Wood Movement in a table

Sun, Sep 24, 2006, 7:01pm (EDT+4) snipped-for-privacy@Yahoo.com (Toller) doth puzzedly queryeth: Questions:

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.

JOAT You'll never get anywhere if you believe what you "hear". What do you "know"?.

- Granny Weatherwax

It *is* pretty basic. The trouble is... you're mistaken. Perhaps you need to spend more than a minute thinking about it.

Wood expands -- hole becomes elongated across the grain. Wood shrinks -- hole contracts across the grain. Diameter of hole along the grain does not change.

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' =

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 situation altogether.

- Owen -

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.

Or, think about it for two minutes and come to the opposite, correct conclusion!

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 with it.

One wood with very little movement is mesquite.

todd

Classic!