On Thu, 27 Jan 2005 20:26:36 -0500, WoofWoof
wrote:
wrote:
On Wed, 26 Jan 2005 13:41:39 -0500, WoofWoof
wrote:
Doug Miller wrote:
Is there really a significant difference in thermal expansion rates for
plastic or aluminum vs. cast iron? Granted, the bar will get wider as it warms
up, but so will the slot. Does it *really* make a difference?
And what about expansion or shrinkage driven by changes in moisture content?
That's gotta be at least an order of magnitude greater than any thermal
changes in dimension, and obviously does not affect plastic or aluminum.
I doubt that thermal expansion is a significant factor over the likely
temperature range. Expansion/contraction due to humidity changes can be
a far more significant problem with wood.
I do see significant thermal expansion in the rails of my table saw.
it's enough that I can't trust the scale on my biesemeyer. now, that's
a lot more metal than a miter bar, but it's real. OTOH, the baltic
birch ply sliders on my sleds don't give me any problems with humidity
changes.
Well, to be fair the thread was on mitre bars and I was commenting on
the expansion of the significant (3/4") dimension but lets see:
The thermal expansion coefficients:
steel is ~ .000006 in/in/degF
aluminum ~ .000012 in/in/degF
UHMW ~ .00011 in/in/degF
wood ~ .000003 in/in/degF (cross-grain) **
(** I found several references for this suprisingly small value for wood
- less than steel. It seems to confirm that the major problem with wood
rails would be humidity rather than thermal expansion).
So for a 3/4" mitre bar over a 30 degF temp. range the expansion would be:
steel = .000135" (.000006 x 30 x .75)
aluminum = .00027"
UHMW = .0025"
wood = .00007"
For a cast-iron/steel table, the diferential expansion of the bar to
slot would be:
steel = 0"
aluminum = .000135"
UHMW = .0024"
wood = .000065"
For an aluminum table, the differential expansion would be:
steel = .000135"
aluminum = 0"
UHMW = .0022"
wood = .0002
Over this temperature range, I would suggest that only UHMW *might*
(just barely) be noticeably affected by thermal expansion/contraction.
(I still feel that humidity would be the controlling factor for wood. I
don't offhand have any hygroscopic data for UHWW ... some plastics -
nylon for instance - absorb a fair amount of water and can swell
noticeably with humidity changes).
As far as your front rail goes, assuming it's ... what, 4 ft long? The
expansion in that case (over the same temperature range) would be:
steel rail = .0086"
aluminum rail = .017"
I assume the scale is thin and bonded to the rail so the rail would
control the expansion. I'm not familiar with the Biesemeyer so I don't
know the material for sure but the expansion is almost 20 thou for
aluminum (but over the total length ... proportionally less for shorter
distances). Whether that's a problem in your work, only you can say.
(I think, in my case, my innate lack of craftsmanship would swamp it (:-) ).
the rail is about 6' long. and I live in Arizona. I'll figure the
shop temp varies close to 100 degrees from a cold day in the winter to
a hot day in the summer. that would result in a total variance of
..043, or about 3/64".
I'm definitely seeing more than that, so there's probably something
else going on. for instance, right now it's reading about 1/16" short
in the 36" range. it's not particularly cold weather right now- brisk,
but nice.
hey, and thanks for digging up those numbers.
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