Machinery's Handbook, 31st Edition
1474 ELECTRICAL DISCHARGE MACHINING EDM operation, if the overcut, wear, and finish are satisfactory, machining speed can best be adjusted by slowly decreasing the off time setting in small increments of 1 to 5 μ s until machining becomes erratic, then returning to the previous stable setting. As the off time is decreased, the machining gap or gap voltage will slowly fall and the working current will rise. The gap voltage should not be allowed to drop below 35 to 40 volts. Metal Removal Rates (MRR): The amount of metal removed in any EDM process depends largely on the length of the on time, the energy/spark, and the number of sparks/ second. The following data were provided by Poco Graphite, Inc., in their EDM Technical Manual . For a typical roughing operation using electrode positive polarity on high- carbon steel, a 67 percent duty cycle removed 0.28 in 3 /hr(4.59 cm 3 /hr). For the same material, a 50 percent duty cycle removed 0.15 in 3 /hr (2.46 cm 3 /hr), and a 33 percent duty cycle for finishing removed 0.075 in 3 /hr (1.23 cm 3 /hr). In another example, shown in the top data row in Table 1, a 40 percent duty cycle with a frequency of 10 kHz and peak current of 50 amps was run for 5 minutes of cutting time. Metal was removed at the rate of 0.8 in 3 /hr (13.11 cm 3 /hr) with electrode wear of 2.5 percent and a surface finish of 400 μ inch Ra. When the on and off times in this cycle were halved, as shown in the second data row in Table 1, the duty cycle remained at 40 percent, but the frequency doubled to 20 kHz. The result was that the peak current remained unaltered, but with only half the on time the MRR was reduced to 0.7 in 3 /hr (11.47 cm 3 /hr), the electrode wear increased to 6.3 percent, and the surface finish improved to 300 μ inch Ra. The third and fourth rows in Table 1 show other variations in the basic cycle and the results. Table 1. Effect of Electrical Control Adjustments on EDM Operations
Metal Removal Rate Electrode Wear (%) (in 3 /hr) (cm 3 /hr)
Surface Finish ( μ in Ra)
On Time ( μ s)
Off Time ( μ s)
Frequency (kHz)
Peak Current (Amps)
40 20 40 40
60 30 10 60
10 20 20 10
50 50 50 25
0.8 0.7 1.2
2.5 6.3 1.4 2.5
400 300 430
13.1 11.47 19.66 4.59
0.28 350 The Recast Layer: One drawback of the EDM process when used for steel is the recast layer, which is created wherever sparking occurs. The oil used as a dielectric fluid causes the EDM operation to become a random heat-treatment process in which the metal surface is heated to a very high temperature, then quenched in oil. The heat breaks down the oil into hydrocarbons, tars, and resins, and the molten metal draws out the carbon atoms and traps them in the resolidified metal to form the very thin, hard, brittle surface called the recast layer that covers the heat-affected zone (HAZ). This recast layer has a white appearance and consists of particles of material that have been melted by the sparks, enriched with carbon, and drawn back to the surface or retained by surface tension. The recast layer is harder than the parent metal and can be as hard as glass, and must be reduced or removed by vapor blasting with glass beads, polishing, electrochemical or abrasive flow machining, after the shaping process is completed, to avoid cracking or flaking of surface layers that may cause failure of the part in service. Beneath the thin recast layer, the HAZ, in steel, consists of martensite that usually has been hardened by the heating and cooling sequences coupled with the heat-sink cooling effect of a thick steel workpiece. This martensite is hard and its rates of expansion and contraction are different from those of the parent metal. If the workpiece is subjected to heating and cooling cycles in use, the two layers are constantly stressed and these stresses may cause formation of surface cracks. The HAZ is usually much deeper in a workpiece cut on a sinker than on a wire machine, especially after roughing, because of the increased heating effect caused by the higher amounts of energy applied.
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