I recently found a set of metal gears that will fit my Mini-Mill. I
know the standard plastic gears will strip out if the Mill tooling is
pushed too far ( which I did once). I would like to put a shear pin in
the drive system and I wonder if any of you have done it? What kind of
shear pressure would be best?

I suggest trial and error. Start with a small diameter AL. Run machine and
see what it takes to break it. Use the data to estimate how much more
strength you need.

Karl