Chip Morphology Characterization

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CHIP MORPHOLOGY CHARACTERIZATION AND MODELINGIN MACHINING HARDENED 52100 STEELS

Mason D. Morehead and Yong Huang & Department of MechanicalEngineering, Clemson University, Clemson, SC

Jian Luo & School of Materials Science and Engineering, Clemson University,Clemson, SC

& Hard machining is attracting more and more attention as an alternative to grinding in finishmachining some hardened steels. The saw-toothed chips formed in hard machining have their ownunique characteristics. The saw-toothed chip morphology is of great interest since the understandingof the saw-toothed chip morphology and its evolution in machining helps unveil hard machiningchip formation mechanisms as well as facilitate hard machining implementation into industry. Inthis study, the effect of tool wear and cutting conditions on the saw-toothed chip morphology wasexamined in machining 52100 hardened 52100 bearing steel. It was found that the chip dimen-sional values and segmentation frequency were affected by tool wear and cutting conditions whilethe chip segmentation angles were approximately constant under different tool wear and cuttingconditions. The shear band spacing has also been predicted at the same order of magnitude asthe measurement, and improved spacing modeling accuracy is expected if the cutting process infor-mation can be better predicted first.

Keywords Chip morphology, Cutting condition, Hard turning, Saw-toothed chip,Tool wear

INTRODUCTION

Finishing machining of hardened steels offers many technical and econ-omical advantages over conventional grinding such as shorter setup=toolchange times, higher material removal rates, and less negative environmen-tal implications. Thus it has gained increasing attention in many industrialsettings (Konig et al., 1993; Tonshoff et al., 2000; Huang, 2002). Recently,this hard machining practice has been made possible through the develop-ment of ultrahard cutting materials such as polycrystalline cubic boronnitride (CBN). The chips formed in machining hardened steels, along with

Address correspondence to Yong Huang, Department of Mechanical Engineering, ClemsonUniversity, Clemson, SC 29634. E-mail: [email protected]

Machining Science and Technology, 11:335–354Copyright # 2007 Taylor & Francis Group, LLCISSN: 1091-0344 print/1532-2483 onlineDOI: 10.1080/10910340701567289

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other difficult to cut alloys such as titanium-based alloys (Hua and Shivpuri,2004) and nickel-based alloys (Shaw and Vyas, 1993), are typically saw-toothed or segmented. This phenomenon occurs generally in machininghardened steels (> 50 HRC) when undeformed chip thickness is greaterthan 20mm (Tonshoff et al., 2000). The formation of such saw-toothedchips, which are of great research interest, is generally considered the resultof a highly nonlinear and dynamic plastic deformation process that has beenshown to adversely affect cutting forces, machine deflection and vibration,tool wear, and surface finish (Ueda and Matsuo, 1982; Davies et al., 1996).

While most hard machining research thus far has focused on chip forma-tion mechanisms and tool wear characterization, it is of great interest to studythe effect of tool wear on the evolution of chip morphology and the effect ofcutting conditions on chip morphology. It has been observed that the chipdimension changes as tools wear (Davies et al., 1996) and cutting conditionschange (Poulachon and Moisan, 2000). However, there is still no systematicstudy of chip morphology characterization in hard machining. The objectiveof this study aims to better define and further characterize different chipdimensions as functions of tool wear and cutting conditions in hard machin-ing. The paper first gives background for this study. Experimental setup anddesign are then discussed, along with the experimental observations on thechip morphology. Furthermore, predicted segmentation spacing is comparedwith the experimental results and discussion is given. Finally, conclusions andrecommendations for the future research are presented. Such chip mor-phology knowledge will help better understand and model the hard machin-ing process, making hard machining a viable technology.

BACKGROUND

Chips produced in machining most metals and alloys can be generallyclassified into four distinct categories based on their geometric shapes:flow, wavy, saw-toothed=segmented, and discontinuous (Komanduri andBrown, 1981). It should be noticed that different approaches have alsobeen pioneered to characterize chips in machining other nonmetallicengineering materials besides the aforementioned chip classification.Chips produced from machining advanced materials such as ceramicsand reinforced composites may be quite different in physical nature thanthose observed in metal machining. For example, Shih et al. (2004)developed a classification system to define 7 types of chips obtained inmachining nonmetallic elastomers under different cutting conditions.

Flow type chip arises in machining of ductile materials and is classifiedby its uniform cross-section. Wavy chips occur when the shear angle oscil-lates widely causing fluctuations in cutting forces and chip thickness(Shaw and Vyas, 1993). Both the flow and wavy chips are considered

336 M. D. Morehead et al.

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continuous as there are no gross fracturing but only signs of uneven strainsin the chip. When there are cracks or fracture in the chip formation, thechip may be discontinuous or saw-toothed=segmented. Discontinuous chipformation is common in machining brittle materials at low cutting speeds(Tonshoff et al., 2000), and these chips are classified by their nearlyidentical and discontinuous segments (Shaw and Vyas, 1993) that areentirely separated by their broken segments. Saw-toothed chips, a commonname for segmented chips, are semicontinuous and have zones of low shearstrain (continuous portion) and high shear strain (discontinuous portion).

Saw-toothed chips, unlike conventional flow type or continuous chips,show areas of intense shear strain in cyclic form causing sharp segmentsto be distributed along the free side of the chip. While the exact chip for-mation mechanism of saw-toothed chips is still not clearly known, there aretwo main conflicting chip formation mechanisms: catastrophic strain loca-lization (Davies et al., 1996, 1997) and fracture=crack generation (Shawand Vyas, 1993; Vyas and Shaw, 1999).

For the catastrophic strain localization mechanism, strain localization,or sometimes called adiabatic shear, takes place when thermal softeninggoverns the combined effects of strain and strain rate hardening. This com-monly takes place when the strain rates occur higher than a significantvalue governed by the properties of a material. Once this value is reached,the chip segment is pushed along the direction of strain and a new segmentis formed. For the fracture=crack generation mechanism, the segmentationof chips is initiated through gross periodic fractures occurring from theworkpiece free surface to the cutting tool tip. For this situation, the fractureinitiates at the workpiece surface and continues down toward the tool tipuntil the compressive stresses formed from the tool tip are strong enoughto stop the crack. These stresses result in plastic deformation near the tooltip that can be compared to the common flow-type chip (Shaw and Vyas,1993). As the segment is released from the crack, compressive stressesbegin to build up and gross fracture occurs again in the same manner.

Regardless of the saw-toothed chip formation mechanisms, the objectiveof this study is to study the relationship of saw-toothed chip morphology andthe effect of both tool wear and cutting conditions on chip morphology inhard turning. It has been observed that the overall chip shape evolves as toolwears in machining hardened steels. This shape evolution can be appreci-ated in Figure 1. Due to the interesting chip morphology of hard turningchips, the saw-toothed chip formation has been studied in terms of theeffect of cutting speed on segmentation dimension (Davies et al., 1996,1997; Poulachon and Moisan, 2000) and segmentation frequency (13,14), and the effect of feed on segmentation dimension (Vyas and Shaw,1999; Poulachon and Moisan, 2000) and segmentation frequency (Poula-chon and Moisan, 2000; Becze and Elbestawi, 2002). However, there still is

Chip Morphology in Machining Hardened Steels 337

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no systematic study about the effect of cutting conditions as well as toolwear on chip morphology (chip dimensional and angular information)and segmentation frequency. This study aims to better understand the chipmorphology and its evolution through a systematic study via hard hard turn-ing of hardened steels. Besides better understanding toward chip formationmechanisms, the chip morphology can be a good and convenient indicatorof tool wear in hard machining. Also, the knowledge on the chip mor-phology evolution can improve manufacturing productivity where tool wearand workpiece dimensional accuracy need to be closely monitored.

EXPERIMENTAL SETUP AND DESIGN

Hardened 52100 bearing steel with a hardness of 62 HRC was chosenfor experimental studies because of its wide use in both industry andresearch fields. Facing operations were performed dry using a Hardinge6=45 computer numerical control lathe. CBN cutting tools (KennametalKD050) with a negative 5-degree rake angle and a negative 20-degree cham-fer angle were used. Tool flank wear was measured using an optical micro-scope, and average cutting forces were measured using a Kistler 9257Bdynamometer where the force fluctuation due to the saw-toothed chip for-mation was ignored. To characterize chip segmentation morphology, steelchips were collected, mounted in epoxy, and polished to reveal theircross-sections. Since the collected chips were three-dimensional in natureinside the epoxy mold, the chip samples were carefully polished to identifyand clearly present the chip cross-section of interest. An optical microscopeand a Hitachi S-3500N scanning electron microscope (SEM) were used tocharacterize saw-toothed chip cross-sections.

Two experimental designs were created to study and quantify chip mor-phology during machining. The first design (Scenario A), as shown inTable 1, aimed to research the effect of tool wear on chip segmentation

FIGURE 1 Hardened 52100 steel chips taken using (a) fresh PCBN tool and (b) worn PCBN tool after12 minutes at speed of 1.52 m=s, feed 0.0762 mm=rev, and depth of cut 0.203 mm. Due to the chip shapedifference, the pictures were taken from the different angles.


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morphology. Tool wear and chip morphology information were collectedthroughout the whole tool life until the tool reached a tool life criterionof 0.2 mm or had a jump of cutting forces due to the catastrophic toolbreakage (Huang, 2002).

The second design (Scenario B), as shown in Table 2, aimed to researchthe effect of cutting conditions on chip segmentation morphology usingnew tools. Cutting conditions were varied to study the effect of cuttingspeed, feed, and depth of cut on cutting forces and chip morphology indi-vidually. Both cutting forces and chip morphology information werecollected. Cutting conditions were selected based on recommendationsfor typical finishing processes (Huang and Liang, 2005).

CHIP MORPHOLOGY CHARACTERIZATION IN HARD TURNING

Chip Morphology

Figure 2a shows a representative SEM picture of a collected chip etchedwith a 2% Nital solution for 10 seconds. The relatively white portion of thechip cross-section is the chip white layer formed in hard machining. Asshown in Figure 2b, chip morphology is characterized by its dimensionalvalues in terms of saw-tooth spacing (a1), saw-tooth height (a2), chip heightof continuous portion (a3), shear band spacing (Lc), half shear bandwidth (h) and angular values in terms of crack initiation angle (h1) and

TABLE 2 Cutting Conditions for Scenario B

Cutting speed(m=s)

Feed(mm=rev)

Depth of cut(mm)

Scenario B1 (effect of cutting speed) 1.52 0.114 0.2032.29 0.114 0.2033.05 0.114 0.203

Scenario B2 (effect of feed) 1.52 0.076 0.2031.52 0.114 0.2031.52 0.152 0.203

Scenario B3 (effect of depth of cut) 1.52 0.114 0.1521.52 0.114 0.2031.52 0.114 0.254

TABLE 1 Cutting Conditions for Scenario A

Cutting speed(m=s)

Feed(mm=rev)

Depth of cut(mm)

Scenario A1 1.52 0.076 0.203Scenario A2 2.29 0.114 0.203


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saw-toothed chip included angle (h2). The aforementioned chip dimen-sional information for each design was measured and averaged, and onlythe mean values are presented here. Since Lc can be easily determinedby a1 and h1 as Lc ¼ a1 sin h1, only a1 and h1 are discussed in this section.More information about the Lc prediction is discussed later.

Effect of Tool Wear

Figure 3 shows the tool wear progressions of the experimental scenariosA1 and A2. Since the measured tool wear progression profiles are similar tothe documented results (Davies et al., 1996; Poulachon et al., 2001; Huangand Liang, 2004), the experiments were not repeated for the same design

FIGURE 3 Tool wear progressions of Scenario A.

FIGURE 2 (a) SEM image of a saw-toothed chip cross-section and (b) Segmentation dimensions forsaw-toothed chips.


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conditions. The more aggressive cutting conditions (Scenario A2) led to ahigher tool wear rate, and the cutting test stopped when a large force jumpdue to tool breakage was observed around 3.75 minutes.

Segmentation geometry evolution with machining time is shown inFigure 4. It is found that both the saw-tooth spacing (a1) and height (a2)decreased as the tool wore (Figures 4a and 4b) while the chip height ofcontinuous portion (a3) increased (Figure 4c). The tooth height was largerthan the continuous portion height when the tool was new; however, thecontinuous portion height turned to be larger at a certain tool wearcondition. All a1, a2, and a3 changed significantly faster under aggressivecutting conditions when comparing such measurements for Scenarios A1and A2. The overall chip thickness, including the segmented and continu-ous portions of the chip, is determined by simply adding a2 and a3. Thischip thickness ða2 þ a3Þ is shown in Figure 4(d) and had an overall increaseafter a slight decrease as the tool wore. The sudden changes between cut-ting time 10 and 12 minutes were due to tool microchipping at the endof tool life.

FIGURE 4 Evolution of (a) saw tooth spacing (a1), (b) saw tooth height (a2), (c) chip height of saw-toothed chip continuous portion (a3), and (d) chip thickness (a2 þ a3) of Scenario A.


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The differences in both feed and velocity under Scenarios A1 and A2are accounted for the differences of a1s and a2s at the beginning of themachining tests. However, it is found that the chip continuous portionheight a3 at the beginning of machining is less sensitive to the change ofcutting conditions. The preceding observations will be examined furtherlater when discussing the effect of cutting conditions respectively.

Figure 5 shows the evolution of segmentation angles (crack initiation angle(h1) and saw-toothed chip included angle (h2)) as the tool wears. There are nonoticeable angle changes of the segmentation angles over the whole cuttingtests, and both tests had similar segmentation angles. The crack initiation angle(h1) was approximate 35 degrees over the whole test, while the included angle(h2) remained constant around 45 degrees. The included angle values (45�)are also comparable with other documented studies in machining AISI 4340steel (45�) (Shaw and Vyas, 1993), facing chromium carbide hardened steel(45�) (Ren et al., 2001), and milling AISI D2 tool steel (47�) (Becze andElbestawi, 2002). It is concluded here that the included angle of steel saw-toothed chips is around 45� regardless of cutting conditions, types of machiningoperations, and tool wear. A complete understanding of the saw-toothed chipformation mechanism is expected to explain this observation in the future.

For conventional machining, the cutting ratio can be determined bydividing the undeformed chip thickness by the average chip thickness orthe cutting speed (V ) by the chip velocity (Vc). It was found that the cuttingratio can be approximated by the ratio of the mean tooth spacing (a1) tothe mean tooth height (a2) (Vyas and Shaw, 1999). Then, the cutting ratioin hard machining can be approximated as follows:

r ¼ a1

a2¼ Vc

Vð1Þ

FIGURE 5 Segmentation angles evolution of Scenario A.


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This cutting ratio can be further used to determine the segmentationfrequency f , which is a ratio between cutting speed and segmentationspacing in hard machining as follows (12):

f ¼ Vc

a1¼ V

a2ð2Þ

Figure 6 shows the segmentation frequency information of Scenario A.An increasing trend of chip segmentation frequency as functions of cuttingtime can be seen for both scenarios, and both scenarios showed compara-ble magnitudes of frequency at the beginning (�60 kHz) and the end ofcutting (�130 kHz). It will be of interest to study in the future whetherthe thermal effect under the worn tool condition is responsible for higherfrequency segmentation as observed, leading to continuous-like chips.

Effect of Cutting Speed

As observed in (Chou and Evans, 1999; Huang and Liang, 2005), theeffect of cutting speed on cutting forces was less significant as seen inFigure 7. However, the tooth spacing, a1, and the mean tooth height, a2,did increase with cutting speed, while the continuous portion of the chip,a3, and the overall chip thickness showed a slight oscillation with a smallincrease followed by a large decrease. It is found that the tooth heightwas always larger than the height of the continuous portion when the toolwas new. Figure 7c shows that the segmentation frequency increased withthe cutting speed, which agrees with the other observations in machininghardened steel (HV0.02 ¼ 630) (Ekinovic et al., 2004) and grade 2 commer-cially pure titanium (Sheikh-Ahmad et al., 2004). However, it was reported

FIGURE 6 Chip segmentation frequencies of Scenario A.


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that there is a linear relationship between segmentation frequency andcutting speed (Ekinovic et al., 2004; Sheik-Ahmad et al., 2004), which is dif-ferent from the observations in this study.

Segmentation angles (h1 and h2) maintained 35� and 45�, respectively,while changing the cutting speeds, which is similar to the observations dur-ing the tool wear progression. Since the same constant segmentation angleswere also observed while changing feed rate and depth of cut, the followingtwo sections will not discuss the observations of segmentation angles.

Effect of Feed

As seen from Figure 8, when the feed increased, all cutting forcesincreased accordingly except the feed force at the low feed rates(0.076 mm=rev), which is attributed to the ploughing effect in finish hardturning with shallow cuts (Huang et al., 2003). Figure 8b shows the toothspacing (a1) increased approximately 20mm for each 38 mm increase infeed. This increase in tooth spacing proved to be the most significantwhen changing cutting conditions, thus making the feed rate have the most

FIGURE 7 The effect of cutting speed on (a) cutting forces, (b) chip morphology, and (c) segmen-tation frequency.


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influence over the tooth spacing dimension. The height of each segment(a2) oscillated with the feed increase, while the chip height of the continu-ous flow portion (a3) decreased. It is found that the tooth height was alwayslarger than the continuous portion height when the tool was new. As shownin Figure 8c, the increase of feed caused the segmentation frequency todecrease. This decreasing trend in segmentation frequency as a functionof feed agrees with the other observations (Poulachon and Moisan, 2000;Becze and Elbestawi, 2002).

Effect of Depth of Cut

Figure 9 describes the effect of depth of cut on cutting forces, segmen-tation dimensions, and segmentation frequency. It can be seen that cuttingforces increased with depth of cut although the relationship is not linear asexpected. As the depth of cut increased from 0.152 mm to 0.254 mm, allthree cutting forces increased at approximately the same rate. Both thechip segmentation spacing (a1) and the height of chip continuous portion(a3) slightly increased, however, the chip tooth height (a2) and overall chipthickness decreased slightly first then increased. Unlike the effect of cutting

FIGURE 8 The effect of feed on (a) cutting forces, (b) chip morphology, and (c) segmentationfrequency.


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speed and feed rate on the segmentation frequency, the chip segmentationfrequency oscillated while the depth of cut changed.

SEGMENTATION PREDICTION AND DISCUSSION

Shear Band Spacing Prediction

Segmentation geometry prediction during saw-toothed chip formationhas been studied by numerous researchers since the late 1990s (Davies et al.,1996; Elbestawi et al., 1996; Poulachon and Moisan, 2000; Becze and Elbestawi,2002; Molinari et al., 2002). Shear band spacing Lc is of great interest heresince it helps discover the saw-toothed chip formation mechanism, and thisspacing can be modeled using a linear thermal softening strain rate hardeningconstitutive model based on the adiabatic shear band theory (Wright and Ock-endon, 1996; Molinari, 1997; Molinari et al., 2002) as follows:

Lc ¼ 2pm2kcð1� h

h0aÞ2h2

0

ð1þ 1mÞb

2ð _ccÞ3sa2

!1=4

ð3Þ

FIGURE 9 The effect of depth of cut on (a) cutting forces, (b) chip morphology, and (c) segmentationfrequency.


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where m is the strain-rate sensitivity, k is the heat conductivity, c is the heatcapacitance per unit mass, a is the thermal softening parameter, h0 is theinitial temperature, b is the Taylor-Quinney coefficient representingthe fraction of plastic work transformed into heat, and the stress s0 andthe strain rate _cc are determined by Equations 4 and 5, respectively:

s ¼ s0 1� hh0

a

� �_cc_cc0

� �m

ð4Þ

_cc ¼ V

2h

cos acosð/� aÞ ð5Þ

where s0 is the reference stress, _c0c0 is the reference strain rate, / is the shearangle (/ ¼ 90� � ðh1 þ aÞ ¼ 30� and is treated as constant for simplicity), ais the tool rake angle (�25�), and h represents the one half of the width ofthe actual adiabatic shear band. Although the chip white layer can be easilyseen from Figure 2a, the shear band width of hard turning chips is still dif-ficult to quantify. The empirical regression approach was used to model theshear band width (2 h) (Becze and Elbestawi, 2002); however, to simplify themodel process, the shear band width is assumed as 1=17th of the shear bandspacing Lc here based on an experimental observation in machiningtitanium (Molinari et al., 2002). Some other physical properties ofhardened steel for Lc modeling are listed as follows (Batra, 1987; Davieset al., 1997; Molinari, 1997): m ¼ 0.025, h0 ¼ 27�C, a ¼ �25�, k ¼ 49.2W=m�C, b ¼ 0.9, c ¼ 0.46 kJ=kg�C, s0 ¼ 930 MPa, _cc0 ¼ 104=s, and a ¼0.00552=�C. Since it is still a challenge to accurately predict the temperatureinformation h along the shear band, h is assumed to be around 500�C(Huang and Liang, 2004) to simplify the Lc prediction. Actually, Equation3 is not as sensitive to the h value because the thermal softening effect (a)is relatively small. Molinari et al. (2002) even ignored the effect of shearzone temperature in estimating Lc when machining Ti-6Al-4V. The strainhardening effect is ignored here for simplicity, and a future study willconsider the effect of strain hardening as discussed in (Molinari, 1997).

The experimentally obtained shear band spacing can be easily calculateusing the simple geometrical relationship Lc ¼ a1 sin h1obtained fromFigure 2b. The predicted and actual values of Lc for all cutting scenarioscan be seen in Table 3. Chip dimensions were taken for this analysis withnew tools under all cutting scenarios using new tools.

It is found from Table 3 that for most cutting tests the model overesti-mates Lc in machining AISI 52100 bearing steel except Scenario B1(V ¼ 3.05 m=s), but the model does predict the same order of the shearband spacing measurements. It is expected that accurate shear band width


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(2 h) measurement or modeling may improve model-based predictionaccuracy since this study uses Lc=17 to represent the actual shear bandwidth. Although the chip formation in hard machining is still controversial,the adiabatic shear band theory based shear band spacing predictionprovides a reasonable way to capturing the chip morphology in hardmachining, which may help understand the chip formation mechanismin the future, vice versa.

Effect of Cutting Conditions on Chip Dimensions

From Figures 7–9, it is found that the segmentation spacing a1

increased with cutting speed, feed rate, and=or depth of cut. It is also foundthat the tooth height a2 was always larger than the continuous portionheight a3 for the investigated cases when the tool was new.

To better appreciate the observations of this study, the chip dimensionsare compared with other published observations (Poulachon and Moisan,2000). Segmentation dimensions (a1; a2, and a2 þ a3) are normalized bydividing by undeformed chip thickness, which is simplified as feed hereeven for oblique cutting configurations. Comparable cutting conditionsas (Poulachon and Moisan, 2000) are selected, normalized, and comparedwith the observations in this study as shown in Figures 10 and 11 to appreci-ate the effect of cutting speed and feed rate.

For the effect of cutting speed, it is found in this study that segmen-tation spacing a1 and saw-tooth height a2 increased with cutting speed.The overall chip thickness (a2 þ a3) remained approximately constant,meaning that the continuous chip portion a3 decreased with cutting speed.However, Poulachon and Moisan (2000) found that a1 oscillated withcutting speed and both a2 and a2 þ a3 decreased with cutting speed.

TABLE 3 Experimental and Predicted Adiabatic Shear Band and Pacings

Lc measurement(mm)

Lc prediction(mm) Error percentage

Scenario A1 (�1min) 30.09 48.27 60.43Scenario A2 (�1min) 38.42 42.60 10.88Scenario B1 (V ¼ 1.52 m=s) 37.09 56.55 52.46Scenario B1 (V ¼ 2.29 m=s) 38.42 42.60 10.88Scenario B1 (V ¼ 3.05 m=s) 40.84 35.92 �12.05Scenario B2 (Feed ¼ 0.076 mm) 31.01 49.39 59.26Scenario B2 (Feed ¼ 0.114 mm) 37.09 56.55 52.46Scenario B2 (Feed ¼ 0.152 mm) 49.95 70.83 41.80Scenario B3 (DOC ¼ 0.152 mm) 32.96 51.72 56.93Scenario B3 (DOC ¼ 0.203 mm) 37.09 56.55 52.46Scenario B3 (DOC ¼ 0.254 mm) 36.50 55.86 53.05


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For the effect of feed rate, this study finds that segmentation spacing a1

increased linearly with feed rate since the slope of a1 is almost zero inFigure 11a, and the slopes of both a2 and a2 þ a3 generally decreased indi-cating a2 and a2 þ a3 do not increase as fast as a1 increases. Poulachon andMoisan (2000) also found the similar effect of feed rate on the chip dimen-sions as seen from Figure 11. Although there are chip dimension valuedifferences between Poulachon and Moisan’s work (2000) and this study,it is believed to be due to the effect of different depths of cut used sincedepth of cut also affects the undeformed chip thickness in oblique hardturning (Huang and Liang, 2003).

Elbestawi et al. (1996) developed a model to predict the overall chipthickness as a function of the rake angle (a) and the included angle (h2)only, which indicates that the overall chip thickness should be constantin this study since both a and h2 remained constant for all cutting scenarios.However, this model is not well justified by the observations here andanother documented study (Poulachon and Moisan, 2000).

It should be noted that cutting conditions have the same effect on Lc asa1 because Lc ¼ a1 sin h1 ¼ a1 sin 35�. For simplicity, the effect of cutting

FIGURE 10 A comparison with Poulachon and Moisan (2000) of data trends of normalizedsegmentation dimensions vs. cutting speed for (a) a1, (b) a2, and (c)a2 þ a3.


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conditions on Lc is not further elucidated here. If Lc can be accuratelyestimated as discussed earlier, it is expected that a1 can also be accuratelypredicted.

Segmentation Angles

It was found that the segmentation angles (h1 and h2) remain constantin machining hardened steels and Ti alloys under different cutting con-ditions (Nakayama et al., 1988; Elbestawi et al., 1996). The critical shearangle (Nakayama et al., 1998; Elbestawi et al., 1996), which correspondsto 90�� (h1þ a) in this study, was found to remain constant at �33�

degrees during saw-toothed chip formation under different cutting con-ditions and rake angles. This shear angle value (33�) agrees well with themeasurements in this study (30�).

Elbestawi et al. (1996) concluded that the crack initiation angle h1 staysconstant and is not affected by varying cutting conditions if the workpiecematerial is very hard, brittle, and homogeneous (Elbestawi et al., 1996), whichagrees the observation in this study. The included angle h2, which was 45� in

FIGURE 11 A comparison with Poulachon and Moisan (2000) of data trends of normalizedsegmentation dimensions vs. feed for (a) a1, (b) a2, and (c) a2 þ a3:


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this study, has also been found around constant 45� over a variety of cuttingconditions and cutting materials (Nakayama et al., 1988). However, this anglewas measured around 52–57�, in machining AISI 1550 case-hardened steel(HRC ¼ 60) using ceramic tools (instead of CBN tools) (Elbestawi et al., 1996).

Ratio of Chip Continuous Portion

It was found that the ratio of the height of chip continuous portion tothe overall chip thickness to be constant at approximately 0.4 in machininghardened AISI 1550 steel under different cutting conditions (Elbestawiet al., 1996). This ratio can be estimated easily using Equation 6 and someresults are shown in Table 4. It is found that the measurements in this studyalso led to a ratio value around 0.4.

rmax ¼a3

a2 þ a3ð6Þ

Segmentation Frequency

Segmentation frequencies can range from 3.8 kHz to 250 kHz in hardmachining (Becze and Elbestawi, 2002; Ekinovic et al., 2004). It is foundthat chip segmentation frequency increased with cutting time (Figure 6)and cutting speed (Figure 7c), but decreased with feed rate (Figure 8c).However, there is no definite conclusion on the effect of depth of cut onsegmentation frequency since frequencies oscillated when depth of cutincreased (Figure 9c). This decreasing trend in segmentation frequencyas a function of feed agrees with other observations by Poulachon andMoisan (2000) and Becze and Elbestawi (2002).

Although the segmentation frequency values are well above naturalfrequencies of most machine elements, rigid machining centers are still recom-mended for hard machining to allow for the tool ¼ workpiece engagement/

TABLE 4 Ratios of Chip Continuous Portion

rmax

Scenario A1 (�1min) 0.45Scenario A2 (�1min) 0.44Scenario B1 (V ¼ 1.52 m=s) 0.44Scenario B1 (V ¼ 2.29 m=s) 0.42Scenario B1 (V ¼ 3.05 m=s) 0.33Scenario B2 (Feed ¼ 0.076 mm) 0.48Scenario B2 (Feed ¼ 0.114 mm) 0.44Scenario B2 (Feed ¼ 0.152 mm) 0.33Scenario B3 (DOC ¼ 0.152 mm) 0.41Scenario B3 (DOC ¼ 0.203 mm) 0.44Scenario B3 (DOC ¼ 0.254 mm) 0.40


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exist shock and the workpiece material non-uniformity-induced cuttinginstability (Vyas and Shaw, 1999).

CONCLUSIONS

In this study, the effect of tool wear and cutting conditions on thesaw-toothed chip morphology was examined in machining hardened 52100bearing steel. This research is of interest because of its relevance to increasinghard machining implementations as a quicker, cleaner, and practical alterna-tive to finish grinding. Chip morphology was measured and quantified toappreciate the effect of tool wear, cutting speed, feed rate, and depth of cut.Chip dimensions in terms of segmentation spacing, saw-tooth height, chipcontinuous portion height, overall chip thickness, shear band spacing andchip segmentation angles in terms of crack initiation and chip included angleswere either measured using microscopes or indirectly deduced. Cutting forcesand segmentation frequency were also collected during the experiments. Thefollowing conclusions can be drawn based on this study, and they can beapplied for vision-based tool wear condition monitoring:

1. Mean saw-tooth segmentation spacing (a1) and tooth height (a2)decrease as flank wear progresses while the chip height of continuousportion (a3) increases;

2. Segmentation spacing a1 always increases with cutting speed, feed rate,and=or depth of cut. The tooth height a2 is always larger than the con-tinuous portion height a3 for the investigated cases when the tool wasnew. However, there are no monotonic relationships among cutting con-ditions and other chip dimensional values except a1;

3. Both crack initiation angle (h1) and included angle of saw-toothed chip(h2) remain approximately constant as 35� and 45�, respectively, underdifferent tool wear and cutting conditions;

4. Segmentation frequency increases with tool wear and cutting speed,decreases with feed, and oscillates with depth of cut; and

5. Shear band spacing (Lc) of the saw-toothed chip can be reasonablyestimated based on cutting configuration and workpieces’ mechanicaland thermal properties.

Further chip morphology research in hard machining is believed to helpunveil the segmentation chip formation mechanisms as well as promote hardmachining to be a viable technology. Some future work is of interest as follows:

1. It will be of interest to study whether the thermal effect under the worntool condition is responsible for higher frequency segmentation asobserved, leading to continuous-like chips;


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2. The chip segmentation mechanism should be clearly identified since theeffect of cutting conditions on machining process information dependson the chip segmentation mechanism;

3. Some inconclusive observations such as the effect of depth of cut onchip segmentation frequency should be explained in terms of machin-ing physics and material behaviors; and

4. The shear band spacing model should be enhanced such as includingthe strain hardening effect to increase its predictive capability.

ACKNOWLEDGMENTS

The authors would like to thank Mr. Yu Long, Mr. Michael Justice, andDr. JoAn Hudson of Clemson University for their help. Financial supportfrom the South Carolina Space Grant Consortium is also highlyappreciated.

REFERENCES

Batra, R.C. (1987) Effect of material parameters on the initiation and growth of adiabatic shear bands.Inter. J. of Solids Structures, 23: 1435–1446.

Becze, C.E.; Elbestawi, M.A. (2002) A chip formation based analytic force model for oblique cutting.Inter. J. of Machine Tools and Manufacture, 42: 529–538.

Chou, Y.K.; Evans, C.J. (1999) White layers and thermal modeling of hard turned surfaces. Inter. J. ofMachine Tools and Manufacture, 39: 1863–1881.

Davies, M.A.; Burns, T.J.; Evans, C.J. (1997) On the dynamics of chip formation in machining hardmetals. Annals of the CIRP, 45: 25–30.

Davies, M.A.; Chou, Y.; Evans, C.J. (1996) On chip morphology, tool wear and cutting mechanics in fin-ish hard turning. Annals of the CIRP, 45: 77–82.

Ekinovic, S.; Dolinsek, S.; Jawahir, I.S. (2004) Some observations of the chip formation process and thewhite layer formation in high speed milling of hardened steel. J. of Machining Science and Technology,8: 327–340.

Elbestawi, M.A.; Srivastava, A.K.; El-Wardany, T.I. (1996) A model for chip formation during machiningof hardened steel. Annals of the CIRP, 45: 71–76.

Hua, J.; Shivpuri, R. (2004) Prediction of chip morphology and segmentation during the machining oftitanium alloys. J. of Materials Processing Technology, 150: 124–133.

Huang, Y. (2002) Predictive Modeling of Tool Wear Rate with Application to CBN Hard Turning. Ph.D.Thesis, Georgia Institute of Technology, Atlanta, GA.

Huang, Y.; Liang, S.Y. (2003) Force modeling in shallow cuts with large negative rake angle and largenose radius tools–application to hard turning. Int. J. of Advanced Manufacturing Technology, 22:626–632.

Huang, Y.; Liang, S.Y. (2004) Modeling of CBN tool flank wear progression in finish hard turning. ASMEJ. of Manufacturing Science and Engineering, 126: 98–106.

Huang, Y.; Liang, S.Y. (2005) Modeling of cutting forces under hard turning conditions considering toolwear effect. ASME J. of Manufacturing Science and Engineering, 127: 262–270.

Komanduri, R.; Brown, R.H. (1981) On the mechanics of chip segmentation in machining. J. of Engin-eering for Industry, Trans. ASME, 103: 33–51.

Konig, W.; Berktold, A.; Koch, K.F. (1993) Turning vs. grinding-A comparison of surface integrity aspectsand attainable accuracies. Annals of the CIRP, 42: 39–43.


Dow

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ded

By:

[EP

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form

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(ES

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r Titl

es o

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Con

sorti

um] A

t: 22

:32

2 O

ctob

er 2

007

Molinari, A. (1997) Collective behavior and spacing of adiabatic shear bands. J. of Mech. Phys. Solids, 45:1551–1575.

Molinari, A.; Musquar, C.; Sutter, G. (2002) Adiabatic shear banding in high speed machining of Ti-6Al-4V: experiments and modeling. Inter. J. of Plasticity, 18: 443–459.

Nakayama, K.; Arai, M.; Kanda, T. (1988) Machining characteristics of hard materials. Annals of the CIRP,37: 89–92.

Poulachon, G.; Moisan, A.L. (2000) Hard turning: chip formation mechanisms and metallurgicalaspects. Trans. of the ASME, 122: 406–412.

Poulachon, G.; Moisan, A.; Jawahir, I.S. (2001) Tool-wear mechanisms in hard turning with polycrystal-line cubic boron nitride tools. Wear, 250: 576–586.

Ren, X.J.; James, R.D.; Brookes, E.J.; Wang, L. (2001) Machining of high chromium hardfacingmaterials J. of Materials Processing Technology, 115: 423–429.

Shaw, M.C.; Vyas, A. (1993) Chip formation in the machining of hardened steel. Annals of the CIRP, 42:29–33.

Sheikh-Ahmad, J.Y.; Quarless, V.; Bailey, J.A. (2004) On the role of microcracks on flow instability in lowspeed machining of CP titanium. Machining Science and Technology, 8: 415–430.

Shih, A.J.; Luo, J.; Lewis, M.; Strenkowski, J.S. (2004) Chip morphology and forces in end milling ofelastomers. Trans. of the ASME, 126: 124–130.

Tonshoff, H.K.; Arendt, C.; Amor, B. (2000) Cutting of hardened steel. Annals of the CIRP, 49: 547–566.Ueda, N.; Matsuo, T. (1982) An analysis of saw-toothed chip formation. Annals of the CIRP, 31: 81–84.Vyas, A.; Shaw, M.C. (1999) Mechanics of saw-tooth chip formation in metal cutting J. of Manufacturing

Science and Engineering, 121: 163–172.Wright, T.W.; Ockendon, H. (1996) Research note: A scaling law for the effect of inertia of the

formation of adiabatic shear bands. Inter. J. of Plasticity, 12: 927–934.


Chip Morphology Characterization

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