[Synchro909]

My father was an excellent toolmaker and engineer/machinist. At one time, he worked for a company that amongst many other things, used to draw wire. the process was to pull a thicker piece of wire through a die with a slightly smaller hole in it. The wire stretched as it came through the die and you ended up with a smaller diameter wire. The dies obviously had to be very hard to last long. They used carbide. Now, the challenge! Try drilling even a round hole in that, then try making a square or rectangular hole in it, then polish it. He did many. (Imagine making the hole then polishing it for the finest strand of wire you have ever seen. They went down to less than 0.001")
I was intrigued, even as a kid to see him do it. It was a slow process. A mask was made of an insulating material with an appropriate hole in it. The mask was clamped to the surface of the block of carbide and a moving electrode set in motion in the hole. It would wander randomly around within the hole in the mask, making sparks as it went. Gradually the carbide was eroded till the hole went right through. Then came the arduous task of polishing the inside of the hole. Electric Discharge Erosion is not new but sometimes, even now is the only way. A friend once broke off a tap in a hole. EDE was the only way to save the component. (with a of of patience)
A square hole in mild sheetmetal? NO PROBLEM.