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I'm curious about this mainly because i saw it being mentioned on woodworking.SE's posts, and some other sites.

Essentially, I always thought that wood could have "wood" movement, especially over time (e.g: aging) or when other factors such as moisture or molds are taken into account.

Taking into account the example I mentioned above (aging, moisture, mold) is it still true that wood movement is unlikely? Even longitudinally?

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  • Hi, welcome to Woodworking. You've read this both here and elsewhere, why are you doubtful? – Graphus May 11 at 8:46
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    Just to check, you're aware that wood does move plenty except in length, it's just that the wording of the final sentence isn't absolutely clear, "Even logitudinally?" to a native English speaker doesn't read correctly in context. – Graphus May 11 at 8:55
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TL;DR: It depends, but generally we can ignore longitudinal movement.

The idea is that compared with the movement across the grain movement along the grain is much smaller, and is often negligible for the purposes of planning for that movement in a build.

This is especially true at the scales woodworkers work with. I'm sure examples of longitudinal movement across giant spans in wood bridges meant that at that scale it would have to be taken into consideration. But this is generally not the case for most woodworking.

But, the cross-grain movement is so large it is the one we often care about most, or at all. But it is moving on those other axes as well. At the end of the day some of this movement might be less than the margin of accuracy we can get when working wood, in which case we simplify and say it doesn't move along that axis.

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  • It's not in comparison with the movement across the grain, it's in absolute terms! – Graphus May 11 at 15:52
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    @Graphus I disagree. Wood moves in all directions as it pleases. It's just that we only care about the gross measurement changes only along some directions. The point being that we can disregard, in most cases, movement along the grain. But it's not like moisture and temp only affects a single axis. The wood moves; it moves comparatively little along the grain even as it moves more across the grain. – jdv May 11 at 20:07
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    This reference says that longitudinal expansion/contraction is typically 0.1 to 0.2%, versus roughly 4 to 9% for tangential and radial movement (all for a 6% change in moisture content), depending on the wood species. extension.purdue.edu/extmedia/fnr/fnr-163.pdf – Mark May 12 at 2:44

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