Consider that a slip clutch would be capable of causing un-noticed loss of
sync during threading. I think an easily replaceable shear element would be
much preferable.
Don Young
"Bob Engelhardt" wrote in message
...
Good news, bad news. Good news is that it wasn't a gear that broke, but a
spring/split pin holding the gear on. 'Course it was down inside the
apron and more work to get to than about anything else on the lathe!

I was surprised than the pin broke - just sheared right off. It was small
(3/32, .094, 2.4mm) and not hard, but still ...

The bad news is that there was also damage in the quick change box. A
couple of gears there had been repaired with brazed-in teeth and one of
those teeth bent. I plan on replacing them (the gears, not the teeth).

I've started experimenting with shear keys. I have some plastic that
looks promising. One test key 3/16 square x 1" long took 70 foot lbs to
shear it. I'm going to start with real small versions and work up till I
get a size that won't shear under normal conditions. Then hopefully it
will shear under damaging ones.

I thought of another approach to gear train protection (for this lathe
anyhow). The first gears off the forward-reverse toggle are two gears on
a jack shaft, keyed together. Instead of keying them, one could sandwich
some friction material between them and create a slip clutch. It might be
kind of difficult to get a consistent slip point, but it might be worth
experimenting with.

Bob