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3.2. Study of the chips

Chip types in machining are determined by the combined eects of workpiece and

tool materials, cutting parameters, and tool geometry. Usually, the type of chip and

the undersurface morphology are a direct indicator of frictional conditions at thetool/chip interface. ess friction and wear corresponds to curlier chips and a smoother

undersurface morphology !"3#. Chips were collected after the tests at "$$ m/min. %ig.

2shows S&' images of the chips for the four cutting tools at low (left) and high (right)

magni*cations. +he lower magni*cation images reeal some tearing of the chips,

which were ad-acent to the leading tool edge due to the small depth of cut and large

nose radius used in the machining test. n the higher magni*cation images, the

smoothness of the chips undersurfaces can be obsered. C+" and C+2 were

preiously studied by 0awada1+omkiewick !"# in the cutting of low chromium alloy

steel, the chips of which showed similar smooth undersurfaces. arpat and

45el !"6# showed similar geometries by 37 *nite elemental analysis (%&8). n the

present study, chips produced by each cutting tool looked similarly smooth and no

concluding remarks could be obtained directly from these morphological

obserations.

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%ig. 2. 'orphology by S&' at low (left) and high (right) magni*cation of the chips from the

four dierent cutting tools9 (a) C+", (b) C+2, (c) C+3, and (d) C+.

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+he CC: is a parameter de*ned as the ratio of the deformed chip thickness to its

undeformed thickness !";#. CC: is a direct indication of the frictional condition on the

tool surface, which helps eith an increase in the cutting speed to about

$?6$ m/min, the CC: increases dramatically to a peak of about .26 (C+3 and C+)

or 6 (C+" and C+2). %urther increases in the cutting speed lead to a gradual decrease

in CC: towards the preiously stated low speed CC: alues. n general, chips

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produced by C+2 had the highest alues of CC: whereas chips produced by C+3 had

the lowest CC: alues. +hese results could indicate greater adhesion between the c1

@A cutting tool and the workpiece material, thus accelerating the wear rate more

than for the 8l2B3+iC and multilayer coated carbide tools.

%ig. = shows S&' images of chip shapes for a cutting speed of =6 m/min, a feed rate

of $.$2 mm/tooth, and a depth of cut of $.6 mm under dry, wet, and A 2 machiningconditions. n A2 coolant machining, shorter chips are obtained compared to those in

dry and wet machining. +his may be attributed mainly due to better penetration of

A2 into the tool?chip interface, resulting in the reduction of the cutting temperature.

8t a lower temperature, the chip cannot promote curl due to increased chip hardness

and lower ductility. %urther, the colour of the chips obtained under the three

machining conditions is also dierent. n dry cutting, the chips produced are dark

blue in colour, which was caused by the e


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%ig. =. S&' photographs of chips at cutting speed of =6 m/min, feed rate of $.$2 mm/tooth

and depth of cut of $.6 mm for the (a) dry machining, (b) wet machining, and (c)

A2 machining cases.

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%ig. E shows S&' images of the chip morphology for a cutting speed of "26 m/min, a

feed rate of $.$2 mm/tooth, and a depth of cut of $.6 mm under dry, wet, andA2 machining conditions. n dry cutting, large serrated teeth are obtained indicating

the heay shearing action at the cutting 5one. n the case of wet machining also,

similar serrated teeth are produced due to the intensie shearing action. +his can be

attributed to the fact that conentional coolant does not proide eectie cooling and

lubrication at the tool?chip interface. Foweer, smaller serrated teeth are produced

under A2 machining, indicating lower shearing forces compared to those under dry

and wet machining. +his is because, the penetration of A2 into the tool?chip

interface results in the formation of a nitrogen cushion, which reduces the friction.

%ig. E. S&' micrographs of chips at cutting speed of "26 m/min, feed rate of $.$2 mm/tooth

and depth of cut of $.6 mm for the (a) dry machining, (b) wet machining, and (c)

A2 machining cases.

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%ig. . Chip morphology as a function of cutting speed and feed in the orthogonal cutting of

+i?;8l?G alloy of 33$ FG. 8ll scale bars in (b) are "$$ Hm.

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%ig. 6. S&'s of the free surface of chips formed from +i?;8l?G alloy under the conditions

listed. +he arrows indicate the chip Iow direction.

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%ig. ;. (a) amellae on the free surface of a continuous chip produced from a tool steel of 6F:C with cutting speed, . (a) , (b) . (c) J%oldsK on

the free surface of a continuous chip produced from the same steel (as in (a)) of L F:C

with . (d) llustration of the manner in which the depth of cut increases shortly

after initial tool engagement. (e) amellae on the free surface of a continuous chip produced

from +i?;8l?G alloy, . (f) %olds on the free surface of a continuous

chip produced from +i?;8l?G alloy, .

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%ig. =. S&'s of a localised shear band in a +i?;8l?G chip, , . +he arrows point to localised shear bands. Aote the elongation of the (light) M1phase

lamellae in (b).

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%ig. E. Shear surfaces on the underside of saw1tooth segments9 (a) ,

(b), ,fNE$Hm. +he arrows indicate the chip Iow direction

3.3. Chip analysis

Chips, formed during the two machining methods with the feed rate of $." mm/tooth,

were analy5ed (%ig. 3a?d). %rom the *gure it is obsered that the chips formed at

6$ m/min during machining under room temperature condition hae ery clear

secondary serrated teeth formed at the free edge. +his is related to higher amplitude

of chatter at 6$ m/min. @ut chips formed at "6$ m/min hae less pronounced

secondary serrated teeth at the free edge. +his also indicates the absence of chatter

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at this speed. Serrated teeth are also formed at the free edge of the chip during

machining under preheated condition.

%ig. 3. S&' top iews of chips formed during9 (a) room temperature machining and (b)during preheated machining at cutting speed, V c N 6$ m/minO and (c) room temperature

machining, and (b) preheated machining at cutting speed,V c N "6$ m/min. %eed

rate, F 5 N $." mm/tooth, 7BC N " mm. 'agni*cation9 $P.

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Qrimary serrated teeth are also obsered under both the cutting conditions. +he teeth

formed are more pronounced in the case of preheated machining. +he freDuencies of

the primary and the secondary serrated teeth are calculated from the S&'

micrographs taking into consideration of the coeRcient of chip shrinkage, cutting

speed and magni*cation of the picture. t is obsered from %ig. that the primary

chip serration freDuency is higher at room temperature machining.Cutting of ductile materials consists of plastic deformation in small areas. +he cutting

elocity aects the temperature, chip formation, mechanics and metallurgy in cutting

processes based on the phenomena described at the beginning of Section 2. >ith

increasing cutting speeds the time for material deformation in a cutting process

decreases and the deformation takes place in a limited area. +he material shearing

process produces signi*cant amounts of heat in the shear 5one. +he temperatures

occurring in the small scale shear 5one in high speed cutting aect the whole process

including tool wear, material behaiour and friction!"# and !"=E#.

+he limiting factor as regards cutting elocity for many workpiece materials is the

tool wear and resulting tool failure. +he main reason for these problems is that the

temperature increases asymptotically with cutting speed approaching the workpiece

material melting temperature. Foweer, the cutting speed range where the ultimate

temperatures are reached is material1speci*c. +itanium alloys, for e


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+he temperature in the shear 5one has a radical inIuence on chip formation. >hen

cutting steel or aluminium alloys with low speeds, continuous chip formation takes

place in accordance with the shear plane model. 8s the speed increases, the aerage

chip compression ratio decreases, the shear angle increases and a change to higher

shear localisation is identi*ed as shown in %ig. ;. 8t high cutting speeds, thin shear

bands may be obsered in comparison to conentional cutting speeds. t isanticipated that the higher strain rates will cause an increase in the shear force at

the shear 5one. Since the shear time decreases with increasing cutting speeds, the

temperature gradients within the shear bands become higher and lead, therefore, to

a higher shear localisation !2"#, !L2# and !"EE#.

%ig. ;. nIuence of the cutting speed on chip formation.

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>ith further increases in the cutting speed the material failure mechanism changes.

+he temperature and the chip formation mechanism hae a radical inIuence on the

cutting power and hence the process forces. Shear localisation and material failure

mechanisms lead to a change in chip formation resulting in a reduction of processforces !"EE#. 8s regards the measurement of process forces at high cutting speeds, it

is necessary to consider the dynamic behaiour of the measurement deice in order

to aoid measurement errors !"6L#.

+he characteristics of the cutting force behaiour with increasing cutting speed hae

been sub-ect to some initial fundamental inestigations !;# and !LE#. +he

corresponding cutting power can be subdiided into a constant fraction of power

reDuired for the high speed cutting range and a ariable fraction depending on the

cutting speed !"EE#. +he point of inIection of the ariable power fraction is then

de*ned as the transition elocity v FSC. +his shift cutting speed can be calculated on

the basis of mechanical and thermal properties for arious materials9

("$)

where T M ? melting temperature, Rm ? tensile strength, c p ? speci*c heat capacity, η ?thermal conductiity and ρ ? density.

%ig. = shows the shift speed for dierent materials. Siems !";;# enhances the

correlation between chip formation mechanism and cutting forces. +he cutting forces

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decrease with higher cutting speeds for all materials inestigated. %or materials with

a change in chip formation from continuous to segmented this decrease begins with

initiation of segmentation.

%ig. =. 'aterial speci*c de*nition of the FSC range.

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+he cutting forces approach a minimum for these metals. 'aterials with continuous

chip formation, howeer, e


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way (%ig. =(b)) as compared to the ones generated by thenew coated tool. n this case, the ma-or modes of chip Iowbecame sliding and sticking. +he chips were caught by therake face and released due to a high shearing force. naddition, the undeformed chip thickness was ery small($.$; mm). +herefore, a way chip surface was formed. 8s

the cutting length grew, the shape of the chips had nosigni*cant changeO howeer, rough JJearsKK on the undersideof the chips were isible (%ig. =(c)?(e)). 8s the tool wearincreased, the cutting edge became dull and more plowingaction, instead of cutting, was inoled in the cuttingprocess. +his resulted in the formation of a large number of big JJearsKK (%ig. =(e)). 7uring chip formation, there werethree phenomena inoled. +he *rst one was materialstrain hardening due to the large plastic deformation of thechip material. +he second was thermal softening due to ahigh cutting temperature. +he third one was the Duenchingeect of the chip by the compressed air blast under theintermittent cutting operation. +he Duenching took placewhen the actual temperature at the tool/chip interface wasaboe the austeni5ation temperature for F"3 tool steel. 8sa result, ery hard and brittle chips were formed with astructure that consisted of untempered martensite withretained austenite. 8t dierent wear stages, the dominantphenomenon changed. 8s the tool became worn, the combinedeect of strain hardening and Duenching resultedin brittle chips. +herefore, lots of cracks were formed(%ig. =(c)?(e)).

+he S&' images of the chip cross sections are presentedin %ig. E. 8gain, curly (%ig. E(a)), Iat (%ig. E(b)), way(%ig. E(c) and (d)) and brittle chips (%ig. E(e)) wereobsered from the low magni*cation images correspondingto the cutting lengths of $.6, 3$, ;$, "$$ and "6$ m,respectiely. @ased on the higher magni*cation images,continuous chip formation was obsered at a cutting lengthof $.6m (%ig. E(a)). 8s the tool was gradually worn, theregular saw1tooth chips for hard machining were formed.

+hese are shown in %ig. E(b)?(e). t was also found that asthe tool wear increased, the chip segmentation freDuencydecreased, plastic deformations in the primary shear 5onebecame larger and the crack propagation from the freesurface to the tool tip got deeper (%ig. E(b)?(e)). %ig. Lshows the typical saw1tooth chip cross section that wasobtained from this study. %our 5ones were obsered, i.e.,"white layer 5one due to a phase transformationassociated with a Duenching phenomenonO 2deformation

5one (underside of the chip at the secondary shear 5one)due to frictionO 3shear plane/5one due to primary shearingO lower deformed5one due to saw1tooth chipformation. 8gain, this result further con*rms the combinedeect of strain hardening, thermal softening and Duenchingon chip formation. %rom the S&' images with highmagni*cation (%ig. "$), it was found that the secondaryshear 5one became wider as the cutting length increased,which indicated that the frictional force became higher.


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8s stated earlier, the chip temperatures were estimated( +able ) based on the relationship between the color of chips and chip temperatures.Thecauseforthesaw-toothchipformationduringHSMAISI 1045 steel %ormicroscopicobseration,thechipshaebeencollected andembeddedintoresin.+hecrosssectionwasmechanically polished withaluminumo


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