I am building a fairly large table and asked about dealing with the wood
movement a few days ago.
After much thought I have decided to securely fasten down the middle of the
aprons perpendicular to the grain and fasten the ends of those aprons down
The aprons parallel the grain have to be able to move .8". I will glue
fiberglass dowels into them, and have the dowels ride in blocks attached to
the table. Does this seem reasonable?
1) As the wood expands, will the holes in the blocks get smaller or bigger?
I am guessing the hole will get larger, but I am not certain about that.
2) What woods have minimal movement. Would nice oily teak be good? I have
some scraps of that laying around. (I have some lignum vitae also, but that
seems like overkill.)
Sun, Sep 24, 2006, 7:01pm (EDT+4) snipped-for-privacy@Yahoo.com (Toller) doth
1) As the wood expands, will the holes in the blocks get smaller or
bigger? I am guessing the hole will get larger, but I am not certain
2) What woods have minimal movement. <snip>
1. Think about it for a minute. Before you ask next time. Wood
expands - smaller holes. Wood shrinks - larger holes. Pretty basic if
you think about it. Or aren't trolling. This post is reminiscent of
one of those troll dopplegangers.
2. Petrified wood.
Try google, or the archives.
You'll never get anywhere if you believe what you "hear". What do you
- Granny Weatherwax
It *is* pretty basic. The trouble is... you're mistaken. Perhaps you need to
spend more than a minute thinking about it. <g>
Wood expands -- hole becomes elongated across the grain. Wood shrinks -- hole
contracts across the grain. Diameter of hole along the grain does not change.
Doug Miller (alphageek at milmac dot com)
So I can't maintain a good fit. If I use a fiberglass dowel, the hole will
expand and contract on the dowel, in one dimension.
If I use a wood dowel, the hole and dowel will expand and contract evenly in
one direction, but unevenly in the other.
Maybe a fiberglass dowel running through a plywood hole...
Yeah, I know I am making this more complicated than it has to be.
I'm sitting here thinking about it and what you are saying doesn't make any
sense to me.
Here's my reasoning:
Suppose you have a square piece of wood that's a thousand layers across, in
both transverse and longitudinal directions. Suppose further that there's a
central hole that's five hundred layers across. Square hole or circular, I
don't think it matters.
If each layer starts out at t units in thickness, then the total span s of
the hole is 500t. Now let the wood shrink by delta (d) in each layer. The
new layer thickness is t' = t - d. What's the span of the hole now? s' 500( t - d ). Therefore the hole is SMALLER by 500d, not larger.
To take care of different shrinkage rates depending on orientation with the
grain, just use a different value for d. Use sines and cosines to your
heart's delight for orientations on an angle. You're still going to get a
different result from what you just said.
We heat bearings to make them larger for long enough to slip them over an
axle. They cool and shrink onto a tight fit around the axle. Same
mechanical principle, just a different mechanism.
As wood shrinks the hole will expand if you constrain the edges of the wood
containing the hole. But then you're facing a split, and that's a different
- Owen -
Yes. Make sure to attach those blocks to the table with screws not
glue, though. Otherwise it might be a little hard to get the dowels
into the holes, or remove the top if you want to at a future date.
Pretty sure you are right.
Are you talking about for the block? Why do you want minimal movement?
I think you should use the same wood as in the apron, so that as the
apron moves (and with it, so does the dowel), the hole will move along
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