"Mike Henry" wrote in message
...
I've a used 1/2" MT2 end mill holder that I'd like to bore out to 3/4".
The
problem is that the holder seems to be hardened throughout. A small file
won't scratch the bore of the holder. It seems that my options would be
to
bore it out as-is with carbide or anneal it and bore. I suspect that
grinding would take forever and am not real thrilled with the idea of
grinding on the lathe, though I do have a decent toolpost grinder.

For annealing, I have available a toaster oven (500 °F) or gas barbecue, a
propane torch, or a 1-hour round trip to a friend with a small heat
treating
oven. If scale is likely to be a problem with annealing, what is the
best
way to deal with that? I don't much care about the appearance of the end
mill holder end, but don't want to screw up the MT2 end.

Does anyone have any thoughts on the best approach?


I'm not familiar with that particular holder, Mike, but I'd be cautious
about opening it up for a larger shank unless the exterior is fairly large
diameter. If it compares favorably with the typical 3/4" holder, I
wouldn't hesitate, but it could prove a mistake if you put it to work with a
slender body. Screws may not hold, and it may offer chatter that a more
robust one wouldn't. I'm assuming it looks similar to a cat 30,40 or 50
holder.

Assuming there's no good reason to not open it up, the first thing you need
to determine is if it is hardened through. It may be induction hardened,
which often controls heat treat depth. If that be the case, you may get
through the hard layer and it will machine better, although you would then
be faced with a soft interior when finished. Regardless of how hard it
may be, I think I'd hesitate to anneal it in any way, not being able to
control where to stop, and scaling.

Grinding the size larger would be the ideal way to go, assuming it is
hardened, and it wouldn't be much of a job on a proper internal grinder,
where you have more power and rigidity and can keep the thing cool while
grinding. If push gets to shove and you can't do it otherwise, you may be
able to bore near finish size, then grind for size and finish. Carbide
should cut it, even if it's in the low 60's, Rc scale. I'm not suggesting
it would be easy, just possible. One thought could be using a carbide end
mill held in your toolpost, not the tailstock. Just a thought.

Luck. Tell us how it turns out.

Harold