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While metal machinists’ straightedges are ideal for setting
up woodworking machinery, their high-tolerance accuracy and
weight are overkill for most woodworking tasks. For centuries,
shop-made wooden straightedges handled many layout and testing
chores, and the tools are still useful today.
Wooden straightedges are lightweight, simple to make in any
size and can be easily trued with common workshop tools. While
straightedges that resemble the machinists’ versions – essentially
a skinny rectangle with parallel edges – are useful, these tend
to go out of square quickly as the seasons change. However,
by understanding a bit about the way wood works, you can make
a wooden straightedge that is both accurate and durable.
Have you ever wondered why old wooden-bodied smoothing planes
have a boat-like shape? This so-called ‘coffin shape’ exposes
a lot more end grain to the atmosphere than a plain rectangular
shape does. Because wood absorbs and gives off moisture much
faster through its end grain than the other parts of the board,
a coffin-shaped plane will respond quickly to changes in humidity,
which will help prevent it from warping through uneven moisture
exchange.
This same principle helps keep a wooden straightedge true. With
a rectangular straightedge, the ends will shrink and swell faster
than the middle, requiring you to true the tool more often.
If you make one long edge of your straightedge curved or tapered
however, you’ll expose more of its end grain to the atmosphere.
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