These problems were intrinsic to the early days of EDMing and continued up through 1990. In 1991, the first compensating power supply circuitry was used for fine finishing on the skim cuts.
Previously, rough cutting was done with DC power for maximum speed and material removal. Fine or skim cutting was then done, at different power settings on an AC electrolysis-limiting power supply, to finish the dimension and, most often, to cover the damage done to the work-piece in roughing.
A true anti-electrolysis (AE) power supply can be used for rough and skim cutting, thus minimizing surface degradation, the material's susceptibility to rust and corrosive action, plus the overall improvements in accuracies and total time required to finish parts.
Since the mid-80s, literally all EDM manufacturers have addressed the finishing power supply issue, with various solutions offered. This is in sharp contrast to the "early" days of EDM promotion, when speed, speed and more speed were the goals.
Briefly, let's get back to the basics of this critical aspect of EDM.
What Is Electrolysis?
For the practical application of EDM, electrolysis is the production of chemical changes by the passage of an electrical current through an electrolyte, that is, a nonmetallic electrical conductor through which current is carried by the movement of agitated ions.
In wire EDMing, stray energy in the dielectric fluid, produced by the cutting process itself, interacts with contaminants in the flushing fluid to disrupt the surface of the workpiece.
The chief result of this process in all materials is an increased heat-affected zone, or white layer, on the surface. Depending upon the workpiece material being cut, the visible results of this action will vary, as described above.
The current-carrying EDM wire commonly discharges particles as well as produces the cutting action on the workpiece. The stray current, once thought inevitable, causes detrimental surface effects such as:
bluing of titanium,
cobalt binder depletion of carbide,
anodic oxidation of aluminum,
rusting of ferrous materials, and
eventual micro-cracking of all materials.
This last effect had prohibited increased use of wire EDMing in medical, aerospace, aircraft and ordnance applications because that condition would render parts either unsafe or inoperable to the specifications required.
Meeting the Challenge
Thus, the challenge facing EDM builders was to engineer a power supply that would minimize, even eliminate, the interaction of the stray current and contaminants on the workpiece surface. Various builders have taken various tacks to solving this problem. Mitsubishi EDM, for example, combined voltage modulation, advanced transistor pulse circuitry and state-of-the art sensors, plus software improvements to interface the cutting program and actual condition protocol, to develop its patented AE power supply. This combination of technologies indicates the complexity of the problem and the effort required to make progress in this area. However, the benefits are substantial--so substantial that this same builder has made its AE power supply standard equipment on its new X Generation wire EDMs.
Other methods are available, many of which have excellent characteristics, as they all aim at two important goals: namely, to eliminate or dramatically reduce surface degradation while, simultaneously, maintaining productivity on the machine.
Depending upon the EDM wire being used, brass or coated, surface finishes down to 0.5-mm Rmax are now attainable with no significant loss in speed. Doing certain types of wire work will always put a premium on an operator's work rate, and only the individual shop's particular needs can dictate the optimum conditions that should be employed.
However, unlike in the past, EDMers now enjoy more choices and thus fewer compromises when balancing speed versus accuracy and finish. To some extent, largely as the result of better power supplies and control circuitry, shops really can "have it all," or close to it.
Electrical discharge machining, as the name implies, creates a certain amount of stray current in the dielectric fluid, by definition. Literally, the instant such current interacts with any contaminants in the solution, surface degradation on the workpiece begins.
Titanium will begin to turn blue, an action caused not by heat, as some suspect, but electrolysis. Aluminum undergoes an anodic oxidation. All iron-based materials simply begin to rust. Sintered materials such as carbides suffer surface degradation, the result of cobalt binder depletion.