On the new type variable stiffness surface grinding machine

Journal of Mechanical Working Technology, 17 (1988)387 - 396 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

387

ON THE NEW TYPE VARIABLE STIFFNESS SURFACE GRINDING MACHINE

K. SATO 1, Y. SAKAI 1 and Y. NAKANO 2 I D e p a r t m e n t of Mechanical Engineering, The I n s t i t u t e of Vocational Training, 1960 Aihara, S a g a m i h a r a - c i t y 229 (Japan) 2Faculty of Engineering, C h i b a University, 1-33, Yayoi-cho C h i b a - c i t y 260 (Japan)

SUMMARY This study deals with a new type surface grinding machine which can give various grinding performances by c h a n g i n g i t s s t a t i c s t i f f n e s s a n d d a m p i n g p r o p e r t i e s f r e e l y . A s t h e f i r s t s t e p , a p r o t o t y p e of s u c h a v a r i a b l e s t i f f n e s s s u r f a c e g r i n d i n g m a c h i n e is m a n u f a c t u r e d a n d i t s f e a s i b i l i t y is i n v e s t i g a t e d b y conducting t h e simulation grinding using this machine. The m a c h i n e has a double column s t r u c t u r e consisting of a movable column and a fixed additional column. In addition, it has several coiled springs and a damping e l e m e n t b e t w e e n t h e two column so as to c h a n g e its s t a t i c stiffness and damping p r o p e r t i e s . The static stiffness can be changed by tightening or loosening the clamping force of t h e coiled spring a t t a c h m e n t s . The damping p r o p e r t i e s c a n be changed by adjusting the force acting upon a friction plate. The results obtained are summarized a s f o l l o w s : (1) T h e p r o t o t y p e of a variable stiffness surface grinding machine can change its static stiffness and damping property freely. Therefore, a surface grinding m a c h i n e having various grinding p e r f o r m a n c e s m a y be produced according to this method. (2) The results of t h e simulation grinding s h o w t h a t t h e s t a t i c s t i f f n e s s a n d t h e d a m p i n g p r o p e r t y of t h e s u r f a c e g r i n d i n g m a c h i n e will b e p r i n c i p a l f a c t o r s a f f e c t i n g g r i n d i n g p e r f o r m a n c e s s u c h as m a c h i n e v i b r a t i o n s , s u r f a c e f i n i s h , g r i n d i n g b u r n , a n d g r i n d a b i l i t y a n d l i f e of grinding wheels. INTRODUCTION With a recent effects

of

trend

a stiffness

grinding p e r f o r m a n c e the static

stiffnesses

toward and

more precise

damping

ceramics static

the

and damping properties

utilization

a grinding

machine

of commercially

according to their machine industrial

grinding machines

its

However

builders(ref.

materials change

1).

s u c h as n e w

toward

a higher

maintain

that

life of grinding wheels are likely to arise when steel

is g r o u n d w i t h s u c h

a machine

these

workers

the

upon

the

available surface

On t h e o t h e r h a n d , t h e s k i l l e d g r i n d i n g w o r k e r s

grinding burn and short problems,

of

of h a r d a n d b r i t t l e

m a d e t h e d e s i g n of s u r f a c e

stiffness.

property

grinding works,

a r e b e c o m i n g more i m p o r t a n t problems than before.

g r i n d i n g m a c h i n e s v a r y , in g e n e r a l , Recently,

and versatile

having high stiffness.

make

search

for

more

As c o u n t e r m e a s u r e s suitable

grinding

for

wheels

according to t h e conditions in trial and e r r o r m e t h o d s in work shops (ref. 2). Furthermore,

in t h e c a s e

of balancing

a grinding

0378 - 3804 / 88 / $ 03.50 ©1988 Elsevier Science Publishers B.V.

wheel,

the

lower

the

388 s t i f f n e s s and damping of the

machine are, the higher the a c c u r a c y of t h e In-Situ

wheel

Therefore,

balancing

becomes.

the

low

static

stiffness

and

the

low

da,"nping of the machine are desirable for a c c u r a t e wheel balancing. The objective of this study is to develop a new type s u r f a c e grinding machine which can give the most optimum grinding p e r f o r m a n c e s accurate

w h e e l b a l a n c i n g by f r e e l y c h a n g i n g i t s s t a t i c

as w e l l as t h e stiffness

and

most

damping

p r o p e r t y a c c o r d i n g to t h e g i v e n c o n d i t i o n s . As t h e f i r s t s t e p , a p r o t o t y p e o f a s u c h a m a c h i n e has b e e n m a n u f a c t u r e d

and i t s c h a r a c t e r i s t i c s

are investigated

experimentally.

DYNAMIC

BEHAVIOR

MACHINE

AND

ITS

OF COMMERCIALLY GRINDING

The i m p u l s e r e s p o n s e s o f t h r e e three

different

machine

tool

AVAILABLE

SURFACE

GRINDING

PERFORMANCE surface grinding machines

builders

are

shown

in Fig.l.

The

manufactured vertical

by

impulsive

e x c i t a t i o n s w e r e g i v e n a t t h e w h e e l h e a d , and t h e v e r t i c a l d i s p l a c e m e n t at t h i s point was measured with an a c c e l e r a t i o n pickup. The w a v e f o r m s o f the damped free vibrations show that according

to the

the

machine

static

stiffnesses

builders.

builder A is 15 N/IJm and its natural machine of builder B,

The static

the

damping

stiffness

of the

C , the

static

stiffness

vary

machine

the s t a t i c s t i f f n e s s is 35 N/IJm and t h e natural

of the machine of builder A, but

for

of the

frequency

is a l m o s t

the

the damping is considerably high.

E x p e r i e n c e s of skilled grinding workers can he s u m m a r i z e d They maintain that

properties

frequency is approximately 75 Hz;

is 250 Hz; f o r t h e m a c h i n e o f b u i l d e r s a m e as t h a t

and

as s h o w n in Fig. l.

the machine of builder C has e x c e l l e n t c h a r a c t e r i s t i c s

Fig, I, Examples of Impulse responses of surface grlndtng machines,

on

389 t h e w h o l e in t h e c a s e of g r i n d i n g s t e e l s . But, in t h e c a s e of g r i n d i n g of hard and b r i t t l e m a t e r i a l s such as c e r a m i c s , t h e m a c h i n e of b u i l d e r B is b e t t e r

than

t h e o t h e r s . On t h e o t h e r hand, t h i s m a c h i n e is l i k e l y to p r o d u c e g r i n d i n g burn in t h e c a s e

of grinding

of hardened

steels

and the

wheel

l i f e is s h o r t .

In

addition, such high stiffness and damping of the machine reduce the amplitude of the wheelhead

vibration

caused

by the

unbalance

of the

grinding wheel,

thereby

making precise wheel balancing difficult. Thus, each of the surface grinding machines shown in Fig.1 has several merits and d e m e r i t s in its g r i n d i n g c h a r a c t e r i s t i c s

and o p t i m u m w o r k i n g c o n d i t i o n s .

It

may be concluded that an ideal grinding machine is the machine which can provide various grinding characteristics

and w o r k i n g c o n d i t i o n s by f r e e l y c h a n g i n g

its

static stiffness and damping property according to requirements. PRINCIPLE OF VARIABLE STIFFNESS SURFACE GRINDING MACHINE Figure

2 shows the principle

grinding machine.

The m a c h i n e

of the proposed is c o n s t r u c t e d

surface grinding machine having a rather equipped

w i t h an a d d i t i o n a l

low s t a t i c

fixed column

Clamping

stiffness

stiffness.

The m a c h i n e is

between the two columns,

F r i c t i o n plate I!

6rlndtng~ w heel x~ . Movable column

I

surface

at the back of the conventional

c o l u m n ( m o v a b l e c o l u m n ) . The m a c h i n e c o m p r i s e s ,

Wheelhead

variable

on t h e basis of a c o n v e n t i o n a l

i Bed

Fig. 2. Principle of variable stiffness surface grinding rnaclline,

a

890 mechanism

c o n s i s t i n g of s e v e r a l

plate

so as t o c h a n g e

static

stiffness

coiled

its overall

can be adjusted

springs,

static

clamping

stiffness

by t i g h t e n i n g

and

bolts and a friction

damping

property.

or l o o s e n i n g t h e c l a m p i n g

on t h e coiled springs installed b e t w e e n the movable column and t h e The damping property can be adjusted

by changing t h e

the dry friction plate. The manufactured Figure 4 shows the structural The spring constant

components

prototype

normal

machine

of t h i s p r o t o t y p e

of t h e c o i l e d s p r i n g u s e d is 1.4 N / p m .

c o i l e d s p r i n g a r e u s e d in all. T h e s t a t i c

stiffness

The bolts

fixed column.

force

acting

upon

is s h o w n in F i g . 3 . machine

in d e t a i l .

S e v e n s e t s of t h i s

of the wheelhead

in v e r t i c a l

d i r e c t i o n c a n b e c h a n g e d s t e p w i s e in 32 s t e p s r a n g i n g f r o m 2.5 t o 15 N / ~ m . In a d d i t i o n , by r e p l a c i n g

the coiled springs with the ones having different

constants,

stiffness

motor,

the static

a balancer

can

be changed

in a w i d e r

a n d a b a l l b e a r i n g s h o w n in Figs.

range.

An

spring electric

3 and 4 are used

Fig, 3, Prototype of vartabIe stiffness surface grtndlng machlne,

in t h e

39.1

Coiled spring

Clamping bolt

C1amplng bolt Push rod

Coiled spring

Coiled spring Friction plate

Movable column Adj us t ing

Motor

screw

r

/

Clamping bolt

l~ / Fixed" Coiled spring column

Total w e i g h t

: 400 Kg

Height of colunm

: 400 nun

J

Wheelhead T Boll bearing

Length of w h e e l h e a d : 260 m m Spring constant of coiled spring

S p e e d of m o t o r spindle

J

Adjusting screw

: 1.4 N / ~ m

Static stiffness of w h e e l h e a d (Vertical direction) : 2.5~15

!

N/~m

: 2200 rpm

Shaft

Bed

Fig. q, Detail of variable stiffness surface grinding machine,

392 simulation

grinding.

The first

and the

second

compose

unbalance of a grinding wheel and t h e third is a simulator

a simulator

for

an

for a grinding wheel.

O n e s e t a m o n g t h e c o i l e d s p r i n g s is a l w a y s f i x e d so as t o k e e p t h e a t t i t u d e

of

t h e movable column. EXPERIMENTAL METHODS AND RESULTS The e x p e r i m e n t s were conducted using t h e p r o t o t y p e

machine on t h e following

items: (1) V a r i a t i o n of t h e f r e q u e n c y

and impulse responses of the machine when the

s t a t i c s t i f f n e s s and damping property were varied. (2) Dynamic behaviors of t h e m a c h i n e in various simulation grinding. Figure machine

5 shows the variation by changing

characteristics It seems t h a t

its static

of t h e f r e q u e n c y

dynamic stiffness.

must be useful

for precise

that

prototype

various

this f e a t u r e

I t is s e e n

of t h e

of t h e m a c h i n e c a n b e o b t a i n e d by c h a n g i n g i t s s t a t i c

period of the natural

stiffness.

responses

wheel balancing. When t h e

frequency of the m a c h i n e coincides with t h a t

of t h e wheel

Without damping

10

lc2

|

Z

(1) Coiled springs 1 and 2

(i)/1 (3)

E

(2) Coiled springs 1 to 3 (3) Coiled springs I to 4

c-

iG

(4) Coiled springs 1 to 5 (5) Colled springs 1 to 6

-2

10

0

20

40 Frequency

60

80

100

Hz

Fig, 5, Variation of frequency responses of prototype machine by changing Its static stiffness.

393

rotation, the vibration amplitude increases, resulting in the improvement of the sensitivity of the wheelhead to an unbalance. This has been confirmed by a balancing test with the device developed by the authors (ref.3). Figure 6 shows the v a r i a t i o n of the frequency responses by changing its damping.

It is seen t h a t t h e developed

various damping characteristics caused

damping d e v i c e a r e

of the machine.

effective

In t h i s c a s e ,

mainly by t h e dry f r i c t i o n p l a t e and t h e s t r u c t u r a l

in providing

the damping are

damping of t h e coiled

springs. Figure 7 shows t h e d y n a m i c b e h a v i o r of t h e w h e e l h e a d and t h e normal grinding force during a simulation grinding using the prototype

machine

s h o w n in F i g s . 3

a n d 4. In t h i s c a s e , s i n c e t h e g r i n d i n g w h e e l h a s b e e n w e l l b a l a n c e d ,

there

is

not any f o r c e d v i b r a t i o n due to t h e wheel unbalance. A d y n a m o m e t e r for measuring grinding forces was mounted

on t h e w o r k t a b l e

w h i c h c a n s l i d e on t h e

roller

guides. A block gauge was m o u n t e d on t h e d y n a m o m e t e r i n s t e a d of a workpiece. The top The

figures in Fig.7

show

the

corresponding

impulse

responses

of

the

wheelhead.

e x p e r i m e n t s were c o n d u c t e d under t h e following conditions ; depth of cut

2 pro, s t a t i c

stiffness

of t h e w h e e l h e a d :

4 N/lira, t a b l e s p e e d :

50 m m / s e c ,

10

(i)

i(1)

z

1

(2) c o

i0

(3) -2

i0 0

20

40

Frequency

60

80

i00

Hz

Fig, 6, Variation of freqency responses of prototype , machlne by changing l t s damplng,

:

100

894

I

Low damping

Medium damping

High damping

Fig, 7, Variation of grinding force and displacement by changing damping of prototype machine (Wtthout wheel unbalance)

Fig, 8, Variation of grinding force and displacement by changing damping of prototype machine (With wheel unbalance : 8 g.n]m),

895 m m / s e c , 200 m m / s e c , damping: low, medium, high. As shown in t h e figure, when the ball bearing (grinding wheel) makes contact gauge

(workpiece)

excitations occur.

are

and s e p a r a t e s applied

It is a l s o s e e n

amplitude

of

to

the

that

the e x c i t e d

d a m p i n g is, t h e s m a l l e r

from block

the

of 8 g . m m

w i t h t h e l e a d i n g e d g e of t h e b l o c k

trailing edge of t h e block gauge,

gauge

higher

the vibration

at

the

and

the

v i b r a t i o n s becomes.

results under the conditions w h e r e unbalance

the

the

table

On

rotational

damping is,

the o t h e r

amplitude

t h e forced

free speed

the

hand,

becomes.

speed of

Figure

f i g u r e , t h e w a v e s d u e to t h e w h e e l u n b a l a n c e

2200 r p m .

vibrations larger

the

v i b r a t i o n is produced

step the

higher

the

8 shows

the

by t h e

wheel

As s h o w n in t h i s

is s u p e r i m p o s e d

on t h e v i b r a t i o n

waves shown in Fig.7. Figure under

9 shows

the

experimental

the conditions shown

in Fig.7

results using

the

of s i m u l a t i o n newly

grinding conducted

developed

variable

stiffness

g r i n d i n g m a c h i n e . T h e u p p e r f i g u r e s show t h e i m p u l s e r e s p o n s e s of t h e m a c h i n e and the

l o w e r f i g u r e s show

wheelhead static

during

stiffness

the

simulation

grinding

grinding.

It

forces

displacements that

of t h e

a change

in

the

a n d d a m p i n g p r o p e r t y of t h e m a c h i n e c a n p r o d u c e t h e n e w t y p e

grinding

machines

addition,

it is s e e n f r o m t h e l o w e r f i g u r e s t h a t

which

are

of the machine is, the larger

Static stiffness : low Damping : low

Flg, 9,

and the

is c o n f i r m e d

able

to

give

various

grinding

performances.

the higher the static

the normal grinding f o r c e becomes.

It is also seen

[Static stiffness : high Static stiffness : medium l IDamping : meditm Damping high 1

Experimental results of simulation grlndtng prototype machlne.

In

stiffness

using

896

that

the

higher

the

damping

of the

machine

structure

is,

the

sooner

the

vibration of t h e machine dies out. On t h e b a s i s of t h e a b o v e - m e n t i o n e d stiffness and the

damping

factors

its

burns,

affecting surface

property

grinding

finish,

of

results,

i t is s u p p o s e d t h a t

a surface

performances

and grindability

grinding

such

and

as

life

machine

machine

of

grinding

the static

are

important

vibrations, wheels

grinding

in

actual

grinding work Iref.4).

CONCLUSIONS The o b t a i n e d results are s u m m a r i z e d as follows: (1)

A prototype

of a variable

manufactured. property.

It c a n

stiffness

change

surface

freely

On t h e b a s i s of t h i s f a c t ,

its

grinding

static

it is p o s s i b l e

machine

stiffness to d e v e l o p

and

has been damping

a new

type

surface grinding machine which can give various grinding p e r f o r m a n c e s . (2)

An a r b i t r a r y selection of t h e natural

frequency

of the

machine

enables

more

precise In-Situ wheel balancing. 13) The

simulation

property

grinding

of a s u r f a c e

shows

that

the

grinding machine

static

stiffness

are principal

and

factors

the

damping

which affect

its grinding performances.

ACKNOWLEDGMENTS The

authors

would

like t o t h a n k

Mr. M. Y a m a g u c h i ,

General

Manager

of

Technical D e p a r t m e n t of Nagase Iron Works Co., Ltd. and Prof. Dr. K. Unno of t h e Institute

of V o c a t i o n a l T r a i n i n g

for the valuable

advices

in c a r r y i n g

out

this

study. REFERENCES 1 2 3 4

R.L. Kegg: I n d u s t r i c a l P r o b l e m s in G r i n d i n g , A n n a l s o f t h e C I R P , V o l . 3 2 / 2 , (1983) pp.559-561. N. Shinozaki, K. Unno: KENSAKU SAGYO, NIKKAN KOGYO SHINBUN, Tokyo, 1982, pp.38. (in Japanese) K. S a t o , Y. S a k a i a n d Y. N a k a n o : A New T y p e B a l a n c e r f o r G r i n d i n g W h e e l s W h i c h Is O p e r a t e d M a n u a l l y While W h e e l Is R o t a t i n g , P r o c . 6 t h I C P E , 1987, pp.500-505. Y. N a k a n o , H. K a t o a n d A. U e t a k e : G e n e r a t i o n o f C h a t t e r M a r k s in S u r f a c e Grinding by Using Horizontal Spindle-Reciprocating Table Type Machines, Bull. of JSPE, 19, 4 (1985) pp.266-272.