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Dressing of Metal Bond Grinding Wheels by Electro-Discharge Machining
On-Machine Trueing/Dressing of Metal Bond Grinding Wheels by Electro-Discharge Machining K. Suzuki, Inst. of Ind. Science, Univ. of Tokyo; T. Uematsu, Toyama Prefectural College of Technology; and T. Nakagawa, Inst. of Ind. Science, Univ. of Tokyo Received on March 4, 1987 - Accepted by the Editorial Committee
SU?NARY: This paper describes a newly proposed method for carrying uut effective and precise trueing and dressing of metal bond grinding wheels on an NC grinding machine o r a machining center by utilizing wire or die sinking electro-discharge machining(EDM). The main feature of this method is that a simple ED unit is mounted on the NC table of the grinding machine. Thereby trueing for eliminating the eccentricity of wheels and for generating or correcting profiled wheels is carried out by the help of NC movement of the table, keeping the grinding wheel on a machine spindle. In order to prevent the influence of wear of a block electrode in die sinking EDM, another new method is proposed for shaping or correcting the form of the electrode with a cutting tool on the grinding machine by the help o f NC movement of the table. The superior effects of the proposed methods have been confirmed in the case of grinding o€ ceramic material with cast iron fiber bond diamond wheels. Key words: grinding of ceramics, trueing, dressing, metal bond grinding wheel, cast iron fiber bond diamond wheel, profiled wheel, electro-discharge machining, on-machine electro-discharge trueingldressing, grinding center, machining center
1.INTRODUCTION
2 .PROPOSAL OF ON-MACHINE ELECTRO-DISCHARGE TRUEINGIDRESSING METHOD
Highly efficient and accurate grinding of extremely hard ceramic material may be realized by selecting a suitable diamond grinding wheel and an ideal grinding machine with high rigidity and many NC (numerical control) functions/l,2.3/. The authors have succeeded in obtaining very high material removal rate of Q,,'=7,500mn~/mm/ min for silicon nitride c-eramics by the combination of a very tough cast iron fiber bond diamond grinding wheel (CIB-F wheel hereinafter) and a revised machining center(MC hereinafter) or a grinding center(GC)/4,5/. When highly accurate griding of ceramic parts is performed on an NC grinding machine f o r a long time, automatic trueing and dressing are required at appropriate times to provide correct shape and sharp cutting edge respectively to the grinding wheel. But there is no appropriate method for trueing/dressing metal bond diamond wheels. Conventional stick grindstones o r rotating grindstones (C,GC,WA) are generally used for that purpose. Though this method is simple and convenient, the high wear rate of the grindstones leads to some disadvantages such as decrease in dimensional and configurational accuracies of diamond wheels. Accordingly this method can not be applied to profiled wheels and automatic trueingldressing operation. The authors propose a new trueingldressing method utilizing electro-discharge machining(EDM)/6,7,8/, named "On-machine electro -discharge trueing/dressing method". This paper describes the principle and the effects of the method, the profile generation of diamond wheels, and further "On-machine shaping of an electrode by cutting".
2.1 Principle o f the method
Fig. 1
Profiled metal bond wheels have conventionally been formed by dle sinking E M on a separate machine. But when the wheels are mounted onto the spindle of a grinding machine, eccentricity or Here the authors unbalance of the wheels is liable to occur. propose "On-machine electro-discharge(ED) trueingldressing method" for metal bond wheels as schematically illustrated in I n the method a trueing unit with an electrode is mounted on a table of an NC grinding machine such as GC or MC and trueingldressing of grinding wheels on the machine spindle is performed by utilizing EDM.
m.
2.2 Features of the method The "On-machine ED trueing/dressing method" is expected to have many features as follows: (1) The method can be applied to all electrically conductive metal bond wheels. (2) Either of wire EDM and die sinking E M can be utilized, (3) Bonding material is rhnoved by EDM while the wheel is mounted on the spindle of a grinding machine so that highly precise trueing is accomplished. ( 4 ) Machining force is so small that precise trueing and dressing are possible even f o r mounted wheels with small diameter and cutting wheels with small thickness. (5) Trueing and dressing suitable for grain sizes are carried out only by changing ED conditions, (6) Precise trueing and dressing including profile generation can he accomplished on the grinding machine by utilizing the NC movement of a GCIK. (7) Complicated servo control function on the side of an ED device i s unnecessary because NC movement on the side of a grinding machine is utilized. This results in a very simple ED power source.
Schematic illustration of On-machine electro-discharge(ED1 trueingldressing method.
f
Electric power source
Spindle head Wheel shaft
Wire
guw/!
Metal bond grinding wheel
6/ Fig.2
W G e electrode
Schematic illustration of On-machine wire ED trueing/ dressing method.
Annals of the CIRP Vol. 36/1/1987
Fig3
On-machine wire ED trueing/dressing unit on the NC table of a machining center.
115
3.
ON-MACHINE YIRE ED TRUEING/DRESSING
*
CIB-F wheel(ds30,#600,conc.l0O)
*
vd=100m/min,lsb-20~m,Ahd=O U-100V,I-80A,ti=to=lps
*
In the method using wire E m . a wire electrode runs on a grooved guide as illustrated i n As the wear of the wire electrode is negligibly small, highly accurate trueing is accomplished.' In addition, the combination of the wire electrode and the NC movement of the grinding machine makes the genaration and the correction of desired profiled wheels possible.
m.
3.1 Experimental equipments and conditions
shows the experimental On-machine wire ED truerng unit mounted on the NC table of a vertical type XC. A brass wire electrode (W.5mm dia.) runs along a wire guide(m32mm dia.) made of alumina ceramic at a speed ofvwe=150m/min. The grinding wheel on the spindle of the MC rotates at an appropriate speed. In the experiment CIB-F wheels ( @ 3 0 m m dia., grain size:#600, conc.100) were used. From the viewpoint of machining efficiency. the wire electrode was made a negative pole and a brush contacting with the wheel shank was made a positive pole. For the sake of convenience in the practical use, grinding fluid (Johnson's JC-707, x50) was used in common as working fluid for ED trueing with no problem, The experimental conditions are listed in T a b l e s 1 and 2.
0 fd-30
A fd'50
( I : ED spark stops)
-201 I
I
I
I
20
0
1
I
Time
I
1
60
40
80
(s/mm)
la) Influence of wheel feedrace. M G N - 1 0 for w i r e E D M MG-30 for N C die sinking E D M . (Makino M i l l i n g Machine Co.,Ltd.)
Power source
Grinding machine
Vertical type machining center
Grinding wheel
Cast Iron Fiber bond diamond wheel d 30-b 5,grain size:#600,conc.100
* CIB-F
-
wheel(ds30,#600,conc.l~)
* vd=lOOm/min, fd=lOmm/min,lsb=20Um,Ahd=O * U-lOOV,ti=to=lUs (v: ED spark stops)0
8
0
(TMC4OV, 3.7kW) I-80A (SintoEogio Ltd. 1
I=60A
JC-707,x50(Johnson Co.,Ltd.) (used in common w i t h grinding) s p e c i f i c resistance=0.8x10 ohm-cm
Liquid for ED
Electrode
Wire:d=0.5mm
brass, 5lock:graphite
5 20
0
s "[
a
-2ot Table 2
~
Conditions for ED trueing/dressing.
0
20
I
l
60
40
t
l
l
1
80
T i m e (s/mm)
(b) Influence of electric current. Set-back
Removed thick./pass: Ahdr(!m)
Voltage
:
U=60-180V
Max. working gap
Fig.4
: smx(um)
Removed thicknesdpass: Ahdr
b 0
Removed thickness: hdr
Set-back: Isb-20Um
Max. working gap: Smax'h&+isb
(a) ED conditions and trueing ability The relations between ED conditions and work removal rate in ED trueing of mounted wheels were investigated. The distance (amount of set-back:lsb) between the initial wheel surface and the wire electrode was made Isb=ZOUm. The wheel was fed in the axial direction (distance-5mm) at a constant speed fd. A s Fias.l*(a) and &I show, the removal rate o r the inclination of the curves in these figures i s much infIuenced by the working current I, but is slightly influenced by the feedrate fd of the wheel. FiR.5 shows that the maximum allowable working gap Smax (=lsb t hdr) can be made larger when the peripheral speed vd of the wheel and the feedrate fd of the wheel are lowered. For example, when the values of fd=lh/miII and vd=2jm/min are chosen, the electrode gap up to l3Oum is allowed. This result means that lower Vd and fd are recommended in case of coarse abrasives to prevent the cutting-off of the wire through the contact with the wheel during ED trueing.
I
I
1
I
50
100
150
200
Wheel peripheral speed Vd (rnlmin)
3.2 Experimental results of On-machine wire ED trueing
116
Relations between ED conditions and work removal rate in wire ED trueing (width of wheek5mm).
Fig3
Relation between wheel speed and maximum allowable working gap.
( b ) Eliminatina effect for eccentricity The effect of wire ED trueing on the elimination of the eccentricity of a grinding wheel was investigated. A grinding wheel having a deflection le, which is two times of eccentricity, of 0.08mm purposely was trued. The wire was made approach t o the wheel by a predetermined value Ahd(=5 o r 1.25~m) per pass. shows the result in case of wheel speed vd=100m/min. The deflection le of a wheel decreases with the number of wire passes at any value of working current I. When the current I is 80A, the decrease of the deflection was saturated at a rather higher value, but in case of a higher wheel speed vd=2O(lm/min, the
* *
*
d e f l e c t i o n decreased up t o a lower v a l u e ( 2 5 ~ m ) . When t h e working c u r r e n t of I=40A was a d o p t e d , t h e d e f l e c t i o n decreased t o 1 3 ~ 1 , and f i n a l l y when t h e c u r r e n t was f u r t h e r lowered t o 1-IOA, t h e d e f l e c t i o n became 3pm. T h i s f i n a l v a l u e of e c c e n t i r i c i t y i s n e a r l y e q u a l t o t h e s p i n d l e accuracy of t h e MC used. This means t h a t t h e t r u e i n g accuracy i n t h e p r e s e n t method a t t a i n s t o t h e accuracy of t h e g r i n d i n g machine used. The above r e s u l t s s u g g e s t t h a t i t is necessary t o reduce ED c o n d i t i o n s step b y s t e p f o r accomplishing p r e c i s e ED t r u e i n g . When Lhe i n i t i a l d e f l e c t i o n is n o t so l a r g e , low working c u r r e n t should be adapted from t h e beginning.
CIB-F wheel(ds30,#600,COnc.1~) vd=lOh/min ,f =lorn/min, &d=5m UP100V, ti"t0'P"S
( I : ED s p a r k s t o p s )
3.3 Experimental r e s u l t s of On-machine w i r e ED d r e s s i n g
( a ) Observations of wheel s u r f a c e shows observed s u r f a c e of a wheel a f t e r ED t r u e i n g under some c o n d i t i o n s . When t h e e l e c t r i c c u r r e n t is l a r g e , s u f f i c i e n t amount of p r o t r u s i o n of a b r a s i v e g r a i n s can be o b t a i n e d , though t h e m a t r i x of a g r i n d i n g wheel i s r a t h e r rough and some a b r a s i v e s a r e released. B u t under f i n i s h i n g ED c o n d i t i o n s 01 smaller e l e c t r i c c u r r e n t , t h e m a t r i x s u r f a c e is comparatively smooth and s u f f i c i e n t a b r a s i v e s a r e found remaining, though t h e h e i g h t of p r o t r u s i o n is n o t s a t i s f a c t o r y . Accordingly, i t is d e s i r a b l e t o change ED c o n d i t i o n s i n t o o n e s f o r f i n i s h i n g i n t h e f i n a l s t a g e of ED t r u e i n g , thereby high q u a l i t y of d r e s s i n g can be c a r r i e d o u t a t t h e same time.
Number of passes Fig.6
Elimination of wheel eccentricity.
(b) Comparison of d r e s s i n g e f f e c t Dressing e f f e c t was compared between t w o k i n d s of CIB-F wheels: one was d r e s s e d by t h e wire ED d r e s s i n g method, and t h e o t h e r was dressed manually with a s t i c k grindstone(WA). Both wheels had been t r u e d with a r o t a t i n g grindstone(C) beforehand. shows t h e change of g r i n d i n g f o r c e f o r s i l i c o n n i t r i d e ceramics. No d i f f e r e n c e can be s e e n between t h e two wheels, and t h i s means t h a t w i r e ED d r e s s i n g h a s a s a t i s f i e d e f f e c t . FiQ.9 shows t h a t t h e g r i n d i n g a b i l i t y or s h a r p n e s s of a CIB-F wheel is recovered when On-machine wire ED d r e s s i n g i s a p p l i e d t o t h e wheel whose g r i n d i n g a b i l i t y h a s been reduced d u r i n g g r i n d i n g of s i l i c o n n i t r i d e ceramics. 4.
Wheel surface a f t e r ED dressing. Fig.7 ~d,30,#100/120,conc.100, vd=1OOm/min,fd=IOmm/min,U-IOOV,ti-t,=1U~~
* CIB-F
L
wheel(ds30,#170/200,conc.lM))
* v,=136m/min,fW=20mm/min, h=O.Smm, b = 5 m I 2 200 - * S i l i c o n n i t r i d e ceramics(Hv1700)
-
c
150-
r4 o)
0
[Wire ED d r e s s i n g ] vd=136m/min,fd=10mm/min
U=100V,I=80A, ti=to~ll-lS
0 '+l
01
a
r
A
2
a
ON-MACHINE ED TRUEISG W I T H A BLOCK ELECTRODE
4.1 S e l e c t i o n of s u i t a b l e e l e c t r o d e Advantages of u s i n g a wire e l e c t r o d e i n t h e On-machine ED t r u e i n g a r e ( 1 ) h i g h accuracy owing t o n e g l i g i b l y s m a l l amount of e t e c t r o d e wear and (2)easy g e n e r a t i o n of p r o f i l e d wheels by u t i l i z i n g NC movement of a g r i n d i n g machine. On t h e o t h e r hand, t h e wire ED t r u e i n g method h a s some d i s a d v a n t a g e s such a s (1)low trueing e f f i c i e n c y which comes from t h e r e s t r i c t i o n of average c u r r e n t and t h e e x i s t e n c e of non-discharging zone a t t h e time of p r o f i l i n g j u s t l i k e " a i r c u t " observed i n metal c u t t i n g process and ( 2 ) t h e l i m i t of generated p r o f i l e s which comes from t h e r e s t r i c t i o n of wire d i a m e t e r . Though a block e l e c t r o d e has a problem of e l e c t r o d e wear, i t h a s s m e advantages s u p e r i o r t o a w i r e e l e c t r o d e i n t h e p o i n t s of ED e f f i c i e n c y and p r o f i l e g e n e r a t i o n . So On-machine ED t r u e i n g w i t h a block e l e c t r o d e would be p r o f i t a b l e i f appropriate countermeasures a r e devised.
100-
t
.rl
2
50-
4
[Dressed with GC-stickstone]
6
! i
w
I
Fig.8
I
I
I 1
Comparison of dressing e f f e c t between wire ED dressing and conventional dressing methods.
* *
I
*
-z
\
CIB-F wheel(ds30mm. #170/200,conc. 100) vS=l36m/min ,fw=20m/min, h=5.5mm, b-5mm S i l i c o n n i t r i d e ceramics(Hvl700)
c
r4
[Dressed w i t h stickstone]
[ED d r e s s i n g , condition-I]
[ED d r e s s i n g , condition-II]
U
0
500
1000
Grinding distance (mm) Fig.9
Recovery of grinding ability by wire ED dressing.
NC table
Fig.10
Schematic illustration of On-machine shaping of block electrode and On-machine ED trueing by the formed electrode.
117
Fig.12 Profiled CIB-F wheel by On-machine ED trueing with a formed graphite electrode.
Fig.11
On-machfne shaping of graphite electrode the NC table of machining center.
material on
4 . 2 Proposal of On-machine shaping of formed electrodes f o r precise trueing It i s apprehended that On-machine ED trueing with a block electrode may cause an Inaccuracy because of electrode wear and mounting error onto a trueina unit. The electrode wear can be restrai&d considerably by oprimizing ED conditions, but this is not enough for highly precise trueing. AS a CountermeasUre for the above apprehension, "On-machine electrode shaping method" illustrated in Fin.14 is proposed. I,, this new method, in case of trueing of a profiled grinding wheel , a cylindrical electrode material mounted o n a rotating shaft of a trueing unit is machined at firsi with a cuttlg tool
Thus a mounted on the spindle of an ?.lC (the spindle is clamped). desired profile is shaped on the electrode material by utilizjng
NC movement of the MC.
4 . 3 Experiments of ED trueing with On-machine shaping of electrode Fin.ll shows the state of shaping a graphite electrode shows the state of on-machine profile material on an MC. F&J+l It generation on a CIB-F wheel. with the above formed electrode. will be understood that sharply recessed grooves, which are difficult to be formed with a wire electrode, are also easily formed hy this method 4 . 4 Grinding of ceramics with profiled wheels When the profiled wheels made by means of above ED trueing method are combined with NC movement of a grinding machine(GC/MC), considerably complicated parts can be formed, shows a silicon nitride ceramic sample formed with three different profiled wheels.
5 . CONCLUSIONS Two types of "On-machine trueing/dressing methods" utilizing EDM were proposed for t h e sake of efficient and accurate grinding ot ceramics with metal bond diamond grinding wheels. The first method utilizes vire EDM with negligibly small amount of electrode wear and makes precise trueing anti dressing of p r o f i l e d wheels possible by the help of WC movement of a grinding machine The second method utilizes die sinking EDM with a block electrode which provides high EDM efficiency. The problem of electrode wear is solved by "On-mach~neelectrode shaping method" in which shaping o r form correction of the electrode is carried out by NC machining operation of the grinding machine. It has been confirmed that the "On-machine ED trueing/dressing method" shows sufficient effects o n l y with a simple electrode unit and with a simple electrical power source. Authors expect that the "On-machine ED trueingldressing method'' will contribute t o the achievement of highly efficient and precise grinding of ceramics and hard metals.
.
Next a grinding wheel is selected instead of the cutting tool by an automatic tool changer and ED trueing of this wheel is performed with the formed electrode. If the amount o f electrode wear exceeds an allowable tolerance, the electrode can be machined again with the above cutting tool for correction. By adopting this method, the reduction of trueing accuracy due to mounting error and electrode wear is eliminated, agid that. it becomes unnecessary to prepare a formed electrode beforehand. The use of a rotating electrode also contributes to the improvement of the trueing accuracy. In case of a straight wheel with a flat surface, a fixed ACKXOWLEDGEMENTS electrode is shaped to have a flat surface with a rotatjng end The authors wish to thank Prof. T.Masuzawa(lnst. of Ind. mill cutter. N.Mohri (Toyota Technological Sci., Univ. of Tokyo) and Prof. Inst.) for their valuable advic.es. The authors also express their thanks to H/S Sintokogio Ltd., Yakino Yilling Machine Co.Ltd., Sodick Co.Ltd.. Johnson Co.Ltd.,Sumitomo Wiring System Co.Ltd., Lor preparing the experimental Maruto Instruments Co.Ltd. rquipments, and t o Mr.T. Yanase(Toyama Pref. College of Technology) and Mr.S.Asano(Stanley Electric Co.Ltd.) for conducting experiments.
Fig.13
Silicon nitride ceramic sample ground with CIB-F wheels formed by On-machine
118
ED trueing method.
REFERENCES 1. Suzuki,K. Uematsu,T. Nakagawa,T.,1985, Grinding of ceramics on machining center(1st report), Preprint of Spring Conf. of .JSPE,809-812(in Japanese) 2. Uematsu,T .Suzuki,K. Nakagawa,T.,1985, Grinding of ceramics on machining center(3rd report), Preprint of Autumn Conf. of JSPE,265-'268(in Japanese) 3 . Nakagawa,T. Suzuki,K. Uematsu.T., 1985, Three dimensional creep feed grinding of ceramics by machining center, Proc. of Winter Annual Meeting of ASME, Vo1.17, 1-8 4 . Nakagawa,T. Suzuki,K. Uematsu,T., 1986, Highly efficient grinding of ceramics and hard metals on grinding center, Annals of the CIRP V01.35/1, 205-210 5 . Uematsu,T. Suzuki,K. Narita,T. Nakagawa,T., 1987, Grinding of ceramjcs on machining center(9th report), Preprint of Spring Conf. of JSPE(in Japanese) 6 . Suzuki,K. >iohri,X. Uematsu,T. Nakagawa,T., 1985, ED trueing method with t v i n electrodes, Preprint of Autumn Conf. of .ISPE, 575-578(in Japanese) 7. Suzuki,K. Uematsu,T. et al, 1986,Grinding of ceramics on of JSPE, machining center(6th report), Preprint of AuLumn Conf. 657-660( in Japanese) 8. Suzuki,K. Uematsu,T. ei al, 1987, Grinding of ceramics on machining center(7th report), Preprint of Spring Conf. o f .JSPE(in Japanese)
SU?NARY: This paper describes a newly proposed method for carrying uut effective and precise trueing and dressing of metal bond grinding wheels on an NC grinding machine o r a machining center by utilizing wire or die sinking electro-discharge machining(EDM). The main feature of this method is that a simple ED unit is mounted on the NC table of the grinding machine. Thereby trueing for eliminating the eccentricity of wheels and for generating or correcting profiled wheels is carried out by the help of NC movement of the table, keeping the grinding wheel on a machine spindle. In order to prevent the influence of wear of a block electrode in die sinking EDM, another new method is proposed for shaping or correcting the form of the electrode with a cutting tool on the grinding machine by the help o f NC movement of the table. The superior effects of the proposed methods have been confirmed in the case of grinding o€ ceramic material with cast iron fiber bond diamond wheels. Key words: grinding of ceramics, trueing, dressing, metal bond grinding wheel, cast iron fiber bond diamond wheel, profiled wheel, electro-discharge machining, on-machine electro-discharge trueingldressing, grinding center, machining center
1.INTRODUCTION
2 .PROPOSAL OF ON-MACHINE ELECTRO-DISCHARGE TRUEINGIDRESSING METHOD
Highly efficient and accurate grinding of extremely hard ceramic material may be realized by selecting a suitable diamond grinding wheel and an ideal grinding machine with high rigidity and many NC (numerical control) functions/l,2.3/. The authors have succeeded in obtaining very high material removal rate of Q,,'=7,500mn~/mm/ min for silicon nitride c-eramics by the combination of a very tough cast iron fiber bond diamond grinding wheel (CIB-F wheel hereinafter) and a revised machining center(MC hereinafter) or a grinding center(GC)/4,5/. When highly accurate griding of ceramic parts is performed on an NC grinding machine f o r a long time, automatic trueing and dressing are required at appropriate times to provide correct shape and sharp cutting edge respectively to the grinding wheel. But there is no appropriate method for trueing/dressing metal bond diamond wheels. Conventional stick grindstones o r rotating grindstones (C,GC,WA) are generally used for that purpose. Though this method is simple and convenient, the high wear rate of the grindstones leads to some disadvantages such as decrease in dimensional and configurational accuracies of diamond wheels. Accordingly this method can not be applied to profiled wheels and automatic trueingldressing operation. The authors propose a new trueingldressing method utilizing electro-discharge machining(EDM)/6,7,8/, named "On-machine electro -discharge trueing/dressing method". This paper describes the principle and the effects of the method, the profile generation of diamond wheels, and further "On-machine shaping of an electrode by cutting".
2.1 Principle o f the method
Fig. 1
Profiled metal bond wheels have conventionally been formed by dle sinking E M on a separate machine. But when the wheels are mounted onto the spindle of a grinding machine, eccentricity or Here the authors unbalance of the wheels is liable to occur. propose "On-machine electro-discharge(ED) trueingldressing method" for metal bond wheels as schematically illustrated in I n the method a trueing unit with an electrode is mounted on a table of an NC grinding machine such as GC or MC and trueingldressing of grinding wheels on the machine spindle is performed by utilizing EDM.
m.
2.2 Features of the method The "On-machine ED trueing/dressing method" is expected to have many features as follows: (1) The method can be applied to all electrically conductive metal bond wheels. (2) Either of wire EDM and die sinking E M can be utilized, (3) Bonding material is rhnoved by EDM while the wheel is mounted on the spindle of a grinding machine so that highly precise trueing is accomplished. ( 4 ) Machining force is so small that precise trueing and dressing are possible even f o r mounted wheels with small diameter and cutting wheels with small thickness. (5) Trueing and dressing suitable for grain sizes are carried out only by changing ED conditions, (6) Precise trueing and dressing including profile generation can he accomplished on the grinding machine by utilizing the NC movement of a GCIK. (7) Complicated servo control function on the side of an ED device i s unnecessary because NC movement on the side of a grinding machine is utilized. This results in a very simple ED power source.
Schematic illustration of On-machine electro-discharge(ED1 trueingldressing method.
f
Electric power source
Spindle head Wheel shaft
Wire
guw/!
Metal bond grinding wheel
6/ Fig.2
W G e electrode
Schematic illustration of On-machine wire ED trueing/ dressing method.
Annals of the CIRP Vol. 36/1/1987
Fig3
On-machine wire ED trueing/dressing unit on the NC table of a machining center.
115
3.
ON-MACHINE YIRE ED TRUEING/DRESSING
*
CIB-F wheel(ds30,#600,conc.l0O)
*
vd=100m/min,lsb-20~m,Ahd=O U-100V,I-80A,ti=to=lps
*
In the method using wire E m . a wire electrode runs on a grooved guide as illustrated i n As the wear of the wire electrode is negligibly small, highly accurate trueing is accomplished.' In addition, the combination of the wire electrode and the NC movement of the grinding machine makes the genaration and the correction of desired profiled wheels possible.
m.
3.1 Experimental equipments and conditions
shows the experimental On-machine wire ED truerng unit mounted on the NC table of a vertical type XC. A brass wire electrode (W.5mm dia.) runs along a wire guide(m32mm dia.) made of alumina ceramic at a speed ofvwe=150m/min. The grinding wheel on the spindle of the MC rotates at an appropriate speed. In the experiment CIB-F wheels ( @ 3 0 m m dia., grain size:#600, conc.100) were used. From the viewpoint of machining efficiency. the wire electrode was made a negative pole and a brush contacting with the wheel shank was made a positive pole. For the sake of convenience in the practical use, grinding fluid (Johnson's JC-707, x50) was used in common as working fluid for ED trueing with no problem, The experimental conditions are listed in T a b l e s 1 and 2.
0 fd-30
A fd'50
( I : ED spark stops)
-201 I
I
I
I
20
0
1
I
Time
I
1
60
40
80
(s/mm)
la) Influence of wheel feedrace. M G N - 1 0 for w i r e E D M MG-30 for N C die sinking E D M . (Makino M i l l i n g Machine Co.,Ltd.)
Power source
Grinding machine
Vertical type machining center
Grinding wheel
Cast Iron Fiber bond diamond wheel d 30-b 5,grain size:#600,conc.100
* CIB-F
-
wheel(ds30,#600,conc.l~)
* vd=lOOm/min, fd=lOmm/min,lsb=20Um,Ahd=O * U-lOOV,ti=to=lUs (v: ED spark stops)0
8
0
(TMC4OV, 3.7kW) I-80A (SintoEogio Ltd. 1
I=60A
JC-707,x50(Johnson Co.,Ltd.) (used in common w i t h grinding) s p e c i f i c resistance=0.8x10 ohm-cm
Liquid for ED
Electrode
Wire:d=0.5mm
brass, 5lock:graphite
5 20
0
s "[
a
-2ot Table 2
~
Conditions for ED trueing/dressing.
0
20
I
l
60
40
t
l
l
1
80
T i m e (s/mm)
(b) Influence of electric current. Set-back
Removed thick./pass: Ahdr(!m)
Voltage
:
U=60-180V
Max. working gap
Fig.4
: smx(um)
Removed thicknesdpass: Ahdr
b 0
Removed thickness: hdr
Set-back: Isb-20Um
Max. working gap: Smax'h&+isb
(a) ED conditions and trueing ability The relations between ED conditions and work removal rate in ED trueing of mounted wheels were investigated. The distance (amount of set-back:lsb) between the initial wheel surface and the wire electrode was made Isb=ZOUm. The wheel was fed in the axial direction (distance-5mm) at a constant speed fd. A s Fias.l*(a) and &I show, the removal rate o r the inclination of the curves in these figures i s much infIuenced by the working current I, but is slightly influenced by the feedrate fd of the wheel. FiR.5 shows that the maximum allowable working gap Smax (=lsb t hdr) can be made larger when the peripheral speed vd of the wheel and the feedrate fd of the wheel are lowered. For example, when the values of fd=lh/miII and vd=2jm/min are chosen, the electrode gap up to l3Oum is allowed. This result means that lower Vd and fd are recommended in case of coarse abrasives to prevent the cutting-off of the wire through the contact with the wheel during ED trueing.
I
I
1
I
50
100
150
200
Wheel peripheral speed Vd (rnlmin)
3.2 Experimental results of On-machine wire ED trueing
116
Relations between ED conditions and work removal rate in wire ED trueing (width of wheek5mm).
Fig3
Relation between wheel speed and maximum allowable working gap.
( b ) Eliminatina effect for eccentricity The effect of wire ED trueing on the elimination of the eccentricity of a grinding wheel was investigated. A grinding wheel having a deflection le, which is two times of eccentricity, of 0.08mm purposely was trued. The wire was made approach t o the wheel by a predetermined value Ahd(=5 o r 1.25~m) per pass. shows the result in case of wheel speed vd=100m/min. The deflection le of a wheel decreases with the number of wire passes at any value of working current I. When the current I is 80A, the decrease of the deflection was saturated at a rather higher value, but in case of a higher wheel speed vd=2O(lm/min, the
* *
*
d e f l e c t i o n decreased up t o a lower v a l u e ( 2 5 ~ m ) . When t h e working c u r r e n t of I=40A was a d o p t e d , t h e d e f l e c t i o n decreased t o 1 3 ~ 1 , and f i n a l l y when t h e c u r r e n t was f u r t h e r lowered t o 1-IOA, t h e d e f l e c t i o n became 3pm. T h i s f i n a l v a l u e of e c c e n t i r i c i t y i s n e a r l y e q u a l t o t h e s p i n d l e accuracy of t h e MC used. This means t h a t t h e t r u e i n g accuracy i n t h e p r e s e n t method a t t a i n s t o t h e accuracy of t h e g r i n d i n g machine used. The above r e s u l t s s u g g e s t t h a t i t is necessary t o reduce ED c o n d i t i o n s step b y s t e p f o r accomplishing p r e c i s e ED t r u e i n g . When Lhe i n i t i a l d e f l e c t i o n is n o t so l a r g e , low working c u r r e n t should be adapted from t h e beginning.
CIB-F wheel(ds30,#600,COnc.1~) vd=lOh/min ,f =lorn/min, &d=5m UP100V, ti"t0'P"S
( I : ED s p a r k s t o p s )
3.3 Experimental r e s u l t s of On-machine w i r e ED d r e s s i n g
( a ) Observations of wheel s u r f a c e shows observed s u r f a c e of a wheel a f t e r ED t r u e i n g under some c o n d i t i o n s . When t h e e l e c t r i c c u r r e n t is l a r g e , s u f f i c i e n t amount of p r o t r u s i o n of a b r a s i v e g r a i n s can be o b t a i n e d , though t h e m a t r i x of a g r i n d i n g wheel i s r a t h e r rough and some a b r a s i v e s a r e released. B u t under f i n i s h i n g ED c o n d i t i o n s 01 smaller e l e c t r i c c u r r e n t , t h e m a t r i x s u r f a c e is comparatively smooth and s u f f i c i e n t a b r a s i v e s a r e found remaining, though t h e h e i g h t of p r o t r u s i o n is n o t s a t i s f a c t o r y . Accordingly, i t is d e s i r a b l e t o change ED c o n d i t i o n s i n t o o n e s f o r f i n i s h i n g i n t h e f i n a l s t a g e of ED t r u e i n g , thereby high q u a l i t y of d r e s s i n g can be c a r r i e d o u t a t t h e same time.
Number of passes Fig.6
Elimination of wheel eccentricity.
(b) Comparison of d r e s s i n g e f f e c t Dressing e f f e c t was compared between t w o k i n d s of CIB-F wheels: one was d r e s s e d by t h e wire ED d r e s s i n g method, and t h e o t h e r was dressed manually with a s t i c k grindstone(WA). Both wheels had been t r u e d with a r o t a t i n g grindstone(C) beforehand. shows t h e change of g r i n d i n g f o r c e f o r s i l i c o n n i t r i d e ceramics. No d i f f e r e n c e can be s e e n between t h e two wheels, and t h i s means t h a t w i r e ED d r e s s i n g h a s a s a t i s f i e d e f f e c t . FiQ.9 shows t h a t t h e g r i n d i n g a b i l i t y or s h a r p n e s s of a CIB-F wheel is recovered when On-machine wire ED d r e s s i n g i s a p p l i e d t o t h e wheel whose g r i n d i n g a b i l i t y h a s been reduced d u r i n g g r i n d i n g of s i l i c o n n i t r i d e ceramics. 4.
Wheel surface a f t e r ED dressing. Fig.7 ~d,30,#100/120,conc.100, vd=1OOm/min,fd=IOmm/min,U-IOOV,ti-t,=1U~~
* CIB-F
L
wheel(ds30,#170/200,conc.lM))
* v,=136m/min,fW=20mm/min, h=O.Smm, b = 5 m I 2 200 - * S i l i c o n n i t r i d e ceramics(Hv1700)
-
c
150-
r4 o)
0
[Wire ED d r e s s i n g ] vd=136m/min,fd=10mm/min
U=100V,I=80A, ti=to~ll-lS
0 '+l
01
a
r
A
2
a
ON-MACHINE ED TRUEISG W I T H A BLOCK ELECTRODE
4.1 S e l e c t i o n of s u i t a b l e e l e c t r o d e Advantages of u s i n g a wire e l e c t r o d e i n t h e On-machine ED t r u e i n g a r e ( 1 ) h i g h accuracy owing t o n e g l i g i b l y s m a l l amount of e t e c t r o d e wear and (2)easy g e n e r a t i o n of p r o f i l e d wheels by u t i l i z i n g NC movement of a g r i n d i n g machine. On t h e o t h e r hand, t h e wire ED t r u e i n g method h a s some d i s a d v a n t a g e s such a s (1)low trueing e f f i c i e n c y which comes from t h e r e s t r i c t i o n of average c u r r e n t and t h e e x i s t e n c e of non-discharging zone a t t h e time of p r o f i l i n g j u s t l i k e " a i r c u t " observed i n metal c u t t i n g process and ( 2 ) t h e l i m i t of generated p r o f i l e s which comes from t h e r e s t r i c t i o n of wire d i a m e t e r . Though a block e l e c t r o d e has a problem of e l e c t r o d e wear, i t h a s s m e advantages s u p e r i o r t o a w i r e e l e c t r o d e i n t h e p o i n t s of ED e f f i c i e n c y and p r o f i l e g e n e r a t i o n . So On-machine ED t r u e i n g w i t h a block e l e c t r o d e would be p r o f i t a b l e i f appropriate countermeasures a r e devised.
100-
t
.rl
2
50-
4
[Dressed with GC-stickstone]
6
! i
w
I
Fig.8
I
I
I 1
Comparison of dressing e f f e c t between wire ED dressing and conventional dressing methods.
* *
I
*
-z
\
CIB-F wheel(ds30mm. #170/200,conc. 100) vS=l36m/min ,fw=20m/min, h=5.5mm, b-5mm S i l i c o n n i t r i d e ceramics(Hvl700)
c
r4
[Dressed w i t h stickstone]
[ED d r e s s i n g , condition-I]
[ED d r e s s i n g , condition-II]
U
0
500
1000
Grinding distance (mm) Fig.9
Recovery of grinding ability by wire ED dressing.
NC table
Fig.10
Schematic illustration of On-machine shaping of block electrode and On-machine ED trueing by the formed electrode.
117
Fig.12 Profiled CIB-F wheel by On-machine ED trueing with a formed graphite electrode.
Fig.11
On-machfne shaping of graphite electrode the NC table of machining center.
material on
4 . 2 Proposal of On-machine shaping of formed electrodes f o r precise trueing It i s apprehended that On-machine ED trueing with a block electrode may cause an Inaccuracy because of electrode wear and mounting error onto a trueina unit. The electrode wear can be restrai&d considerably by oprimizing ED conditions, but this is not enough for highly precise trueing. AS a CountermeasUre for the above apprehension, "On-machine electrode shaping method" illustrated in Fin.14 is proposed. I,, this new method, in case of trueing of a profiled grinding wheel , a cylindrical electrode material mounted o n a rotating shaft of a trueing unit is machined at firsi with a cuttlg tool
Thus a mounted on the spindle of an ?.lC (the spindle is clamped). desired profile is shaped on the electrode material by utilizjng
NC movement of the MC.
4 . 3 Experiments of ED trueing with On-machine shaping of electrode Fin.ll shows the state of shaping a graphite electrode shows the state of on-machine profile material on an MC. F&J+l It generation on a CIB-F wheel. with the above formed electrode. will be understood that sharply recessed grooves, which are difficult to be formed with a wire electrode, are also easily formed hy this method 4 . 4 Grinding of ceramics with profiled wheels When the profiled wheels made by means of above ED trueing method are combined with NC movement of a grinding machine(GC/MC), considerably complicated parts can be formed, shows a silicon nitride ceramic sample formed with three different profiled wheels.
5 . CONCLUSIONS Two types of "On-machine trueing/dressing methods" utilizing EDM were proposed for t h e sake of efficient and accurate grinding ot ceramics with metal bond diamond grinding wheels. The first method utilizes vire EDM with negligibly small amount of electrode wear and makes precise trueing anti dressing of p r o f i l e d wheels possible by the help of WC movement of a grinding machine The second method utilizes die sinking EDM with a block electrode which provides high EDM efficiency. The problem of electrode wear is solved by "On-mach~neelectrode shaping method" in which shaping o r form correction of the electrode is carried out by NC machining operation of the grinding machine. It has been confirmed that the "On-machine ED trueing/dressing method" shows sufficient effects o n l y with a simple electrode unit and with a simple electrical power source. Authors expect that the "On-machine ED trueingldressing method'' will contribute t o the achievement of highly efficient and precise grinding of ceramics and hard metals.
.
Next a grinding wheel is selected instead of the cutting tool by an automatic tool changer and ED trueing of this wheel is performed with the formed electrode. If the amount o f electrode wear exceeds an allowable tolerance, the electrode can be machined again with the above cutting tool for correction. By adopting this method, the reduction of trueing accuracy due to mounting error and electrode wear is eliminated, agid that. it becomes unnecessary to prepare a formed electrode beforehand. The use of a rotating electrode also contributes to the improvement of the trueing accuracy. In case of a straight wheel with a flat surface, a fixed ACKXOWLEDGEMENTS electrode is shaped to have a flat surface with a rotatjng end The authors wish to thank Prof. T.Masuzawa(lnst. of Ind. mill cutter. N.Mohri (Toyota Technological Sci., Univ. of Tokyo) and Prof. Inst.) for their valuable advic.es. The authors also express their thanks to H/S Sintokogio Ltd., Yakino Yilling Machine Co.Ltd., Sodick Co.Ltd.. Johnson Co.Ltd.,Sumitomo Wiring System Co.Ltd., Lor preparing the experimental Maruto Instruments Co.Ltd. rquipments, and t o Mr.T. Yanase(Toyama Pref. College of Technology) and Mr.S.Asano(Stanley Electric Co.Ltd.) for conducting experiments.
Fig.13
Silicon nitride ceramic sample ground with CIB-F wheels formed by On-machine
118
ED trueing method.
REFERENCES 1. Suzuki,K. Uematsu,T. Nakagawa,T.,1985, Grinding of ceramics on machining center(1st report), Preprint of Spring Conf. of .JSPE,809-812(in Japanese) 2. Uematsu,T .Suzuki,K. Nakagawa,T.,1985, Grinding of ceramics on machining center(3rd report), Preprint of Autumn Conf. of JSPE,265-'268(in Japanese) 3 . Nakagawa,T. Suzuki,K. Uematsu.T., 1985, Three dimensional creep feed grinding of ceramics by machining center, Proc. of Winter Annual Meeting of ASME, Vo1.17, 1-8 4 . Nakagawa,T. Suzuki,K. Uematsu,T., 1986, Highly efficient grinding of ceramics and hard metals on grinding center, Annals of the CIRP V01.35/1, 205-210 5 . Uematsu,T. Suzuki,K. Narita,T. Nakagawa,T., 1987, Grinding of ceramjcs on machining center(9th report), Preprint of Spring Conf. of JSPE(in Japanese) 6 . Suzuki,K. >iohri,X. Uematsu,T. Nakagawa,T., 1985, ED trueing method with t v i n electrodes, Preprint of Autumn Conf. of .ISPE, 575-578(in Japanese) 7. Suzuki,K. Uematsu,T. et al, 1986,Grinding of ceramics on of JSPE, machining center(6th report), Preprint of AuLumn Conf. 657-660( in Japanese) 8. Suzuki,K. Uematsu,T. ei al, 1987, Grinding of ceramics on machining center(7th report), Preprint of Spring Conf. o f .JSPE(in Japanese)