> > 1. A homogenous piece of metal, evenly heated, will expand 
> > proportionally, incuding any voids.
> > 2. A pipe, with uniform walls, will expand when the 
> interior is heated 
> > (gun barrel).
> > 3. A pipe, with a wall thickness greater than $magic, the hole will 
> > contract when the interior is heated.  At some point, the hole will 
> > start expanding again.
> 
> Can you guys give us a definitve answer whether it is 
> possible that a hole in metal decreases in diameter when 
> heating it up, given the right circumstances?
> 
> Thanks a lot.
>    Gerhard - Saturday, 08/12/06 14:35:46 EDT
> ---------------------------------------
> Gerhard: In a material like steel that is a poor conducter of 
> heat the hole should close up when only the area around it is 
> heated, as the rest of the large plate will confine the 
> expanding material, and since the hole is the only place for 
> it to go, it should become a little smaller. This is theory, 
> 
> Anyway, I'm just applying physical principles and don't have 
> any real experience with that situation. But it seems that a 
> number of people that do have that experience agree with my theory. 
> 
> Gerhard
> 
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We can call $magic $TuringPoint if you like.

Your ball bearing & glue model sounds good.  I'd replace with glue with
rubber though.  Anyway, under heat, the atoms are behaving like little
children holding onto their parents hands - bouncing all over the place, but
not getting very far.  They're very determined to go somewhere.

Lots of "well, it should happen..." from those blacksmiths  :)  The
machinists all say it gets bigger.  I wonder what the physicists, material
science guys & the metallurgists think?

For the guy who said he tried it and it works, I'd take his answer with a
large grain of salt.  So you heat up sheet metal to close holes?  Remember,
the sizes we are talking about require a 1/1000mm inside micrometer (or
similar tools) to accurately measure the change in size.  Close up a hole
when the change in size is only 0.007mm per mm at 600 degrees?  What's he
smoking and can I have some!

Still, the ball bearing test is a good one.  Downside is it sounds easy to
do, but isn't.  I don't have the equipment to do that test.  Drilling the
hole is the problem, like the ball bearing, it needs to be perfect.  Drills
don't drill round holes (slightly oval) so you need a lathe.  A big one.
And a machinist capable of boring a deep 10mm hole to 1/1000mm accuracy,
with a mirror finish.  A blacksmith won't have the equipment either.  But it
can be done.

The comment about getting bulges (distortions) out of metal near holes by
heating it sounded odd too.  Thanks for the link BTW, it's been a while
since I've stumbled over that site.

I initially thought of placing a pyrex rod in the hole, and running a
current (lots of amps!) thru the metal (electrodes nears the hole).  Pyres
because it's non-conductive and low thermal expansion (3 vs. 12 for steel).
The rod will snag if the hole gets smaller.

Dropping a ball bearing thru it is obvious in hindsight, that's how they
demonstrate this to high school science classes.

Tony

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