ch21 fundamentals of cutting2

8/9/2019 Ch21 Fundamentals of Cutting2

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Chapter 21

Fundamentals of Machining


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Introduction to Cutting - CommonIntroduction to Cutting - Common

Machining OperationsMachining Operations

Figure 21.1 Some examples of common

machining operations.

Cutting processes remove material

from the surface of a workpiece by

producing chips.

Turning, in which the workpiece is

rotated and a cutting tool removes a

layer of material as the tool moves to the

left.Cutting off:in which the cutting tool

moves radially inward and separates the

right piece from the bulk of the blank.

Slab milling: in which a rotating cutting

tool removes a layer of material from thesurface of the workpiece.

End milling:in which a rotating cutter

travels along a certain depth in the

work-

piece and produces a cavity.


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Figure 21.2 Schematic illustration of the

turning operation showing various

features.

In the turning process, illustrated

the cutting tool is set at a certain

depth of cut (mm) and travels to

the left with a certain velocity as

the workpiece rotates. The feed,

or feed rate, is the distance thetool travels horiontally per unit

revolution of the workpiece

(mm!rev). This movement of the

tool produces a chip, which movesup the face of the tool.

Introduction to Cutting The TurningIntroduction to Cutting The Turning

OperationOperation


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Two-DimensionalTwo-Dimensional

Cutting rocessCutting rocess

Figure 21.3 Schematic illustration of a

two-dimensional cutting process, also

called orthogonal cutting !a" #rthogonal

cutting with a well-defined shear plane,

also $nown as the Merchant Model. %ote

that the tool shape, depth of cut, to, and the

cutting speed, V, are all independentvaria&les, !&" #rthogonal cutting without a

well-defined shear plane.


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Introduction to cuttingIntroduction to cutting

" Compare !igs" #$"# and #$"%&

and note that:

#eed in turning is e$uivalent to t%

&epth of cut in turning is

e$uivalent to width of cut

(dimension perpendicular to the

page) in the idealied model.

These relationships can be

visualied by rotating #ig. '%.

C by *%o.


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!actors Influencing Machining Operations!actors Influencing Machining Operations


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" Independent 'ariables in the cutting process:

Tool material, coatings and tool condition.

Tool shape, surface finish, and sharpness. orkpiece material, condition, and temperature.

Cutting parameters, such as speed, feed, and depth of cut.

Cutting fluids.

The characteristics of the machine tool, such as its stiffness anddamping.

orkholding and fi+turing.

" Dependent 'ariables:

Type of chip produced.

#orce and energy dissipated in the cutting process.

Temperature rise in the workpiece, the chip, and the tool.

ear and failure of the tool.

urface finish produced on the workpiece after machining.

!actors Influencing Machining Operations!actors Influencing Machining Operations


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Orthogonal cuttingOrthogonal cutting

Chip !ormation b( ShearingChip !ormation b( Shearing

Figure 21.' !a" Schematic illustration of the &asic mechanism of

chip formation &( shearing. !&" )elocit( diagram showing angular

relationships among the three speeds in the cutting *one.

+ The tool has a rakeangle of , and a

relief (clearance)

angle.

+ The shearing

process in chipformation (#ig.

'./a ) is similar to

the motion of cards

in a deck sliding

against each other.


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T)E MEC)*+ICS O! C)I !O,M*TIO+-T)E MEC)*+ICS O! C)I !O,M*TIO+-

O,T)OO+*. C/TTI+O,T)OO+*. C/TTI+

+ The ratio of to!tcis known as the cutting ratio, r, e+pressed as0

+ Chip thickness is always greater than the depth of cut

+ Chip compression ratio0 reciprocal of r. It is a measure of how

thick the chip has become compared to the depth of cut.

('.)

+ The cutting ratio is an important and useful parameter for

evaluating cutting conditions. ince the undeformed chip

thickness, to, is a machine setting and is therefore known, the

cutting ratio can be calculated easily by measuring the chip

thickness with a micrometer.


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O,T)OO+*. C/TTI+O,T)OO+*. C/TTI+" The shear strain, 1, that the material undergoes can be e+press as0

('.')

" 2arge shear strains are associated with low shear angles, or low or negative

rake angles." hear strains of 3 or higher in actual cutting operations.

" &eformation in cutting generally takes place within a very narrow

deformation one4 that is, d 5 6C in #ig. /.'-a is very small.

" Therefore, the rate at which shearing takes place is high." hear angle influences force and power re$uirements, chip thickness, and

temperature.

" Conse$uently, much attention has been focused on determining the

relationships between the shear angle and workpiece material properties

and cutting process variables.


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O,T)OO+*. C/TTI+O,T)OO+*. C/TTI+" 7ssuming that the shear angle ad8usts itself to minimie the cutting force,

or that the shear plane is a plane of ma+imum shear stress.

('.)

9 is the friction angle and is related to the coefficient of friction, :, at the tool

; chip interface (rake face)0

o #rom


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O,T)OO+*. C/TTI+O,T)OO+*. C/TTI+" #rom #ig. '%., since chip thickness is greater than the depth of cut, the

velocity of the chip, Bc, has to be lower than the cutting speed, B.

" Conservation of mass0

('.3)

('.)

" Bsis the velocity at which shearing

takes place in the shear plane.

+ #rom the velocity diagram (#ig. '%./b), we obtain the


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T0ES O! C)IS ,OD/CED I+ MET*.-T0ES O! C)IS ,OD/CED I+ MET*.-

C/TTI+C/TTI+

1" Continuous

#" 2uilt-up Edge

%" Serrated or Segmented

3" Discontinuous

7 chip has two surfaces0

.6ne that is in contact with the tool face (rake face). This surface is

shiny, or burnished.

'.The other from the original surface of the workpiece. This surface

does not come into contact with any solid body. This surface has a8agged, rough appearance (#ig. '%.), which is caused by the

shearing mechanism shown in fig. '%./a.


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Figure 21. asic t(pes of chips produced in orthogonal metal cutting, their schematic representation, and

photomicrographs of the cutting *one !a" continuous chip with narrow, straight, and primar( shear *one/ !&"

continuous chip with secondar( shear *one at the chip-tool interface/ !c" &uilt-up edge/ !d" segmented ornonhomogeneous chip/ and !e" discontinuous chip. Source 0fter M.C. Shaw, .. right, and S. alpa$4ian.

+ #ormed with ductile

materials at high cutting

speeds and!or high rake

angles (#ig. '.3a).

+ &eformation of the

material takes place alonga narrow shear one,

primary shear one.

+ CCs may, because of

friction, develop a

secondary shear one at

tool;chip interface (#ig.'.3b).

+ The secondary one

becomes thicker as tool;

chip friction increases.+ In CCs, deformation may

also take place along awide primary shear one

with curved boundaries

(#ig. '.b).


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T0ES O! C)IS ,OD/CED I+ MET*. -T0ES O! C)IS ,OD/CED I+ MET*. -

C/TTI+: CO+TI+/O/S C)IS 4CC5C/TTI+: CO+TI+/O/S C)IS 4CC5

" The lower boundary is below the machined surface, sub8ecting

the machined surface to distortion, as depicted by the distorted

vertical lines.

" This situation occurs particularly in machining soft metals at

low speeds and low rake angles.

" It can produce poor surface finish and induce residual surface

stresses.

" 7lthough they generally produce good surface finish, CCs are

not always desirable.


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C/TTI+: 2uilt-up-Edge Chips 42/E5C/TTI+: 2uilt-up-Edge Chips 42/E5

" =D


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2uilt-up Edge2uilt-up Edge

Figure 21.5 !a" 6ardness distri&ution with a &uilt-up edge in the cutting *one !material, 311 steel".

%ote that some regions in the &uilt-up edge are as much as three times harder than the &ul$ metal of

the wor$piece. !&" Surface finish produced in turning 137 steel with a &uilt-up edge. !c" Surface

finish on 1718 steel in face milling. Magnifications 1x. Source Courtes( of Metcut 9esearch

0ssociates, :nc.

(b)

(c)


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C/TTI+: Serrated ChipsC/TTI+: Serrated Chips

" errated chips0 semi-continuous chips with ones of low and

high shear strain (#ig. '.3e).

" Aetals with low thermal conductivity and strength that

decreases sharply with temperature, such as titanium, e+hibit

this behavior.

" The chips have a saw-tooth-like appearance.


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C/TTI+: Discontinuous Chips 4DCs5C/TTI+: Discontinuous Chips 4DCs5

" &Cs consist of segments that may be firmly or loosely

attached to each other (#ig. '.3f ).

" &Cs usually form under the following conditions0

. =rittle workpiece materials'. orkpiece materials that contain hard inclusions and

impurities, or have structures such as the graphite flakes in

gray cast iron.

. Bery low or very high cutting speeds./. 2arge depths of cut.

3. 2ow rake angles.

. 2ack of an effective cutting fluid.

F. 2ow stiffness of the machine tool.


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" =ecause of the discontinuous nature of chip formation, forces

continually vary during cutting.

" >ence, the stiffness or rigidity of the cutting-tool holder, the

orkholding devices, and the machine tool are important in

cutting with both &C and serrated-chip formation


C/TTI+: Discontinuous Chips 4DCs5C/TTI+: Discontinuous Chips 4DCs5


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Chip 2rea6erChip 2rea6er

Figure 21.; !a" Schematic illustration of the action of a chip &rea$er. %ote that the chip&rea$er decreases the radius of curvature of the chip and eventuall( &rea$s it. !&" Chip

&rea$er clamped on the ra$e face of a cutting tool. !c"


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Chips roduced in TurningChips roduced in Turning

Figure 21.8 Chips produced in turning !a" tightl( curled chip/ !&" chip hits wor$piece and

&rea$s/ !c" continuous chip moving radiall( awa( from wor$piece/ and !d" chip hits tool

shan$ and &rea$s off. Source: 0fter


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C/TTI+: Chips 2rea6ers 4C2s5C/TTI+: Chips 2rea6ers 4C2s5

" ith soft workpiece materials such as pure aluminum or

copper, chip breaking by such means is generally not effective.

" Common techni$ues used with such materials, include

machining at small increments and then pausing (so that a chip

is not generated) or reversing the feed by small increments.

" In interrupted cutting operations, such as milling, chip

breakers are generally not necessary, since the chips already

have finite lengths because of the intermittent nature of the

operation.


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T)E MEC)*+ICS O! O2.I7/E C/TTI+T)E MEC)*+ICS O! O2.I7/E C/TTI+

" Chip in #ig. '.*a f lows up the

rake face of the tool at angle c(chip flow angle), which is

measured in the plane of the

tool face.

" 7ngle n, the normal rake

angle, is a basic geometric

property of the tool. This is theangle between the normal o to

the workpiece surface and the

line oa on the tool face.

" The workpiece material

approaches the tool at a

velocity B and leaves thesurface (as a chip) with a

velocity BcFigure 21.= !a" Schematic illustration of cutting with an

o&li>ue tool. %ote the direction of chip movement. !&" ?op

view, showing the inclination angle, i,. !c" ?(pes of chips

produced with tools at increasing inclination angles.


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"


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,ight-hand Cutting Tool and Insert,ight-hand Cutting Tool and Insert

Figure 21.17 !a" Schematic illustration of right-hand cutting tool. ?he various

angles on these tools and their effects on machining are descri&ed in Section

23.3.1 0lthough these tools traditionall( have &een produced from solid tool-steel

&ars, the( have &een replaced largel( with !&" inserts made of car&ides and other

materials of various shapes and si*es.


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C/TTI+ !O,CES *+D O8E,C/TTI+ !O,CES *+D O8E,

" Cutting force, #c, acts in the direction of cutting speed, B, and

supplies energy re$uired for cutting.

" Thrust force, #t , acts in a direction normal to cutting velocity,

perpendicular to E. The resultant force, ? can be resolved into

two components 0

#riction force0 #, along the tool-chip interface

@ormal force0 @, perpendicular to it.

# 5 ? sin 9 ('.Ga)

@ 5 ? cos 9 ('.Gb)? is balanced by an e$ual and opposite force along the shear plane and

is resolved into a shear force, #s, and a normal force, #n

#s 5 #c cos ; #t sin ('.*)

#n 5 #c sin J #t cos ('.%)


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Cutting !orcesCutting !orces

Figure 21.11 !a" Forces acting on a cutting tool during two-dimensional cutting. %ote

that the resultant force, R, must &e collinear to &alance the forces. !&" Force circle to

determine various forces acting in the cutting *one.


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" The ratio of # to @ is the coefficient of friction, :, at the tool-

chip interface, and the angle 9 is the friction angle.

('.)

" The coefficient of friction in metal cutting generally ranges

from about %.3 to '.

tan

tanfriction,oftCoefficien

tc

ct

FF

FF

N

F

+==



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Thrust !orceThrust !orce

" If the thrust force is too high or if the machine tool is not

sufficiently stiff, the tool will be pushed away from the surface

being machined.

" This movement will, in turn, reduce the depth of cut, resulting

in lack of dimensional accuracy in the machined part, 7s the

rake angle increases and!or friction at the rake face decreases,

this force can act upward.

" This situation can be visualied by noting that when : 5 %

(that is, 9 5 %), the resultant force, ?, coincides with thenormal force, @.

" In this case, ? will have a thrust-force component that is

upward.


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C/TTI+ !O,CES *+D O8E, -C/TTI+ !O,CES *+D O8E, -

owerower


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,ange of Energ( ,e9uirements in Cutting Operations,ange of Energ( ,e9uirements in Cutting Operations


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"


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"


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TEME,*T/,E I+ C/TTI+TEME,*T/,E I+ C/TTI+

" The main sources of heat generation are the primary shear

one and the tool-chip interface.

" If the tool is dull or worn, heat is also generated when the tool

tip rubs against the machined surface.

" Cutting temperatures increase with0

. strength of the workpiece material

'. cutting speed

. depth of cut" Cutting temperatures decrease with increasing specific heat

and thermal conductivity of workpiece material.


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" The mean temperature in turning on a lathe is proportional to the

cutting speed and feed0

Aean temperature Bafb ('.*)

" a and b are constants that depend on tool and workpiece

materials, B is the cutting speed, and f is the feed of the tool.

" Aa+ temperature is about halfway up the face of the tool.

" 7s speed increases, the time for heat dissipation decreases and

temperature rises

Tool Material a B

Car&ide 7.2 7.12

6SS 7. 7.3;

TEME,*T/,E I+ C/TTI+TEME,*T/,E I+ C/TTI+


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Temperatures in Cutting oneTemperatures in Cutting one

Figure 21.12 ?(pical temperature

distri&ution in the cutting *one. %ote the

severe temperature gradients within the

tool and the chip, and that the wor$piece is

relativel( cool. Source 0fter


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Temperatures De'eloped in Turning ;#1$$ SteelTemperatures De'eloped in Turning ;#1$$ Steel

Figure 21.13 ?emperatures developed in turning 2177 steel !a" flan$ temperature

distri&ution and !&" tool-ship interface temperature distri&ution. Source 0fter . ?.

Chao and . @. ?rigger.


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TOO. .I!E: 8E*, *+D !*I./,ETOO. .I!E: 8E*, *+D !*I./,E

" Conditions that would cause tool wear0

. >igh localied stresses

'. >igh temp

. liding of chip along the rack face/. liding of the tool along the machined surface

" The rate of wear depends on0

. Tool and workpiece materials

'. Tool shape

. Cutting fluids

/. Erocess parameters

3. Aachine tool characteristics


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" The Cutting tool should be replaced when:

. urface finish of the workpiece is poor

'. Cutting force increases significantly

. Temperature increases significantly/. Eoor dimensional accuracies


8ear atterns on Tools8ear atterns on Tools Fi 21 1 ! " Fl $


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8ear atterns on Tools8ear atterns on Tools Figure 21.1 !a" Flan$wear and crater wear in a

cutting tool/ the tool

moves to the left as in

Fig. 21.3. !&" )iew of the

ra$e face of a turningtool, showing various

wear patterns. !c" )iew

of the flan$ face of a

turning tool, showing

various wear patterns.

!d" ?(pes of wear on a

turning tool 1. flan$wear/ 2. crater wear/ 3.chipped cutting edge/ '.

thermal crac$ing on ra$eface/ . &uilt-up edge/ 5.

catastrophic failure. !See

also Fig. 21.18."Source

Courtes( of ennametal,

:nc.


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" BTn5 C (Taylor


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Ta(lor Tool .ife E9uationTa(lor Tool .ife E9uation

VTn = C

VTndxf

y= C

?a(lor A>uation


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Effect of 8or6piece )ardness and MicrostructureEffect of 8or6piece )ardness and Microstructure

on Tool .ifeon Tool .ife

Figure 21.15 Affect of wor$piece hardness and microstructure on tool life in turning ductile cast iron. %otethe rapid decrease in tool life !approaching *ero" as the cutting speed increases. ?ool materials have &een

developed that resist high temperatures, such as car&ides, ceramics, and cu&ic &oron nitride, as will &e

descri&ed in Chapter 22.


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Tool-life Cur'esTool-life Cur'es

Figure 21.1; ?ool-life curves for a

variet( of cutting-tool materials.

?he negative inverse of the slope

of these curves is the exponent nin the ?a(lor tool-life e>uation andCis the cutting speed at TB 1

min, ranging from a&out 277 to

17,777 ft.min in this figure.


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*llowable 8ear .and*llowable 8ear .and

" B= in fig '.3c for various machining conditions is given in

table './

" 6ptimum cutting speed


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Crater 8earCrater 8ear

" 6ccurs on the rake face (fig '.3a, b, and d and fig '.G)

" #actors influencing crater wear0

. temp at tool-chip interface

'. chemical 7ffinity between tool and workpiece materials. factors for flank wear

" &iffusion mechanism, movement of atoms across tool-chip

interface.

" &iffusion rate increases with temp, so increasing crater wear(fig '.*)


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T(pes of 8ear seen in Cutting ToolsT(pes of 8ear seen in Cutting Tools

Figure 21.18 !a" Schematic illustration of t(pes of wear o&served on various cutting tools. !&"

Schematic illustrations of catastrophic tool failures. 0 wide range of parameters influence these wear

and failure patterns. Source Courtes( of ). C. )en$atesh.


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,elationship between Crater-8ear ,ate and,elationship between Crater-8ear ,ate and

*'erage Tool-Chip Interface Temperature*'erage Tool-Chip Interface Temperature

Figure 21.1= 9elationship &etween crater-wear rate and average tool-chip interface

temperature 1" 6igh-speed steel, 2" C-1 car&ide, and 3" C- car&ide !see ?a&le 22.'".

%ote how rapidl( crater-wear rate increases with an incremental increase in temperature.Source 0fter . ? Chao and . @ ?rigger.


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Cutting Tool Interface and ChipCutting Tool Interface and Chip

Figure 21.27 :nterface of a cutting

tool !right" and chip !left" in

machining plain-car&on steel. ?he

discoloration of the tool indicates

the presence of high temperatures.Compare this figure with the

temperature profiles shown in Fig.

21.12. Source Courtes( of . .

right.



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ChippingChipping

" Chipping0 small fragments from the C< breaks away Nsudden

loss of materialO . =rittle CT like ceramic.

" Two main causes0

. Aechanical shock0 (i.e., impact due to interrupted cutting, as in

turning a splined shaft on a lathe).

'. Thermal fatigue0 (i.e., cyclic variations in the temperature of the

tool in interrupted cutting)

" Thermal cracks normal to the cutting edge of the tool (fig '.Ga)

" Chipping may occur in a region in the tool where a small crack or

defect already e+ists

" >igh Jve rake angles can contribute to chipping

" ItPs possible for crater wear region to progress toward the tool tip,

weakening the tip and causing chipping



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eneral obser'ations on tool weareneral obser'ations on tool wear" &ue to decreasing in yield strength from high temp during cutting, tools may

soften and undergo plastic deform" This type of deformation generally occurs when machining high-strength

metals and alloys.

" Therefore, tools must be able to maintain their strength and hardness at

elevated temperature.

" ear groove or notch on cutting tools is due to0

. This region is the boundary where chip is no longer in contact with the tool

'. This boundary known as &6C line, oscillates because of inherent variations

in the cutting operation and accelerates the wear process

. This region is in contact with the machined surface from the previous cut/. ince a machined surface may develop a thin work-hardened layer, this

contact could contribute to the formation of the wear groove

" 2ight cuts should not be taken on rusted workpieces.


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Tool Condition MonitoringTool Condition Monitoring

1" Direct techni9ues" optical measurement of wear

" done using toolmaker microscope

1" Indirect methods

" correlation of tool condition with process variables0 forces,

power, temp rise, surface finish, and vibration

*" *coustic emission techni9ue 4*E5

" Dtilies a pieo-electric transducer attached to tool holder

" The transducer picks up acoustic emissions that result from the

stress waves generated during cutting.

" =y analying the signals, tool wear and chipping can be

monitored


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2" Transducers are installed in original machine tools" Continually monitor tor$ue and forces during cutting

" ignals are pre-amplified and microprocessor analyses andinterprets their content

" The system is capable of differentiating the signals that comefrom tool breakage, tool wear, a missing tool, overloading ofthe machine, or colliding machine comp

" The system also auto compensate for tool wear and thus

improve dim accuracyC" Monitoring b( tool-c(cle time

" In C@C e+pected tool life in entered into the machine controlunit, when it is reached, the operator makes the tool change.


Tool Condition Monitoring Indirect methodsTool Condition Monitoring Indirect methods

. th T l D t.athe Tool D namometer


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.athe Tool D(namometer.athe Tool D(namometer


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S/,!*CE !I+IS) *+D S/,!*CES/,!*CE !I+IS) *+D S/,!*CE


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S/,!*CE !I+IS) *+D S/,!*CE

I+TE,IT0I+TE,IT0

(a) (b)

Figure 21.21 Machined surfaces produced on steel!highl( magnified", as o&served with a scanning electron

microscope !a" turned surface and !&" surface produced&( shaping. Source Courtes( of @. ?. lac$ and S.

9amalingam.

?ubbing generates heat

and induce residual stresses

causing surface

damage

&6C should be greater

than the radius of thecutting edge.

the built-up edge has the

greatest influence on

surface finish. Figure 21.21

Dull Tool in Orthogonal MachiningDull Tool in Orthogonal Machining


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Dull Tool in Orthogonal MachiningDull Tool in Orthogonal Machining

#igure '.'' chematic illustration of a dull tool with respect to the depthof cut in orthogonal machining (e+aggerated). @ote that the tool has a

positive rake angle, but as the depth of cut decreases, the rake angle

effectively can become negative. The tool then simply rides over the

workpiece (without cutting) and burnishes its surface4 this action raises

the workpiece temperature and causes surface residual stresses.


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M*C)I+*2.IT0M*C)I+*2.IT0

" Aachinability of a material is defined in terms of / factors0

. # and I of the machined part

'. tool life

. force and power re$

/. chip control" Tool life and #0 most important factors in machinability

" Machinabilit( ratings

based on a tool life, T 5 %min

standard material is 7II ' steel (resulfuried), given a

rating of %%

for a tool life of % min, this steel should be machined at speed

of %% ft!min

M*C)I+*2.IT0


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machinabilit( of steelsmachinabilit( of steels

" Improved by adding lead and sulfur (free machining steels)

" ,esulfuri


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leaded steelsleaded steels

" >igh Q of lead in steels solidifies at the tip of manganese sulfide

inclusions

" in non-resulfuried steels, lead takes the form of dispersed fine

particles

" Eb acts as solid lubricant because of low shear strength

" hen temp is high, Eb melts in front of the tool, acting as a li$uid

lubricant

" =ismuth and tin are possible substitutes for lead in steel

"Calcium-Deo?idi


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Stainless steelsStainless steels

" 7ustentic steels (%% series) are generally difficult to machine

" Chatter can be a problem, need machine tools with high

stiffness

" #errite steels have good machinability

" Aartensistic steels are abrasive, tend to form =D


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Effects of other elements in steelEffects of other elements in steel

" 7l and i is always harmful because they combine with 6 toform aluminium o+ide and silicates, which are hard and

abrasive

" C and An have various effects on machinability, depending on

their composition." Elain low carbon steels (R %.3Q C) can produce poor # by

forming =D

ch21 fundamentals of cutting2

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