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1st July 2016, 09:20 AM #1Novice
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expansion & contraction in wood question
Hi,
I've been reading on expansion and contraction in wood and how a woodworker has to plan for this. I understand that boards cup, bow etc and how this can be planned out, I also know that it expands and contracts more on its width. I guess what im asking is, how do you deal with expansion at joints? or is it no an issue because (e.g. dovetails) the joint isnt very large?
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1st July 2016, 03:09 PM #2GOLD MEMBER
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Yep, generally you need to consider differences in expansion and contraction (e&c) in planning every joint.
As you point out timber e&c's a considerable amount across the grain, but very little along the grain. Therefore, e&c along the grain is usually treated as though zero and ignored. Likewise e&c is usually ignored for man-made boards (plywood, mdf, pyneboard) because its just not enough to be significant.
So the rule is you never join a board with grain running crosswise (crossgrain) to a board with timber running along it length (longrain) without making special allowance for the obvious difference in expansion and contraction. A breadboard end is an example of just such an allowance. Table buttons are another.
Dovetails work because both of the boards you are joining are aligned the same way wrt the grain. This means both the pin and tail boards expand and contract in the same direction and by roughly the same amount - so there is no damage done.
Mortise and tenon joints work, even though you are usually joining long to cross grain, because the mortise is usually fairly small. Likewise dowel joints.
However, you shouldn't rely on size to save a badly designed joint.
When I first started woodworking I built a number of items with scant regard for e&c and the result was they slowly deconstructed over time. It wasn't that anything split or buckled, more just that the glue joints failed to hold up long term due to the stresses applied to them.Apologies for unnoticed autocomplete errors.
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