Jon Elson wrote:

I started this thread when I *DID* break a toolholder! But, it clearly was
*NOT* steel at all, but some kind of cast iron material, from the look of
the break.

Cast iron, if indeed it was cast iron, is extremely weak in tension. A
toolholder works as a cantilever beam, tension on top, compression on
the bottom. There are grades of cast iron that aren't too bad but I
wouldn't expect to see them in a cheap toolholder. Also, IIRC, you said
3/8". For the same loading, 3/8" will stress the material almost 2 1/2
times as much as 1/2".

Ted