EDM Mould Polishing - A short history
During the war someone came up with an idea. They already had a machine that used one electrical polarity in the workpiece and the opposite polarity in a meltable electrode. When the two came together the electrode melted and they called it a welder.
Somebody had the idea that if they could come up with an electrode that would not burn up or melt away they might be able to spark off and blast a hole though those broken taps. It worked. They called it "The Tap Buster" and from this primitive device evolved the very sophisticated CNC machines of today.
Now about the blessing and the curse... The EDM machine has revolutionised the mould making industry, no question about it. It has cut down on the time needed to build a given mould and it has increased the potential complexity of moulds.
It has increased the strength of moulds by allowing less sectioning. Less sectioning means that there is less chance of a mould "blowing apart" from injection pressure and flashing or, at the very least, showing section lines.
Before the use of the EDM machine, the polisher had a relatively easy job. It was not too difficult to get polishing stones into any slot or configuration a mill cutter could reach. What couldn't be machined in the solid block was sectioned and fitted. Most of the components could be stoned and polished with the sections apart. So much for the good old days.
The second problem is the EDM finish itself. This finish, no matter how fine it is, is a series of pits. Shooting plastic into an EDM finish is a little like shooting it into a sponge. A orbiting EDM machine can do some weird things and make it very tough to get at the finish.
In deep and narrow slots it is essential to get the finish out or the part will not release. And the slots are getting narrower. One sixteenth of an inch was once considered a narrow slot, now one thirty-second is commonplace. It can really test the ingenuity of the mould polisher.