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A Fundamental Study on a Horizontal EDM
A Fundamental Study on a Horizontal EDM M.Kunieda; Toyko University of Agriculture and Technology; T. Masuzawa (2); Institute of Industrial Science, University of Tokyo Received on January 14,1988
ABSTRACT A HoriruriLal EDM is proposed f o r a more productive and accurate technique than a conventional vertical EDM. Experimental and analytical investigations proved the following characteristics of the horizontal EDN. Eroded particles can easily flow out of the working gap aa a result of buoyancy of bubbles. This effect is further significant when both the electrode and workpiece are rotated synchronously. The 'rotary HEM' is most effective at an optimal number of revolutions. Moreover, the rotation assures better machining accuracy. In order to demonstrate the existence of the optimal number, theoretical investigations on the particle movement were carried out. KEY WORDS ED?I Sinking, Horizontal EDM, Vertical EDM, Rotary HEDN, Flushing, Bubble, Contamination, Eroded Particles, Working Gap
1. Introduction E M sinking requires a smooth outflow of eroded particles and decomposed dielectric from the working gap. Since these conductive particles influence the breakdown strength [ l ] and their increasing concentration causes reduced stock removal rate and disastrous arcing. One of the conventional methods to avoid gap contamination is pressure flushing or suction flushing. However, it is not always possible to make holes for flushing in electrodes, and even if possible, a uniform flow rate of the dielectric is difficult to The periodical lifting of the obtain over a large working area. electrode is another conventional method but too often lifting lowers the removal rate and the stagnation points at the bottom gap of the electrode cannot be avoided where arcing is apt to occur. A self-flushing system which is based on a pumping effect by a special movement of the electrode was proposed by Masuzawa et al. [2] Kremer et a1.[3] reported superior productivity of ultrasonics imposed EDM where ultrasonic vibration helps particles flow out of the gap smoothly. In this paper, a much simpler method is proposed. The direction of the main axis of the EDM process is simply changed from the conventional vertical to horizontal. It is described that not only the stability of machining but also the accuracy of the final workpiece can be improved by adding a synchronous revolution of both the electrode and workpiece.
On the other hand, in the case of a Horizontal EDM (HEDY) particles are expected to rise along the frontal gap, and the clear dielectric is sucked from the lower side as shown in Fig.l(b). This means there are no stagnation points along the machining gap. With a high discharge energy, particles might be deposited in the lower lateral gap because the eroded particles are comparatively large and heavy. However, since they do not need to climb up the lateral wall as in a VEDM. they can easily drop out into a dielectric bath. Therefore, it seems that there are intrinsic differences in the gap appearance between the Vertical Ehi and Horizontal EDM. Incidentally, in a HEDM, the concentration of particles may have a tendency to become large in the upper portion of the working gap causing arcing and short circuiting there. The gap distance also increases in this region and this results in a lower However, accuracy of the final workpiece than in a Vertical EDM. these problems of the H E M can be solved by designing special equipment which can revolve both the elecLrode and workpiece around the same rotation axis at equal speed and direction because turning of the vector of gravity around the axis of the electrode alters the outflow direction of the particles and averages the contamination. The following are the results of the experiments and a discussion on the HEDM method.
2 . Cenernl View of the Horizontal EDM
3 . Experiiiiental Equipment
Since the suspended particles in the working gap are conductors, a too large concentration is harmful to the spark initiation. However, it is known that under Some appropriate conditions normal discharges can continue during the early stages of machinine. without soecial flushinns. This indicates that the -. rapid expansion of a bubble seems to have the function of blowing off the particles. However, it is the macroscopic behavior of bubbles as a group to which this paper especially pays attention since the bubbles usually occupy a large portion of the working area of the gap. in the case of a conventional vertical As shown in Fig.l(a), EDM (VEDM). the buoyancy of bubbles causes a rising flow of the dielectric fluid with which the particles flow out of the gap. However, the direction is opposite to the entrance of the clear dielectric, and heavy particles are likely to accumulate on the bottom. Moreover, since stagnation points are inevitable in the bottom area, nuclei of arcing are thought to grow there.
Figure 2 shows the equipment which was specially made to investigate the Characteristics of a HEDM. The equipment is of a 3-axis NC machine ED22, Installed on the work Atable schematic illustration of the(maklno equipment is Generator GPC30P). shown in Fig.3. The dieset to which the electrode and workpiece
.
I
I .
Workpiece (a) Vertical EDM Fig.1
(b) Horizontal EDM
Comparison of the gap appearance between a VEDM and HEDM
Annals of the ClRP Vol. 37/1/1988
Fig.2
Experimental equipment for a HEhi fixed on the work table of the ED machine
187
when t h e l i f t i n g d i s t a n c e w a s 0.14mm. t h e eroded d e p t h of t h e HEDM exceeded 25mm. w h i l e t h e l i m i t of eroded d e p t h was 6mn f o r t h e VEDM These phenomena may be e x p l a i n e d as f o l l o w s . I n t h e HEDM immediately a f t e r t h e e l e c t r o d e jumps, t h e b u b b l e s go up q u i c k l y followed by t h e r i s i n g flow of t h e suspended p a r t i c l e s . However, t h e e f f e c t of t h e l i f t i n g is u n s a t i s f a c t o r y i n t h e VEDM because t h e l i f t i n g d o e s n o t e l i m i n a t e t h e s t a g n a t i o n p o i n t of t h e VEDM. Considering t h e f a c t t h a t t h e spark erosion is i n t e r r u p t e d during l i f t i n g , t h e H E M is advantageous f o r improving t h e s t o c k removal r a t e s i n c e t h e l i f t i n g d i s t a n c e and f r e q u e n c y r e q u i r e d t o a v o i d a r c i n g can be s m a l l e r t h a n i n t h e VEDFl.
.
Electrode holder of ED machine
a.
4 E E
Coil spring
Work t a b l e
c -n
3
0 0 0
Fig.3
m
Schematic i l l u s t r a t i o n of t h e equipment
: 2 W
X i n d i c a t e s a s t o p of machining caused by
1
a r e f i x e d i s s u p p o r t e d by a s h a f t , and t h e s h a f t is r o t a t e d by a dc motor ahove t h e f l u i d s u r f a c e u s i n g a t i m i n g b e l t and p u l l e y s . By u s i n g bushes made of i n s u l a t o r , p l a t e 1 on which t h e e l e c t r o d e i s a t t a c h e d is e l e c t r i c a l l y i n s u l a t e d from p l a t e 2 on which t h e workpiece is a t t a c h e d . Moreover, p l a t e 1 c a n s l i d e smoothly along the guideposts i n t h e horizontal direction. The compression s p r i n g s between t h e two p l a t e s pushes p l a t e 1 a g a i n s t t h e s t o p p e r which is f i x e d t o t h e e l e c t r o d e h o l d e r of t h e ED machine. An e l e c t r i c a l c u r r e n t i s s u p p l i e d t h r o u g h t h i s s t o p p e r r e g a r d l e s s of t h e r e v o l u t i o n of t h e d i e s e t . The s p a r k gap is c o n t r o l l e d by a servo-mechanism which d r i v e s t h e work t a b l e horizontally. With t h i s equipment t h e e f f i c i e n c v of a s i m p l e HEDY without r e v o l u t i o n and a r o t a r y HEDM can be i n v e s t i g a t e d . It is noted t h a t t h e s h a p e of t h e e l e c t r o d e i s a r b i t r a r y s i n c e both t h e e l e c t r o d e and workpiece a r e r o t a t e d s i m u l t a n e o u s l y .
0
30
60
Fig.5
120
90
Spark erosion time
min
I n f l u e n c e of t h e number of r e v o l u t i o n s on t h e machining a b i l i t y of HEM. ie:30A, te:300us, t :4Ous, Copper electrode:d130rnn? (t), Workpiece:S45C(-).
4. Stock Removal R a t e
4.2 Optimal number of r e v o l u t i o n s
4 . 1 E f f i c i e n c y without. r e v o l u t i o n
In t h e n e x t s t e p t h e e f f e c t of r e v o l u t i o n i n a r o t a r y HEM was i n v e s t i g a t e d . F i g u r e 5 shows t h e i n f l u e n c e of t h e number of r e v o l u t i o n s on t h e e f f i c i e n c y of t h e s p a r k e r o s i o n . The d i a m e t e r of t h e c y l i n d r i c a l copper e l e c t r o d e was 30mm, and t h e workpiece The c u r v e of Orpm i n d i c a t e s a ’ s i m p l e was carbon steel (S45C). HEDM’ (HEDM w i t h o u t r e v o l u t i o n ) . As t h e number of r e v o l u t i o n s i n c r e a s e s , t h e l i m i t of t h e eroded d e p t h a l s o i n c r e a s e s . In the r e g i o n of 1 t o 5rpm t h e l i m i t d e p t h t a k e s t h e maximum number. A further i n c r e a s e makes t h e s i t u a t i o n worse. This r e s u l t I n t h e case i n d i c a t e s t h e r e is an o p t i m a l number of r e v o l u t i o n s . of t h e VEDN, i t was i m p o s s i b l e t o e r o d e t h e workpiece w i t h o u t l i f t i n g of t h e e l e c t r o d e under t h e same g e n e r a t o r c o n d i t i o n s .
FirsL, t h e e f f e c t s of a HEDM w i t h o u t r e v o l u t i o n were examined. The e x p e r i m e n t a l r e s u l t s i n Fig.4 show t h e d i f f e r e n c e of t h e s t o c k rcmoval r a t e between a VEDM and HEDM. A cylindrical copper e l e c t r o d e of 20mm d i a m e t e r was used t o e r o d e carbon s t e e l (S45C). When l i f t i n g of t h e e l e c t r o d e was l.Omm, t h e r e v a s no d i f f e r e n c e i n t h e removal r a t e . The e f f e c t s of t h e l i f t i n g seemed t o he so l a r g e t h a t t h e working gap w a s k e p t c o m p a r a t i v e l y c l e a r for b o t h t h e VIEBY and HEDM. On t h e c o n t r a r y , though i t is not i n d i c a t e d i n t h e f i g u r e , t h e o c c u r r e n c e of d i s a s t r o u s a r c i n g i n t e r r u p t e d t h e c o n t i n u a t i o n of machinmg a t t h e eroded depth Of only 2.5mm f o r both c a s e s when no l i f t i n g was g i v e n . However,
25
4 20
-
O.lrpm
E E
c Y
E
15
n
-
c IJ
0
FL
m
W W
e
W
0
.L
3
10
“
L i f t i n g of electrode
-
W
5-
e
0 VEDM 0 VEDV
0.14mm
Q 0
1.0 mm
A
W
0.14mm
HEBY A HEDM
2
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k
1 . 0 mm
/
Here a r c i n g i n t e r r u p t e d machining I
I
I
I
50
100
153
200
u
~~
0 Fig.4
le:30A, te :300p, to :40ps , E1ectrode:Copper 420mm(+), Workpiece:S45C(-).
188
250
Spark erosion t i m e min Comparison of t h e s t o c k removal r a t e between a HEDM and VEDM.
X i n d i c a t e s a s t o p of machining caused by arcing
120
181
S p a r k e r o s i o n time Fig.6
min
Change of t h e o p t i m a l number of r e v o l u t i o n s when t h e d i s c h a r g e c u r r e n t d e c r e a s e s . ie:24A.
4.3 Influence of parameters on the optimal number of revolutions Some tests were carried out to find the influence of working conditions on the variaLion of the optimal number of revolutions. Fjgure 6 shows the results obtained under Lhe same conditions as Fig.5 except that the peak current was decreased from 30A to 24A. The optimal number of revolutions is found to decrease to the vicinity of 0.1 to 1.Orpm. The next test was performed under the same conditions a s Fig.5 except that the diameter of the electrode was decreased from 30mm to 20mm. The results are shown in Fig.7. It is evident thaL the optimal number of revolutions increased to 7.5rpm. The next section deals with the reason for the existence of the optimal value and its variation.
particle just on the point A , it keeps standing still against the absolute space but not against the rotating surfaces of the electrode and workpiece. 5 . 2 Influence of the number of revolutions on the contamination of
the working gap If the contour of the elecLrode is overlapped on Fig.9, the points from which particles flow out of the frontal gap are identified. For simp1icit.y. the electrode is assumed to be is cylindrical and the normalized coordinate system X I - Y ' considered by dividing all the values of coordinates by the radius R of the electrode.
Y
Workpiece
e
l
Electrode
\
X indicates a stop of machining caused by arcing
W
~~
0
30
15
Spark erosion time Flg.7
5.
60
45
Change of the optimal number of revolutions when the electrode diameter decreases. Copper electrode:@Omm
Qualitative Investigation on Xumber of Revolutions
I
min
the Existence of
Fig.8
W
Schematic illustration of a rotary H E M
the Optimal
5.1 Simulation of particle movement in the working gap A schematic illustration of a rotary HEDH is shown in Fig.8. Both the electrode and workpiece rotate around the same rotation axis in the same direction and at an equal number of revolutions. In order to analyze the particle movement in the working gap. a coordinate system 9-Y is defined as shown in Fig.8. This coordinate system is fixed against the absolute space and its origin 0 is placed on the rotation axis. Since the gap distance is very small and the number of revolutions w is the same for both the electrode and workpiece, the viscosity of the dielectric fluid tends to revolve the particles around the rotation axis at the same number of Therefqrs. a s shown in Fig.9, the particle+Pi has revolutions w . a peripheral veQcity wxr around the origin 0. Here, ri is a position vector OPi. In the next step, it may be assumed that if there is no revolution, the dielectric fluid will go straight up in the +positive direction of the Y-axis, and that the velocity vector v is constant and equal all over the frontal gap. Therefore, the particle PI also hss the velocity vector ?. Consequently, the velocity vector VI of the particle PI is obtained by + ++ + V,=wxr 1 tv. (1)
Y
\
\
The value of w is so small that the centrifugal f9rce p y not+ be taken into account. If the absolute values of r , w and v are defined as r , w and :r respectively, Eq.(l) becomes
/ I
/
Since ?l=(dx/dt,dy/dt),
By solring Eq.(3).
I
Irw
ifl=rw(-sine, cose)+(o. v) =( -wy , v+wx).
dx/dt= -wy.
\
/
(3)
dy/dt= Y+WX.
/
I I
the path of the particle P1 is expressed as
(x+v/w)2ty2=(r
cos8+v/w)2+(r
sine)'.
(4)
Equation(4) indicates that the particle moves along a circle of ehich the cenLcr is A(-v/w,O) and the radius i.s PIA. Since v/w is constant the center of revolution f o r another particle P2 should be also point A . Therefore, all particles move on concentric circles around the center, A . Noreover, it is found As for the that every particle has an equal angular velocitv Y .
Fig.9
Velocity vector of particles on the frontal gap surface.
189
Y'
A s shown i n F i g . 1 0 , t h e v a l u e -v/Rw (X' c o o r d i n a t e o f t h e p o i n t A ) d e t e r m i n e s t h e movement p a t h o f p a r t i c l e s and t h e d i s t r i b u t i o n o f t h e c o n c e n t r a t i o n o f p a r t i c l e s . The c o n c e n t r a t i o n of p a r t i c l e s a t a c e r t a i n p o i n t was c a l c u l a t e d by i n t e g r a t i n g t h e f o r m a t i o n of a p a r t i c l e p e r u n i t time p e r u n i t a r e a w i t h time. The i n t e g r a t i o n was c a r r i e d o u t a l o n g t h e p a t h w i t h i n t h e f r o n t a l surface. Here, t h e f o r m a t i o n rate p e r u n i t area w a s assumed t o be u n i f o r m on t h e e n t i r e s u r f a c e . F i r s t of a l l , t h e e f f e c t i v e n e s s o f r e v o l u t i o n is d i s c u s s e d u s i n g P i g s . l O ( a ) and ( b ) . F i g u r c lO(o) shows t h c s i t u a t i o n v = O , t h a t is, t h e casc o f a s i m p l e HEDM. Vithout revolution, t h e u p p e r r e g i o n s o f t h e f r o n t a l g a p s u r f a c e s are a l w a y s e x p o s e d t o a high l e v e l of contamination. T h e r e f o r e , i t seems a r c i n g is l i a b l e t o occur i n t h e upper p o r t i o n . I n f a c t , sometimes, black marks o f c a r b o n were o b s e r v e d on t h e u p p c r p a r t of t h e e r o d e d b o t t o m s u r f a c e o f t h e w o r k p i e c e i n t h e s i m p l e HEDH, where a r c i n g must h a v e o c c u r r e d . On t h e c o n t r a r y , i f t h e r e is a r e v o l u t i o n (Fig.lO(b)), a c e r t a i n p o i n t Q on t h e e r o d e d s u r f a c e r e v o l v e s around t h e o r i g i n . T h e r e f o r e , i t p a s s e s t h r o u g h n o t o n l y t h e polluted region but a l s o t h e clean region, a l t e r n a t e l y . I n tt1i.s w a y , t h e a v e r a g e d c o n t a m i n a t i o n l e v e l t o v h i c h t h e p o i n t Q is exposed becomes c o m p a r a t i v e l y s m a l l , and t h e s p a r k e r o s i o n c a n b e very s t a b l e . N e x t , t h e r e a s o n f o r t h e e x i s t e n c e o f t h e o p t i m a l number o f r e v o l u t i o n s is d i s c u s s e d . For s i m p l i c i t y . boLh v and R are assumed t o b e k e p t c o n s t a n t . When w is i n c r e a s e d t o make v/Rv less t h a n a u n i t ( F i g . l O ( c ) ) , p o i n t A is i n c l u d e d w i t h i n t h e c i r c l e of t h e w o r k i n g g a p . T h u s , t h e h a t c h e d a r e a is c r e a t e d i n t h e circle where p a r t i c l e s c o n t i n u e r e v o l v i n g i n d e f i n i t e l y a r o u n d t h e p o i n t A and n e v e r f l o w o u t o f t h e w o r k i n g g a p . Thus, i n t h i s a r e a t h e c o n c e n t r a t i o n i n c r e a s e s i n f i n i t e l y as time g o e s on. Of c o u r s e , t h e r e i s no s u c h s i t u a t i o n p r a c t i c a l l y s i n c e p a r t i c l e s m i g h t be e x h a u s t e d r a d i a l l y by s u c c e s s i v e e x p l o s i o n s . Even so, a t l e a s t i t i n d i c a t e s t h e b e g i n n i n g o f an i n s t a b i l i t v o f d i s c h a r g e . I f a f u r t h e r i n c r e a s e o f w makes v/Rw less t h a n 0.5, t h e r o t a t i o n c e n t e r o f t h e e l e c t r o d e a n d w o r k p i e c c is f o r c e d t o b e i n c l u d e d w i t h i n t h e h a t c h e d area. In this situation, the v i c i n i t y o f t h e r o t a t i o n c e n t e r on t h e s u r f a c e s o f b o t h t h e electrode and workpiece c a n n o t e s c a p e from an infinite c o n t a m i n a t i o n i n s p i t e of t h e r e v o l u t i o n s and d i s a s t r o u s a r c i n g becomes i n e v i t a b l e . T h i s d i s c u s s i o n i s s u p p o r t e d by t h e f a c t t h a t b l a c k m a r k s o f a r c i n g were o b s e r v e d a t t h e c e n t e r of t h e e r o d e d b o t t o m when t h e number of r e v o l u t i o n s was more t h a n t h e optimal value. I n t h i s way. a q u a l i t a t i v e e x p l a n a t i o n is p o s s i b l e f o r t h e e x i s t e n c e of t h e o p t i m a l number o f r e v o l u t i o n s . The above discussion is u s e f u l f o r explaining the experimental r e s u l t s previously mentioned i n s e c t i o n 4.3. The i n f l u e n c e o f t h e peak c u r r e n t is e x p l a i n e d a s f o l l o w s . Since a low peak c u r r e n t c a u s e s a d e c r e a s e of g a p d i s t a n c e and f o r m a t i o n rate o t bubbles, t h e r i s i n g v e l o c i t y v a l s o d e c r e a s e s . I n ordci t o k e e p v/Rw a t t h e o p t i m a l v a l u e , w must d e c r e a s e . In t h e same way, t h e e f f e c t o f t h e d i a m e t e r c a n be e x p l a i n e d . When t h e d i a m e t e r o f t h e e l e c t r o d e i s small, v/Rw must b e k e p t c o n s t a n t by i n c r e a s i n g W. C o n s e q u e n t l y , a smaller w i s s u i t a b l e f o r a small c u r r e n t and a l a r g e r w is d e s i r a b l e f o r a t h i n e l e c t r o d e .
X'
A
X'
a(-v/Rw,O)
Y'
6. Accuracy
- X'
The r o u n d n e s s o f t h e e r o d e d h o l e was measured when a 30mm d i a m e t e r c y l i n d r i c a l e l e c t r o d e was u s e d . The peak c u r r e n t was 20A. The l i f t i n g d i s t a n c e o f t h e e l e c t r o d e was 1.5mm. The r e s u l t s show t h a t t h e r o u n d n e s s is 80Pm f o r t h e VEDM, 120Pm f o r t h e s i m p l e HEDM and 60um f o r t h e r o t a r y HEDN. Since the l a t e r a l g a p d i s t a n c e o f t h e s i m p l e HEDM i s l a r g e i n t h e u p p e r p o r t i o 7 , t h e a c c u r a c y measurement o f t h e s i m p l e HEM shows t h e w o r s t r o u n d n e s s . The e f f e c t i v e n e s s of r e v o l u t i o n i s o b v i o u s from t h e s e r e s u l t s . 7. C o n c l u s i o n s The d i s c h a r g e c a n b e s t a b i l i z e d i n a H o r i z o n t a l EbY b e c a u s e t h e g a p c o n t a m i n a t i o n is small a s a r e s u l t of buoyancy of b u b b l e s which make a r i s i n g f l o w of d i e l e c t r i c f l u i d o v e r a l l t h e working gap. However, i n a s i m p l e HEDH i t was found t h a t d i s a s t r o u s a r c i n g is a p t t o o c c u r i n t h e u p p e r p o r t i o n o f t h e f r o n t a l g a p , and t h a t t h e a c c u r a c y of t h e f i n a l w o r k p i e c e i s n o t v e r y good. T h e r e f o r e , t h e r o t a r y HEDM was p r o p o s e d and t e s t e d . The r e s u l t s show t h a t t h i s method i m p r o v e s e r o s i o n e f f i c i e n c y and a c c u r a c y .
4
Acknowledgements The a u t h o r s w i s h t o e x p r e s s t h e i r g r a t i t u d e t o Messrs. H. Chosho and N. Nebashi o f Tokyo U n i v e r s i t y of A g r i c u l t u r e & T e c h n o l o g y , and Mr. Y. Aoyama o f Tokyo Denki U n i v e r s i t y , f o r t h e i r e x t e n s i v e c o n t r i b u t i o n s toward t h i s s t u d y .
X'
References 1) Bommeli B., F r e i C. and R a t a j s k i A . . 1 9 7 9 , On t h e I n f l u e n c e of M e c h a n i c a l P e r t u r b a t i o n on t h e Breakdown of a Lj.quid D i r l e r L r i c , J . of E l e c t r o s t a t i c s , 7 , 123-144. 2 ) Masuzawa T. and Heuvelman J.. 1983, A S e l f - F l u s h i n g Method w i t h S p a r k - E r o s i o n Machining, A n n a l s o f t h e CIRP, Vol. 32/1/1983, 109-111. 3 ) Kremer D., B a z i n e G . , Moisan A . , B e s s a g u e t L . , Astier A . and Thanh N . K . . 1983, U l t r a s o n i c H a c h i n i n g Improves EDM Technology, P r o c . o f t h e ISEM 7 . 67-76.
190
(d) 0 < Fig.10
& 5 0.5
I
Changes of g a p c o n t a m i n a t i o n a s p e r t h e v a l u e of v/Rw.
ABSTRACT A HoriruriLal EDM is proposed f o r a more productive and accurate technique than a conventional vertical EDM. Experimental and analytical investigations proved the following characteristics of the horizontal EDN. Eroded particles can easily flow out of the working gap aa a result of buoyancy of bubbles. This effect is further significant when both the electrode and workpiece are rotated synchronously. The 'rotary HEM' is most effective at an optimal number of revolutions. Moreover, the rotation assures better machining accuracy. In order to demonstrate the existence of the optimal number, theoretical investigations on the particle movement were carried out. KEY WORDS ED?I Sinking, Horizontal EDM, Vertical EDM, Rotary HEDN, Flushing, Bubble, Contamination, Eroded Particles, Working Gap
1. Introduction E M sinking requires a smooth outflow of eroded particles and decomposed dielectric from the working gap. Since these conductive particles influence the breakdown strength [ l ] and their increasing concentration causes reduced stock removal rate and disastrous arcing. One of the conventional methods to avoid gap contamination is pressure flushing or suction flushing. However, it is not always possible to make holes for flushing in electrodes, and even if possible, a uniform flow rate of the dielectric is difficult to The periodical lifting of the obtain over a large working area. electrode is another conventional method but too often lifting lowers the removal rate and the stagnation points at the bottom gap of the electrode cannot be avoided where arcing is apt to occur. A self-flushing system which is based on a pumping effect by a special movement of the electrode was proposed by Masuzawa et al. [2] Kremer et a1.[3] reported superior productivity of ultrasonics imposed EDM where ultrasonic vibration helps particles flow out of the gap smoothly. In this paper, a much simpler method is proposed. The direction of the main axis of the EDM process is simply changed from the conventional vertical to horizontal. It is described that not only the stability of machining but also the accuracy of the final workpiece can be improved by adding a synchronous revolution of both the electrode and workpiece.
On the other hand, in the case of a Horizontal EDM (HEDY) particles are expected to rise along the frontal gap, and the clear dielectric is sucked from the lower side as shown in Fig.l(b). This means there are no stagnation points along the machining gap. With a high discharge energy, particles might be deposited in the lower lateral gap because the eroded particles are comparatively large and heavy. However, since they do not need to climb up the lateral wall as in a VEDM. they can easily drop out into a dielectric bath. Therefore, it seems that there are intrinsic differences in the gap appearance between the Vertical Ehi and Horizontal EDM. Incidentally, in a HEDM, the concentration of particles may have a tendency to become large in the upper portion of the working gap causing arcing and short circuiting there. The gap distance also increases in this region and this results in a lower However, accuracy of the final workpiece than in a Vertical EDM. these problems of the H E M can be solved by designing special equipment which can revolve both the elecLrode and workpiece around the same rotation axis at equal speed and direction because turning of the vector of gravity around the axis of the electrode alters the outflow direction of the particles and averages the contamination. The following are the results of the experiments and a discussion on the HEDM method.
2 . Cenernl View of the Horizontal EDM
3 . Experiiiiental Equipment
Since the suspended particles in the working gap are conductors, a too large concentration is harmful to the spark initiation. However, it is known that under Some appropriate conditions normal discharges can continue during the early stages of machinine. without soecial flushinns. This indicates that the -. rapid expansion of a bubble seems to have the function of blowing off the particles. However, it is the macroscopic behavior of bubbles as a group to which this paper especially pays attention since the bubbles usually occupy a large portion of the working area of the gap. in the case of a conventional vertical As shown in Fig.l(a), EDM (VEDM). the buoyancy of bubbles causes a rising flow of the dielectric fluid with which the particles flow out of the gap. However, the direction is opposite to the entrance of the clear dielectric, and heavy particles are likely to accumulate on the bottom. Moreover, since stagnation points are inevitable in the bottom area, nuclei of arcing are thought to grow there.
Figure 2 shows the equipment which was specially made to investigate the Characteristics of a HEDM. The equipment is of a 3-axis NC machine ED22, Installed on the work Atable schematic illustration of the(maklno equipment is Generator GPC30P). shown in Fig.3. The dieset to which the electrode and workpiece
.
I
I .
Workpiece (a) Vertical EDM Fig.1
(b) Horizontal EDM
Comparison of the gap appearance between a VEDM and HEDM
Annals of the ClRP Vol. 37/1/1988
Fig.2
Experimental equipment for a HEhi fixed on the work table of the ED machine
187
when t h e l i f t i n g d i s t a n c e w a s 0.14mm. t h e eroded d e p t h of t h e HEDM exceeded 25mm. w h i l e t h e l i m i t of eroded d e p t h was 6mn f o r t h e VEDM These phenomena may be e x p l a i n e d as f o l l o w s . I n t h e HEDM immediately a f t e r t h e e l e c t r o d e jumps, t h e b u b b l e s go up q u i c k l y followed by t h e r i s i n g flow of t h e suspended p a r t i c l e s . However, t h e e f f e c t of t h e l i f t i n g is u n s a t i s f a c t o r y i n t h e VEDM because t h e l i f t i n g d o e s n o t e l i m i n a t e t h e s t a g n a t i o n p o i n t of t h e VEDM. Considering t h e f a c t t h a t t h e spark erosion is i n t e r r u p t e d during l i f t i n g , t h e H E M is advantageous f o r improving t h e s t o c k removal r a t e s i n c e t h e l i f t i n g d i s t a n c e and f r e q u e n c y r e q u i r e d t o a v o i d a r c i n g can be s m a l l e r t h a n i n t h e VEDFl.
.
Electrode holder of ED machine
a.
4 E E
Coil spring
Work t a b l e
c -n
3
0 0 0
Fig.3
m
Schematic i l l u s t r a t i o n of t h e equipment
: 2 W
X i n d i c a t e s a s t o p of machining caused by
1
a r e f i x e d i s s u p p o r t e d by a s h a f t , and t h e s h a f t is r o t a t e d by a dc motor ahove t h e f l u i d s u r f a c e u s i n g a t i m i n g b e l t and p u l l e y s . By u s i n g bushes made of i n s u l a t o r , p l a t e 1 on which t h e e l e c t r o d e i s a t t a c h e d is e l e c t r i c a l l y i n s u l a t e d from p l a t e 2 on which t h e workpiece is a t t a c h e d . Moreover, p l a t e 1 c a n s l i d e smoothly along the guideposts i n t h e horizontal direction. The compression s p r i n g s between t h e two p l a t e s pushes p l a t e 1 a g a i n s t t h e s t o p p e r which is f i x e d t o t h e e l e c t r o d e h o l d e r of t h e ED machine. An e l e c t r i c a l c u r r e n t i s s u p p l i e d t h r o u g h t h i s s t o p p e r r e g a r d l e s s of t h e r e v o l u t i o n of t h e d i e s e t . The s p a r k gap is c o n t r o l l e d by a servo-mechanism which d r i v e s t h e work t a b l e horizontally. With t h i s equipment t h e e f f i c i e n c v of a s i m p l e HEDY without r e v o l u t i o n and a r o t a r y HEDM can be i n v e s t i g a t e d . It is noted t h a t t h e s h a p e of t h e e l e c t r o d e i s a r b i t r a r y s i n c e both t h e e l e c t r o d e and workpiece a r e r o t a t e d s i m u l t a n e o u s l y .
0
30
60
Fig.5
120
90
Spark erosion time
min
I n f l u e n c e of t h e number of r e v o l u t i o n s on t h e machining a b i l i t y of HEM. ie:30A, te:300us, t :4Ous, Copper electrode:d130rnn? (t), Workpiece:S45C(-).
4. Stock Removal R a t e
4.2 Optimal number of r e v o l u t i o n s
4 . 1 E f f i c i e n c y without. r e v o l u t i o n
In t h e n e x t s t e p t h e e f f e c t of r e v o l u t i o n i n a r o t a r y HEM was i n v e s t i g a t e d . F i g u r e 5 shows t h e i n f l u e n c e of t h e number of r e v o l u t i o n s on t h e e f f i c i e n c y of t h e s p a r k e r o s i o n . The d i a m e t e r of t h e c y l i n d r i c a l copper e l e c t r o d e was 30mm, and t h e workpiece The c u r v e of Orpm i n d i c a t e s a ’ s i m p l e was carbon steel (S45C). HEDM’ (HEDM w i t h o u t r e v o l u t i o n ) . As t h e number of r e v o l u t i o n s i n c r e a s e s , t h e l i m i t of t h e eroded d e p t h a l s o i n c r e a s e s . In the r e g i o n of 1 t o 5rpm t h e l i m i t d e p t h t a k e s t h e maximum number. A further i n c r e a s e makes t h e s i t u a t i o n worse. This r e s u l t I n t h e case i n d i c a t e s t h e r e is an o p t i m a l number of r e v o l u t i o n s . of t h e VEDN, i t was i m p o s s i b l e t o e r o d e t h e workpiece w i t h o u t l i f t i n g of t h e e l e c t r o d e under t h e same g e n e r a t o r c o n d i t i o n s .
FirsL, t h e e f f e c t s of a HEDM w i t h o u t r e v o l u t i o n were examined. The e x p e r i m e n t a l r e s u l t s i n Fig.4 show t h e d i f f e r e n c e of t h e s t o c k rcmoval r a t e between a VEDM and HEDM. A cylindrical copper e l e c t r o d e of 20mm d i a m e t e r was used t o e r o d e carbon s t e e l (S45C). When l i f t i n g of t h e e l e c t r o d e was l.Omm, t h e r e v a s no d i f f e r e n c e i n t h e removal r a t e . The e f f e c t s of t h e l i f t i n g seemed t o he so l a r g e t h a t t h e working gap w a s k e p t c o m p a r a t i v e l y c l e a r for b o t h t h e VIEBY and HEDM. On t h e c o n t r a r y , though i t is not i n d i c a t e d i n t h e f i g u r e , t h e o c c u r r e n c e of d i s a s t r o u s a r c i n g i n t e r r u p t e d t h e c o n t i n u a t i o n of machinmg a t t h e eroded depth Of only 2.5mm f o r both c a s e s when no l i f t i n g was g i v e n . However,
25
4 20
-
O.lrpm
E E
c Y
E
15
n
-
c IJ
0
FL
m
W W
e
W
0
.L
3
10
“
L i f t i n g of electrode
-
W
5-
e
0 VEDM 0 VEDV
0.14mm
Q 0
1.0 mm
A
W
0.14mm
HEBY A HEDM
2
0
k
1 . 0 mm
/
Here a r c i n g i n t e r r u p t e d machining I
I
I
I
50
100
153
200
u
~~
0 Fig.4
le:30A, te :300p, to :40ps , E1ectrode:Copper 420mm(+), Workpiece:S45C(-).
188
250
Spark erosion t i m e min Comparison of t h e s t o c k removal r a t e between a HEDM and VEDM.
X i n d i c a t e s a s t o p of machining caused by arcing
120
181
S p a r k e r o s i o n time Fig.6
min
Change of t h e o p t i m a l number of r e v o l u t i o n s when t h e d i s c h a r g e c u r r e n t d e c r e a s e s . ie:24A.
4.3 Influence of parameters on the optimal number of revolutions Some tests were carried out to find the influence of working conditions on the variaLion of the optimal number of revolutions. Fjgure 6 shows the results obtained under Lhe same conditions as Fig.5 except that the peak current was decreased from 30A to 24A. The optimal number of revolutions is found to decrease to the vicinity of 0.1 to 1.Orpm. The next test was performed under the same conditions a s Fig.5 except that the diameter of the electrode was decreased from 30mm to 20mm. The results are shown in Fig.7. It is evident thaL the optimal number of revolutions increased to 7.5rpm. The next section deals with the reason for the existence of the optimal value and its variation.
particle just on the point A , it keeps standing still against the absolute space but not against the rotating surfaces of the electrode and workpiece. 5 . 2 Influence of the number of revolutions on the contamination of
the working gap If the contour of the elecLrode is overlapped on Fig.9, the points from which particles flow out of the frontal gap are identified. For simp1icit.y. the electrode is assumed to be is cylindrical and the normalized coordinate system X I - Y ' considered by dividing all the values of coordinates by the radius R of the electrode.
Y
Workpiece
e
l
Electrode
\
X indicates a stop of machining caused by arcing
W
~~
0
30
15
Spark erosion time Flg.7
5.
60
45
Change of the optimal number of revolutions when the electrode diameter decreases. Copper electrode:@Omm
Qualitative Investigation on Xumber of Revolutions
I
min
the Existence of
Fig.8
W
Schematic illustration of a rotary H E M
the Optimal
5.1 Simulation of particle movement in the working gap A schematic illustration of a rotary HEDH is shown in Fig.8. Both the electrode and workpiece rotate around the same rotation axis in the same direction and at an equal number of revolutions. In order to analyze the particle movement in the working gap. a coordinate system 9-Y is defined as shown in Fig.8. This coordinate system is fixed against the absolute space and its origin 0 is placed on the rotation axis. Since the gap distance is very small and the number of revolutions w is the same for both the electrode and workpiece, the viscosity of the dielectric fluid tends to revolve the particles around the rotation axis at the same number of Therefqrs. a s shown in Fig.9, the particle+Pi has revolutions w . a peripheral veQcity wxr around the origin 0. Here, ri is a position vector OPi. In the next step, it may be assumed that if there is no revolution, the dielectric fluid will go straight up in the +positive direction of the Y-axis, and that the velocity vector v is constant and equal all over the frontal gap. Therefore, the particle PI also hss the velocity vector ?. Consequently, the velocity vector VI of the particle PI is obtained by + ++ + V,=wxr 1 tv. (1)
Y
\
\
The value of w is so small that the centrifugal f9rce p y not+ be taken into account. If the absolute values of r , w and v are defined as r , w and :r respectively, Eq.(l) becomes
/ I
/
Since ?l=(dx/dt,dy/dt),
By solring Eq.(3).
I
Irw
ifl=rw(-sine, cose)+(o. v) =( -wy , v+wx).
dx/dt= -wy.
\
/
(3)
dy/dt= Y+WX.
/
I I
the path of the particle P1 is expressed as
(x+v/w)2ty2=(r
cos8+v/w)2+(r
sine)'.
(4)
Equation(4) indicates that the particle moves along a circle of ehich the cenLcr is A(-v/w,O) and the radius i.s PIA. Since v/w is constant the center of revolution f o r another particle P2 should be also point A . Therefore, all particles move on concentric circles around the center, A . Noreover, it is found As for the that every particle has an equal angular velocitv Y .
Fig.9
Velocity vector of particles on the frontal gap surface.
189
Y'
A s shown i n F i g . 1 0 , t h e v a l u e -v/Rw (X' c o o r d i n a t e o f t h e p o i n t A ) d e t e r m i n e s t h e movement p a t h o f p a r t i c l e s and t h e d i s t r i b u t i o n o f t h e c o n c e n t r a t i o n o f p a r t i c l e s . The c o n c e n t r a t i o n of p a r t i c l e s a t a c e r t a i n p o i n t was c a l c u l a t e d by i n t e g r a t i n g t h e f o r m a t i o n of a p a r t i c l e p e r u n i t time p e r u n i t a r e a w i t h time. The i n t e g r a t i o n was c a r r i e d o u t a l o n g t h e p a t h w i t h i n t h e f r o n t a l surface. Here, t h e f o r m a t i o n rate p e r u n i t area w a s assumed t o be u n i f o r m on t h e e n t i r e s u r f a c e . F i r s t of a l l , t h e e f f e c t i v e n e s s o f r e v o l u t i o n is d i s c u s s e d u s i n g P i g s . l O ( a ) and ( b ) . F i g u r c lO(o) shows t h c s i t u a t i o n v = O , t h a t is, t h e casc o f a s i m p l e HEDM. Vithout revolution, t h e u p p e r r e g i o n s o f t h e f r o n t a l g a p s u r f a c e s are a l w a y s e x p o s e d t o a high l e v e l of contamination. T h e r e f o r e , i t seems a r c i n g is l i a b l e t o occur i n t h e upper p o r t i o n . I n f a c t , sometimes, black marks o f c a r b o n were o b s e r v e d on t h e u p p c r p a r t of t h e e r o d e d b o t t o m s u r f a c e o f t h e w o r k p i e c e i n t h e s i m p l e HEDH, where a r c i n g must h a v e o c c u r r e d . On t h e c o n t r a r y , i f t h e r e is a r e v o l u t i o n (Fig.lO(b)), a c e r t a i n p o i n t Q on t h e e r o d e d s u r f a c e r e v o l v e s around t h e o r i g i n . T h e r e f o r e , i t p a s s e s t h r o u g h n o t o n l y t h e polluted region but a l s o t h e clean region, a l t e r n a t e l y . I n tt1i.s w a y , t h e a v e r a g e d c o n t a m i n a t i o n l e v e l t o v h i c h t h e p o i n t Q is exposed becomes c o m p a r a t i v e l y s m a l l , and t h e s p a r k e r o s i o n c a n b e very s t a b l e . N e x t , t h e r e a s o n f o r t h e e x i s t e n c e o f t h e o p t i m a l number o f r e v o l u t i o n s is d i s c u s s e d . For s i m p l i c i t y . boLh v and R are assumed t o b e k e p t c o n s t a n t . When w is i n c r e a s e d t o make v/Rv less t h a n a u n i t ( F i g . l O ( c ) ) , p o i n t A is i n c l u d e d w i t h i n t h e c i r c l e of t h e w o r k i n g g a p . T h u s , t h e h a t c h e d a r e a is c r e a t e d i n t h e circle where p a r t i c l e s c o n t i n u e r e v o l v i n g i n d e f i n i t e l y a r o u n d t h e p o i n t A and n e v e r f l o w o u t o f t h e w o r k i n g g a p . Thus, i n t h i s a r e a t h e c o n c e n t r a t i o n i n c r e a s e s i n f i n i t e l y as time g o e s on. Of c o u r s e , t h e r e i s no s u c h s i t u a t i o n p r a c t i c a l l y s i n c e p a r t i c l e s m i g h t be e x h a u s t e d r a d i a l l y by s u c c e s s i v e e x p l o s i o n s . Even so, a t l e a s t i t i n d i c a t e s t h e b e g i n n i n g o f an i n s t a b i l i t v o f d i s c h a r g e . I f a f u r t h e r i n c r e a s e o f w makes v/Rw less t h a n 0.5, t h e r o t a t i o n c e n t e r o f t h e e l e c t r o d e a n d w o r k p i e c c is f o r c e d t o b e i n c l u d e d w i t h i n t h e h a t c h e d area. In this situation, the v i c i n i t y o f t h e r o t a t i o n c e n t e r on t h e s u r f a c e s o f b o t h t h e electrode and workpiece c a n n o t e s c a p e from an infinite c o n t a m i n a t i o n i n s p i t e of t h e r e v o l u t i o n s and d i s a s t r o u s a r c i n g becomes i n e v i t a b l e . T h i s d i s c u s s i o n i s s u p p o r t e d by t h e f a c t t h a t b l a c k m a r k s o f a r c i n g were o b s e r v e d a t t h e c e n t e r of t h e e r o d e d b o t t o m when t h e number of r e v o l u t i o n s was more t h a n t h e optimal value. I n t h i s way. a q u a l i t a t i v e e x p l a n a t i o n is p o s s i b l e f o r t h e e x i s t e n c e of t h e o p t i m a l number o f r e v o l u t i o n s . The above discussion is u s e f u l f o r explaining the experimental r e s u l t s previously mentioned i n s e c t i o n 4.3. The i n f l u e n c e o f t h e peak c u r r e n t is e x p l a i n e d a s f o l l o w s . Since a low peak c u r r e n t c a u s e s a d e c r e a s e of g a p d i s t a n c e and f o r m a t i o n rate o t bubbles, t h e r i s i n g v e l o c i t y v a l s o d e c r e a s e s . I n ordci t o k e e p v/Rw a t t h e o p t i m a l v a l u e , w must d e c r e a s e . In t h e same way, t h e e f f e c t o f t h e d i a m e t e r c a n be e x p l a i n e d . When t h e d i a m e t e r o f t h e e l e c t r o d e i s small, v/Rw must b e k e p t c o n s t a n t by i n c r e a s i n g W. C o n s e q u e n t l y , a smaller w i s s u i t a b l e f o r a small c u r r e n t and a l a r g e r w is d e s i r a b l e f o r a t h i n e l e c t r o d e .
X'
A
X'
a(-v/Rw,O)
Y'
6. Accuracy
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The r o u n d n e s s o f t h e e r o d e d h o l e was measured when a 30mm d i a m e t e r c y l i n d r i c a l e l e c t r o d e was u s e d . The peak c u r r e n t was 20A. The l i f t i n g d i s t a n c e o f t h e e l e c t r o d e was 1.5mm. The r e s u l t s show t h a t t h e r o u n d n e s s is 80Pm f o r t h e VEDM, 120Pm f o r t h e s i m p l e HEDM and 60um f o r t h e r o t a r y HEDN. Since the l a t e r a l g a p d i s t a n c e o f t h e s i m p l e HEDM i s l a r g e i n t h e u p p e r p o r t i o 7 , t h e a c c u r a c y measurement o f t h e s i m p l e HEM shows t h e w o r s t r o u n d n e s s . The e f f e c t i v e n e s s of r e v o l u t i o n i s o b v i o u s from t h e s e r e s u l t s . 7. C o n c l u s i o n s The d i s c h a r g e c a n b e s t a b i l i z e d i n a H o r i z o n t a l EbY b e c a u s e t h e g a p c o n t a m i n a t i o n is small a s a r e s u l t of buoyancy of b u b b l e s which make a r i s i n g f l o w of d i e l e c t r i c f l u i d o v e r a l l t h e working gap. However, i n a s i m p l e HEDH i t was found t h a t d i s a s t r o u s a r c i n g is a p t t o o c c u r i n t h e u p p e r p o r t i o n o f t h e f r o n t a l g a p , and t h a t t h e a c c u r a c y of t h e f i n a l w o r k p i e c e i s n o t v e r y good. T h e r e f o r e , t h e r o t a r y HEDM was p r o p o s e d and t e s t e d . The r e s u l t s show t h a t t h i s method i m p r o v e s e r o s i o n e f f i c i e n c y and a c c u r a c y .
4
Acknowledgements The a u t h o r s w i s h t o e x p r e s s t h e i r g r a t i t u d e t o Messrs. H. Chosho and N. Nebashi o f Tokyo U n i v e r s i t y of A g r i c u l t u r e & T e c h n o l o g y , and Mr. Y. Aoyama o f Tokyo Denki U n i v e r s i t y , f o r t h e i r e x t e n s i v e c o n t r i b u t i o n s toward t h i s s t u d y .
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References 1) Bommeli B., F r e i C. and R a t a j s k i A . . 1 9 7 9 , On t h e I n f l u e n c e of M e c h a n i c a l P e r t u r b a t i o n on t h e Breakdown of a Lj.quid D i r l e r L r i c , J . of E l e c t r o s t a t i c s , 7 , 123-144. 2 ) Masuzawa T. and Heuvelman J.. 1983, A S e l f - F l u s h i n g Method w i t h S p a r k - E r o s i o n Machining, A n n a l s o f t h e CIRP, Vol. 32/1/1983, 109-111. 3 ) Kremer D., B a z i n e G . , Moisan A . , B e s s a g u e t L . , Astier A . and Thanh N . K . . 1983, U l t r a s o n i c H a c h i n i n g Improves EDM Technology, P r o c . o f t h e ISEM 7 . 67-76.
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(d) 0 < Fig.10
& 5 0.5
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Changes of g a p c o n t a m i n a t i o n a s p e r t h e v a l u e of v/Rw.