Paper International Journal of Electrical Machining, No.I3, January 2008

Near-Dry EDM Milling of Mirror-Like Surface Finish

Jia Tao*, Albert J. Shih*, Jun Ni* (Received on May 27, 2007)

* Department of Mechanical Engineering, University of Michigan, Michigan 48109 USA

Abstract This study investigates near-dry EDM milling as a finishing process to achieve a mirror-like surface finish. A liquid-gas mist mixture is the dielectric medium delivered through a rotating tubular electrode in the near-dry EDM milling. Near-dry EDM exhibits the advantage of good machining stability and smooth surface finish at low discharge energy input. An EDM power generator is modified by adding an electrical resistance to enable the reduction of surface roughness. Effects of dielectric fluid, electrode material and pulse energy on material removal rate and surface roughness are studied. The mirror-like surface finish with 0.32 /lm Ra is achieved using the kerosene mist and copper infiltrated graphite electrode with 100 /lJ pulse energy. Key words: Near-dry EDM, surface finish, mist dielectric medium. l. INTRODUCTION the machined debris from the useful powders and Electrical discharge machining (EDM) is a maintaining a constant powder concentration. prevalent nontraditional preCISIOn machining Near-dry EDM, first explored by Tanimura et al. method. The EDM machined surface is composed 11), utilizes the liquid-gas mixture as the dielectric of discharge craters overlapping each other. To fluid. It is found to be beneficial for the finishing achieve a super surface finish, ultra low pulse process with good machining stabilityI2). Better energyl)-4) and powder mixed dielectric (PMD) surface finish was achieved in near-dry EDM than 5 EDM )-IO) are two existing methods. Both that of wet and dry EDM at the same energy level. methods have their limitations. It is hypothesized that the liquid particles dispersed Luo et al. l),2) demonstrated a super finished EDM in the gas medium has a similar mechanism to surface with 0.04 /lm Ra by reducing the pulse promote the finishing process as the additive energy with a very low discharge duration of tj less powders in PMD EDM. The dielectric disposal of 3 than 0.2 /lS. Egashira et a1. ) applied the ultralow near-dry EDM is cleaner than that of PMD EDM. discharge energy, less than 3 nJ, to conduct In addition, the EDM milling configuration uses a machining with the assistance of ultrasonic small tubular electrode to scan over the large vibration. Okada et a1. 4) studied the working area. In this case, the stray capacitance, radio-frequency plasma to produce a surface which increases with the overlapping area between roughness of 0.15 /lm Ra on aluminum. The the electrode and workpiece, is no longer a disadvantage of the low pulse energy method is the constraint on the machining area, as it did in low material removal rate (MRR) and long conventional die-sinking EDMI). machining cycle due to the frequent abnormal In this study, the near-dry EDM milling is discharges resulting from the narrow gap distance. investigated to understand the effect of dielectric The PMD EDM has been recognized as a more fluid, electrode material and pulse energy level. practical finishing process to generate very fine Benefits and potentials of near-dry EDM milling as 5 surface finish at relatively high MRR ),6). Powders the finishing process are identified and discussed. such as graphite, silicon and aluminum suspended in the dielectric help to stabilize the machining 2. EXPERIMENTAL SETUP process at low pulse energy by increasing the The near-dry EDM milling experiments were discharge gap distance, decreasing the stray conducted on a CNC die-sinking EDM machine, capacitance and dispersing the discharge pulses 7),8). Vanguard 150H from EDM Solutions at -Elk Grove However, the use of powder increases the Village, Illinois, US. Figure 1 shows the setup of machining cost and the consequent toxic disposal the test bed. A rotary spindle, Rotobore causes an environmental concern9),IO). In practical RBS-IOOO, with through-spindle flushing capability applications, the powder suspended dielectric is mounted on the EDM head. The mist mixture is circulation system is also challenged by separating delivered by a pulse mist spray system, AMCOL

-29- 6000 precision applicator, as in Figure 1(b), which To reduce the pulse energy for smooth surface was originally adopted in the minimum quantity finish, the EDM power generator was modified. A lubrication (MQL) machining. The input liquid power resistor was serially connected in to the flow is set at 5 ml/min. The tested mist dielectrics discharge circuit, as shown in Figure 2(a). The and electrode materials are listed in Table 1. applied resistance lowered the output discharge Negative polarity, i.e. electrode as cathode, is used energy. The resistance was varied in the experiment in the experiment, due lower wear on cathode at from 1 k!1 to 50 k!1 in order to reduce the pulse low discharge pulse duration and smoother energy. The current and voltage waveforms were discharge crater on anode. monitored using an Agilent Infiniium 54833A digital oscilloscope. As shown in Figures 2(b) and

(c), the discharge current, ie, was reduced from 4 to 1 A, in a sampled pulse. It is also observed the open voltage, uo, also drops from 80V to 60V. This is not an expected output by inserting the resistance. It could results from the measurement error or leakage current. Since the open voltage is not a main factor influencing the pulse energy and resultantly crater morphology, this question is set aside in this paper. (a) (b) AISI H13 tool steel was the work material. The Figure 1. Rotary spindle and mist dispenser. surface roughness was measured using a Taylor Hobson Form Talysurf Intra profilometer with a 2 Table 1. Mist dielectrics and electrode materials. flm stylus tip radius. The workpiece was weighed Mist dielectric Electrode materials before and after machining to quantify the MRR. An Olympus PME 3 optical microscope was used to Gas Air Copper inspect the machined surface and discharge crater. medium Nitrogen Graphite Liquid Water Copper infiltrated 3. RESULTS AND DISCUSSION medium Kerosene graphite 3.1 Effect of dielectric fluid Figure 3 compares the surface roughness, MRR and micrographs of the machined surfaces of four different near-dry EDM dielectric fluids. The surface finish is about the same, between 0.85 to 0.9 Ilm Ra, for the four dielectric fluids. EDM head Although previous study l2) reported that the water mist with nitrogen generates the smoothest surface ss----+t------l finish, the long machining time induces severe '------' electrolysis corrosion and deteriorated surface (a) finish on the water mist machined surface, as -Voltage -·-Current I -voltage .. - Current: shown in Figure 3. The individual discharge 120 f 12 120 ' 12 crater created by the water mist is finer than those ~ 60 1r1 _ I w/o 60 ,h ,e""" , ~ ""':1' l .. of kerosene mist, but the electrolysis causes the ~OtJ "" ' 60 . 6 e blackened and rugged surface. The nitrogen ~-;:~ t_J\/'~J : ;:~ ] r----~ I: j medium was effective in preventing the water ~ ~ - l"-- .- - -f -180 .,...-,--.- -3-180 ~ . -3 electrolysis by isolating the oxygen content at high I2 0.0 2.0 4.0 6.0 8.0 0.0 2.0 4.0 6.0 8.0 pulse energy input ). It was also reported by ll Time (IlS) Time (Ils) Tanimura ) that water did not cause any (b) (c) electrolysis problem in near-dry EDM. However, Figure 2. Power generator modification, (a) as the discharge gap distance decreases and the schematic of the modified circuit and discharge machining time increases with the reduction of wave form measurement, (b) pulse before pulse energy, the arcing and short circuiting occur modification, and (c) pulse with 10 k!1 resistor more frequently and provoke the water electrolysis inserted. in this study. This indicates that the electrolysis is

-30- an obstacle to implement the water mist for super 3.2 Effect of electrode material finishing near-dry EDM. The results of surface roughness and MRR for the three electrode materials in near-dry EDM are shown in Figure 4. The copper infiltrated graphite 1 0.4 electrode has the advantage of good surface finish ~Ra <> MRR -r:: 0.9 0.3 :g and moderate MRR over the graphite and copper e- M electrode materials. ~0.8 0.2 E ca .§. Among the three materials, graphite has the a:: 0.7 0::: highest MRR but the surface finish is the worst. 0.1 0::: :E As shown in Figure 5(a), the machining marks are 0.6 0 observed as deep craters with deposition of Kerosene Kerosene Water Water mist overflowed molten material at the crest. This mist with mist with mist with with air nitrogen air nitrogen indicates that a large amount of heat is generated in (a) the discharge melting the material, but the bubble explosion force is not strong enough to eject all the molten material. Such discharge pulse is close to the status of arcing. The frequent arcing could have been induced by the flaking of graphite electrode due to the thermal shock.

1.5 1.00 ~Ra OMRR Kerosene mist with air Kerosene mist with 1.2 0.80 C nitrogen E 0.9 0.60 J .E: E ca 0.6 0.40 .§. et r:t:: 0::: 0.3 0.20 :E 0 0.00 Graphite Copper Copper infultrated Water mist with air Water mist with nitrogen Figure 4. Machining performance of different (b) electrode materials (ti = 4 Ils, ie = 4 A, Ue = 45 V, Figure 3. (a) Surface roughness and MRR and (b) kerosene mist with air). micrographs of the discharge craters for four near-dry EDM dielectric fluids (ti = 4 Ils, ie = 4 A, Ue = 45 V, copper infiltrated graphite electrode).

Kerosene mist exhibits the advantages of shiny surface, high MRR and moderate surface finish. Even though kerosene mist generates larger and deeper craters than water mist, it could be (a) Graphite (b) Copper overcome by further reducing the pulse energy to infiltrated graphite yield the desired smooth surface. Comparing the gas media in kerosene mists, air obviously enhances the MRR while the surface is slightly rougher than that of nitrogen. The nitrogen medium, which was 12 used for preventing oxidation or electrolysis ), does not contribute much to improve the surface finish, since little oxidation or electrolysis occurs while using the kerosene mist. Considering the (c) Copper high MRR and easy availability of air, kerosene Figure 5. Optical micrographs of discharge craters mist with air is chosen as the dielectric fluid for generated by three electrode materials further investigation.

-31- The copper electrode generates a clean surface, (1) as in Figure 5(c), but the MRR is unacceptably low.

At low pulse energy input, the gap distance between 2.0 2.5 the electrode and workpiece is small. This -.-Ra ~MRR C requires a servo system with a high sensitivity to 1.6 2.0 ·e E 1.2 1.5 ..,~ maintain the proper gap distance. Otherwise, the ..=;. system will switch between the open and short .. E- a:: 0.8 1.0 circuit and thus lower the MRR significantly. o:: 0.4 0.5 0:: When machining using the copper electrode, this :lE situation occurs frequently. 0.0 0.0 500 0 The copper infiltrated graphite electrode 1500 1000 combines the advantages of both copper and Pulse energy (j.lj) graphite electrode materials, as shown in Figure (a) 5(b). The graphite content, once dispersed into the discharge gap at a comparatively small amount, can help to initiate the discharge at low pulse energy input. The copper content increases the thermal conductivity, which relieves the thermal shock on the electrode. Copper also has enough toughness to hold the electrode material together in Step 1 (1400 ~J) Step 2 (700 ~J ) near-dry EDM. The arcing can thus be reduced by reducing the flaking of graphite powder. Smooth discharge craters, as shown in Figure 5(c), are generated. The copper infiltrated graphite electrode is used in the following experiments for further surface finish improvement.

Step 3 (300 ~J ) Step 4 (100 ~J ) 3.4 Mirror-Like Machined Surface (b) Figure 6 demonstrates the machined pockets with Figure 7. (a) Surface roughness and MRR and (b) the mirror-like surface finish using a tubular optical micrographs of the discharge craters electrode with 3 mm outside diameter. Reflection generated at different finishing steps. of the electrode tip can be observed. The surface finish is smoothed effectively as the pulse energy decreases. As shown in Figure 7(b), the small pulse energy reduces the depth of the discharge crater and the molten material overflow. Currently, the best surface finish achieved is 0.32 ~m Ra with a pulse energy of 100 ~J by applying a 50 kO resistor in the discharge circuit. At this energy level, the test with wet EDM was failed Figure 6. Illustration of the mirror-like machined because of low machining stability. Frequent surface. electrode servo retraction was observed. This is because an extremely small discharge gap distance The four steps from roughing to final finishing is required to initiate the discharge in liquid are summarized in Figure 7. The pulse energy is kerosene medium. High sensitivity and quick reduced by applying higher resistance as the response of the servo system are thus necessary for machining proceeds. The discharge current and maintaining the small gap distance. Otherwise, discharge voltage are monitored using an the machining status could easily deteriorate to the 1 oscilloscope. Since ie and Ue alters, the pulse switching between the open and short circuit }. energy is calculated using Equation (1) to characterize the pulse energy for each step. 3.5 Discussion The current near-dry EDM result is compared

-32- with work by Pecas and Henrique1o>, who also Technology Program. Discussions with Dr. Yuefeng studied the EDM polishing of AISI H13 tool steel. Luo of Federal Mogul are greatly appreciated. They reported that with a 32 cm2 machining area, 0.50 J.1m Ra is the limit for conventional EDM using REFERENCES the kerosene dielectric while the silicon powder 1) Y.F. Luo and C.G. Chen: Effect of a Pulsed mixed dielectric is able to lower the surface Electromagnetic Field on the Surface Roughness in Superfinishing EDM, Precision Engineering, roughness to 0.20 J.1m Ra with the pulse energy less Vol. 12, No. 2 (1990) 97-100. than 10 J.1J. The polishing by near-dry EDM 2) Y.F. Luo, Z.Y. Zhang and C.Y. Yu: Mirror milling already exceeds the conventional Surface EDM by Electric Field Partially Induced, die-sinking EDM and is promising when compared Annals of Cl RP, Vol. 37, No. 1 (1988) 179-18l. to PMD EDM, if the same low pulse energy is used. 3) K. Egashira, A. Matsugasako, H. Tsuchiya, and The potential of near-dry EDM has not yet been M. Miyazaki: Electrical Discharge Machining fully explored due to the limitation of the minimum with Ultralow Discharge Energy, Precision pulse duration. In this study, the EDM generator Engineering, Vol. 30, No. 4 (2006) 414-420. is only capable of producing 4 J.1s pulse duration, as 4) A. Okada, J.A. McGeough, D. MacMillan and B. shown in Figure 2. In general, pulse duration Flynn: Machining Characterstics of EDM by close to or lower than 1 J.1S is expected for the Radio-Frequency Plasma, Annals of CIRP, Vol. finishing EDM1),IO). With lower pulse duration, 55, No. 1 (2006) 167-170. 5) H. Narumiya, N. Mohri, N. Saito, H. Ootake, Y. the expansion of the plasma channel can be Tsunekawa, T. Takawashi and K. Kobayashi: restricted, resulting in smaller discharge crater size EDM by Powder Suspended Working Fluid, and finer feature. In this study, the discharge Proceedings of 9th International Symposium on crater depth is reduced but the crater diameter Electromachining (lSEM IX) (1989) 5-8. remains about the same due to the constant pulse 6) N. Mohri, N. Satio and M. Higashi: A New duration, as shown in Figures 2(b) and (c). As Process of Finish Machining of Free Surface by l l2 reported by Luo ) and Tao and Shih ), among the EDM Method, Annals of CIRP, Vol. 40, No. 1 three factors influencing the pulse energy, the pulse (1991) 207-210. duration has the most significant effect on surface 7) Y.S. Wong, L.C. Lim, I. Rahuman and W.M. Tee, finish in super finishing EDM. Therefore, by Near-Mirro-Finish Phenomenon in EDM Using Powder-Mixed Dielectric, Journal of Materials reducing the pulse duration, the surface finish is Processing Technology, Vol. 79, No. 1-3 (1998) expected to be further reduced in near-dry EDM. 30-40. 8) F.L. Zhao, Z.Z. Lu, H. Wang and Z.Q. Qian: 4. CONCLUSIONS Research on Effecting Mechanism of Particles Near-dry EDM as a super finishing process was in Powder-Mixed EDM, Journal of Dalian explored in this study. Kerosene mist with air, University of Technology, Vol. 45, No. 5 (2005) coupled with a copper infiltrated graphite electrode 668-67l. was found to produce the best overall results in 9) S.H. Yeo, H.C. Tan and A.K. New: Assessment near-dry EDM finishing. Advantages and of Waste Streams in Electric-Discharge potentials of near-dry EDM milling were identified. Machining for Environmental Impact Analysis, The near-dry EDM had good machining stability Proceedings of the Institution of Mechanical and its surface finish was very good at relatively Engineers, Part B: Journal of Engineering long pulse duration and high pulse energy level. Manufacture, Vol. 212, No. B5 (1998) 393-40l. 10) P. Pecas and E. Henriques: Influence of Silicon Further exploration of the near-dry EDM milling Powder-Mixed Dielectric on Conventional as a super finishing process is being carried out at Electrical Discharge Machining, International the University of Michigan. Lowering of the pulse Journal of Machine Tools and Manufacture, Vol. duration « 0.5 J.1s) and pulse energy « 10 IlJ) will 43, No. 14 (2003) 1465-147l. be implemented to drive towards even better 11) T. Tanimura, K. Isuzugawa and I. Fujita: surface finish. The electrode wear mechanism will Development of EDM in the Mist, Proceedings be studied and compensated for the precision EDM of 9th International Symposium on milling process. Effect of gas-liquid composition on Electromachining (lSEM IX) (1989) 313-316. MRR and surface roughness will be investigated. 12) 1. Tao and AJ. Shih: Dry and Near-Dry Electrical Discharge Milling Processes, ISEM ACKNOWLEDGEMENTS XV (2007) 275-280. This research is sponsored by the NIST Advanced

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