On 05/29/2010 09:34 PM, Ignoramus6711 wrote:
Sorry for a bunch of posts at once. I worked all day long (almost) and
gave 1500 lbs of old junk to a scrapper.

Never had this problem before and I REALLY do not want to screw up.

The head of the Bridgeport CNC mill is attached to the turret with
four socket head cap screws, probably 1/2". They have a 3/8" hex hole.

All my attempts to unscrew them failed. I used a nice Unbrako L shaped
key.

Cannot do it without leverage.

With leverage (pipe) the key twists so much, that I really do not want
to push my luck.

I made a tool today from one 3/8" L shaped key: I drilled a
perpendicular hole close to the end of a pipe, and welded the straight
end of the key into that hole. So I can use the pipe as a lever to
turn the key without as much length to twist.

-. (L key)
pipe |
============|=

^ this end will go into the SHCS

I have no idea how will the key behave after having been welded.

I also have hex keys that are driven with a 1/2" breaker bar or an
impact. I am really afraid that it may somehow break the SHCS, which
would be terrible considering its job (holding the mill head).

Any comments?

i

Heat, cool, apply penetrating oil and wait, bang on it as you torque on
it. Repeat. I've never done the heating thing on a heat-treated screw
-- with cruddy old grade 3 bolts or equivalent it often helps a _lot_ to
get the bolt up to red or even orange heat.

Basically, rust is strong, but rust is brittle. If you can break the
rust that's holding it in there, then it'll come right out. All the
temperature cycling and beating on it and impact wrench stuff is to
break the rust. Penetrating oil weakens the rust and keeps it from
reforming bonds (does anyone know why?) so not only will it help
directly, but it also helps that if the rust particles are all
surrounded by penetrant when you break them, they'll stay broken.

With old cars, sometimes the best thing to do with a stuck bolt is to
twist the head off, get the part that it's holding down off, then figure
out what to do with the stub. That's a pretty extreme solution, though
-- usually the above is what you tell yourself after you torque on it
and feel that sick feeling of metal giving way.

If I were in your shoes and I stripped out the socket in the bolt, I
wouldn't hesitate to arc weld a right-sized bolt in there -- that'll
both give you plenty of meat to get a wrench on, and it'll give you some
pretty extreme temperature cycling on the bolt you're trying to get out.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com