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InvestigationofChipFormationCharacteristicsinOrthogonalCuttingofGraphite

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Investigation of Chip Formation Characteristics in Orthogonal Cutting ofGraphiteL. Zhou ab; C. Y. Wang b; Z. Qin b

a Faculty of Electromechanical Engineering, Guangdong Polytechnic Normal University, Guangzhou,China b Faculty of Electromechanical Engineering, Guangdong University of Technology, Guangzhou,China

Online publication date: 16 December 2009

To cite this Article Zhou, L., Wang, C. Y. and Qin, Z.(2009) 'Investigation of Chip Formation Characteristics in OrthogonalCutting of Graphite', Materials and Manufacturing Processes, 24: 12, 1365 — 1372To link to this Article: DOI: 10.1080/10426910902997399URL: http://dx.doi.org/10.1080/10426910902997399

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Materials and Manufacturing Processes, 24: 1365–1372, 2009Copyright © Taylor & Francis Group, LLCISSN: 1042-6914 print/1532-2475 onlineDOI: 10.1080/10426910902997399

Investigation of Chip Formation Characteristicsin Orthogonal Cutting of Graphite

L. Zhou1�2, C. Y. Wang

2, and Z. Qin

2

1Faculty of Electromechanical Engineering, Guangdong Polytechnic Normal University, Guangzhou, China2Faculty of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, China

Graphite becomes the prevailing electrode material in electron discharge machining (EDM) currently. This article aims to reveal the chipformation characteristics of graphite by orthogonal cutting experiments. The results showed that semicontinuous chip, crushed particle chip, andfractured block chip formation were identified as three major types of graphite chip formation. The transitions of chip formations were highlydependent on the depth of cut. The chips produced in different type of chip formation exhibited different surface fractography. Three types ofchip size distribution corresponded well to three types of chip formation. The surface roughness and cutting force increased prominently with thedepth of cut increasing. The cutting force response in each type of chip formation can be identified by the fluctuation extent and waveform ofcutting force.

Keywords Chip formation; Graphite; Orthogonal cutting.

1. Introduction

In recent years, istropic graphite has been used widely inelectron discharge machining (EDM) applications of die andmould to manufacture products in the fields of automobile,home appliances, communications, and electronic industry.It has the advantage of the fine-grained structure and highermechanical strength over common molded graphite andsintered graphite. Compared with copper, istropic graphitehas better machinability and less thermal deformation inmechanical machining, and less electrode wear, higherremoval rate, and heat resistance in EDM. Due to its lowerdensity, graphite can be stuck to each other by use of specialadhesive to produce large size electrodes with complicatedshapes. Therefore, istropic graphite becomes the prevailingelectrode material over copper in EDM, especially formanufacturing complicated mould cavities with narrow anddeep slots or microholes.Graphite is a special brittle material with inconsistent

polycrystal microstructure. It has many interior micro-defects such as micropores and microcracks. So its actualmechanical strength is much lower than theoretical strength.These defects can result in unacceptable cracking of graphiteelectrodes during machining. In the cutting process ofgraphite, chips are not like the strip ones produced by plasticflow in metal cutting. Graphite chips are mainly producedby cutting impact, crush, and flaking off actions of cuttersin the form of brittle fractured chips or dust. Therefore,graphite machining has its special characteristics totallydifferent from those of metal cutting [1, 2].

Received October 2, 2008; Accepted December 30, 2008Address correspondence to C. Y. Wang, Faculty of Electromechanical

Engineering, Guangdong University of Technology, Guangzhou, China;E-mail: cywang@gdut.edu.cn

Several previous researches have reported the cuttingcharacteristics of some graphite materials. König [3] foundthat in high speed milling of graphite, crushed fracturesoccurred to the graphite material at the tool tips with finechips and microcraters; the cracks extended downwardsahead of the tool tip and grew to the workpiece surfaceresulting in fracture chips formation. Masuda et al. [4]observed that in turning process of sintered graphite, a crackwas produced and extended along the cutting direction, andthen some workpiece materials were crashed into chips asthe tool pushing forward. Sato and Nakayama [5] found thatin turning sintered graphite, the graphite particles smallerthan 250�m accounted for the majority of total weight,and the proportion of big particles would increase withfeed rate increasing. The authors [6, 7] reported that inhigh speed milling of isotropic graphite, many fracturedcraters of various depths were caused on the machinedsurface; the chip shapes appeared irregularly, and chips likeblock, strip, sphere, and flake were observed; cutting toolssuffered severe wear due to the highly abrasive nature ofthe graphite. However, it is necessary to do further in-depthstudy on the characteristics of graphite chip formation andthe machined surface and cutting forces related so as to geta comprehensive understanding of graphite chip formation.In this article, orthogonal cutting experiments were

conducted and observed in situ to study the characteristics ofgraphite chip formation. The micrographs of graphite chipswere examined. The chip size distributions were analyzed.The variations of machined surfaces and cutting forces withchip formation transitions were investigated.

2. Experimental method and procedure

Experimental setup is shown in Fig. 1. Orthogonal cuttingexperiments were performed on a CNC milling center(J1VMC40M, China). Inserts (TaeguTec SPGN120404)

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1366 L. ZHOU ET AL.

Figure 1.—Schematic of orthogonal cutting experiments.

were fixed by a specially designed tool holder clamped ontothe spindle as cutters (Fig. 1). This holder is composed ofa shank, a seat, and a small platen. The cutter moved withthe spindle to feed along the X-direction of machine.The workpiece material is fine-grained graphite of grade

ISO-63 (Toyo Tanso Inc.) with average grain size of 5�m,density of 1.82g/cm3, shore hardness of 80, and flexuralstrength of 79MPa. The typical fractographies of graphiteproduced by standard shear and tensile tests are shown inFigs. 1(a) and (b). Graphite materials were pre-machinedto slices of 1mm in thickness, 50mm in width, and 20mmin height. Graphite slice was clamped by a vice on apiezoelectric dynamometer mounted on the working table.Cutting speed was set to a slow constant velocity v of

10mm/min to enable easy visualization of cutting process.Depths of cut ap were set to 0.02–0.24mm at interval of0.02mm so as to study the chip formation changes undervarious depths of cut.In situ observations of the orthogonal cutting processes

were conducted by use of a stereo microscope (TaikeXT53022-CTV, China) with CCD camera (Panasonic WV-CP460/CH). The recorded videos of chip formation werepost-processed by replaying at playing speed of 25 fpmwith media player software. Typical instantaneous framesof chip formation were captured to analyze the dynamiccharateristics of graphite chip formation. Horizontal cuttingforces FH and vertical cutting forces FV were measured andrecorded using a dynamometer (YDM-III99).The graphite chips were collected and observed by SEM

(JSM-6380). Magnified digital images of the collectedgraphite chips were processed to measure the size andnumber of chips so as to evaluate the size distribution ofgraphite chips. The chip size was presented as the equivalentdiameter of a circle which can just cover the chip. Themachined surfaces of graphite were observed by ScanningElectronic Microscope (SEM JSM-6380) and measured byLaser Scanning Confocal Microscope (Olympus LSCMFV1000S).

3. Results and discussions

3.1. Chip FormationAccording to the recorded videos of cutting process and

chip morphology observations under various depths of cut,

Figure 2.—Semicontinuous chip formation: ap = 0�02mm, �o = 5�,v = 10mm/min.

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CHIP FORMATION OF GRAPHITE 1367

Figure 3.—Micrograph of a semicontinuous “rag” chip: ap = 0�02mm,�o = 5�, v = 10mm/min.

graphite chip formation can be identified and classified intothree major types: semicontinuous chip, crushed particlechip, and fractured block chip formation.

3.1.1. Semicontinuous Chip Formation. Semicontinuouschip is a special kind of chip, which usually occurs atshallow depths of cut. As shown in Fig. 2, at smalldepth of cut of 0.02mm, a semicontinuous chip like a“strip” was generated ahead of the tool rake face, flowingout smoothly like chips in metal cutting. From Fig. 3,

it can be noticed that semicontinuous chip is full of flawsand composed of some small fragments loosely attachingtogether. It looks very coarse and incompact, and seemseasy to smash by slight exterior load. The cutter served as ascrape ploughing graphite along the workpiece surface. Asthe cutter advanced, the “strip” chip gradually curled into abig agglomerate, as illustrated in Fig. 2(c).Figure 4 shows the typical micrographs of semi-

continuous chips from different view directions. Thecamber-like lateral view of chip [Fig. 4(a)] indicates itsflexibility and continuity in microscale and thinner thicknessthan the depth of cut. As can be seen from Fig. 4(b),the chip surface viewed in direction A bears interlayerdelamination fractography similar to the shear fractographyin Fig. 1(b). In Figs. 4(c) and (d), it can also be observedthat the chip surfaces are rather smooth with parallel tracesof shear fracture of graphite. Because graphite is proneto delaminate between interlayers, and its failure undercompression is basically in shear [8], it can be concludedthat semicontinuous chips are commonly peeled off bycompression-induced shear.

3.1.2. Crushed Particle Chip Formation. Crushedparticle chip formation is a type of discontinuous chipformation under moderate depths of cut. Figures 5(a) and (b)shows that the graphite material ahead of the cutter wascrushed into small particles, which were composed of someirregularly shaped particles and fine dusts. Crushed particlechips are prone to accumulate gradually in front of the cutteras shown in Fig. 5(c).

Figure 4.—Micrographs of the small fragments of semicontinuous chips: ap = 0�02mm, �o = 5�, v = 10mm/min. (a) Lateral view; (b) View in direction Aindicated in Fig. 4(a); (c) View in direction B indicated in Fig. 4(a); and (d) Englargement of region C in Fig. 4(c).

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1368 L. ZHOU ET AL.

Figure 5.—Crushed particle chip formation: ap = 0�06mm, �o = 5�,v = 10mm/min.

The micrographs of crushed particle chips shown inFigs. 6(a) and (b) reveal that some parts of chip surfacesare of fracture features similar to the shear fractographyin Fig. 1(b), while other parts appear the honeycombedintergranular fracture induced by tensile stress as shown inFig. 1(a). There are still several microcracks visible on thechip surfaces. All these suggest that the particle chips arefragmented mainly by the mutual action of compression-induced shear fracture and tensile fracture.

3.1.3. Fractured Block Chip Formation. Fractured blockchip formation is another typical discontinuous chipformation, which is different from the above crushedparticle chip formation in the size and initiation of the chips.At 0.1 s of the cutting process, the graphite material aheadof the tool was crushed into small particles (indicated byarrow A in Fig. 7(a)) when the tool penetrated into theworkpiece. At 0.2 s, the material in a small area aheadof the tool tip were crushed and compacted with furtherdisplacement of cutter. The stress filed in the rest loadedmaterial was built up, and a crushed zone (indicated byarrow B in Fig. 7(b)) came into being with a big initial crack(indicated by arrow C in Fig. 7(b)) ahead of it. As the toolmoved forward, the crushed zone was squashed as shown byarrow B in Fig. 7(c) at 0.3 s. Then some fine powder chips

Figure 6.—Micrographs of crushed particle chips with residual microcracks:ap = 0�06mm, �o = 5�, v = 10mm/min.

might be dislodged from the crushed zone, while some othersmight still accumulate between the cutter and the loadedworkpiece material with the shape like a wedge as shown byarrow B in Figs. 7(c) and (d) at 0.3–0.4 s. At the same time,the crack was opened by the lift effect of the crushed zoneand propagated upwards to the free surface directly or aftertraveling downwards into the material to a certain depth asindicated by arrow D in Figs. 7(c) and (d). Then a fractureblock chip (indicated by arrow E in Fig. 7(e) at 0.5 s) wasproduced. After the fractured block chip departed from thetool rake face at 0.6 s, there were still some chips remainedon the tool tip as shown by arrow B in Fig. 5(e), which werethe residual fragments of the crushed zone.The fractured block chip was always rectangle-like in

shape as shown in Fig. 7, and the chip thickness was oftengreater than the depth of cut because of the propagation ofthe initial crack below the cutting plane. Figure 8 showsthe micrographs of fractured block chips. As can be seen,there are two different types of fractured surfaces: one typeis represented by regions A1 and A2 in Fig. 8 with thetensile fractography, and another is exhibited by the areaswith shear fractography indicated by regions B1 and B2in Fig. 8.Based on the above observations and discussions of

fractured block chip formation of graphite, it is obvious thatit is similar to the chip formation of other typical brittlematerials [9]. In rock cutting the rock fails mainly by tensilestress, and the crushed zone is formed due to the shearsover the slip lines [10]. So it can be considered that in

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CHIP FORMATION OF GRAPHITE 1369

Figure 7.—Fractured block chip formation: ap = 0�24mm, �o = 5�, v = 10mm/min. (A = small particles, B = crushed zone, C = initial crack, D = crackpropagation, and E = fractured chip.)

fractured block chip formation, the major crack is formedand propagated under a tensile stress as a result of themutual action of compression and bending by the cutter,and the regions B1 and B2 are shaped after the crushedzone was squashed by compression-induced shear.

3.2. Graphite Chip Size DistributionFrom the SEM micrographs shown in Fig. 9, with the

depth of cut increasing from 0.02mm to 0.14mm, the chipgeometry increased significantly in size from about 10�mto 630�m. The length-based size probabilities of graphite

Figure 8.—Micrographs of fractured block chips: ap = 0�14mm, �o = 5�, v = 10mm/min. (a) and (b) Typical fractured block chips; (c) Enlargement of regionA2 in Fig. 8(b); and (d) Enlargement of region B1 in Fig. 8(a).

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1370 L. ZHOU ET AL.

Figure 9.—Transformation of graphite chip geometry with depths of cut increasing: �o = 5�, v = 10mm/min. (a) ap = 0�02mm; (b) ap = 0�04mm;(c) ap = 0�06mm; (d) ap = 0�08mm; (e) ap = 0�10mm; and (f) ap = 0�14mm.

chips at various depths of cut are shown in Fig. 10. The sizeintervals were set between the values of 20, 40, 60, 80, 100,and 200�m. It is obvious that the proportions of mean sizechips and big size chips increased markedly with the depthof cut increasing, and the chips accounting for the biggestproportion also shifted in a trend from small to big size.From Fig. 10, three principal types of chip size distributioncan be identified as following. (i) When the depth of cutis 0.02mm, the small chips of size 20–40�m were in themajority, and little chip was observed above 100�m in size.(ii) When the depth of cut increased to 0.06mm, the biggestshare still belonged to those small chips of size 20–40�m,however some bigger chips in size of 100–200�m couldalso be found in the overall collected chips, suggesting thatthe chips were produced in a mixed manner of some smallsize chips and a few major chips. (iii) When the depth ofcut increased to 0.08mm, the biggest share switched to thebig chips of size 60–80�m, and little portion of chips werein size less than 40�m, which means that the chips weremainly composed of major chips.These alterations of chip size distributions support the

in situ observations of chip morphologies under various

depths of cut. Different chip size distribution is in closeassociation with different failure mode of graphite. Chipsize distributions i, ii, and iii correspond well to thesemicontinuous chip, crushed particle chip, and fracturedblock chip formation, respectively.

3.3. Morphology and Surface Roughnessof the Machined SurfaceThe SEM micrographs of machined surfaces of graphite

under various depths of cut [Figs. 11(a)–(d)] show thatthe machined surfaces at most depths of cut are composedof some smaller or bigger fracture craters. The 3D profilemicrographs and roughness Ra of the machined surfacestaken by LSCM are shown in Figs. 11(e)–(h). The fracturecraters observed by SEM are represented by various greyareas. The darker the grey areas appear, the deeper thecraters, and the coarser the surfaces are. In such a way, themachined surfaces of graphite are visualized very clearly. Itis apparent that the machined surface appears rather smoothwith little visible craters at the depth of cut of 0.02mm.With the depth of cut increasing, fracture craters begin toform and become bigger and deeper. At the depth of cut

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CHIP FORMATION OF GRAPHITE 1371

Figure 10.—Chip size distribution under various depths of cut: �o = 5�,v = 10mm/min.

of 0.14mm, there is a deep crater on the machined surfaceas shown in Figs. 11(d) and (h), which was caused bypropagation of the initial crack downward into the materialbelow the depth of cut. It can also be found in Fig. 11 thatthe surface roughness Ra of the machined surface increasedobviously with the depth of cut increasing. Therefore,good surface finish can be achieved in semicontinuouschip formation under shallow depths of cut; while rapidmaterial removal rate can be achieved in discontinuous chipformation by increasing the depth of cut.

3.4. Cutting ForcesThe variations of cutting forces with depth of cut

(Fig. 12) shows that the maximum forces FHmax andFVmax increased with the increase of depths of cut andthe mean forces FH0 and FV0 changed little. The cuttingforces fluctuated apparently in graphite cutting (Fig. 13).Associated with the videos of chip formation processesrecorded simultaneously with cutting forces measurement,

Figure 12.—Variations of cutting forces with depths of cut.

it is obvious that the variations of cutting forces were indeep association with the changes of chip morphologies.Different cutting response can be identified by the certainwaveforms of force curves. When cutting under smalldepths of cut, most graphite materials were compressedand sheared into semicontinuous chips with little materialcracking, so the cutting force fluctuated little [Fig. 13(a)].The irregular waves of cutting force like zigzags [Fig. 13(b)]are related to the formation of crushed particle chips atmoderate depths of cut. Cutting force fluctuated drasticallywhen fractured chips were produced [Fig. 13(c)]: the greatoscillation of cutting force was induced by brittle fracturechipping, and those small wave crests ahead of the highestpeak forces corresponded to the small fragments before thebig fracture chip formed. During the whole cutting process,the cutter was in intermittent contact with the graphitematerial, so the cyclic cutting impact on the tool tip wasincreased undoubtedly. Therefore, the smaller the depths of

Figure 11.—SEM and 3D profile micrographs of machined surfaces: �o = 5�, v = 10mm/min.

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1372 L. ZHOU ET AL.

Figure 13.—Cutting force curves in various types of chip formation: (a) in semicontinuous chip formation; (b) in crushed particle chip formation; and (c) infractured block chip formation.

cut, the less the cutting forces fluctuate, and the more stablethe cutting process. Cutting stability can be improved bydecreasing the depths of cut to lessen the cutting force andits fluctuation amplitude.

4. Conclusions

(1) Three primary types of chip formation were observed ingraphite cutting. Semicontinuous chips were producedby compression-induced shear at shallow depth of cut.With the depth of cut increasing, the chip formationmode transited smoothly to crushed particle chipformation, and finally changed to fractured block chipformation.

(2) The machined surfaces are mainly composed of somesmaller or bigger fracture craters. The greater the depthof cut was, the bigger and deeper the craters wereproduced, and the coarser the machined surface became.

(3) The waveform and fluctuation of cutting force wereinfluenced significantly by the depth of cut. The cuttingforce with specific fluctuation characteristics ocurred inthe specific type of chip formation. Surface finish andcutting stability can be improved with the depth of cutreducing.

Acknowledgments

The authors would like to thank the financial supportfrom National Science Foundation of China (No. 50605008)and Natural Science Fund of Guangdong Province (No.8451063301001813).

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