Chip formation in the machining of hardened steel

Chip Formation in the Machining of Hardened Steel M C Shaw (1) A Vyas, Arizona State University, Tempe, Arizona/USA Received on January 4,1993 Abstract With the avai1abilit.y o f polycrystalllne cubic boron nitride (PCBN) i t i s possible t o machine vend hard gears, etc at speeds of (60-150 m/min = 200-500 fpm) When this i s done using PCBN tools i n face milling, Chip formation i s of a cyclic saw toothed type This type of chip formation i s reviewed i n relation t o other types of cylic and noncyclic chip formation The root cause o f high frequency, saw toothed chip formation i s found to be periodic gross Shear fracture extending from the free surface of the chip toward the tool t.ip and not adiabatic shear as commonly believed Keywords: Cutting, Cubic boron nitride (CBN), Chip formation Introduction With the appearance of superhard cutting tool materials i t i s possible t o machine work materials such as case carburized gears a f t e r heat treatment rather than by grinding (Hodgson and Trendler,1901; Schwarzhofer and Kaelin, 1986; Koenig et al, 1990) In the course of a general study of this possibility some very interesting cyclic chips have been obtained i n the practical cutting speed range s i m i l a r t o ones described i n the literature when machining materials of lower hardness at very high speeds In order t o optimize such machining operations, i t i s important t o understand the chip forming mechanics of these cyclic chips i n fundamental terms Before discusssing experimental results obtained when face m i l l i n g case carburized steel specimens w i t h polycrystalline cubic boron nitride (PCBN) tools, i t i s useful to review cyclic chip formation from a broad point of Vie’w Cyclic Chip Formation Within a short time a f t e r Merchant (1941, 1945) published his world famous model of continuous chip formation (Fig la), i t was suggested by several authors that a l l chips not behave i n accordance w i t h this model I f the work i s relatively s o f t and not prestrain hardened, chip formation w i l l involve a pieshaped zone (Fig lb) and an even more extensive shear zone i f the radius a t the tool t i p (p) i s large relative to the undeformed chip thickness a (Fig Ic) Fig I Chip formation for -flow- type chips a ) concentrated shear model f o r Precoldworked softmaterial b! Pie shaped shear zone for soft annealed material c) More extensive shear zone w i t h subsurface plastic f l o w f o r tool w i t h rounded t i p a :: continuously across the w i d t h of the chip but are separated by regions undergoing subfracture plastic flow These microcracks are subsequently rewelded w i t h further deformation Evidence f o r this i s the fact that the mean shear stress-on the shear plane increases w i t h normal stress (Merchant, 1945) which would not be the case l f only plsstic f l o w were involved Further evidence i s the presence of the ends of localized microfracture planes on the back of a “flow !ype” chip IFig.2) Since there i s no evidence of microfracture on the side surface of a continuous “flow” type chip, i t has been generally incorrectly assumed that no fracture i s involved One of the f i r s t papers concerning cyclic Chip formation where the chip i s continuous but i s alternately thick and thin and rnOVeS in a stick-slip fashion up the face of the tool was described by Bickel (1954) a t the ClRP General Assembly of that year Bickel used a high frequency flash lamp t o produce a series of pictures showing the development of what i s generally referred to as a wavy chip !Fig 3) This was the sit.uation f o r a relatively s o f t material machined at relatively high but practical cutting speeds f o r the HSS and carbide tools then i n use As the chip was fot-med, the shear angle gradually decreased w i t h the chip stationary on the tool face u n t i l the component of force along the tool face was sufficient t o cause the chip t o move up the tool face as the shear angle increased This was followed by the chip again Coming to rest and’a repetition of the cycle 01 variation In shear angle, forces and chip thickness With this type of chip there was no gross fract.ure (i.e fracture where t.he plane of fracture extends clear across the ‘width of the chip) and when viewed from the side, i t had the same appearance as a continuous “flow” type chip except that there were signs of greater and less strain having taken place I n the thin and thick chip vicinities respectively (Fig.3) Similar cyclic chips were described by Eugene (1957) a short time after Bickel Albrecht !I9621 has presented a discussion of wavy chip formation that involves the Cycllc variation of shear angle but no gross fracture b1 rn It was also soon discovered that a great deal o f cutting involved cyclic chip formation The rnechanics of this type of chip formation has been thoroughly viewed by Komanduri and Brown (198 1) and some important observations concerning this topic have been published by Nakayania (1974, 1988) Those interested i n cyclic chip formation w i l l find these three papers a valuable starting point Only observs:ions that tixtend the concepts presented i n these papers or s h i f t the emphasis w i l l be presented here Fig Back (free) surface of ‘flow’ type chip showing ends of rnicrorfracture planes Fig Wavy chip A l l types of metal cutting involves fracture Even the forma?ion o f continuous (so-called f l o w type) chips (Fig la) involves extensive localized microfractures that not extend Annals of the ClRP Vol 42/1/1993 29 A I I other cyclic Chip tormation involves periodic gross fracture that extends clear across the width of the Chip Periodic gross fracture may begin at the tool t i p or at !he free back surface of the chip The f i r s t type which leads t o relatively small drsconnected segments i s generally termed discontinuous chip formation Discontinuous Chip Formation Figure shows cyclic chip formatjun where gross fracture originates periodically from the tool t i p (A) The numbers under the sketches are motion picture frame numbers The l a s t sketch on the l e f t i s a composite showing how a single chip segment i s formed I n this case, the chip does not slide over the tool face as i t is formed but r o l l s down upon t.he tool face as the center of the chip i s extruded upward When frame 40 i s reached, the free surface of the chip i s tangential t o the t.ool face and tool face f r i c t i o n i s essentially zero at the tool tip same f o r a reasonably homogeneous work mat.erial Figure shows the situation when the fracture curve extends below the line of tool t i p travel This i s then a source of surface roughness for discontinuous chip formation and the finished surface w i l l consist of alternale unburnished (dull) and burnished (shiny) regions when the finished surface i s viewed from above (Fig 6b) cast iron and unleaded 70130 or 60/40 brasses are materials that tend t o give discontinuous chips that are easily disposed of It i s found that the stick-slig freauency of segment formation i s influenced i n a minor way by the stiffness of the tool-work machine tool system Saw T o o t h Chip F o r m a t i o n Still another type of cyclic Chip i s obtained w i t h cold worked 60/40 brass Figure 7a shows a Chip root f o r such a material while Figure 7b i s a diagrammatic interpretation In t h i s case, perlodic gross fracture occurs at the free surface where normal stress on the shear plane i s zero and runs clear across the surface t o the tool t i p In Figure 7a a new crack i s l u s t about t o occur and run t o the tool tip Nakayama (1988) has given good reasons why FG i s parallel t o the resultant force (R) on the tool Fig Cyclic chip formation f o r Beta brass(after Cook et al; 1954) Figure shows the elastic stress pattern when a concentrated horizontal force P is acting at the tool point (tool face f r i c t i o n essentially zero) OF i s a line of constant shear stress direction and since the magnitude of the shear stress rncreases as point i s approached, OG o r a related line curving upward from and t o the left should be the fracture surface f o r discontinuous chip formation The importance of tool face f r i c t i o n being essentially zero when a new crack forms at i s that tool face f r i c t i o n a t would give r i s e t o a compressive stress there that would tend t o prevent crack i n i t i a t i o n a t A i n Figure The condition that determines when a new gross crack forms at A i s when the shear stress a t A i s high enough and the normal stress i s l o w enough consistent w i t h the shear strength of the material at A to cause fracture P Fig Elastic stress pattern f o r cutting force P and zero tool face f r l c t i o n force at t l p of Sharp t o o l l a f t e r Marlelloti, 1941) Fig Situation when crack running from A t o E ext.endS below path 01 cutting edge A-C a) side view b) top view showing crack region (gray) and burniqhed cut region (shiny) Eased on the foregoing sequence of events leading to discontinuous chip formation i t IS t o be expected, as observed, that the size and shape o f each segment will be auuroxlmately the 30 Fig a) Optical Photomicrograph o f partially formed continuous chip of60/40 cold worked brass (rolled to 60% reduction i n area before cutting) Rake angle = -150; undeformed chip thickness O.16mm (.0063in), cutting speed = 0.075 m min-1(2.95 ipm) (Nakayama, Toyama Universit-y) b) diagrammatic representaion of a) As soon as a crack runs from B t o A i n Figure 7b material i s displaced f r o m cross hatched region 1.0 w i t h further advance of the tool This forces the block of material A B W outward t o i t s rinal position EFGD w i t h essen!ially no deformation except f o r that associated w i t h f r i c t i o n sliding along AB, DG and the tool face AH In the case o f Figure 7, f r i c t i o n on the tool face is relatively low and the resultant force on !he tool i s approximately horizontal and the fracture angle i s 45O The fact that the direction o f the cold worked f l o w lines i n Figure 7aremains unchanged i n the uncut material and i n the bulk of the chip i s consistent w i t h the foregoing mechanism (Nakayama 1988) The work done i n t h i s type of chip formation w i l l involve frictional sliding resistance along planes AB and and tool face AH plus a relatively small amirunt o f extrusion related energy associated w i t h the displacement of material from t o (Fig 7b) A new fracture plane w i l l develop when the total sliding distance i s sufficient t o again produce the fracture stress a t the free surface The dotted planes 8'A' etc are subsequent fracture planes which w i l l be uniformly spaced f o r a homogeneous work material It should be noted that i n Figs the sliding that occurs i s l i k e that of a f r i c t i o n slider w i t h essentially zero subsurface plastic flow Fig.8 Optical photomicrographs of Coninuirus titanium chips a) st low cut.ting speed of 25 mm/min ( ipm) b) a t relatively high cutting speed o f 53 m/min (175 fpm) c i diagrammatic interpretation of b) (after Shaw etal, 1954) Figure 8a shows a titanium chip produced at low speed where a saw toothed chip i s produced due t o shear fracture surfaces running periodically i r o m the free surface t o the tool t i p i n the manner of Figure Here the major expenditure of energy i s again involved i n overcoming sliding f r i c t i o n along the tool face and fracture surfaces L i t t l e evidence of plastic f l o w i s evident w i t h i n the segments Figure b shows the chip produced from the same material but a t a more normal cutting speed 153 m/min (175 fpm)] Several differences between 8a and 8b are evident: There i s considerable secondary deformation along the tool face i n Figure 8b but not i n 8a The secondary deformation along the tool face i s very inhomogenenus i n Figure 8b There i s evidence of considerable subsurface deformation along the fracture surfaces i n Figure 8b C o m p o s i t e Chip F o r m a t i o n Figure 8c shows a diagrammatic interpretation of Figure 8b I n this case, gross fracture extends only part way to the tool tip The gross fracture crack w i l l be stopped at a point where the compressive stress on the shear plane reaches a ralue sufficiently high t o stop the crack This rneans that the layer below the stopped crack must be removed by the flow type cutting mechanism while the upper part involves sliding frictional resistance Deformation in the secondary shear zone along the tool face i s very inhomogeneous There i s evidence of unusually large plastic strain i n the secondary shear zone along extensions of the gross shear cracks (indicated by dots in Figure 8c) This suggests that weakening associated w i t h the generation of noncontinuous microcracks along the dotted crack extensions shown in Figure 8c i s responsible for the concentrated shear bands i n the secondary shear zone The shear bands eviden! i n Figure Rb l i e along the shear fracture planes and are evident when materials having l o w thermal properties (conductivity and volume specific heat) are cut at relatively high speeds A t high speeds of sliding, considerable thermal energy develops and w i t h l o w thermal properties, this results i n thermal softening and considerable subsurface f l o w along the sliding f r i c t i o n (fracture) surfaces These concentrated shear bands are frequently referred t o as being due t o adiabatic shear While this i s true enough regarding the end result, adiabatic shear i s not the root cause of this type o f cyclic Chip formation but merely evidence of the presence of high thermal energy due t o high speed sliding along already formed periodic fracture surfaces The cyclic formation of these fracture surfaces i s the actual cause of the instability and not adiabatic shear as suggested by Shaw et al (1954) Saw tooth chips are obtained w i t h (Fig 8b) and without (Fig 8a! the presence of adiabatic shear When a titanium rrlloy i s cut at extremelu low speed (-0.001 rn/min = 0.004 fpm) continuous chips of the type shown i n Figlire w i t h individual segments welded together are obtained !Komanduri and Turkovich, 1981) In this case, periodic fracture begins at the tool tip However, a t more practical speeds but as l o w as 25 mm/min [ I ipm]) chip format.ion i s as shown i n Figure w i t h periodic lractiure st-arting at the free surface as suggested by Nakayams ( 988) Kornanduri and tiis associates have obtained turning chips very s i m i l a r to that of Figure 8b when cutting materials having l o w thermal properties a t very high speeds Figures 9a and 9b are two examples Figure 9a is a saw toothed chip produced when machining AlSl 4340 steel of moderate hardness (% = 325 Kg/mm*) at a high speed 250 m/min (800 fpmi This i s seen to be similar t o Figure 8b except !hat the very high speed of sliding along the extensions of the gross fracture surfaces !dots i n Fig 8c) and along the tool face results i n what appears t o be melting as indicated by the whit.e [unet-ched ) bends i n Figure 9a Figlure 9a shows periodic melting at points A, 6, C, etc along the ton1 face As Komanduri has suggested, the speed of the chip luctuates periodically and w i l l have i t s maximum velocity j u s t a f t e r gross fracture occurs This appears t o correspond t o the regions of greatest depth o i melting where the r a t e of heat g e n m t i o n will be B maximum A t higher cutting speeds than that pertaining i n Figure 9a an w e n thicker white layer was evident along the tool face Figure 9b shows a saw toothed chip produced when turning a nickel base tlirbine alloy at what i s high speed f o r this material The chip of Figure 9b i s very s i m i l a r t.o !.hose o f Figures 8b and 9a From Figure 9c i t i s evident that when periodic gross cracks not penetrate a l l the way t o the t.ool tip, two t-ypes of chip formrjtion are superimposed - that COrreSpOnding to Figure 7b where sliding f r i c t i o n i s predominant and that corresponding t o very nonhomogeneous f l o w type chip iormat.ion w i t h extensive secondary shew flow along the tool face Lindberg and Lindstrom (1983) studied saw toothed chip formation of A l S l 1035 steel and found that saw toothed chips were not formed even at very high speeds if the undeforrned chip thickness (feed) had a low value For examDle, saw toothed chips were formed at a frequency of about 14000 Hertz at a cutting Speed of 150 m/min (490 fpm), a feed of 0.315 mmirev (0.012 ipr)and a depth of cut of rnm (0.080 in) but continuous chips were formed at the same speed and depth of cut when the feed was reduced to 0.100 mm/rev (0.004 ipr) Since the highest natural frequency of any of the components of the tool-work-machine tool systern i s only about 1000 Hz, i t follows that the stiffness o f the System shoulU have no influence on the frequency of segment formation This has been found t o be so for a l l cases of saw tooth chip formation (Komanduri et 81, 1982) nelting This Is not the f i r s t t i m e what i s believed to be a molten layer has been observed i n metal cutting Schaller (1962) in studying the machining of specially deoxidized steels having a low tendency t o cause cratering o f carbide tools at high cutting speeds has observed a nonetching white layer along the tool face (Fig 10) This appears t o be a material that has melted and then cooled so rapidly that an unresolvable grain size o r none at a l l (amorphous) develops In this case relatively low melting ternary (Si$-AI,O,-CuO inclusions spread over the tool face and act as a diffusion barrier Fig U p t i i a l photomicrographs of continuous saw toothed chips produced when machining d i f f i c u l t materials a! high speel a) AlSl 4340 steel (Hg = 325 z HRC = 34) turned w i t h AI,O,/Ti ceramic tool at cutting speed of 250 m/min !800fprn), feed of O.Smm/rev.(O.O I8 ipr),depth of clut of 3.75 mm (0.1Soin), rake angle of - 5O no cutting fluid (after Kornanduri et al, 1982) b! Solution t-reated and aged lnconel 718 (y=300 s ) nickel based turbine alloy turned w i t h A1,0, /TIC ceramic tool at cut.ting speed of 92 m i m i n (300 fpm), feed of O.2Omm (0.008 ipr), dept.h of cut of 2.5 mm (0.100 in!, rake angle, -5O no c.utt.ing fluid (after Komanduri and Schroeder, 1’3861 31 DeSalvo and Shaw (1969) have investigated the possibilities of hydrodynamic action w i t h such a situation and have shown that what at first glance appears to be a f i l m that i s inclined i n the wrong direction f o r positive hydrodynamic pressure development w i l l actually give positive pressure This becomes clear by reference t o Figure I Figure I la shows the classical slider bearing w i t h a stationary inclined pad and an extensive member moving w i t h a velocity V to the l e f t while Figure l b shows the chip moving parallel t o the stationary tool Figure ICi s the kinematic equivalent of Ilb which is seen t o be identical t o Ila These chips are seen t o h a w t.he same appearance as those of I'igure including the following: 0 periodic gross cracks extending part way from free slurface of chip t.o tool t.ip very Iit-tle evidence of plastic f l o w i n t.he "teeth' of the chip heavy plastic f l o w i n ?he region of the chip below the extent of gross cracks and along the tool face white unetched layers along ?he too! face and gross frac!ure surfaces Fig 10.Formation of layer on tool face when turning specially deoxidized steel at high speed (after Schaller, 1962) Venuvinod e t a l (1983) have also found a structureless w h i t e layer when using an externally driven rotary tool to turn m i l d steel The presence of such a f i l m led to l o w force levels which were attributed to hydrodynamic action No such f l u i d f i l m s were found f o r materials having higher thermal conductivity (Cu, Al, brass) The unetched white layers i n Figure represent a third case i n metal cutting where molten layers appear t o be involved No legitimate evidence of melting i n grinding exists even though the specific energy in fine grinding i s more than an order o f magnitude greater than that f o r metal cutting (Shaw, (1984) A t cutting speeds even higher than those f o r Figures 9a and 9b the chips are no longer continuous (Komanduri e l al, 1982) but consist of individual segments This i s apparently due t o melting of a continuous layer separating individual segments Fig 12 a) Optical photomicrograph of continuous saw toothed chip produced when face m i l l i n g case carburized steel (61 kl at cutting speed of 500 fpm (152 m/min), feed of 0.0 10 i p r (0.25 mm/rev), depth of cut 0.010 i n (0.25 mm), rake angle of - 7,and using no cutting fluid b) Scanning electron micrograph o f portion of a) at 5x the magnification Chips produced a t other feeds and speeds were s i m i l a r t o those of Figiure 12 The material i n the white layer along the tool face in Figure 128 i s seen t o be essentially wihout structure This a) 6) Fig 1 a) Classical hydrodynamic slider bearing w i t h f l u i d layer decreasing i n thickness i n direction of motion b) Inclined 'fluid' layer between stationary tool and solid chip surface moving parallel t o tool face c) kinematic equivalent of b) which i s the same as a) was even found t o be the case in SEN micrographs of the white layer at 3500~ Figure 8c holds equally w e l l f o r Figures 9, 8b and 12 When the gross cracks extend only part way t o the tool t i p there are t w o shear angles Q: 9, = the gross fracture plane shear angle (459) Milling H a r d Case C a r b u r i z e d Steel When case carburized A l S l 8620 steel (b= 61 and 0.050 i n case depth ~ 5mm) is subjected t o a plane m i l l i n g operation under the following conditions, saw-toothed chips very s i m i l a r t o those in Figure are obtained (Fig 12): Cutting speed: 500, 200 fpm (152, m/min) Depth of cut: 0.010, 0.005 i n (0 25, 0.13 mm) Feed: 0.0 10, 0.005 i p r (0.25, 0.13 mm/rev) Rake angle: -7 Tool: Five PCBN inserts each 0.500 x 0.188 i n (12.5 x 4.80mm), 0.031 i n (0.78mm) nose radius, i n cutter diameter (76mm) 80"x100" diamond shaped inserts w i t h 100' corner used 32 +2 = the plastic defomation shear angle for the f l o w type chip formation region ( @ < Q,) Also, i n Figure Eic, p i s the spacing of successive gross fracture planes on the work surface and pc i s the corresponding spacing on the chip (p>p,) The cutting r a t i o for such a composite model w i l l be r = pJp (1) As Nakayama !I3881 has shown, the resultant force on the tool face i s as shown in Figures 8a and and the included angle at the t i p of each "tooth" should be 4S0 as indicated in Figure C o n c l u d i n g Remarks Considerably more information may be extracted by examination of a saw tooth chip than from a cant-inuous f l o w type LhiP but t o consider t h i s would carry the present, diSclJSsion too f a r ailellj The main objective of t h i s paper was t o demonstrate !hat Saw tooth chips are obtained no! onlq when a highly cold wYorked b r i t t l e material i s machined even a t l o w Speeds [Fig f o r brass) a a d i f f i c u l t t o machine material w i t h low thermal properties (k, pc) i s niachirted over a wide range of speeds (Fig 8, Ti) and Fig 9b (Ni base alloy a a somewhat d i f f i c u l t to machine material and moderate hardness i s machined at very high Speed (Fig.98, AlSl 4340 steel) but also when a very nard b i t t l e material i s machined at relatively l o w speed (Fig 12 - case caburized hard steel) A second objective was t o show the relationship of saw tooth chip formation t o other modes of cyclic chip lormation as well as t o f l o w type Chips A l l types o f chip formation involve fracture as w e l l as plastic flow The f l o w type chip involves localized microfracture and rewelding i n conjunction w i t h plastic f l o w &haw et al, 1991) I t i s important t o keep this i n mind when attempting analytical simulation of any chip forming process Use of the Von Mises f l o w criterion i s inadequate as a constitutive relation even f o r f l o w type chip formation and i s particularly inadequate f o r cyclic Chips, since i t does not take the important fracture aspect i n t o account An important result i s that what appears t o be a liquid layer of chip rnaterial i s formed along the tool face when a very hard b r i t t l e material such as hardened case carburized steel i s macnineu at ordinary speeds This means that the contact area between Chip and tool w i l l be 100% of the apparent area of contact This coupled w i t h the very high temperature involved (M.P of work material) greatly increases the likelihood of crater wear on the tool face Polycrystalline CBN i s w e l l suited t o the machining of superhard work materials because of i t s hardness, chemical stability i n contact wit.h high tmperature iron and i t s outstanding thermal properties Komaridut-1, R ; Schroe0er.T A.; Hayra, J., von Turkovich, F.; and Florn, D.G 119823 On the Ca+astrophic Shear Instability i n High Speed Machining of an AlSl 4330 St.eel, J Ena Ind (Trans.ASME) 104- I2 - I I Komandluri, R.; Flom, G.,and Lee, m !1'?851 Highlights of DARPA Advanced M a m n i n g Research Program, J Eno f o r InU (Trans ASME) 107 325-335 Komanduri, R and Schroeder, T.A (19863 On Shear Instability i n machining a Nickle-Iron Base Superalloy J Ena for Ind (Trans ASME) 108 93- I 00 Lindberg, B and Llndstrorn, R (19833, Measruements of the Segmentation Frequency i n the Chip Formation Process, Annals of CIRP.32/1, 17-20 Martelotti,M.E (1941) An Analysis of the Milling Procss TRANS ASME,63.8,677 Merchant, M E (1945) Mechanics of the Metal Cutting Process J ADD^ Phus 16.267(a) 318(b) Nakayama, K (1974) The Formation of Saw-tooth Chip, & ylternat Conf on Produc., Tokyo, 572-577 Nakayama, K (1988) Machining Characteistics of Hard Materials, Annals of C R P U Schaller, E (1962) Beitrag zur Untersuchung Yon Spannungen und dynamischen Vorgangen i n der Grenzschicht zwischen Wergzeug und Span bei der StahlZerSDannUng m i t Hartrnetallwerkzeugen QEna Dissertation T.H Aachen Schwerzhofer, R.P and Kaelin, A (1986) Finish Cuting of Case Hardened Gears, Annals of CIRP.35/1 45-50 S h W M.C.; Dirke, S 0.; Smith, P A,; Cook, N H.; Loewen, E.G and Yang, C.T.( 1953) Machining Titanium, unoub NIT reDoct Shaw, M.C (1984) Slurface Melting in Grinding Operations? Annals Of ClRP.33/ I , 2 1-223 Shaw, M.C.; Janakiram, M and vyas A (199 ) The Role of Fracture i n Metal Cutting Chip Formaion, Proc NSF Grantees CON on Pesian and M a n u f c t u r m SutsemS, Dub by SME, Dearborn, MI 359-366 Venuvinod, P.D; Lau,W 5.; and Reddy P.N (1983) On the Formation of a Fluid F i l m at the Chip-Tool Interface i n Rotary Machining, Annals of C l R P , a - References A1brecht.P (1962) Self Induced Vibrations i n Metal Cutting, IND (Trans ASME) 84,405 Bicke1,E (1 954) Hochlrequenten Zeitlupenaufnahmen (Spanbildung) Annals of CIR P 3.90-9 Cook,N.H.; Finnie, 1.; and Shaw, M C (19543 Discontinuous Chip Formation, Trans ASME 153-162 DeSalvo, G.J and Shaw, M.C 11968) Hydrodynamic Action at ChipTool Interface, Advances i n m.Tool Pergamon P r e s s 96 1-97 Ernst, H J and Merchant, M C (1941), Chip Formation, Friction, and Finish, Trans Am SOC.Metals 29,299 Eugene, E (1957) Etude Experimentale sur I'lnfluence Conjointe de la Pente D'Affutage de I'Outil et de la Vitesse de Coupe sur les Modalites de l a Formation de Copeau, Annals of CIRP A 121 Hodgson, T and Trendler, P.H.H (198 I ) Turning Hardened Tool Steel r m w i t h Cubic Boron Nitride Inserts, &&&J&, Koenig, W.;Klinge, M.; Link, R (1990) Machining Hard Materials w i t h Geomtrically Defined Cutting Edges - Field of Applications and Limitations, Annals of CIRP 59/1.61-6 Komanduri, R and Brown, H (1981) On the Mechanics of Chip Segmentation i n Machining J Ena for lnd (Trans ML 33-51 a m, Kornanduri, R and yon Turkovich ( I New 0bSerVat.ions on the Mechanism of Chip F o r f d l o n when Machining Tttnium Alloys 179-188 Komanduri,R; Schroeder, T.A., H a p , J; von Turkovich, 6.F; (198 ) New obsevations on the Mechanism of Chip Formation When Machining Titanium Alloys, W e a r 179- 18s 33 ... 7aremains unchanged i n the uncut material and i n the bulk of the chip i s consistent w i t h the foregoing mechanism (Nakayama 1988) The work done i n t h i s type of chip formation w i l l involve... involved (M.P of work material) greatly increases the likelihood of crater wear on the tool face Polycrystalline CBN i s w e l l suited t o the machining of superhard work materials because of. .. of cut of rnm (0.080 in) but continuous chips were formed at the same speed and depth of cut when the feed was reduced to 0.100 mm/rev (0.004 ipr) Since the highest natural frequency of any of