Machining Hard Materials with Geometrically Defined Cutting Edges Field of Applications and Limitations W Konig (1) M Klinger, R Link; Lehrstuhl fur Technologie der Fertigungsverfahren, WZL, RWTH Aachen/Federal Republic of Germany Received on January 15,1990 Summary The paper deals with means of reducing production time and costs by machining hard materials The cutting processes described are turning, milling, drilling and broaching Tool materials for these tasks require extreme hardness and high compressivestrength combined with adequate high-temperature resistance and a stable chemical structure Surface quality and accuracy-to-size are extremely high, dispensingwith the need for finish grinding Keywords: Hard machining, CBN ,carbides, ceramics Introduction Highly-stressed steel components are frequently hardened to increase their strength and wear resistance Impermissible distortions must be expected, particularly in the case of geometrically complex parts Where there are high demands on workpiece quality, i.e on surface finish and accuracy-to-shapeand-size, the part has to be finished in a highly-tempered or hardened state Hitherto, a grinding process has generally been used to finish materials with hardness values in excess of 60 HRC,but improved knowledge of processes and the consistent exploitation of modern cutting materials now enable cutting processes with a geometrically defined edge to he employed For the first time, for example, the broaching of surface-zone hardened components offers a means of finishing complex internal profiles at economic cost The turning, milling and drillingprocesses are particularly advantageous, since there are no shape restrictions on the corresponding tools, rendering them suitable for a variety of machining tasks and workpiece geometries and in many cases permitting complete processing in a single fixture Higher machining rates as compared to grinding often allow a reduction in production times and costs saving through elimination of heat treatment This assumes, however, that the material can be hardened directly using the forming heat Chip Formation and Surface Integrity In order to specify the potential and requirements of this new production technology in greater detail, it will be necessary to discuss the technological distinctions between hard machining and the machining of soft materials This entails an initial analysis of the chip formation mechanism Owing to the negative tool rake angle, high compressive stresses are created both on the cutting edge and in the material As a result, the material is parted by cracking and plastification, and chips are formed (Fig 2) The chip root in the figure represents a chip segment which has just been produced Owing to the brittleness of the material, the high compressivestress initially leads not to a material flow but to formation of a crack This crack releases the stored energy and thus acts as a sliding surface for the material seement, allowing the segment to be forced out between the parting surface Alternative Production Sequences by Means of Hard Machining In addition investment costs can be saved and door-to-door times shortened by redesigning production sequences, as illustrated in the case of large roller bearing rings (Fig 1) Fig 2: Chip Forming of Care-Hardened Steel (63 HRC) fig 1: Convewiond and optimhed producjon squnCes for (he P&ction of Roller Bearing Rings In this instance, the redesigned finishing process requires fewer production steps than the conventional process, with the additional advantage of energy Annals of the ClRP Vol 3/1/15140 Simultaneously, plastic deformation and heating of the material occur at the leadingedge ofthe cutting tool.Once the chipsegment hasslid away, renewed cutting pressure results in formation of a fresh crack and chip segment The temperature increase needed for plastification of a small section of the chip material issupplied by the heat created in the cuttingprocess.The temperature may be calculated from the applied power, i.e the product of cutting force and speed, and the dissipated heat The latter is determined by the thermal conductivityof the tool and workpiece materials and of the ambient medium The individual chipsegments are linked by the small proportion of the material which is plastically deformed and heated to a high temperature A continuous 61 chip is formed In most cases, the temperature required for rehardening on the underside of the chip is created only by friction on the face The characteristic chip formation process influences the residual stress state of the machined surface This factor is of particular interest, since, in conjunction with accuracy-to-shape and roughness, it decisively determines component behaviour Formation of the chip produces high restoring forces These induce a compressive residual stress state in the workpiece surface zone (Fig.3).An increase in feed raises the compressive residual stress maxima and In comparison to conventional CBN, which possesses good toughness characteristics, a new CBN-based cutting material with a high percentage of metal carbide exhibits a 60Yereduced thermal conductivity, in the same range as for the mixed ceramics It also possesses higher edge strength owing to its finegrained structure (F$ 4) in fine turning tests on hardened 1OOCr6 roller bearing steel, this high metalcarbide cutting material (indexable tip with soldered CBN comer) proved significantly superior to conventional CBN (solid indexable tip) and mixed ceramic In a dry cut at existing machining parameters, the width of wear land is approximately 50' '9 below that of the cutting ceramic used in the tests, with a value of 0.15 mm after roughly two hours' cutting time At the same time, the required surface quality for the large roller bearing rings was not exceeded In addition, a comparison between a chamfered and a sharp-edged cutting edge showed the chamfer to be unfavourable in terms of attainable surface quality For mixed ceramic, which is generally chamfered in order to stabilize the relatively brittle cutting edge, a 0.05 x 20" chamfer represents the closest approximation to a sharpedge a" P g 0.4 0.4 z6 pm f mm mrr F { - ' z g2 ir Fig 3: Residual S t m Curves of Hard-Tmd Won4piecec deepens the affected tone This effect is largely attributable to the high mechanical stress on the workpiece Feed force and friction-induced temperature risewith increasingwear,resultinginaresidual tensile stressin thevicinity of the workpiece surface and an increase in compressive stress in the deeper surface tone I I rCBNwith~!b~~s mixedmamk B l 0.1 g 0.01 0.04 L 40 feed: 80 120 t - 0.08 mm 200 g 0.01 I C I 4' ' I 1' 20 4' 32' 200 1.6m Process-SpecifieRequirements and Results Important appraisal criteria for the finishing performance of a cutting process are attainable surface quality and tool wear, which directly influences the process forces and indirectlyaffects tool life It is therefore necessary to select the optimum cutting material for each specific application 4.1 Turning CBN-basedpolynystallinecutting materialsare now availablefor turning (and also for milling and drilling) in a number of variants and compositions.There are significant differences between individual products in terms of hard material component, intermediate phase and structure These affect wear behaviour and attainable surface quality for the different cutting materials An additional question is the extent to which the behaviour of these cutting materials differs from that of mixed ceramic in the fine turning of hardened material @ inseft work material: 100 Cr 6, hardness 61 - 63 HRC cutting speed: v, = 120 m/min depth of cut: ap = 0.4 mm fed: f 0.08 mm no coolant tool cuning edge anglex, 48O @- - tool life criterion 120 100 Ra - 0,s 80 I- e - m - ' 15 30 cutting time tc 60 200 40 47.6.Hard Turning with High Sped Steel 20 - achieved after tool lives of 120 minutes With mixed ceramics, cutting edge breaksuts lead to extremely rapid wear I Ii:,"~ialll CBN I CBN with carbides I cutting edge 11 chamfered 0,2x20° 11 11 ceramic AhOS+TiC Fig 4: Hard Turning with CBN and Ceramic 62 I chamfered chamfered chamfered sharp ~ ~ I2100 ~ ~ 00~ ~2 ~ ~ A distinct influence on the wear state of the tool can be demonstrated metallographically in the case of surface zone influences in hard turning A change of 2-3pm in the surface zone, not observable when a cooling lubricant is employed, is detectable with a width of wear land of YBmu = 0.3 mm (Fk.7) 0 OEIDU Fig 7: Structural Change in the swfoce Zone of a Had-Turned Workpke 18 27 m 38 broachingleng(h Fig 10: Broaching Cmehardened S h - Influence of Coating allowed for by limiting the undeformed chip thickness to 1Opm.A calibration piece without a lead is needed to improve accuracy-to-size A considerable reduction in wear is achieved by coating the cutting edges Cutting speeds of vC = -20 d m i n are generally used for broaching non-hardened materials In order to attain the results outlined above, however, a cutting speed of at least 40 Wmin is essential This cannot be realized on the majority of existing broaching machines Under these optimized conditions, components with surfaces finished to grinding quality can be produced 4.4 Drilling The use of indexable tip technology in conjunction with CBN and adapted drilling tools hasenabled holes to be drilled in hardened steelswith workpiece hardnesses of approximately 62 HRC A modified indexable tip drill with negative wedge geometry has proven its value in this application It possesses two rhomboid tips secured to the tool by means of countersunk screws The internally-cooled tool achieves drilling depths of up to 1.5 times its diameter of D = 34 nun Fig Milling Cmehardened Slots with CBN Milling Casehardened slots can also be milled with CBN In this case, however, use of a cooling lubricant very rapidly causes cutting edge break-outs (Fig 8) Even in a dry cut, end of tool life is determined by a break-out near the soldered edge; wear is still extremely low at this stage A stable cutting edge, especially in the comer zone, is, however a prerequisite for the successful use of CBN In addition, high cutting speeds are necessary, entailing considerable machine effort whensmall-diameter tools areused, owing to the high speeds of rotation For the application d m i d above, carbides, which are cheaper than CBN, can be used cost-effectively A lower cutting speed of vc = 100 d m i n can be selected The best results are achieved with micrograin carbides without cooling lubricant (Fig 9) No surface zone changes occurred with this material Tool life behaviour when drilling differing steel materials was investigated Identical cutting parameters were selected to enable the influence of various hardness mechanisms and material structures on the machinability of the materials to be appraised (Fig 11) - case-hardening steel surface hardened mmensite maerial hardness : ca.62 HRC depth 01 drilling : I rnm without coolant r e o l d work s t e e m I I i~ii-irdened martensite i 0.2 nitriding steel carbonltrided ca mm sudace contents carLndes n!!!s5m-i m m mm E 0.1 Fig I I : Influence of Maerial Structure on Drilling with CBN BI 0.05 0.02 500 om- 1000 2000 5ooo 10000 mm 20000 cutting length if Fig 9: Increming Tool Life for the Milling of Cawhardened Steel Using Mictvgrain Carbides Broaching Broaching is the most recently-introduced hard machining process using a geometrically defined cutting edge Cutting edge stresses are similar to those encountered in milling Owing to its great toughness, micrograin carbide has proved to be especially suitable for broaching martensitically-hardened materials (Fig 10) 'Ihe high impact loading at cutting edge contact must be Maximum tool life was achieved with a 16MnCrSE casehardening steel The depth of the purely martensitic casehardened zone was roughly nun, and hardness measurements of the zone yielded values of up to 62 HRC at the workpiece surface Taking casehardening depth into account, a drilling depth of = mm or = 1.5 nun (31CrMo12) was selected for the surface-zone hardened materials This ensured that exclusively hard structures were machined A drilling depth of mm was necessarily also selected for the fullyhardened X 100CrMoV51and X210CrW12cold work steels At a cutting speed of vc = 200 d m i n and a feed off = 0.02 mm, a tool life of roughly 100 mm was achieved with X1oocrMoV51 As carbide content and the proportion of coarse carbides increase, maximum tool life is reduced The wear criterion of V B m u = 0.5 mm or cutting edge fracture is reached after only about 20 mm in S6-5-2 high speed steel The machining of thin, ultrahard layers of 31CrMo12 nitride steel is problematical In this case, the hardness increase is produced by the incorporation of carbon atoms (casehardening or carburization) and nitrogen atoms (nitriding) in the metal lattice of the steel structure Owing to the specific parting mechanism involved in the machining of such structures, the negative cutting edge geometry may lead to spalling and breakouts on the centre edge of the drill 63 'rhc fundamental problems of the drilling process stemming from the drop in cutting speed lo vC = Oat the centre are especially evident when drilling hard materials The centre tip can withstand the high compressive stress only if cutting speed IS sufficient For each material there will therefore be ;in optimum cutting speed: the softening of the material caured hy the cutting rpeed or cutting temperature will he adequate for the stahilityof the centre tip, while the abrasive wear L'fl occurring on the peripheral tip remains minimal material diameter drilling depth feed 31 CrMo 12 ( ca 60 HRC ) D= 34 mm I = rnm I = 1.5 rnm Ilnder the conditions described above, high surface qualities and good concentricity values can he achieved for the drilling of fully hardened materials (Fig 12) The required minimum cutting speed is dependent on the type and - : Y m > work material X 210 CrW 12 (ca 60 HRC) cuning material CBN tool inserted drill D = 34 mm drilling depth I = x D leed rale I = 02 m m cunlng edge geometry {r E 300 m 200 z 100 %per k e n 275 285 300 350 cunlng speed (m.mini cutting speed vc 'Cczat989 Fig 12: Drilling f h ~ w rSIC(,/ d with CRN formation of the material structure It is approximately vc = 2x5 m/min for X210CrWI2 cold work steel At lower speeds the centre edge fails due to hreak-out, while at higher values the abrasive wear on the peripheral tip rises sharply Cooling lubricant is essential to cool the tool shaft and transport the chips The lubricant affects wear, however, as demonstrated by the drilling of Ih,blnCrS (Fig 1.3) No differences are detectable in the wear diagram with wear values ranging from 0.54 to 0.56 m m after identical drilling lengths At lower drilling depths, i.e for the removal of hard surface zones, a dry cut is therefore feasible cutting material CBN material 16 MnCr E hardness c a 62 HRC drilling depth I = rnrn cutting s p e e d vc = 200 rnirnin feed f = 0,02rnrn width of wear land without coolant V h a x = 56 rnrn with coolant V h a x = 54 mm Fig 14: Cornpurism of Cutting Marerialr for llrilling 31CrMo12 mately 1.1 m Flank wear VAwas not measurable No damage was detected on !he chisel edge Material-specific optimi7ation of the cutting parameters enabled tool live\ for the drilling of this type of material structure with carbidetipped drills of this kind to he prolonged Fig I S : Drilling Sutfuce-Zone Ilardened Mnterinlr with Curhidc Literature ;I: mm1989 Fig 13: Influence of Cooling Fluid on CuffingEdge U k i r In general the machining of thin, hard layers of the kind encountered with carbonitriding is problematical The depth of hardnesr in 31C:r41(112 steel is less than 0.5 mm.Whisker reinforced ceramic fails after a few steps due to fracture This may be attributed to the stress relationship on penetration of the hardened zone (drilling depth I = 1.S mm, Fig 14) The use of tough CBN withaut carhides ir similarly characterized by extremely short tool lives Carbide tools (Fig 1.5) are hetter suited to this application The figure presents tool lives for the drilling of various curface-zone hardened materials Drilling depth was 30 mm i.e machining continucd in the transitional structure and the soft substrate structure after the hardened zone had been penetrated At a cutting speed of 50 m/rnin and a feed off = 0.08 (douhlc cutting drill:f, = 0.04 mm), break-outs occurred at the corners after a drilling length of approxi- 64 i2: /3/ /4/ ?5! :6: Ackerschott, G.: Grundlagen der Zerspnnung einsatzgehiirteter Stiihle mit geometrisch bestirnmter Schneide doctoral thesis, RITII Aachen 1989 Nakayama K Arai, M., Kanda, T.: Machining Characteristics of Hard Materials, Annals of the CIRP Vol 37/1/1988 pp 89-92 Momper, F.: Mischkeramiken und Bornitrici-Schneidstoffe I d - A n z 14/IYXX, s 26-29 Konig, W., Komanduri, R., Tiinshoff, H.K., Ackerschott G.: Machining of flard Materials Annals of the CIRP, Val 33/2/1984 pp 417-427 K(inig W Iding, M Link K.: Feindrchcn u n d Rohren gehirteter Stahlwerkstoffe IDR 1/89, S 22-33 Abel R.: Harthearbeitung mit Schneidkeramik und Homitrid dima YIXX, S SO 60 Ohtani, T Yokogawa, 11.: The Effects of Workpiecc Hardness on Tool Wear Ch:ir:icteristics, Dull Japan Soc of Prcc Eng., Vol 22 S o (Scpt IYXX), pp 229-231, ~ 17: ... Arai, M., Kanda, T.: Machining Characteristics of Hard Materials, Annals of the CIRP Vol 37/1/1988 pp 8 9-9 2 Momper, F.: Mischkeramiken und Bornitrici-Schneidstoffe I d - A n z 14/IYXX, s 2 6-2 9 Konig,... improve accuracy-to-size A considerable reduction in wear is achieved by coating the cutting edges Cutting speeds of vC = -2 0 d m i n are generally used for broaching non-hardened materials In order... depth of cut: ap = 0.4 mm fed: f 0.08 mm no coolant tool cuning edge anglex, 48O @- - tool life criterion 120 100 Ra - 0,s 80 I- e - m - ' 15 30 cutting time tc 60 200 40 47.6 .Hard Turning with