Inside EDM

by Dave Wilson, Editor

They say that travel broadens the mind. So when I got an invite from Agie and Charmilles Technologies to discover more about the EDM (Electric Discharge Machining) process, I hopped on the plane to Switzerland to investigate.

Initially, I was a little bit sceptical of the whole deal. Knowing a little bit about the technology, I thought that the machines might be a little boring. After all, the basic principle behind such EDM machines dates back to the fifties. Basically, they work in one of two ways: either, they cut metal with a special metal wire electrode that is programmed to travel along a preprogrammed path (in the case of wire EDM), or they form a required shape negatively in a metal using a three dimensional electrode (in the case of die sinking EDM machines).

Surprisingly, however, after wandering for what seemed like an eternity through the plants and sticking my nose into the back of these EDM beasts, I noticed a host of electronic subsystems that would make some of my pals at NASA green with envy. In the back of one of the machines, for example, I saw a large control system based on a 21 slot VME rack running a real time operating system. This was interfaced over a fibre optic cable to a bank of proprietary VME based servo controllers driving a set of AC servo motors that provided the axial movement for the machine.

Also linked up to the VME control system was a complex generator. The real secret sauce of the EDM design, the generator not only provides the current to the wire but also can measure its effect on the workpiece, which it does by sampling the signal delivered at over 100GHz, converting, processing and analysing it with over five on-board DSPs.

According to the designers involved, generating the spark with the correct shape and obtaining such a high sampling rate are two of the toughest design nuts to crack.

Interestingly, in such advanced mechatronic solutions, the response time of the electronics is orders of magnitude faster than the slow servo/ballscrew driven mechanics. To increase the speed, it was indicated that for some applications, marrying a piezoelectric actuator with the servo drives could increase the response time of the system in question.

The latest of the Agie EDM machines has already been successfully employed to produce diesel engine fuel injector nozzles with borings so small that they are hardly visible to the naked eye. Far from being a mundane technology, EDM turned out to be a real design engineer’s dream. Indeed, to gain a full understanding of the EDM process could take most of an engineer’s life. Unfortunately, I only had a couple of days…