Long meandering story follows:
My uncle showed me a sort of huge bolt that he was required to make as a
demonstration in a machine shop class that he took fifty years ago. It was made
in two pieces; a round shaft about two inches in diameter, and a square head in
a correct proportion to it. In order to make it, he machined the shaft and
threaded it on each end; fine threads where it joined the head and coarse
threads for the business end. He machined the head and threaded it inside to
match. He also left the threads on the shaft slightly oversize. He then put the
shaft in the freezer and the bolt head on a hot boiler. When he put them
together, the shaft fit into the bolt head, and once that their temperature
stabilized together they were virtually inseparable. I think he also made a hex
head nut to go with it, but don't quote me on that; doesn't matter anyway.
The point is that his expectation was that the bolt shaft would shrink when it
was cooled, and it apparently did; for it wouldn't fit the head when they were
the same temp. The bolt head apparently expanded in all directions when heated,
making the whole bigger, too. Or did it? Was my uncle wasting his time heating
the bolt head? If holes get bigger when material is cooled, should we be using
the 'ice blue wrench' instead of the 'blue wrench'? If he had cooled them both,
would they have gone together even easier, or would their sizes have remained
proportional to each other and the shaft threads still oversize in relation to
the bolt head?
Just another thought for the discussion.
Jon Larsen
79 MGB, Pageant (ice) Blue
>>> Chris Chandler <spawn@net-link.net> 12/01 5:20 PM >>>
Michael F. Adamson wrote:
> When materials get cold, ice is an exception, they shrink. The atoms
> occupy less space due to their decreased activity. Holes in material
> become larger. A piece of material, say a piston, becomes smaller
> therefore increased clearances.
That's the simple part (the piston that is)... now... take a hole in a
large piece of material. Does it get larger or smaller due to shrinkage
when cold? My memories of a materials class I had a few years ago tell
me that the hole will get bigger because the material shrinks back
toward the center of mass, not towards the empty space in the middle.
Anyone have a more definitive answer than my half-remembered and
possibly wrong memories?
Regards,
Chris
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