My 13x34 Jet(Taiwan) uses 1/8" brass rod shear pins in both the feed shaft
and lead screw couplings where they come out of the Q/C box. I have sheared
the feed screw pin two times over the years, but there has been no gear
damage.

Randy


"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