Machining characteristics of hard materials

Machining Characteristics of Hard Materials Kazuo Nakayama (l), Minoru Arai and Torahiko Kanda; Yokohama National University /Japan Received on January 13,1988 ABSTRACT: The hardening of steel rather lowers the cutting forces in many cases This is the result of high shear angle and the saw-toothed chip formation due to the poor ductility of hard materials ReinforceinenL of A1 alloy with fiber also decreases the cutting forces However, the hard materials wear the cutting tool rapidly and increase the forces, especially thrust force which causes size error The profile of machined surface of hardened steel reflects the profile of cutting edge KEY WORDS: Hard materials, hardened steel, FRM, ceramics, tool wear, surface finish, elastic deformation Introduction The development and improvement of poly-crystalline diamond(PCD), sintered cubic boron nitride(CBN) and ceramic tool materials have been expanding the application of these tools for the machining of hard materials such as hardened steel and ceramics/l/ For the improvement of productivity in machine shop, the shift from grinding to cutting has become one of the main targets in these days From the viewpoint of machining, hard materials are characterized by the following properties: a) High indentation hardness: This requires the cutting tool of much higher hardness (usually more than three times as hard) This also causes strong impact and stress on the small area of tool-work contact at the engage of cutting tool with workpiece b) High abrasiveness: This requires the cutting tool having high resistance against the abrasive wear c) LOW ductility: This causes the formation of sawtoothed chip instead of continuous chip, or even discrete elemental chips In such cases, chip is produced after small plastic deformation Accordingly, the power consumption and cutting forces are relatively low d) High value of the ratio (Hardness/E- Modulus ) : This induces an appreciable amount of local elastic recovery after the cutting tool passes over The size error due to this fact becomes serious for the finishing of hard materials On the other hand, the cutting conditions in the practical machining of hard materials are generally characterized by the following points: a) Thin undeformed chip thickness: In order to minimize the tool wear and size error, fairly small values of the depth of cut and feed rate are taken Large corner radius of cutting tool is also selected As the result, the undeformed chip thickness is very thin, especially in the region of machined surface generation b) Negative rake angle: For the prevention of the chipping of cutting edge, negative rake angle and chamfering are necessary for the cutting tool to machine hard materials As the undeformed chip is very thin, actual cutting is done only at the chamfer in most cases Furthermore, the abrasivenessof work material blunts the cutting edge very easily and makes the actual rake angle more negative fa) Continuous c h i p format ion ;'1 h i p f o r ma % i on I'odels of chip formation Annals of the CIRP Vol 37/1/1988 In these many points, the machining of hard materials is very different from conventional machining Many of our knowledge and theoretical works on the conventional machining cannot be applied For the progress of machining technology f o r hard materials, the machining characteristics of such materials must be examined basically In this paper, the machining characteristics of hard materials (mostly hardened steel) are studied basically in contrast with that of soft and ductile materials Saw-toothed chip formation In the machining of ductile materials, the chip formation is accompanied with very severe plastic deformation at the shear zone, as shown in Fig.l(a) When a work material has not enough ductility, however, the deformation is limited by the crack initiation at the surface where no hydrostatic pressure exists, Fi l(b) This was observed in the machining of work hargdened 70-30 brass/)/ and hardened steel/6/, and the chip thus produced was named "saw-toothed chip" from the shape of its cross-section "Segmental chip"/2.3/ has similar cross-section, but its formation has been attributed to the adiabtic shear which happens intermittently in the high speed machining of metals having poor thermal conductivity such as Ti alloy and austenitic stainless steel These two types of chip together with the "wavy shear type) chip" may be collectively called '*Zimi-continuouschip" "Elemental chip" or separated fragmental chip is produced in the machining of more brittle materials such as gray cast iron and ceramics The saw-toothed chip was shown to be produced when the shear strain on the surface attain an ultimate value yc over which the work material cannot afford In the smplified model the crack is assumed to initiate at point Q When the surface inclination at Q is JI , the shear strain attained by the material in the prscess to reach there is s, Yc = JIc From the geometrical relation in the inclination of crack $c is expressed by (1) figure, the $c = n/4 - JIc = n14 - yc/2 (2) y is considered to be a material constant, the is to be determined only incligation of crack 0, by the work material: Smaller y o r less ductile work material gives larger and: accordingly, thin ner and longer chip This Es quite different from the shear angle theories based on the cutting model of continuous chip formation As ' - .'i.q.2 C r a c k initiation in the fornation of saw-toothed chip 89 Fig.3 shows t h e scheme o f t h e c r o s s - s e c t i o n o f c h i p s produced i n t h e m a c h i n i n g o f h a r d e n e d b e a r i n g s t e e l (HV760) w i t h t h e c u t t i n g t o o l s o f t h r e e d i f f e r e n t -30' and -50' Under t h e c u t t i n g rake a n g l e s , -10' conditions i n d i c a t e d below t h e figure, only the rounded c o r n e r o f t o o l was i n a c t i o n The i n c l i n a t i o n + , was of c r a c k a g a i n s t t h e c u t t i n g d i r e c t i o n , measured a n d i n d i c a t e d i n t h e f i g u r e I t 'can be s e e n t h a t , i n s p i t e o f wide v a r i a t i o n o f r a k e a n g l e , i s almost c o n s t a n t ( a b o u t " ) T h i s r e s u l t s u p p o r t s t%e a b o v e r e l a t i o n E q ( ) Since these c h i p s had non-uniform c r o s s - s e c t i o n , r e a s o n a b l e measurement o f t h e i r t h i c k n e s s was n o t possible Then, t h e mean v a l u e o f c h i p t h i c k n e s s r a t i o C h was s u b s t i t u t e d by t h e c h i p l e n g t h r a t i o ( c h i p l e n g t h / l e n g t h o f c u t ) , which is a l s o i n d i c a t e d i n t h e f i g u r e I t is t o b e n o t i c e d t h a t a l l o f t h e s e c h i p s i n d i c a t e v e r y l a r g e v a l u e o f C over , wherea s , i n t h e u s u a l machining o f m e t a l s Ch i s less t h a n e v e n w i t h p o s i t i v e r a k e t o o l s Actile I n ( b ) and ( c ) o f F i g , t h e s h e a r a n g l e , shear s t r a i n o f c h i p y c , a n d t h e s h e a r s t r e s s on s h e a r p l a i n T~ c a l c u l a t e d u s i n g t h e s e f o r c e s and t h e c h i p t h i c k n e s s measured a r e shown The i n c r e a s e o f h a r d n e s s i s s e e n t o i n c r e a s e b o t h and T S These v a r i a t i o n s of and T~ g i v e o p p o s i t e e f f e c t s on t h e c u t t i n g f o r c e s These two were a l m o s t b a l a n c e d i n t h e c a s e of 0" r a k e tool, whereas t h e e f f e c t o f s u r p a s s e d t h a t o f T~ and d e c r e a s e d t h e f o r c e s S i m i l a r t e s t on 0.45% C s t e e l o f v a r i o u s h a r d n e s s up t o HV540 i n d i c a t e d t h e d e c r e a s e o f c u t t i n g f o r c e s wi t h t h e i n c r e a s e of hardness even i n t h e c a s e of 0' r a k e t m l When a n n e a l e d s t e e l was work-hardened by c o l d f o r g i n g , s h e a r a n g l e i n c r e a s e d and c u t t i n g f o r c e s d e c r e a s e d I t was a l s o found t h a t , i n t h e c u t t i n g t e s t o f f i b e r A1 reinforced metal (FRM), t h e r e i n f o r c e m e n t of a l l o y wi t h v a r i o u s c o n t e n t s of f i b e r s ( S i c whisker and A f i b e r ) l o w e r e d t h e c u t t i n g f o r c e s c o n s i d e r a b l y a s shown i n = E Ch.1 A0 6=36O cutting ratio Ch=l 31 r e a n s h e a r a n g l e 0=46O - -1g.3 Ch=l 5.25' C r o s s - s e c t i o n s of c h i p s p r o d u c e d i p t h e m a c h i n i r r of h a r d e n e d s t e e l Work material: B e a r i n g s t e e l S U J (HV760) Tool: C e r a m i c , rc=0.8 m m , r a k e a n g l e = v a r i e d a mm f = r,m/rev, V.60 m/min I SiCw ~1203 F i O r t h o r o n a l c u t t i n , : 01' rti'; C u t t i n g forces C u t t i n g f o r c e s i n t h e machining o f hard m a t e r i a l s a r e , i n s p i t e of t h e i r hardness, not necessarily h i g h b e c a u s e o f t h e f o l l o w i n g two e f f e c t s : a ) Relatively small p l a s t i c deformation due t o t h e c r a c k f o r m a t i o n m e n t i o n e d a b o v e b ) R e l a t i v e l y s m a l l a r e a of reduces the f r i c t i o n force tool-chip of chip c o n t a c t which I n p r a c t i c e , however, t h e h a r d m a t e r i a l s wear down t h e c u t t i n g t o o l r a p i d l y and i n c r e a s e t h e c u t t i n g f o r c e s , e s p e c i a l l y t h r u s t f o r c e , a s i s shown l a t e r The c u t t i n g t e s t o f 0.25% C steels o f f o u r d i f f e r e n t h e a t t r e a t m e n t s , Fig.4, i n d i c a t e s t h a t , when -20' r a k e tool is u s e d , t h e i n c r e a s e of workpiece hardness d e c r e a s e s b o t h c u t t i n g and t h r u s t f o r c e s When 0' r a k e t o o l i s u s e d , on t h e o t h e r h a n d , t h e f o r c e s a r e almost independent of t h e hardness T o o l : HSS y = O , V=2 m/min h VP' Work m a t e r i a l s : F i b e r r e i n f o r c e d A ? : a l l ) ; ' shows t h e e f f e c t s of r a k e a n g l e on t h e c u t t i n g f o r c e s i n t h e l i g h t c u t t i n g of b e a r i n g steels of two d i f f e r e n t h e a t t r e a t m e n t s T r i a n g u l a r (Al2O3+ T i c ) t y p e r p r a m i c i n s e r t s w i t h 0.8mm c o r n e r r a d i u s were ground t o g i v e t h e f i v e d i f f e r e n t n e g a t i v e r a k e a n g l e s t o t h e rounded c o r n e r s I n t h e f i g u r e , 'the f e e d f o r c e i s n o t shown b e c a u s e i t was v e r y s m a l l i n comparison w i t h c u t t i n g and t h r u s t f o r c e s F o l l o w i n g a r e known from t h i s f i g u r e : a ) I n t h e machining o f hardened s t e e l ( H V 60) l a r g e r negative rake angle increases the cutting force F, o n l y a l i t t l e , w h e r e a s i n c r e a s e s t h e t h r u s t f o r c e r e m a r k a b l y T h i s c a n b e a t t r i b u t e d t o t h e formaFP t i o n o f s a w - t o o t h e d c h i p by t h e h i g h n e g a t i v e r a k e angle I n t h e m a c h i n i n g of a n n e a l e d s t e e l ( H V 2 ) on t h e o t h e r h a n d , b o t h f o r c e s i n c r e a s e w i t h t h e i n c r e a s e of n e g a t i v e rake a n g l e l a k nr k D i e c e 1.5 z 1D *0 a I t = 0.5 A : c u t t i n g force 120,A:thr;st fo;ce 71;; , , , 100 300 500 X o r k p i e c e h a r d n e s s , HV Fig.4 Effects 100 300 500 Workpiece h a r d n e s s H'J 01 I -60° I , I -30' Rake a n g l e y , , , J 00 of v o r k p i e c e h a r d n e s s Work m a t e r i a l : 0.25SC s t e e l h e a t t r e a t m e n t v a r i e d Tool: HSS,rake a n g l e v a r i e d C u t t i n g s p e e d : 20 m/min, Width of c u t : :m' Depth o f c u t : mm(orthgona1 c u t t i n g ) C u t t i n g f l u i d : none 90 '' c Tr Fiz.6 E f f e c t o f n e g a t i v e r a k e a n g l e o n cutting forces Cut,ting c o n d i t i o n s a r e t h e same a s i n Fit: ? Qxcept f o r a = m m b ) The f o r c e r a t i o F / F y i n d i c a t e d i n F i g shows very high value T h f s 1s r e m a r k a b l e e s p e c i a l l y i n t h e m a c h i n i n g o f HV760 s t e e l w i t h h i g h n e g a t i v e r a k e t o o l , which i s t h e c a s e i n t h e p r a c t i c a l machini n g of hardened s t e e l W c 0.a rl I fi m Effects of tool w e a r s C m ,-I c Cutting forces C During t h e machining of hard m a t e r i a l s , t h e h i g h abrasiveness and high cutting temperature cause t h e r a p i d t o o l w e a r , a n d t h e worn f l a n k t o g e t h e r w i t h t h e b l u n t c u t t i n g edge r a i s e s t h e c u t t i n g f o r c e s , especially thrust force This a c c e l e r a t e s furthen wear C c h c * F19.7 i s t h e test r e s u l t on t h e v a r i a t i o n o f c u t t i n g f o r c e s w i t h t h e i n c r e a s e o f a r t i f i c i a l l y made f l a n k wear: I n t h e machining o f hardened ste el (HV720) is noticed, considerable increase i n t h r u s t force F is not w h e r e a s t h e i n c r e a s e i n c u t t i n g f o r & Fv so much I n t h e c a s e of n o r m a l i z e d s t e e l ( H V 2 ) , 0.4 c A m ceramic: i rl %A c m S o LOO I Workpiece hardness H V +A]G.6$ Fig.9 E f f e c t of workpiece hardness H V on t h e c o e f f i c i e n t o f the f r i c t i o n on f l a n k wear l a n d C u t t i n g conditions a r e the same as i n FiC.8 ~'(0.2-0.3) T h i s c a u s e s t h e h i g h t h r u s t f o r c e r e l a t i v e t o t h e c u t t i n g f o r c e i n t h e machining of hard m a t e r i a l s w i t h worn t o o l The i n c r e a s e d t h r u s t f o r c e due t o f l a n k wear c a n b e d i m i n i s h e d t o some e x t e n t by h e e l i n g t h e c u t t i n g t o o l so a s t o i n c r e a s e t h e r e l i e f a n g l e Though t h i s makes t h e r a k e a n g l e more n e g a t i v e , t h e t e s t shown i n F i g i n d i c a t e s t h e a p p a r e n t d e c r e a s e i n t h r u s t f o r c e and p r a c t i c a l l y no c h a n g e i n cutting f o r c e : When VB = O.Zmm,about 30% d e c r e a s e i n t h r u s t f o r c e i s o b t a i n e d by t i l t i n g t h e c u t t i n g t o o l by 8" I 0.1 0.2 k l i d t h o f f l a n k wear land V B m m t'ie.7 Increase of c u t t i n g f o r c e s with t h e width of f l a n k wear land Tool: ceramic(A120-,+TiC), y =-35' yc=0.8nm V=60m/nin f o r HV760 s t e e l 120mImin f o r ~ ~ s2t e0 e l a=O.lmm, f=O.lmn/rev however, o n l y a l i t t l e i n c r e a s e is s e e n forces 4.2 Surface f i n i s h I n t h e m a c h i n i n g of h a r d m a t e r i a l s , BUE i s h a r d l y formed b e c a u s e o f t h e i r p o o r d u c t i l i t y a n d h i g h As the result, the profile c u t t i n g temperature in _I both The c h i p t h i c k n e s s r a t i o was n o t changed by f l a n k wear a s shown i n t h e f i g u r e T h i s means t h a t t h e f o r c e i n c r e m e n t s c a n b e c o n s i d e r e d t o a c t on t h e worn f l a n k Then, t h e normal a n d t a n g e n t i a l stresses on t h e worn f l a n k c a n b e c a l c u l a t e d from t h i s k i n d of test I n some o f t h e r e s u l t s t h u s o b t a i n e d on s e v e r a l work m a t e r i a l s a r e shown i n r e l a t i o n t o t h e i r hardness w, 300 -z P > k 200 a The c o e f f i c i e n t o f f r i c t i o n a t t h e worn f l a n k P ' ( = F ' / F ' ) o b t a i n e d from t h e s e tests a r e shown i n Fig.9, which i n d i c a t e s t h a t t h e n o r m a l i z e d s t e e l h a s t h e adhesive n a t u r e of f r i c t i o n w i t h hi gh P ' (about 0.7) w h e r e a s t h e h a r d m a t e r i a l s have l o w h L 0.1 ," T Width of f l a n k wear land T i alloy(bA1-4V) c Fig.10 Decrease of c u t t i n g f o r c e s by heeling worn tool C m I 0.2 VB m m - C u t t i n g conditions a r e t h e same as i n Fig.7 o f c u t t i n g e d g e i s t r a n s f e r r e d t o t h e machined surf a c e w i t h r e a s o n a b l e a c c u r a c y Then, a s long a s t h e f l a n k wear i s u n i f o r m , i t d o e s n o t i n c r e a s e t h e s u r f a c e r o u g h n e s s With t h e p r o g r e s s o f f l a n k w e a r , however, c u t t i n g edge t e n d s t o g e t rough, and d e t e r i o r a t e s t h e s u r f a c e F i g 1 i s a n example showing t h a t t h e u n i f o r m f l a n k wear u p t o 0.15mm d o e s n o t m a t t e r from t h e s t a n d p o i n t o f s u r f a c e f i n i s h 800 4.3 S i z e e r r o r d u e t o t h r u s t f o r c e Workpiece hardness HV Fig.8 Relation between workpiece hardness HV and the normal and t a n g e n t i a l s t r e s s e s on flank wear land of and if T o o l : P20 f o r T i a l l o y , t r i a n g u l a r tip(rc=0.8mm) PCD f o r ceramics, c i r c u l a r tip(b=13mm) Ceramic f o r s t e e l s , t r i a n g u l a r t i p ( r - m m ) C u t t i n p ( conditions a r e t h e same a s i n Fig.?except f o r t o o l Large t h r u s t f o r c e i n t h e machining o f h a r d m a t e r i a l s shown a b o v e c a u s e s t h e d i m e n s i o n a l e r r o r o f f i n i s h e d p a r t due t o t h e f o l l o w i n g t w o t y p e s of e l a s t i c deformation: 1) E l a s t i c deformation of workpiece-cutting toolmachine t o o l s y s t e m T h i s c a n b e m i n i m i z e d s i m p l y by i n c r e a s i n g t h e r i g i d i t y o f t h e s y s t e m a n d d e c r e a s ing the t h r u s t force 91 Acknowledgement The authors wish to thank Mr.Takashi Nakano of Nippon Kokan K.K for carrying out a part of experimental work when he was in Yokohama National University L- References e /I/ K0enig.W K0manduri.R T0enshoff.H.K 1984, Machining of Hard Materials, Annals of the CIRP, 33/2 :4 17-427 /2/ K0manduri.R von Turk0vich.B.F 1981, New Observations on the Mechanism of Chip Formation When Machining Titanium Alloys, Wear, 69: 179188 /3/ K0manduri.R 1982, Some Clarifications on the Mechanics of Chip Formation When Machining Titanium Alloys, Wear, 76:15-34 x e D: m 111 m ! Theoretical ! f1/8rE /4/ Nakayama,K., 1971, Elastic Deformation of Contact Zone in Grinding, Bull Japan SOC of Precision Eng 4:93-98 /5/ Width of f l a n k n e a r l a n d v B mm Nakayama.K., 1974, The Formation of Satv-toothed Chip, Proc Int'l Conference on Production Engg, Tokyo, 572-577 /6/ Nakayama,K., Taka9i.J Fig.11 E f f e c t o f w i d t h of f l a n k w e a r t h e s u r f a c e r o u g h n e s s Rmax Nakan0.T 1974, Peculiarity in the Grinding of Hardened Steel, Annals of the CIRP, 23/1:89-90 On Cutt.ini: c o r l d i t i o n s a r e t h e s a m e a s i n F ~ I : ~ 2) Local elastic deformation in the region cutting point, as indicated by in Fig.12 near Since hard materials are characterized by the high ratio of (Hardness/E-Modulus) as shown in Fig.13, high stress at the tool-workpiece contact causes appreciable amount of local elastic deformation of the order of to 10 urn When the study on the similar problem in grinding/4/ is applied to cutting, this local elastic deformation can be minimized by a) decreasing the undeformed chip thickness h , b ) increasing the shear angle @ Fig.12 f:odel of e l a s t i c deformation i n cutting region and c) decreasing the contact length at flank, VB 100 Diamond 11 Conclusions 1) Cutting forces in the machining of hard materials are not necessarily high compared with that of soft materials: High shear angle and the formation of sow-toothed chip due to poor ductility lower the forces in spite of the high strength of hard materials ) Tool wear due to the abrasion and high cutting temperature in the machining of hard materials raise the cutting forces, especially thrust force 3) The profile of machined surface of hardened steel reflects that of cutting tool with reasonable accuracy: As long as the tool profile is kept smooth, the tool wear to some extent does not deteriorate the surface finish 10 c5 a Lo 111 W C a h m " ) Since hard materials have high ratio of (Hardness/ E-Modulus), the elastic deformation due to thrust force causes appreciable amount of dimensional error 0.1 10 100 E - m o d u l u s GPO 1000 Fir! R e l a t i o n b e t w e e n h r i r d n e s s a n d R - m o d u l l l : : 92 ... cutting temperature in the machining of hard materials raise the cutting forces, especially thrust force 3) The profile of machined surface of hardened steel reflects that of cutting tool with reasonable... high compared with that of soft materials: High shear angle and the formation of sow-toothed chip due to poor ductility lower the forces in spite of the high strength of hard materials ) Tool wear... Nakano of Nippon Kokan K.K for carrying out a part of experimental work when he was in Yokohama National University L- References e /I/ K0enig.W K0manduri.R T0enshoff.H.K 1984, Machining of Hard Materials,