Re: [CAD_CAM_EDM_DRO] A bit off topic - resistance welding

> From: Hugh Mahlendorf <hmahl@...>
> >There are big monster ones used in sheet metal shops and little
> >baby ones. Typically just 1 volt or so and hundreds or
> >even thousands of amps AC. I have seen them with 1" diameter conductors!

A few months ago, I was called in to have a look at a very large machine that
was used to spot weld aluminum up to about 3/16" thick. I told the shop
foreman over the phone that I didn't know anything about spot welders, but as
I'm the "technician of last resort" around here they said to come over and
have a look anyway. The "arms" that carried the electrodes were made from 4"
diameter copper bars! The machine was really pretty simple. The main power
input was 480Vac, 3 phase and was fused at something like 400 Amps! The power
was directed into a box at the base of the machine wherein lived three
ignitrons (special tubes) each slightly smaller than a 5 gallon pail. These
controlled the current flow through the primary windings of a large
transformer that formed the frame of the machine (about 6' tall x 4' wide x
4' deep). The secondary winding was about one turn of absolutely gigantic
wire! A control panel on the side of the machine (that's what they were
having trouble with) allowed setting the weld parameters such as current,
voltage, weld time and a few others I have forgotten. The operator gave me a
demonstration and when he pushed all the way down on the pedal that clamped
the work between the two electrodes, the control cycled the welding current.
The thing that really got my attention was that when the machine cycled, the
conduit on the wall that led from the fusebox to the machine jumped a little
in it's brackets! The problem they were having was that if the current was
turned up past 50%, the 400A breaker would trip...

The control was made in '78 and moisture had corroded it pretty bad. I
measured a few voltages (+5V and +12V) and I read through the schematics for
about half an hour and then told the foreman that there was no charge for the
visit, and that I didn't think I could help them. I recommended that they get
someone who was intimately familiar with these things out there and that they
would in all likelihood have to replace the control (they had been quoted
$13,000 for this and they hoped I could patch it up). Although I'm usually
adventurous, I didn't really want to make some mistake inside that control
and see those ignitrons explode!

Matt (poor, but wise)