THEORY OF METAL CUTTING 2.ppt

7/24/2019 THEORY OF METAL CUTTING 2.ppt

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By

S. SAMPATH KUMAR

Assistant Professor

Mechanical Engineering Department

SRM Nagar Kattan!"lath"r # $%& '%&

ME (%%) Man"fact"ring Technology

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Unit # & THE*R+ *, META- UTT/N0


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Orthogonal and oblique cutting Classification of cutting tools: single,

multipoint Tool signature for single point cutting tool Mechanics of

orthogonal cutting Shear angle and its significance Chip formation

Cutting tool materials Tool wear and tool life Machinability Cutting

Fluids Simple problems

Sylla1"s

2

TE2T B**KS

! Sharma, "C, A textbook of Production Technology Vol I and II, S. Chand &

Comany !td., "e# #elhi, !$$%

& 'ao, "(, $anufacturing Technology, Vol I & II, Tata $c%ra# ill Publi'hing

Co., "e# (elhi, )**+.


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Metal "tting

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Metal cutting or Machining operation is to produce a desired shape, si)e and

finish of a component by remo*ing e+cess material in the form of chips

Chips may constitute more than -. of initial wor/ piece

Machining processes are performed on metal cutting machines, usingvarious types of cutting tools

Metal cutting process in general should be carried out at high speeds and feeds

with least cutting effort at minimum cost

,actors affecting metal c"tting! "roperties of 0or/ material

& "roperties 1 geometry of cutting tool

2 3nteraction between tool and wor/


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4+ternal

3nternal

threadingforminggroo*ingfacingturning


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Mechanics of Metal "tting5 cutting tool e+erts compressi*e force on the wor/piece which stresses the wor/

material beyond the yield point and therefore metal deform plastically and shears off

"lastic flow ta/es place in a locali)ed

region called the shear plane

Sheared material begins to flow along

the cutting tool face in the form of chips

Flowing chips cause tool wear

5pplied compressi*e force which leads

to formation of chips is called cutting

force

6eat produced during shearing action raises the temperature of the wor/peice,

cutting tool and chips

Temperature rise in cutting tool softens and causes loss of /eenness in cutting

edge

Cutting force, heat and abrasi*e wear are important features in metal cutting


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Cutting tools performs the main machining operation

3t is a body ha*ing teeth or cutting edges on it

They comprise of single point cutting tool or multipoint cutting tools

Types of "tting Tools


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Types of Metal "tting Process

Orthogonal cutting is also /nown as two dimensional metal cutting in which the cutting

edge is normal to the wor/ piece 7angle 8 $-deg9

Oblique cutting is also /nown as three dimensional cutting in which the cutting action

is inclined with the ob by a certain angle called the inclination angle 7angle ; $-deg9


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Single point c"tting tool 3 This type of tool has a effecti*e cutting edge and

remo*es e+cess material from the wor/ piece along the cutting edge

These tools may be left


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>eometry comprises mainly of nose, ra/e face of the tool, flan/, heel and shan/ etc

The nose is shaped as conical with different angles


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Types of hipsChips are separated from the wor/piece to impart the required si)e and shape

The chips that are formed during metal cutting operations can be classified into four

types: (. ontin"o"s chips

'. ontin"o"s chips 4ith 1"ilt5"p e6ge

&. Discontin"o"s or segmental chips.

7. Non homogeno"s chips

(. ontin"o"s chips

Chip is produced when there is low friction between the chip and tool face

This chip has the shape of long string or curls into a tight roll

Chip is produced when ductile materials such as 5l, Cu, MS, and wrought 3ron are

machined

Formation of *ery lengthy chip is ha)ardous to the machining process and the

machine operators


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3t may wrap up on the cutting tool, wor/ piece and interrupt in the cutting operation

3t becomes necessary to deform or brea/ long continuous chips into small pieces

3t is done by using chip brea/ers and this can be an integral part of the tool design

or a separate de*ice

'. ontin"o"s chips 4ith 1"ilt5"p e6ge

0hen high friction e+ists between chip and tool, the chip material welds itself to the

tool face

0elded material increases friction further which in turn leads to the building up a layer upon layer of chip material

=uild up edge grows and brea/s down when it becomes unstable

Chips with build up edge result in higher power consumption, poor surface finish and

large tool wear


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&. Discontin"o"s or segmental chips

Chip is produced in the form of small pieces

These types of chips are obtained while

machining brittle material li/e cast iron, brassand bron)e at *ery low speeds and high feeds

For brittle materials it is associated with fair

surface finish, lower power consumption and

reasonable tool life

For ductile materials it is associated with poor surface finish e+cessi*e tool wear

7. Non5homogeneo"s chips

3t will be in the form of notches and formed due to non


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hip ontrol an6 hip Brea!ers#uring machining high tensile strength materials chips has to be properly

controlled

Carbide tip tools will be used for high speeds which leads to high temperature andproduce continuous chips with blue color

3f the abo*e mentioned chips are not bro/en means it will ad*ersely effect the

machining in following ways,

?Spoiling cutting edge

?'aising temperature

?"oor surface finish

?6a)ardous to machine operator

Two ways are employed to o*ercome all the abo*e drawbac/s

First one is Proper selection of c"tting con6itions an6 secon6 one is

chip 1rea!ersare used to brea/ the chips


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Proper selection of c"tting con6itions

Since the cutting speed influences to the great e+tend the producti*ity of

machining and surface finish, wor/ing at low speeds may not be desirable

3f the cutting speed is to be /ept high, changing the feed and depth of cut is areasonable solution for chip control

hip 1rea!er

There are two types of chip brea/ers

!@ 4+ternal type, an inclined obstruction clamped to the tool face

& 3ntegral type, a groo*e ground into the tool face or bulges formed onto the toolface

clamped


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N*MEN-ATURE *f S/N0-E P*/NT T**-


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,ee6

Bac! ra!e angle 891:

3t is the angle between the face of the tool and a line parallel with base of the tool

measured in a perpendicular plane through the side cutting edge

This angle helps in remo*ing the chips away from the wor/ piece


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Si6e ra!e angle 89s:

3t is the angle by which the face of tool is inclined side ways

This angle of tool determines the thic/ness of the tool behind the cutting edge

3t is pro*ided on tool to pro*ide clearance between wor/ piece and tool so as to

pre*ent the rubbing of wor/< piece with end flan/ of tool

En6 relief angle3t is defined as the angle between the portion of the end flan/ immediately below

the cutting edge and a line perpendicular to the base of the tool, measured at rightangles to the flan/

3t is the angle that allows the tool to cut without rubbing on the wor/< piece

Si6e relief angle3t is the angle that pre*ents the interference as the tool enters the material

3t is the angle between the portion of the side flan/ immediately below the side

edge and a line perpendicular to the base of the toolmeasured at right angles to

the side


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En6 c"tting e6ge angle3t is the angle between the end cutting edge and a line perpendicular to the shan/

of the tool

3t pro*ides clearance between tool cutting edge and wor/ piece

Si6e c"tting e6ge angle3t is the angle between straight cutting edge on the side of tool and the side of the

shan/

3t is also /nown as lead angle

3t is responsible for turning the chip away from the finished surface


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Tool Signat"reCon*enient way to specify tool angles by use of a standardi)ed abbre*iated

system is /nown as tool signature or tool nomenclature

The se*en elements that comprise the signature of a single point cutting tool canbe stated in the following order:

Tool signature -2 4nd relief angle 7%9

D Side relief angle 7B9

4nd cutting edge angle 7!9

% Side cutting edge angle 7!%9A (ose radius 7-B mm9


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Properties of c"tting tool materials

(. Re6 har6ness or Hot Har6ness3 3t is the ability of a material to retain its hardness

at high temperature

'. ;ear resistance3 3t enables the cutting tool to retain its shape and cutting efficiency

&. To"ghness3 3t relates to the ability of a material to resist shoc/ or impact loads

associated with interrupted cuts

lassification tool materials1. Carbon-Tool Steels:

-%

Gow carbon *arieties possess good toughness 1 shoc/ resistance

6igh carbon *arieties possess good abrasion resistance

2. High Speed Steels (HSS):

6igh carbonE little amount Tungsten, Molybdenum, Cr, 1 cobalt to increase

hardness, toughness and wear rHsistance

6igh operating temperatures upto %--oC


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Two types of 6SS ie, is T


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Straight Tungsten Carbide Cobalt >rade: Cast iron, non ferrous alloys, plastics,

wood, glass etc

5lloyed Tungsten Carbide >rade:5ll grades of steel at 2 to D times more speeds

than 6SS!. Cerai" Tools: 5luminium O+ide, Silicon Carbide, =oron Carbide, Titanium Carbide, Titanium

=oride

6igh speed, longer tool life, superior surface finish, (o coolant is required

#. $iaond Tools:

More abrasion resistance

Ised for turning grinding wheels

Ised to produce mirror surface finish

#iamond abrassi*e belts are used to produce T screens

"oly crystalline diamond inserts are bra)ed into cutting edges of circular saws for

cutting construction materials li/e concrete, refractories, stone etc


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Tool -ife

"roperly designed cutting tool is e+pected to perform the metal cutting operation

in an effecti*e an smooth manner

3f a tool is not gi*ing satisfactory performance it is an indicati*e of tool failure

Following are the ad*erse effects obser*ed during operationJ

#uring operation cutting tool may fail due to followingJ

4+tremely poor surface finish on the wor/piece

6igher consumption of power

0or/ dimensions are not produced as specified

O*erheating of cutting tool

5ppearance of burnishing band on the wor/ surface

Thermal crac!ing an6 softening

Mechanical hipping

0ra6"al 4ear


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(.Thermal rac!ing an6 Softening#uring cutting operation lot of heat will be generated due to this cutting tool tipand area closer to cutting edge will become hot

Cutting tool material will be harder up to certain limit 7temperature 1 pressure9,

if it crosses the limit it starts deforming plastically at tip and adacent to the cutting

edge under the action of cutting pressure and high temperature

Tool looses its cutting ability and it is said to ha*e failed due to softening

Main factors responsible for this condition areJ

6igh cutting speed6igh feed rate

More depth of cut

Small nose radius

Choice of wrong tool material

Tool life is 6efine6 as the tie inter%al &or 'hi"h tool 'ors satis&a"torily

bet'een t'o s""essi%e grinding or re-sharpening o& the tool.


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0or/ing temperatures for common tool materials areJ

ar1on tool steels '%%o 5 '


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'. Mechanical hipping

Mechanical chipping of nose an cutting edge of the tool are commonly obser*ed

causes for tool failure

'easons for failure are 6igh cutting pressure, Mechanical impact, 4+cessi*e

wear, too high *ibrations and wea/ tip an cutting edge, etc

This type of failure is pronounced in carbide tipped and diamond tools due to

high brittleness of tool material

hippe6 Tip


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&. 0ra6"al 4ear

0hen a tool is in use for some time it is found to ha*e lost some weight or

mass implying that it has lost some material from it due to wear

;ear locations3

rater 4ear location

,lan! 4ear location

Crater wear

#ue to pressure of the hot chip sliding

up the face of the tool, crater or a

depression is formed on the face of tool

7#uctile materials9

=y diffusion shape of crater formed

corresponds to the shape of underside

of the chip

rater 4ear


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,lan! 4ear

*cc"rs 1et4een tool an6 4or!piece

interface

D"e to a1rasion 1et4een tool flan!

an6 4or!piece

The entire area s"1=ecte6 to flan!

4ear is !no4n as ;EAR -AND 8>B:

occ"rs on tool nose front an6 si6e

relief faces


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Machina!ility"he ma#or factor involved in metal cutting are,

$orces and po%er a!sor!ed

"ool %ear and tool life

&urface 'nish

(imensional accuracy

Machining cost

"his factor depend upon a large varia!les, suchas

)roperty of %or* material"ool geometry

+utting condition

Machine tool rigidity32


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ualitative measure ofmachina!ility

1- "he easy %ith %hich it could !e machined,2- "he life of tool !efore tool failure or re

sharpening

3- "he .uality of machined surface-

4- "he po%er consumption per unit volumeof material removed-

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The machinability may be evaluated asgiven below

/ong tool life at a given cutting speed/o% po%er consumption per unit volume of

material removed-

Ma0imum metal removal per tool resharpening

igh .uality of surface 'nish

ood and uniform dimensional accuracy of

successive partsasy disposa!le chips-

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Cutting FluidsTypes and Applications

"tting ,l"i6s

4ssential in metal


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"tting fl"i6is a type of coolant and lubricant designed specifically formetalwor/ingand machiningprocesses

There are *arious /inds of cutting fluids, which include oils, oil


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Economic A6@antages to Using "tting ,l"i6s

Re6"ction of tool costs

'educe tool wear, tools life longer

/ncrease6 spee6 of pro6"ction

'educe heat and friction so higher cutting speeds

Re6"ction of la1or costs

Tools life longer and require less regrinding, less downtime, reducing

cost per part

Re6"ction of po4er costs

Friction reduced so less power required by machining

h t i ti f 0 6 tti ,l i6


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haracteristics of a 0oo6 "tting ,l"i6

>ood cooling capacity

>ood lubricating qualities

'elati*ely low *iscosity

Stability 7long life9

'ust resistance

(onto+ic

Transparent

(onflammable


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Types of "tting ,l"i6s

Most commonly used cutting uids

ither a.ueous !ased solutions orcutting oils

"hree categories+utting oilsmulsi'a!le oils+hemical synthetic cutting uids

THEORY OF METAL CUTTING 2.ppt

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