use of the tool

radual wear occurs at two locations on a tool

Crater !ear occurs on top ra(e face

Flan- !ear occurs on flan( -side of tool


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5igure 1iagram of worn cutting tool, showing the principal

locations and types of wear that occur


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Crater !ear

Bt consists of a concave section on the tool face formed by theaction of the chip sliding on the surface.

Crater wear affects the mechanics of the process increasing

the actual ra(e angle of the cutting tool and consequently,

ma(ing cutting easier.

t the same time, the crater wear wea(ens the tool wedge and

increases the possibility for tool brea(age.

Bn general, crater wear is of a relatively small concern.


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Flan- !ear

Bt occurs on the tool flan( as a result of friction between themachined surface of the wor( piece and the tool flan(.

5lan( wear appears in the form of so/called wear land and

is measured by the width of this wear land, >+, 5lan( wear

affects to the great e)tend the mechanics of cutting.

Cutting forces increase significantly with flan( wear.

Bf the amount of flan( wear e)ceeds some critical value i.e.

->+ F 0.:G0.6 mm, the e)cessive cutting force may causetool failure.


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Corner 0ear

Bt occurs on the tool corner. Can be considered as a part of the wear land and

respectively flan( wear since there is no distinguished

boundary between the corner wear and flan( wear land.

8e consider corner wear as a separate wear type because

of its importance for the precision of machining.

Corner wear actually shortens the cutting tool thus

increasing gradually the dimension of machined surfaceand introducing a significant dimensional error in

machining, which can reach values of about 0.0'G0.0: mm.


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Fig"re 3

-aCrater wear, and

-bflan( wear on acemented carbide tool,as seen through a

toolma(erHs microscope


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Tool 0ear3 Mechanisms A#hesion !ear3

5ragments of the wor(/piece get welded to the tool surface at hightemperaturesI eventually, they brea( off, tearing small parts of the tool with

them.

A$rasion

3ard particles, microscopic variations on the bottom surface of the chips

rub against the tool surface and brea( away a fraction of tool with them.

2iff"sion !ear

t high temperatures, atoms from tool diffuse across to the chipI the rate of

diffusion increases e)ponentially with temperatureI this reduces the fracture

strength of the crystals.


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5igure Tool wear as a function of cutting time

5lan( wear -58 is used here as the measure of tool wear

Crater wear follows a similar growth curve

Tool 0ear 4s& Time


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Factors affecting Tool life

Tool material

Har#ness

0or- material

+"rface ro"ghness of !or- piece

(rofile of c"tting tool

T%pe of machining operation

C"tting spee# fee# an# #epth of c"t

C"tting temperat"re


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5igure Effect of cutting speed on tool flan( wear -58 for three

cutting speeds, using a tool life criterion of 0.:0 mm flan( wear

Effect of C"tting +pee#


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5igure 4atural log log plot of cutting speed vs tool life‑


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Tool Life

Tool wear is a time dependent process. s cutting

proceeds, the amount of tool wear increases gradually.

Tool wear must not be allowed to go beyond a certainlimit in order to avoid tool failure.

Tool life is #efine# as the time inter4al for !hich tool

!or-s satisfactoril% $et!een t!o s"ccessi4e grin#ing or

re'sharpening of the tool&


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Ta%lor Tool Life E/"ation

This relationship is credited to 5. 8. Taylor -G"J00

C vT n=

8here, v < cutting speedI

T < tool lifeI and

n and C are parameters that depend on feed, depth of cut, wor(

material, tooling material, and the tool life criterion used

n is the slope of the plot

C is the intercept on the speed a)is


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Tool Life 4s& C"tting +pee#

s cutting speed is increased, wear rate increases, so the same wear

criterion is reached in less time, i.e., tool life decreases with cutting speed


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T%pical 5al"es of n an# C in Ta%lor.s Tool

Life E/"ation

Tool material n C (m/min) C (ft/min)

High speed steel:

Non-steel work 0.125 120 350

Steel work 0.125 70 200

Cemented carbide

Non-steel work 0.25 900 2700

Steel work 0.25 500 1500

Ceramic

Steel work 0.6 3000 10,000


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Tool life

>olume of metal removed per minute > m

5 m 6 ... 7 in /

1 < dia of wor(piece, mm

t < depth of cut, mm

f < feed, mm2rev

4 < #%!


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Tool Life

Bf T be the time for tool failure in mins, The total volumeremoved up to Tool 5ailure

olume of material removed up to tool failure

6 888 5 ...


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Tool Near En# of Life

Changes in sound emitted from operation.

Chips become ribbon/li(e, stringy, and difficult to

dispose off.

1egradation of surface finish.

Bncreased power required to cut.

>isual inspection of the cutting edge with magnifying

optics can determine if tool should be replaced.


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Tool life is measured by

>isual inspection of tool edge

Tool brea(s

5ingernail test

Changes in cutting sounds Chips become ribbony, stringy

urface finish degrades

Computer interface says/ power consumption up

/ cumulative cutting time reaches certain level

/ cumulative number of pieces reaches certain value

Tool life is measured by

>isual inspection of tool edge

Tool brea(s

5ingernail test

Changes in cutting sounds Chips become ribbony, stringy

urface finish degrades

Computer interface says

/ power consumption up

/ cumulative cutting time reaches certain level

/ cumulative number of pieces reaches certain value

Operator.s Tool life


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0ear Control The rate of tool wear strongly depends on the cutting

temperature, therefore, any measures which could be

applied to reduce the cutting temperature would reduce

the tool wear as well.

The figure shows the process parameters that influence the

rate of tool wear

dditional measures to reduce the tool wear include theapplication of advanced cutting tool materials, such as

coated carbides, ceramics, etc..


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0ear Control


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C"tting Tool Technolog%

Bt has two principal aspects

:& Tool material

1eveloping materials that can withstand the forces,temperatures and wearing in machining process.

;& Tool geometr%

Optimi&ing the geometry of the cutting tool for the

tool material and for a given operation.


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The cutting tool materials must possess a number of important

properties to avoid e)cessive wear, fracture failure and hightemperatures in cutting.

The following characteristics are essential for cutting materials to

withstand the heavy conditions of the cutting process and to produce

high quality and economical parts

Tool failure modes identify the important properties that a tool

material should possess

Toughness to avoid fracture failure.‑

3ot hardness ability to retain hardness at high temperatures.‑

8ear resistance hardness is the most important property to‑

resist abrasive wear.

CUTTING TOOL MATERIAL+


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CUTTING TOOL MATERIAL+ har#ness at ele4ate# temperat"res -so/called hot hardness so

that hardness and strength of the tool edge are maintained in highcutting temperatures.

To"ghness3 ability of the material to absorb energy without

failing. Cutting is often accompanied by impact forces especially

if cutting is interrupted, and cutting tool may fail very soon if it isnot strong enough.

!ear resistance3 although there is a strong correlation between

hot hardness and wear resistance, latter depends on more than

*ust hot hardness. Other important characteristics include surface finish on the tool, chemical inertness of the tool material with

respect to the wor( material, and thermal conductivity of the tool

material, which affects the ma)imum value of the cutting

temperature at tool/chip interface.


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Car$on +teels Bt is the oldest of tool material. Bt is ine)pensive, easily

shaped, sharpened. The carbon content is 0.6G".:7 with small quantities of

silicon, chromium, manganese, and vanadium to refine

grain si&e.

This material has low wear resistance and low hot hardness.

!a)imum hardness is about 3#C 6$.

=sed for drills taps, broaches, reamers.

Dimited to hand tools and low cutting speed operation. -#edhardness temp. $00K C

The use of these materials now is very limited.


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High +pee# +teel


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High +pee# +teel


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Cemente# Car$i#es

Bntroduced in the "J'0s. These are the most important

tool materials today because of their high hot hardness

and wear resistance.

There may be other carbides in the mi)ture, such astitanium carbide -TiC and2or tantalum carbide -TaC in

addition to 8C.

The main disadvantage of cemented carbides is their low

toughness.

C # C $i# G l


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Cemente# Car$i#es General

(roperties 3igh compressive strength, but low to moderate tensile

strength

3igh hardness -J0 to J: 3#

ood hot hardness

ood wear resistance

3igh thermal conductivity

3igh elastic modulus 600 ) "0‑ ' !%a -J0 ) "06 lb2in$

Toughness lower than high speed steel


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This hard tool material is produced by a powder

metallurgy technique, sintering grains of tungsten carbide

-8C in a cobalt -Co matri) -as the binder, it provides

toughness.

%articles "/: Nm in si&e are pressed M sintered to desired

shape in a 3 $ atmosphere furnace at "::00 C.

mount of cobalt present affects properties of carbide

tools. s cobalt content increases strength, hardness M

wear resistance increases.

Cemente# Car$i#es


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Cemente# Car$i#es

Insert Attachment


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Insert Attachment In spite of more tra#itional tool materials cemente# car$i#es are a4aila$le as

inserts pro#"ce# $% po!#er metall"rg% process&

Inserts are a4aila$le in 4ario"s shapes an# are "s"all% mechanicall% attache#$% means of clamps to the tool hol#er or $ra,e# to the tool hol#er&

The clamping is preferre# $eca"se after an c"tting e#ge gets !orn the insert

is in#eBe#


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T%pes of Cemente# Car$i#es

Two basic types:& Non steel c"tting gra#es ' onl% 0C Co‑ ‑

;& +teel c"tting gra#es ' TiC TaC a##e# to 0C Co‑


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Non +teel C"tting Car$i#e Gra#es‑

• =sed for nonferrous metals and gray cast iron

• %roperties determined by grain si&e and cobalt content

– s grain si&e increases, hardness and hot hardness

decrease, but toughness increases.

– s cobalt content increases, toughness improves at the

e)pense of hardness and wear resistance.


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+teel C"tting Car$i#e Gra#es

=sed for low carbon, stainless, and other alloy steels

– 5or these grades, TiC and2or TaC are substituted for

some of the 8C.

– This composition increases crater wear resistance for

steel cutting, but adversely affects flan( wear resistance

for non steel cutting applications.‑


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Coate# 0C

One advance in cutting tool materials involves the applicationof a very thin coating -G "0 Nm to a A/grade substrate, which

is the toughest of all carbide grades.

Coating may consists of one or more

thin layers of wear/resistantmaterial, such as titanium carbide

-TiC, titanium nitride -Ti4,

aluminum o)ide -l $O' , and2or

other, more advanced materials.

Coating allows to increase

significantly the cutting speed for the

same tool life.tructure of a multi/layer

coated carbide insert


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Coate# Car$i#es

Cemented carbide insert coated with one or more thin layers

of wear resistant materials, such as TiC, Ti4, and2orl $O'

Coating is applied by chemical vapor deposition or physical

vapor deposition.

Coating thic(ness < $.: "'‑ µ m -0.000" to 0.000: in

pplications cast irons and steels in turning and milling

operations.

+est applied at high speeds where dynamic force and thermal

shoc( are minimal.


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Ceramics

%rimarily fine grained l ‑

$O' , pressed and sintered at high pressures and temperatures into insert form with no binder.

• Applications high speed turning of cast iron and steel

• 4ot recommended for heavy interrupted cuts -e.g. rough

milling due to low toughness

• There is no occurrence of built/up edge, and coolants

are not required.

• l $O' also widely used as an abrasive in grinding.


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Ceramics

Two types are available 8hite or cold/pressed ceramics, which consists of only

l $O' cold pressed into inserts and sintered at high

temperature.

+lac( or hot/pressed ceramics, commonly (nown as

cermet -from ceramics M metal. This material consists of

07 l $O' and '07 TiC.

+oth materials have very high wear resistance but lowtoughness, therefore they are suitable only for continuous

operations such as finishing turning of cast iron and steel at

very high speeds.

C


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CermetsCombinations of TiC, Ti4, and titanium carbonitride -TiC4,

with nic(el and2or molybdenum as binders.

ome chemistries are more comple).

Applications3

high speed finishing and semi/finishing of steels, stainless

steels and cast irons.

– 3igher speeds and lower feeds than steel cutting carbide‑

grades

– +etter finish achieved, often eliminating need for grinding.

2i #


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2iamon# • 1iamond is the hardest substance ever (nown of all

materials.

• Dow friction, high wear resistance.

• bility to maintain sharp cutting edge.

• =se is limited because it gets converted into graphite at

high temperature -00 KC. raphite diffuses into iron

and ma(e it unsuitable for machining steels.

• Bt is used as a coating material in its polycrystalline form,or as a single/ crystal diamond tool for special

applications, such as mirror finishing of non/ferrous

materials.


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C $i 1 Nit i#


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C"$ic 1oron Nitri#e 4e)t to diamond, cubic boron nitride -C+4 is hardest

material (nown. #etain hardness up to "000KC.

+y bonding 0.: mm thic( polycrystalline C+4 onto a

carbide substrate through sintering under pressure.

C+4 is used mainly as coating material because it is verybrittle.

Bn spite of diamond, C+4 is suitable for cutting ferrous

materials.

Applications3 machining steel and nic(el based alloys.‑


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%1 and C+4 tools are e)pensive.

!ade by bonding -0.:/".0 mm Dayer of poly crystalline

cubic boron nitride to a carbide substrate by sintering

under %ressure. 8hile carbide provides shoc( resistance C+4 layer

provides high resistance and cutting edge strength.

Cubic boron nitride tools are made in small si&es without

substrate.

C"$ic 1oron Nitri#e

C i l i#


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Lubricants – purpose is to reduce friction… usually oil based

Coolants – purpose is to transport heat… usually water based

1oth lose their effecti4eness at higher c"tting spee#sD

Lubricants – purpose is to reduce friction… usually oil based

Coolants – purpose is to transport heat… usually water based

1oth lose their effecti4eness at higher c"tting spee#sD

C"tting Fl"i#s


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2r% Machining

• 4o cutting fluid is used

• voids problems of cutting fluid contamination,

disposal, and filtration

• %roblems with dry machining – Overheating of the tool

– Operating at lower cutting speeds and production rates to

prolong tool life

– bsence of chip removal benefits of cutting fluids in

grinding and milling


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77

C"tting Fl"i#s• Essential in metal/cutting operations to reduce heat

and friction

• Centuries ago, water used on grindstones

• "00 years ago, tallow used -did not cool

• Dard oils came later but turned rancid

• Early $0th century saw soap added to water

• oluble oils came in "J'6

• Chemical cutting fluids introduced in "JLL


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C"tting Fl"i#s

ny liquid or gas applied directly to machiningoperation to improve cutting performance

Two main problems addressed by cutting fluids

". 3eat generation at shear &one and friction &one

$. 5riction at the tool chip and tool wor( interfaces‑ ‑

Other functions and benefits

–. 8ash away chips -e.g., grinding and milling

–. #educe temperature of wor( part for easier handling

–. Bmprove dimensional stability of wor( part

H G # 2 i M hi i


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79

Heat Generate# 2"ring Machining • 3eat finds its way into one of three places

– 8or( piece, tool and chips

Too much wor!

will e"pandToo much cutting edge will

brea! down rapidly

reducing tool life

#ct as disposable

heat sin!


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80

Heat 2issipation

• Bdeally most heat ta(en off in chips

• Bndicated by change in chip colour as heat causes

chips to o)idi&e.

• Cutting fluids assist ta(ing away heat

– Can dissipate at least :07 of heat created during

machining.

Characteristics of a Goo#


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Characteristics of a Goo#C"tting Fl"i#

Characteristics of a Goo#


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82

Characteristics of a Goo#C"tting Fl"i#

:& Goo# cooling capacit%

;& Goo# l"$ricating /"alities

7& Resistance to ranci#it%

?& Relati4el% lo! 4iscosit%& +ta$ilit% & R"st resistance

& NontoBic

@& Transparent

& Non inflamma$le

Economic A#4antages to Using


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83

Economic A#4antages to UsingC"tting Fl"i#s

Re#"ction of tool costs& – #educe tool wear, tools last longer

Increase# spee# of pro#"ction& – #educe heat and friction so higher cutting speeds

Re#"ction of la$or costs&

– Tools last longer and require less regrinding, lessdowntime, reducing cost per part

Re#"ction of po!er costs&

–

F i f C i Fl i#


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84

F"nctions of a C"tting Fl"i#

(rime f"nctions

– (ro4i#e cooling

– (ro4i#e l"$rication

Other f"nctions

– (rolong c"tting'tool life

– (ro4i#e r"st control

– Resist ranci#it%

F"nctions of a C"tting Fl"i#3


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85

F"nctions of a C"tting Fl"i#3Cooling • !ost effective at high cutting speeds where heat generation

and high temperatures are problems.

• !ost effective on tool materials that are most susceptible to

temperature failures. -e.g., 3• Two sources of heat during cutting action

– %lastic deformation of metal

• Occurs immediately ahead of cutting tool

• ccounts for $2' to '2L of heat

– 5riction from chip sliding along cutting/tool face.


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86


Cooling • 8ater used as base in coolant type cutting fluids.‑

• 8ater most effective for reducing heat by will promoteo)idation -rust.

• 3eat has definite bearing on cutting/tool wear

– mall reduction will greatly e)tend tool life

• 1ecrease the temperature at the chip/tool interface by :0degrees 5, and it will increase tool life by up to : times.



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87

F"nctions of a C"tting Fl"i#3 L"$rication• =sually oil based fluids and are most effective at lower

cutting speeds.

• lso reduces temperature in the operation.

• #educes friction between chip and tool face

– hear plane becomes shorter

– i.e., the area where plastic deformation occurs is smaller

• E)treme/pressure lubricants reduce amount of heat produced

by friction.• E% chemicals of synthetic fluids combine chemically with

sheared metal of chip to form solid compounds -allows chip

to slide

C"tting fl"i# re#"ces friction an#


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88

C"tting fl"i# re#"ces friction an#

pro#"ces a shorter shear plane&


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89

C"tting'Tool Life

• 3eat and friction are prime causes of cutting/tool

brea(down

• #educe temperature by as little as :00 5, life of cutting

tool increases fivefold

• +uilt/up edge

– %ieces of metal weld themselves to tool face

– +ecomes large and flat along tool face, effective ra(e

angle of cutting tool decreased


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90

1"ilt'"p E#ge

+uilt/up edge (eeps brea(ing off and re/forming and result is

poor surface finish, e)cessive flan( wear, and cratering of

tool face.


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92

R"st Control

• 8ater is the best and most economical coolant

– Causes parts to rust

• #ust is o)idi&ed iron

• Chemical cutting fluids contain rust inhibitors


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R f i # Ai +


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Refrigerate# Air +%stem

• nother way to cool chip/tool interface

• Effective, ine)pensive and readily available

• =sed where dry machining is necessary

• =ses compressed air that enters vorte) generation

chamber

– Cooled "000 5 below incoming air

• ir directed to interface and blow chips away

f C i l i#


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T%pes of C"tting Fl"i#s

• !ost commonly used cutting fluids

– Either aqueous based solutions or cutting oils

•

5all into three categories – traight Cutting oils

– Emulsifiable oils or 8ater oluble oils

– Chemical -synthetic cutting fluids

+traight C"tting Oils


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+traight C"tting Oils• 1erived from petroleum, animal, marine or vegetable

substances and may be used straight or in combination.

• Their main function is lubrication and rust prevention.

• They are chemically stable and lower in cost.

• =sually restricted to light duty machining on metals of high

machinability, such as aluminium, magnesium, brass and

leaded steels.

– Two classifications

» Acti4e

» Inacti4e


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I ti C tti Oil


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Inacti4e C"tting Oils

• Oils will not dar(en copper strip immersed in them for 'hours at $"$0 5

• Contained sulfur is natural

–

Termed inactive because sulfur so firmly attached to oil very little released

• 5our general categories

– traight mineral oils, fatty oils, fatty and mineral oil

blends, sulfuri&ed fatty/mineral oil blend

Em"lsifia$le


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Em"lsifia$le


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Merits 2emerits of +%nthetic Fl"i#s


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f %

A#4antages of +%nthetic Fl"i#s


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g f % ood rust control

Resistance to ranci#it% for long perio#s of time

#eduction of amount of heat generated during cutting due

to E)cellent cooling qualities

Donger durability than cutting or soluble oils

4onflammable nonsmo(ing M 4onto)ic QQQQQQ

Easy separation from wor( and chips

Ruic( settling of grit and fine chips so they are not re/

circulated in cooling system

4o clogging of machine cooling system due to detergent

action of fluid

Can leave a residue on parts and tools.


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Machina$ilit%


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• Ease or difficulty with which metal can be machinedwith satisfactory finish at low cost.

• !easured by length of cutting/tool life in minutes or by

rate of stoc( removal in relation to cutting speedemployed.

Machina$ilit%


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G i +t t


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Grain +tr"ct"re

• !achinability of metal affected by its microstructure.

• 1uctility and shear strength modified greatly by

operations such as annealing, normali&ing and stress

relieving.

• Certain chemical and physical modifications of steel

improve !achinability.

– ddition of sulfur, lead, or sodium sulfate

– Cold wor(ing, which modifies ductility

Machina$ilit% In#eB


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Machina$ilit% In#eB

Machina$ilit%3


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Machina$ilit% of Nonferro"s Metals•

Al"min"m – very easy to machine

– but softer grades form +=E poor surface finish⇒

– ⇒ recommend high cutting speeds, high ra(e and relief angles

• 1er%lli"m

– requires machining in a controlled environment

– this is due to to)icity of fine particles produced in machining

• Co$alt'$ase# allo%s

– abrasive and wor( hardening

– require sharp, abrasion/resistant tool materials, and low feeds and

speeds

• Copper

– can be difficult to machine because of +=E formation109

Machina$ilit%3


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Machina$ilit% of Nonferro"s Metals•

Magnesi"m – very easy to machine, good surface finish, prolonged tool life

– Caution high rate of o)idation and fire danger

• Titani"m and its alloys

– have very poor thermal conductivity

– ⇒ high temp. rise and +=E difficult to machine⇒

• T"ngsten

– brittle, strong, and very abrasive

– ⇒ machinability is low

• irconi"m

– ood machinability

– #equires cooling cutting fluid -danger of e)plosion, fire

110

Al i


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Al"min"m

• %ure aluminum generally more difficult to machinethan aluminum alloys

– %roduces long stringy chips and harder on cutting tool

• luminum alloys – Cut at high speeds, yield good surface finish

– 3ardened and tempered alloys easier to machine

– ilicon in alloy ma(es it difficult to machine

• Chips tear from wor( -poor surface

Copper


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Copper

•

3eavy, soft, reddish/colored metal refined from copperore -copper sulfide

– 3igh electrical and thermal conductivity

– ood corrosion resistance and strength

– Easily welded, bra&ed or soldered

– >ery ductile

•

1oes not machine well long chips clog flutes ofcutting tool

– Coolant should be used to minimi&e heat

Copper91er%lli"m


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Copper91er%lli"m

• 3eavy, hard, reddish/colored copper metal with +eryllium

added

– 3igh electrical and thermal conductivity.

– ood corrosion resistance and strength.

– Can be welded.

– omewhat ductile.

– 8ithstands high temperature.

• !achines well

– 3ighly abrasive to 3 Tooling.

– Coolant should be used to lubricate and minimi&e tool wear.

Copper'1ase# Allo%s3 1rass


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Copper 1ase# Allo%s3 1rass

• lloy of copper and &inc with good corrosion

resistance, easily formed, machines, and cast.

• everal forms of brass.

– lpha brasses up to '67 &inc, suitable for cold wor(ing.

– lpha " beta brasses Contain :L7/6$7 copper and used

in hot wor(ing.

• mall amounts of tin or antimony added to minimi&e

pitting effect of salt water.• =sed for water and gas line fittings, tubings, tan(s,

radiator cores, and rivets.

Copper'1ase# Allo%s3 1ron,e


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• lloys of copper and tin which contain up to "$7 of principal alloying element

– E)ception copper/&inc alloys

•

%hosphor/bron&e – J07 copper, "07 tin, and very small amount of phosphorus

– 3igh strength, toughness, corrosion resistance

–

=sed for loc( washers, cotter pins, springs and clutch discs

Copper 1ase# Allo%s3 1ron,e


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• ilicon/bron&e -copper/silicon alloy

– Contains less than :7 silicon

– trongest of wor(/hardenable copper alloys

– !echanical properties of machine steel and corrosion

resistance of copper

– =sed for tan(s, pressure vessels, and hydraulic pressure

lines



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Copper 1ase# Allo%s3 1ron,e• luminum/bron&e -copper/aluminum alloy

– Contains between L7 and ""7 aluminum

– Other elements added

Bron and nic(el -both up to :7 increases strength

ilicon -up to $7 improves machinability

!anganese promotes soundness in casting

– ood corrosion resistance and strength

– =sed for condenser tubes, pressure vessels, nuts and bolts

Effects of


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ff f

Temperat"re an# Friction

• 3eat created

– %lastic deformation occurring in metal during process of

forming chip

– 5riction created by chips sliding along cutting/tool face

• Cutting temperature varies with each metal and

increases with cutting speed and rate of metal

removal

Effects of


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Temperat"re an# Friction• Temperature of metal immediately ahead of cutting tool

comes close to melting temperature of metal being cut.

• reatest heat generated when ductile material of high

tensile strength is cut.• Dowest heat generated when soft material of low tensile

strength is cut.

• !a)imum temperature attained during cutting action.

– affects cutting/tool life, quality of surface finish, rate of

production and accuracy of wor( piece.

Friction


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Friction

• Aept low as possible for efficient cutting action

• Bncreasing coefficient of friction gives greater

possibility of built/up edge forming

– Darger built/up edge, more friction

– #esults in brea(down of cutting edge and poor surface finish

• Can reduce friction at chip/tool interface and help

maintain efficient cutting temperatures if use good supply of cutting fluid.

F t Aff ti + f Fi i h


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Factors Affecting +"rface Finish

• 5eed rate

• 4ose radius of tool

• Cutting speed • #igidity of machining operation

• Temperature generated during machining process

+"rface Finish


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+"rface Finish•

1irect relationship between temperature of wor( pieceand quality of surface finish

– 3igh temperature yields rough surface finish

– !etal particles tend to adhere to cutting tool and form

built/up edge

• Cooling wor( material reduces temperature of

cutting/tool edge

– #esult in better surface finish

thermal aspects in metal cutting

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