OPTIMUM TOOL GEOMETRY OF CBN TOOL FOR CONTINUOUS TURNING OF CARBURIZED STEEL

Int J Mach Tools Manufact Printed in Oreat Britain Vol 29, No 3, pp.403-413 1989 (IBg(~955/xg$3.INI -~ 00 Maxwell Pergamon Macmillan pie O P T I M U M T O O L G E O M E T R Y O F CBN T O O L F O R C O N T I N U O U S T U R N I N G OF C A R B U R I Z E D S T E E L K SmNTANI,* M UEKrt and Y FUJIMURA* (Received 22 January 1988; in final form 14 September 1988) Abstract The optimum geometry of a CBN tool for continuous turning of a carburized steel bar was determined for the angle and width of negative land, the nose radius and the honing radius Fine turning by a tool with such an optimum geometry could be fully substituted for a grinding process on the basis of tool life and roughness of the finished surface Tool wear as a controlling factor of tool life was analyzed Particularly in the initial wear stage, frequent chipping of the cutting edge strongly influenced the life of tools with small negative land angles NOMENCLATURE f L LI L2 NL R~ Rac rH rn l VB ~c w~ feed rate cutting length (spiral) cutting length to reach the wear life limit (VB=0.25 ram) cutting length to reach the surface roughness life limit (R~=0.8 ~m) tool-chip contact length negative land angle average roughaess height calculated average surface roughness honing radius nose radius depth of cut flank wear width cutting speed negative land width rake angle INTRODUCTION A CONSIDERABLEimprovement has been achieved in the heat resistant and wear resistant performance of cutting tools in recent years However, low speed or inefficient machining processes have not yet been improved for the finishing of extremely hard materials such as carburized steel because of the necessity of troublesome grinding If cutting could be substituted for the grinding process, many advantages would be obtained not only in reducing the cost of equipment but also in increasing the flexibility of machining facilities To achieve this substitution, cutting tools capable of machining the hardened steels are required For this purpose a sintered body of c-BN (cubic boron nitride) particles, which have been used as abrasive grains for grinding, can be used as the cutting tool The potential of CBN (sintered c-BN) as a cutting tool, however, has not yet been realized due mainly to lack of fundamental data on its cutting performance and lack of reliability as a tool, particularly in regard to its toughness Experiments with turning of hardened steels using the CBN tool have been carried out by Hodgson and Trendler [1] and Chryssolouris [2, 3] In these studies, the optimum cutting conditions were determined based on the tool wear behaviour during cutting The present study attempts to present the technological implications of effective use o f t h e C B N t o o l f r o m t h e v i e w p o i n t o f o p t i m u m t o o l g e o m e t r y d e s i g n e d for i n c r e a s e d t o o l life a n d i m p r o v e d c u t t i n g p e r f o r m a n c e T u r n i n g using t h e C B N t o o l e n a b l i n g t h e c o n t i n u o u s finishing o f c a r b u r i z e d s t e e l as a s u b s t i t u t e f o r the g r i n d i n g p r o c e s s was also investigated *Department of Mechanical Engineering, Kanazawa Institute of Technology, 7-10hgigaoka, Nonoichi Ishikawa 921, Japan +R&D Laboratories I, Nippon Steel Corporation, 1618 Ida, Nakahara-ku Kawasaki 211 Japan 403 404 K SHINTANI et al Ps VIEW ON Po VIEW ON Pf rH VIEW ON Ps P~ FtG G e o m e t r y of tool tip EXPERIMENTAL PROCEDURES Workpiece and tool materials The workpiece material was carburized Cr-Mo steel (JIS-SCM420) of ~b98x 450 mm The region from the surface to mm in depth was used in the cutting experiments The hardness of the carburized layer was in the range of Hv 600-720 The tool material was cubic boron nitride (c-BN) sintered with binding phases such as TiN and AIN The size of the c-BN particle was about p,m The volume fraction of c-BN was about 60% which is reported to be effective for cutting hardened steels [4, 5] The transverse rupture strength of the tool was about 800 MPa at room and elevated temperatures [6] Tool tip parameters The tip geometry of the tool is shown in Fig Various values of the tool parameters, such as negative land angle NL, negative land width WL, nose radius rn and honing radius rH, were combined for each series of cutting experiments as denoted by (1)-(4) in Table The honing radius was finished by a #800 hand lapper to an accuracy of -0.01 ram Except for the experiments to determine the effect of WL on the tool life, a considerably larger value of WL of 0.2 mm was used, which is larger than those of commercial tools produced in Japan (0.05-0.06 ram) The land acts as the rake face in the present case which is different to the general type of chamfering in [1] and the rake angle is NL plus - °, the - ° being the tool holder attaching angle Continuous Turning of Carburized Steel 405 TABLE EXPERIMENTALSELECTIONOF TOOLTIP PARAMETERSFOR CONTINUOUSCUTTING Tool" parameter Experimental condition (1) (2) (3) (4) Negative land angle, N L (deg.) 15, 25, 30, 35, 40, 45 35 35 35 Negative land width, WE (mm) 0.2 0.06, 0.07, 0.1, 0.15, 0.2 0.2 0.2 Nose radius, rn (mm) 0.8 0.8 0.4, 0.8 1.2, 1.6 0~8 Honing radius, rH (mm) 0.05 0.05 0.05 0.02, 0.035, 0.05, 0.085, 0.1 Cutting conditions and determination of tool wear In order to substitute for grinding, fine cutting conditions were used with feed rate f and depth of cut t chosen as 0.1 mm/rev and 0.1 mm respectively The cutting speed vc was selected as 100 m/min (1.67 m/sec) based on preliminary experiments using tools with NL of 25 ° and 35 ° Tool life was determined by measuring both the flank wear width of the tool, VB, and the average roughness height of the workpiece, Ra, where the cut-off value was 0.8 mm The limiting values of these to determine the life of the tool were set as VB=0.25 mm and R~=0.8 Ixm VB was determined by measuring the mean distance between the initial cutting edge and the front of a worn portion of the tool excluding the wear notch Both measurements were carried out at each cutting length L of 50, 100, 500, 1000, 1500 and 2000 m For L longer than 2000 m, the measurements were made at each 1000 m interval The tool-chip contact length, ec was determined by measuring the scratched length on the painted rake face The profile of the wear crater in the tool was also measured at the same time The wear and failure of the tool were observed using a scanning electron microscope (SEM), while the cutting force was measured and recorded using a tool dynamometer and a digital wave memory scope RESULTS AND DISCUSSION Effects of NL and WL on" the tool life One of the measures of wear resistance, the cutting length to reach the life limit, was defined as L1 The dependent of L1 on NL is shown in Fig 2(a), indicating the maximum wear resistance at an N L of 30-35 ° As seen in the figure, experiments were performed 3-5 times under the same conditions and the results show a significant scatter at NL less than 30 ° The relationship between Ra and the cutting length is shown in Fig Although a rapid increase in the roughness at the initial cutting stage occurred in the tool with an NL of 15°, R~ first decreased for the other tools to the minimum due to the increase in the nose radius by wear, and then increased again by the progress of groove wear in the end cutting edge The variations in the wear patterns can be discerned from the results presented in Fig 4, the variation of the surface roughness profile of the workpiece in the direction normal to the feed mark at different stages in the cutting process As an indication of wear resistance from the surface roughness point of view, the cutting length to reach the life limit was defined as the tool life L2 The relation between L2 and NL is shown in Fig 2(b) which exhibits a similar trend to Fig 2(a), namely that L2 also has a maximum at around NL=35 °, and that the experimental results are more scattered at lower NL values 406 K SHINTAN[et al 15.0 I (a) I I o i I I I / A E i¢ 10.(] ,.d 5.13 r,:O.emm / V~:lOOm/min / , (b)' f :0.1mm/rev L - ' t : 0.1mm ] ~15.( 10.0 / " -4 \ 5.0 I I 20 I I I 30 I 40 50 Negative land angle NL (deg.) FIG Dependence of tool lives Lt(a) and L2(b) on the negative land angle ~" 1.2 ~,: lOOm/min f : Q.1 mm/re v t : O.lmm y 1.0 ~D ~ 0.8 tx tt -~ o.0 gtO £ 0.4 x ,% • ~ 0.2 o 15, WL: m m r,:O.Bmm r , : O.05mm ~ • 15b ~ 25 o 35 • 45 ' d ' 12 ' 1~ C u t t i n g length L (km) ' 2o FIG Variation of average roughness height, R~, with cutting length for various negative land angles L (km) NL15~ NL35 ° 0.04 j J 1.80 J J 4.50 i f J 5.00 j j 5.60 j f J J J 10.00 13.00 FtG Comparison of surface roughness profiles for N L 15° and N L 35° with variation of cutting length Continuous Turning of Carburized Steel 407 r :0.O m m ' 15 rx : 0.05~,.o - ' ~ - ~ A E 10 d o %:100m/min t :0.1 mm 0.o5 z~ O.lO o 0.15 i ~ 0.10 0.15 O.120 0.25 Width of negative land 0.'05 WL (mm) FIG Dependence of tool life (L,) on negative land width Wt_ for various feed rates 20.0 NL:35 ~ 115~.rx :0.05 mm • / Z 10.0 / v~:100 m/rain f:0.1 mm/rev ' t:01mm 8 ~ ° 7.5 Tool l i f e o Li 5.0~- ° 251 • Lz 0.5 t I 1.0 1.5 I 2.O Nose radius r (mm) Flo Dependence of tool lives L, and L on nose radius Figure shows the relationship between L~ and W L at a range of feed rates from 0.05 to 0.15 mm/rev L1 increased as WL initially increased and then reached a constant value for all feed rates• The WL at which L1 reached a constant value corresponds approximately to the value at which /~c becomes constant Effects of r,, and rH on the tool life U n d e r condition (3) in Table 1, the effect of nose radius r on L1 and Lz was investigated and the results are shown in Fig Initially L~ increased as r, was increased and it reached a certain constant value for rn larger than 0.8 mm The low values of L1 which occurred at small r, values were thought to be caused by thermal wear due to the increased temperature rise in the tool tip because of the narrowed chip contact area and the increased chip thickness Such a temperature rise can also be confirmed by the significant progress of groove wear in the side cutting edge due to oxidation as shown in the SEM micrograph of the tip in Fig On the other hand, the maximum value of L2 was exhibited at r , = mm At r, larger than 0.8 mm L2 slightly decreased with increasing r, This may be caused by the severe groove wear in the end cutting edge with an increased rn as shown in Fig Since the slice becomes thin with an increased rn resulting in the so-called size effect, MTM 29:3-H 408 K SHINrANI et al m ,,.,11 FIG SEM micrograph of cutting edge in tool with small nose radius at cutting length of L=5.2 km (NL=35 °, rH=0.05 mm, r,=0.2 ram, vc=100 m/min, f=0.1 mm/rev, t=0.1 ram) groove wear occurs markedly in the end cutting edge Although the calculated average surface roughness (Rac=f2/32r, [7] - 0.78 ~m) is approximately equivalent to the life limit of Ra=0.8 p,m at rn=0.4 mm, the surface roughness at this rn should be greater than the life limit because the experimental value is usually observed to be higher than the calculated one The effect of the honing radius rH on L1 and L2 was investigated under condition (4) in Table As shown in Fig 9, the maximum lives for Lt and L2 were exhibited at rH=0.05 ram Similar to the variation of life with NL shown in Fig 2, the experimental data were scattered significantly at smaller values of rH For the tool with rH =0.10 mm, most of the cut was covered by the honing portion and therefore a shortened life similar to that of the tool with an NL of 45 ° was observed It is apparent from the figure that the optimum honing radius can be considered as 0.05 mm In order to investigate the reason why a significant scattering of experimental data and shorter lives were recorded in the tools with a honing radius smaller than 0.05 mm, the tip of an unused tool with ria=0.02 mm was observed by the SEM as shown in Fig 10 Some cracks were observed in the cutting edge These cracks were induced during grinding of the cutting edge from chips broken from the edge of a size of 0.01-0.03 mm For rH=0.02 mm, these cracks will remain without being eliminated completely during honing Such a chipped edge acts as the starting point for tool wear, resulting in a shortening of the life Optimum tool geometry For maximum cost effectiveness in manufacturing the tools, a thinner tip is desirable for the CBN tool while a negative land is needed to make the rake angle negative for increased toughness in the tool However, the negative land width should be as small as possible because successful treatment of the tool tip will become difficult for large widths According to the experimental results obtained, the optimum parameters of the CBN tool could be specified as in Table 2, including the nose radius and the honing radius Continuous Turning of Carburized Steel 409 II U II ,,,,1 E~ E.~ ~'~ II -aN 1i |~ ,-= ,~ II ~ II r| E E m K SHINTANIet al 410 20.0 I I I • 17.5 I I %: 100 m/min f : 0.1 m m/rev 15.0 12.5 10.0 _~ 7.5 /N \°\" 5.0 Tool life 2.5 oLt ALz I [ I I I 0.02 0.04 0.06 0.08 0.10 0.12 Honing radius rx (mm) FIG Dependence of tool lives Li and L on honing radius rH FIG 10 Chipping observed at cutting edge in tool before use, where NL=35 °, rH=0.02 mm and r.=0.8 ram T A B L E O P T I M U M COMBINATION OF TOOL TIP PARAMETERS FOR CONTINUOUS CU'VI'ING OPERATION Negative land angle NL (deg.) Negative land width WL (mm) Nose radius r, (ram) Honing radius rH (mm) 35 WL>e~ 0.8 0.05 ec: 0.08 0.13 mm Continuous turning with the optimum tool T h e progress of tool wear f r o m cutting is shown for tools with N L of 15-45 ° in Fig 11 as a variation of the cross-sectional profile of the rake face for cutting lengths from to 10 kin T w o different wear behaviour patterns were observed in the tools with N L of 15 ° In the tool d e n o t e d by subscript " a " , which exhibited the shorter life, the cutting edge was already extensively worn at a cutting length of 1.2 km H o w e v e r the tool denoted by subscript " b " and other tools with larger values of N L showed a smaller amount of wear of the cutting edge Especially at an N L of 35 °, which is the o p t i m u m Continuous Turning of Carburized Steel 411 (a) ~ ~.~ % ~ ~ (d) ( e ) ~ ~ ~~ "" -, L (km) * 1.2 • • 3} e=_IO#m - - - - 311 2.0 * - - 4.5 ~ 5.0 -o-: I0.0 7.0 Fro 11 Variation of cutting edge profile with cutting length in tools with various negative land angles after cutting at v¢=100 m/min, t=0.1 mm, f 0.1 mm/rev, rH=0.05 mm and r,=0.8 mm N L value: (a) 15°,, (b) 15°h, (c) 25°, (d) 35° and (e) 45° value, a relatively sharp cutting edge was maintained on the tool even after a cutting length of 15 km (see Fig 8(b)) Two types of tool wear, defined by the decrease in the cutting edge, can be identified: one exhibits a significant backward decrease of the cutting edge as was observed in tool "a" with NL=15 °, while the other retains the cutting edge for larger lengths, L, as was observed in the tool "b" with NL= 15° and the other tools with larger values of NL In the former type of tool wear the rake angle becomes large with the progress of wear without any wear of the lower cutting edge, resulting in a steepening of the tool tip As seen in Fig 11, the rake angle in such tools hardly changes during cutting with any value of NL Although in general the cutting force increases with an increase in NL, the tool "a" with NL= 15° exhibited abnormal behaviour with a significant increase in the force at the initial cutting stage for a small VB, namely that the force was larger than that for the tool with NL=45 ° at VB=0.05 mm The cutting force-time curve of the tool "a" with an NL of 15° just after starting the cutting experiment was recorded as shown in Fig 12 A step-wise increase in the force was observed According to the SEM observation of the tool tip in this case, many adhering particles from the workpiece and scratches (indicated by an arrow in Fig 13) in the rake face due to chipping were observed in a portion of the cutting edge and the flank wear surface Therefore, it can be considered that the chipping must be the operative mechanism of tool wear in the initial stage of the cutting process The chipping is induced by the spalling out of particles from those adhering along the cutting edge just after starting the cutting experiment Then the step-wise increase in the cutting force as shown in Fig 12 must be caused by the change in the geometry of the cutting edge due to the chipping The initial wear of the CBN tool can be summarized as follows In the tools with a sharp cutting edge (small NL), micro-spalling of constituent particles in the tool material occurs readily at the tip The workpiece material tends to adhere in the small cavities 412 K SHINTANI et al Tangential r' = I Cutting length Fro 12 Step-wise increase of cutting force in tool of N L = ° Fro, 13 Chipped cutting edge after short time cutting at L=17 m, NL=15 °, rvt 0.05 mm and r, ffi0.8 mm ~'Workpiece = J Adhesion of workpieee material Micro-spallin8 of t o o l tip -p J Accomodated cracking in tool material Chipped tool FIG 14 Schematic representation of chipping process of CBN tool Continuous Turning of Carburized Steel 413 formed by the micro-spalling On further continued cutting, these adhering particles raise the friction coefficient in the tool-workpiece interface and induce further spallings The adhering particles reduce the amount of tool material on the spallings, but a further enlargement of the cavities results It is thought that the chipping of the tool occurs through several steps of this spalling, giving the cutting force-time curve for the initial stage of cutting as shown in Fig 12 The chipping process of the tool mentioned above is shown schematically in Fig 14 CONCLUSION The optimum tool geometry of a CBN tool for improving both tool life and surface roughness in the continuous fine cutting of carburized steel was determined as follows; a negative land angle of 30-35 °, a negative land width larger than the tool-chip contact length, a nose radius of 0.8 mm and a honing radius of 0.05 mm It was shown that cutting with the above described optimum tool could fully substitute for grinding from the standpoint of tool life and surface roughness Then, according to the experimental observations, wear of the cutting tool as the controlling factor of its life was classified into two stages, namely initial and steady wear In the initial stage, the frequent occurrence of chipping, which was caused by high stresses generated on the tool edge, greatly influenced the tool life, especially in the tools with a small negative land angle REFERENt~ES [1] [2] [3] [4] [5] T HODGSON and P H H TRENDLER, Ann CIRP 30, 63 (1981) G CtaRYSSOLOURIS,J appl Metalworking 2, 100 (1982) G CHRVSSOLOURIS,Ann CIRP 31, 65 (1982) S TARArSU, H SmMOOA and K OXAN1, Int J refract, hard Metals 2, 178 (1983) Y KoNo, A HARA, S YAZU, T UCmOA and Y Morn, Cutting Tool Materials (edited by F W Gorsler), p 281 American Society for Metals, Cleveland, OH (1981) [6] K Smr~TANI M UEgl and Y FUJIMURA,J mater Sci Len 6, 987 (1987) [7] G BOOTHROYO,Fundamentals of Metal Machining and Machine Tools, p 138 Scripta, Washington, D.C (1975) [...]... steps of this spalling, giving the cutting force-time curve for the initial stage of cutting as shown in Fig 12 The chipping process of the tool mentioned above is shown schematically in Fig 14 CONCLUSION The optimum tool geometry of a CBN tool for improving both tool life and surface roughness in the continuous fine cutting of carburized steel was determined as follows; a negative land angle of 30-35.. .Continuous Turning of Carburized Steel 413 formed by the micro-spalling On further continued cutting, these adhering particles raise the friction coefficient in the tool- workpiece interface and induce further spallings The adhering particles reduce the amount of tool material on the spallings, but a further enlargement of the cavities results It is thought that the chipping of the tool occurs... negative land width larger than the tool- chip contact length, a nose radius of 0.8 mm and a honing radius of 0.05 mm It was shown that cutting with the above described optimum tool could fully substitute for grinding from the standpoint of tool life and surface roughness Then, according to the experimental observations, wear of the cutting tool as the controlling factor of its life was classified into... controlling factor of its life was classified into two stages, namely initial and steady wear In the initial stage, the frequent occurrence of chipping, which was caused by high stresses generated on the tool edge, greatly influenced the tool life, especially in the tools with a small negative land angle REFERENt~ES [1] [2] [3] [4] [5] T HODGSON and P H H TRENDLER, Ann CIRP 30, 63 (1981) G CtaRYSSOLOURIS,J... J refract, hard Metals 2, 178 (1983) Y KoNo, A HARA, S YAZU, T UCmOA and Y Morn, Cutting Tool Materials (edited by F W Gorsler), p 281 American Society for Metals, Cleveland, OH (1981) [6] K Smr~TANI M UEgl and Y FUJIMURA,J mater Sci Len 6, 987 (1987) [7] G BOOTHROYO,Fundamentals of Metal Machining and Machine Tools, p 138 Scripta, Washington, D.C (1975) ... The optimum tool geometry of a CBN tool for improving both tool life and surface roughness in the continuous fine cutting of carburized steel was determined as follows; a negative land angle of. .. -p J Accomodated cracking in tool material Chipped tool FIG 14 Schematic representation of chipping process of CBN tool Continuous Turning of Carburized Steel 413 formed by the micro-spalling... 0.13 mm Continuous turning with the optimum tool T h e progress of tool wear f r o m cutting is shown for tools with N L of 15-45 ° in Fig 11 as a variation of the cross-sectional profile of the