Colchester Chipmaster revisited
 

You may remember that I had a problem with the variable speed drive box on
the chipmaster lathe at Scienceworks.
I had resolved to strip out the internals of the variator and replace it
with a shaft that went straight through the box ( thus making it the most
expensive pillow block I have ever seen) and vary the spindle speed with a
frequency inverter.. Yesterday, I decided to have another look at it before
I gutted it. I pulled it apart, and measured the diameter of the six balls.
( See photo at the bottom of this page
http://www.allspeeds.co.uk/kopp/index.htm ).There didn't seem to be any real
wear marks on them, though you could feel a slightly different texture on
the area where they contacted the drive and driven plates. Ther was no more
than 3 tenths of a thou difference in diameter , between the working area
and the part that didn't thoch the drive/ driven shafts.I dismantled the
ball/shaft assembles and turned the balls upside down on the shafts,so the
wear was on a different part of the balls. I put it all back together and
test ran it. Better than it was, but no cigar. There were still speed
ranges where it slipped, then suddenly sped up. As a last resort, I
increased the preload that forces the balls against the input and output
shaft discs. Another test run. Break out the champagne! It ran over the
entire speed range smoothly, and didn't slip even when I put a fairly heavy
cut on. We were told that a new variator would cost about A$6000 and the
alternative ( a variable frequency inverter) was about a thousand dollars.
The great thing about being retired is that you can take the time to do
things the right way!