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 o

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

the profile of cutting edge

deformation

1 Introduction

The development and improvement of poly-crystalline

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/

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 saw-

toothed 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 relative-

ly low

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

characterized by the following points:

a) Thin undeformed chip thickness: In order to mini-

mize 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 sur-

face 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

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

f a ) Continuous c h i p ( b ) S a w - t o o t h e d > h i p '

format ion f o r ma % i o n

;'1<.1 I'odels of chip formation

In these many points, the machining of hard materials

Many of our knowledge and theoretical works on the conventional machining cannot be applied For the

must be examined basically

hard materials (mostly hardened steel) are studied basically in contrast with that of soft and ductile materials

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, how- ever, the deformation is limited by the crack initi- ation at the surface where no hydrostatic pressure exists, Fi l(b) This was observed in the machining

and the chip thus produced was named "saw-toothed chip" from the shape of its 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

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 prscess to reach there is

From the geometrical relation in the figure, the inclination of crack $c is expressed by

the shear angle theories based on the cutting model

of continuous chip formation

-

.'i.q.2 C r a c k initiation in the fornation of saw-toothed chip

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

r a k e a n g l e s , - 1 0 ' -30' and -50' Under t h e c u t t i n g

c o n d i t i o n s i n d i c a t e d below t h e f i g u r e , o n l y t h e

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

o f 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 , + , was

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 b e

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 ,

0 i s almost c o n s t a n t ( a b o u t 3 0 " ) 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 ( 2 )

S i n c e t h e s e c h i p s had n o n - u n i f o r m 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

p o s s i b l e 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 1 0 , where-

a s , i n t h e u s u a l m a c h i n i n g o f Actile m e t a l s Ch

i s less t h a n 0 5 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

c u t t i n g r a t i o C h = l 3 1 Ch.1 .A0 Ch=l 5 6

-

-1g.3 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 2 (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 0 3 m m f = 0 1 r , m / r e v , V.60 m/min

3 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 m a c h i n i n g o f h a r d m a t e r i a l s

a r e , i n s p i t e o f t h e i r h a r d n e s s , n o t n e c e s s a r i l y

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 ) R e l a t i v e l y s m a l l p l a s t i c d e f o r m a t i o n o f c h i p

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 o f t o o l - c h i p c o n t a c t which

r e d u c e s t h e f r i c t i o n f o r c e

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 s t e e l s 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 t o o l i s u s e d , t h e i n c r e a s e o f w o r k p i e c e h a r d n e s s

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 a l m o s t i n d e p e n d e n t o f t h e h a r d n e s s

8

1.5

z 1D

*

0

0

a.

t

I=

0.5

0 A : c u t t i n g f o r c e

X o r k p i e c e h a r d n e s s , HV W o r k p i e c e h a r d n e s s H'J

F i g 4 E f f e c t s 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: H S S , r a k e 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 : 2 0 ':m

Depth o f c u t : 0 1 5 mm(orthgona1 c u t t i n g )

C u t t i n g f l u i d : n o n e

I n ( b ) and ( c ) o f F i g 4 , t h e s h e a r a n g l e s h e a r

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 0 and T S These v a r i a t i o n s

o f 6 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

o f 0 " r a k e t o o l , w h e r e a s t h e e f f e c t o f 0 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

w i t h t h e i n c r e a s e o f h a r d n e s s e v e n i n t h e c a s e o f 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

r e i n f o r c e d m e t a l (FRM), t h e r e i n f o r c e m e n t o f A 1

a l l o y w i t h v a r i o u s c o n t e n t s o f f i b e r s ( S i c w h i s k e r and A 1 2 0 3 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

F i 2 5 O r t h o r o n a l c u t t i n , : 01' rti';

T o o l : HSS y = 5 O , V=2 m/min h 0 1 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 o f b e a r i n g s t e e l s o f 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 c o m p a r i s o n w i t h c u t t i n g a n d 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 m a c h i n i n g o f h a r d e n e d s t e e l ( H V 7 6 0 ) l a r g e r

n e g a t i v e r a k e a n g l e i n c r e a s e s t h e c u t t i n g f o r c e

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

FP 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 forma-

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

a n g l e 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 2 0 )

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 o f n e g a t i v e r a k e a n g l e

l a k n r k D i e c e 1

0

Rake a n g l e y

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

c u t t i n g f o r c e s

C u t , t i n g 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 = 0 1 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 6 shows

v e r y h i g h v a l u e 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 machin-

i n g o f h a r d e n e d s t e e l

4 E f f e c t s o f t o o l w e a r

4 1 C u t t i n g f o r c e s

D u r i n g t h e m a c h i n i n g o f h a r d m a t e r i a l s , t h e h i g h

a b r a s i v e n e s s and h i g h c u t t i n g t e m p e r a t u r e c a u s e

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 e d g e r a i s e s 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 T h i s a c c e l e r a t e s f u r t h e n

wear

F19.7 i s t h e t e s t 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 m a c h i n i n g o f h a r d e n e d s t e e l ( H V 7 2 0 )

c o n s i d e r a b l e i n c r e a s e i n t h r u s t f o r c e F i s n o t i c e d ,

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 i s n o t

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 2 0 )

, + A ] G 6 $

I

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 ~ ~ 2 2 0 s t e 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 i n b o t h

f o r c e s

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

o f t e s t I n w, 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 h a r d n e s s

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 t e s t s 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 a d h e s i v e n a t u r e o f f r i c t i o n w i t h h i g h P '

( a b o u t 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

," 2 -

T 0 T i a l l o y ( b A 1 - 4 V )

C

-m

800

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 i f

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)

P C D 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 - 0 8 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

W

c 0.a

rl

fi

0

s C

,-I

c

C

C

m

m

2 0.4

c

0 A

h

c

c 0

*

m

.i

0

rl

%A

c m

I

ceramic:

Workpiece hardness H V 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 o n

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 m a c h i n i n g o f h a r d

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 1 0 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 c u t t i n g

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 "

4.2 S u r f a c e 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

c u t t i n g t e m p e r a t u r e A s t h e r e s u l t , t h e p r o f i l e

300

-z

P

4

k 200

>

a

0

h

0

_I

Width o f f l a n k wear land VB m m Fig.10 Decrease of c u t t i n g f o r c e s by heeling

C u t t i n g conditions a r e t h e same as i n Fig.7 worn tool

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 sur-

f 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 l o n g 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 e d g e t e n d s t o g e t r o u g h , 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 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 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

L a r g e t h r u s t f o r c e i n t h e m a c h i n i n g 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 deform-

a t i o n :

1 ) E l a s t i c d e f o r m a t i o n o f w o r k p i e c e - c u t t i n g t o o l - machine 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 -

i n g t h e t h r u s t f o r c e

L -

e

2

x 3 -

e

D:

m

111

m

!

2

! T h e o r e t i c a l f 1 / 8 r E

W i d t h o f f l a n k n e a r l a n d v B mm

F i g 1 1 E f f e c t o f w i d t h o f f l a n k w e a r O n

t h e s u r f a c e r o u g h n e s s Rmax

C u t t i n i : 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 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

similar problem in grinding/4/ is applied to cutting,

this local elastic deformation can be minimized

by

b ) increasing the shear angle @ and

c ) decreasing the contact length at flank, VB

5 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 materi-

als

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 accura-

cy: As long as the tool profile is kept smooth,

the tool wear to some extent does not deteriorate

the surface finish

4 ) Since hard materials have high ratio of (Hardness/

E-Modulus), the elastic deformation due to thrust

force causes appreciable amount of dimensional error

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

References

/ I /

/2/

/3/

/4/

/ 5 /

/6/

CIRP, 33/2 : 4 17-4 27

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Titanium Alloys, Wear, 76:15-34

Nakayama,K., 1971, Elastic Deformation of Con-

Precision Eng 4:93-98

Nakayama.K., 1974, The Formation of Satv-toothed

Tokyo, 572-577

Nakayama,K., Taka9i.J Nakan0.T 1974, Peculi-

of the CIRP, 23/1:89-90

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