[TheForge] Re: Shrink fitting steel to wood

[Mike Spencer]

> Ideally,  wooden hub should be made of elm.

You may say that.  I was once given a bunch of elm bolts for firewood,
and took the trouble to haul it all home.  Back before I knew better.
I believe I was able to split a couple of pieces off just to prove who
was master but it took a while and I'm pretty handy with axe, maul and
wedges.


The splitting tendency of elm relates to that of maple of oak about the way
the crumbling tendency of, say, fibreglass grinding wheels relates to that
of crystal wine glasses. :-)

The coefficient of thermalexpansion of steel is around 13 x 10^6. By
my quick calculation, a 4" ring would gain about 1/32" in
circumference (or ca. ten thou in diameter) if heated to 400 deg. F.

I vaguely recall that some old timer told me once that if you heat a
metal band to where it just barely chars or browns the wood it's going
on, then force it on a cool it quickly, that the slight charring makes
for a perfect fit that the heat shrink is enough to make a
difference.  Can't remember my source.  Might have been the guy who
hewed the replacement rafters for Old North Church in Boston 70 or so
years ago.

- Mike

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                 						   /V\ 
mspencer@tallships.ca            						  /( )\
http://home.tallships.ca/mspencer/						  ^^-^^

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