EDM dressing of fine grain super abrasive grinding wheel

Materials Processing Technology ELSEVIER

Journal

of Materials

Processing

Technology

62 (1996)

299-302

EDM Dressing of Fine Grain Super abrasive Grinding Wheel Department

Xiankui WANG, Baoge YING, Weigang LIU of Precision Instruments, Tsinghua University, Beijing 100084, China

Abstract: This paper deals with the EDM dressing of tine grain metal bonded diamond wheels. The researches focus on the truing effects of EDM and the micro-geomorphic analyses of the wheel surface after EDM dressing. The basic relations between the elements of EDM dressing and the dressing effects are discussed and the mechanism of EDM dressing is introduced. J+words:

Superabrasive

grinding

wheel, EDM dressing

1. Introduction Recently, metal bonded fine diamond wheels are applied more and more widely for high precision grinding of hard and brittle materials. But the problem is that the conventional dressing methods are found unsuitable for them. So a new dressing method, the EDM dressing, is researched. The researches focus on the truing effects of EDM dressing of fine grain bronze bonded diamond wheels and the micro-geomorphic analyses of wheel surface after EDM dressing. 2. The EDM

dressing

method

qf EDM dressing The dressing of bronze bonded diamond wheels are quite difficult. The crux of the problem is that the wheels are the 2. I. Pl?nciple

combination of the hardness of diamond grains and the strength of metal bond. So a kind of non-contact dressing method should be used to avoid cutting the hardest grains. In this point of view, the electro-discharge machining is a good selection. In order to master the characteristics of EDM Dressing, an experimental system, showed in Fig. 1, was developed. The specifications of the equipment were listed in table 1. Table 1 Specifications of the equipment machine EDM power source electrod diamond wheel liquid for EDM

for EDM dressing M7120A made by ourselves graphite, bronze 1AUT2 200 10 75 4D W20MlOO ordinary grinding coolant

2.2. Features of the method There are some obvious advantages of EDM dressing method as follows. (1) No special dressing equipment is required. The dressing can be conducted on grinding machines and the power source of conventional EDM machines can be chosen. (2) Dressing with this method is easily changed to in-process dressing. (3) The wheel is dressed directly on spindle of the grinding machine , so there is no assembling error. (4) The EDM dressing can not only be .used to sharpen the wheels, but also true them.

Fig. 1. Device for EDM dressing 0924-0136/96/$1.5.00 0 PII 0924-0136(96)02424-7

1996 Elsevier

Science

S.A.

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reserved

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X. Wang

3. The efficiency of EDM

et al./Journal

of Materials

truing

Processing 3.3.

Experiments show that not only the electric parameters such as current, voltage and pulse frequency etc. affect the EDM truing efficiency, but also some non-electric parameters especially the wheel speed have influence on it. The influence of these factors are concluded as follows.

Technology

Irtjllrerfce

62 (1996)

299-302

duration

ratio

ofpulse

Fig. 4 illustrates the experiments results of relation between truing effkiency and pulse duration ratio. when the pulse duration ratio is below 50%, the truing efficiency will go up with the pulse duration ratio. But when pulse duration ratio is above 50%, the increase of pulse duration ratio make no contribution to the truing efficiency, on the contrary , the efficiency will fall down a few.

3.1, Irfjllrrence of cm-rent

Fig, 2 illustrates the relation between truing efficiency and truing current. It shows that the efficiency has almost a direct relation to the current. But when the current reduces to a certain level, though it is above zero, the truing effkiency will changes to zero. This character is due to that the electro-resistanceof the grinding coolant is not as high as ordinary EDM fluid, so there must be a leak current between the wheel and the tool electrode. When the leak current cannot maintain the lowest discharge voltage between the two sides of the gap, the EDM truing stopped. Electric current changeswith resistance and voltage of the power source. Experiment results show that a voltage of NV-1 OOVis the most suitable.

I

a

3

4m II

. Pulse. duration

Ea itl

m

5J

ratio (%)

Fig. 4. Relation between truing efficiency and pulse duration ratio. 3.4. Influence of wheel speed

The wheel speed is a non-electric parameter, but it affects the truing effkiency to a great extent. According to the experiment result illustrated in Fig. 5, we find that when the speed is below 6OOr/rnin,the efficiency maintains a high level, and if the speed increases above 600r/min, the efficiency will drop. 2

4

6

!3 Current

18

12

14

(A )

Fig. 2. Relation between truing efficiency and electric current.

3.2. Ii,fllrence

of pulse frequency

The relation between the EDM truing efficiency and the pulse frequency is illustrated in Fig. 3. It is obvious that when the kequency is below SKHz, the EDM truing efficiency maintain a high level, and when the frequency is greater than XHz, the etliciency decreaseswhile the frequency increased,

Fig. 5. Relation between truing efficiency and wheel speed.

1s z-3 SD Is 3. 69 78 m w ,ea Ciicukuity Pulse fi-equency ( KHz )

Fig. 3. Relation between truing efficiency and pulse frequency

if the wheel (

m )

Fig. 6. Relation between truing efficiency and the eccentricity of wheel.

X. Wang

3.5. Influence electrode

of initial

eccentticity

of Materials

et al./Journal

and the kind

of tool

Fig. 6 is the results of these experiments. It is obvious that if the eccentricity is bigger, the truing efficiency is higher. The results also show that the copper electrode can achieve a higher efficiency than graphite. But in experiments we found that the electrolysis of the copper electrode cannot be avoided during truing and the electrode surface will be destroyed soon. This will influence the truing accuracy of the wheel. So the graphite electrode is more practice. 4. The accuracy of EDM truing The accuracy of EDM truing is affected by many factors, some of them can be controlled or changed, but others are determined by the equipment and cannot be reduced , for example, the spindle precision of the grinding machine and the balance condition of the wheel.

Processing

Technology

62 (1996)

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Fig. 7 were taken before and after EDM dressing. From the quatemary shaped grain ‘A’ and the rift in the upper-right part, we can fmd that the two pictures cover the same part of the wheel surface. In (a), we can see diamond grain ‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’, ‘G’, ‘I-Tand T. Scanning (b) carefully, we can find the grain ‘A’, !B’, ‘C’,D’, ‘E’,F and ‘G’are still on the surface, but the others cannot be found. The protrusion of grain ‘A’ in (b) is lower than in (a), becauseit is partly covered by metal bond being due to the EDM. Protrusion of grain ‘B’,‘c’ and ‘F’ in (b) seem to become higher, which is due to some bond around the grains is removed by ED. There are also some new grains in (b), such as T and ‘K’. So these conclusionas below can be achieved: (1) The metal bond can be erased and diamond grains can be emerged by EDM dressing. (2) The electro-dischargewill not destroy the diamond grains in the surf&e. (3) The grains may be covered, erased or emerged while the metal bond nearby was erased by electrodischarge according to its size and initial position diving in bond. So a logical conclusion can be obtained that if the removing volume of single ‘pulse is less than the size of grams, the result of EDM dressing will be improved. A coarsegram diamond wheel was adopted to prove this theory. The photographs were illustrated in Fig. 8. From this picture we can see that if the gram size is big enough, the capability of being covered or removed decreases, the protrusion of grains becamehigher.

!%4&poii1lsmdrarmfemoe0fthevA~(~q~y)

(a) Voltage 8OV Current 7SA Frequency 2KHZ Duration ratio 50% 2 1 12 ; 10 2 8 3 6 B 4 b .g- 2 d

0

$-zLI”“““‘-”

4

5

6

7

8

9

10 11 12 13 14

1

2

3

4

lullrkpoitsalcof~~(~~)

(b)Voltage 60V Current 1SA Frequency XHZ Duration ratio 50% Fig. 7. EDM truing accuracy.

(a)

Fig. 7 shows two truing process and the fmal result. IIre EDM condition of (a) in Fig. 7 is large than (b). So the efficiency of (a) is higher. But the final value of eccentricity of (b) is lower. So in the end of truing , a finish EDM condition should be taken. After all the circularity of 3~ is the limited accuracyof our experimental system. 5. The micro-geomorphic

analyses of the wheel surface

5. I. The mechanism oJEDh4 dressing

Ln order to analyze the mechanism of EDM dressing, a trace experiments has been developed. The two SEM photograph in

301

@I Fig. 8. SEM photograph of trace experiment.

X. Wang

et

al./Joournal

of Materials

Processing

Technology

62 (1996)

299-302

6. Conclusions

Pig. 9. EDM dressingof course grain wheel.

5.2.

Choose

of shurpening

parameter

In the dressing of fine grain wheel, a smaller current, smaller pulse duration and higher pulse frequency should be adopted. But the dressing efficiency will drop with the reduce of single pulse energy of EDM. Wheel speed affects the dressed surf&e very much . Future experiments show that when dressing a fine grain metal bonded diamond wheel using EDM, the more smooth the wheel surface is, the more grains remain in the surface and the higher the protrusion is. The surface roughness of the wheel after dressing at various speed were illustrated in Fig. 10. The roughness maintained is very high while the wheel speed is below 6OOr/min, and it will drop when the speed increase above 6OOr/min. So EDM dressing should be processed at the speed above 600rfmin

Fig. IO. Relation between surface roughness of wheel EDM dressing and wheel speed

From all the experiments and researches described above, the conclusions as follows can be achieved: 1. EDM dressing can obtain high truing effkiency while suitable processing parameters are chosen.Generally, the greater current, higher voltage, lower wheel speed and lower frequency can get higher efficiency, while the pulse duration ratio should be 50%. 2. The truing accuracy of EDM dressing is depended not only on the electric parameters but also on the precision of the equipment and balance of the wheel. Generally, in the end of truing a finish EDM condition should be taken. In our system, the obtainable circularity of wheel after truing is 3~. 3. Considering the roughness of wheel surface and truing efficiency, the suitable wheel speed is about 600r/min. 4. While dressing, the grains on wheel surface may be covered, removed or emerged because of its position and the erasing volume of single pulse. In order to obtain a better dressed wheel surface, the removing volume of a single EDM pulse must less than the size of diamond grain.

References

[1] Suzuki,K. Uematsu,T. Nakagawa,T., On Machine Truing /Dressing of Metal Bond Grinding Wheels By ElectroDischarge Machining, Annals of CIRP Vo1.36/1/1987,115118. [2] X.K.WANG, B.G.YING, W.G.LIU, EDM Truing of Fine Grain Metal Bonded Diamond Wheel, Aviation Precision Manufacturing Technology. Vo1.31, No.2,1995, 19-23 (in Chinese) [3] X.K.WANG, B.G.YING, W.G.LIU, The Microcosmic Analysis of EDM Dressing of Fine Grain Metal Bonded Diamond Wheel, Manufacturing Techniques and Machine tools.,Vo1401, No.12, 199513-16 (in Chinese) [4] X.K.WANG, W.G.LIU, B.G.YING, Progres’s of Dressing Techniques of Super abrasives,Abrasives Kr Grinding, No. 3 (Serial No. 81) , 994,30-32. (in Chinese)