Wood Movement on a round table - likely solved! - Woodworking Talk - Woodworkers Forum
 
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post #1 of 3 Old 11-02-2019, 08:33 PM Thread Starter
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Wood Movement on a round table - likely solved!

Hi Again,

I just wanted to follow up on my last post

https://www.woodworkingtalk.com/f9/q...-table-215279/

My original question was whether this assembly would split. Opinion was pretty divided, and after thinking about it some more Iím pretty sure splitting is absolutely inevitable. But I think Iíve found a pretty slick solution. Just wanted to update in case someone finds it useful, or can point out whether I'm missing something obvious.

First off, why it will split. I was having trouble getting my head around just how strong the tensions in the piece will be and whether it would be enough to cause a split, but I think there is a simple way to look at it. Based on the material (mostly Cherry) the outer rim will expand between 1 and 3 % (3% being the absolute worst conditions with a mix of quarter and flatsawn), which will make the radius increase from 30 to about 30.2-30.9 inches. Since the staves length essentially canít stretch, it means that the 1inch hole in the center will be forced to change from a half inch radius to a .7-1.6 inch radius, stretching the circumference of the hole between 40% and 220%. This is orders of magnitude larger than the 1 and 3% it naturally wants to expand. Unless Iím working with rubber, somethings going to give even in the best imaginable situation.

The circumference of the central hole will always be forced to expand more than it Ďwantsí to, but I think
(hope) if its expansion is in the ballpark of the natural expansion, then the piece will avoid splitting. For example, if the hole in the center has a radius of 15 inches, then the expansion of the circumference would only be 2.5%-6%, which is close enough to make me happy.

The solution Iím going to try is to include radial expansion joints (shown in white on the figure). They will be dry joints supported by dowelling to keep the table flat while letting the central hole move as it needs. The joints will seal at 15 inches where I am pretty satisfied the tabletop can take the movement. Iíll include 10" expansion joints from the outside in to account for compression, I donít think these will need to be quite as long (need to check the math on that).

Anyway, wish me luck! Hope this helps someone in the same boat. Iíll post an update once I get through thisÖ but that may be a while.
Thanks again for the feedback.
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post #2 of 3 Old 11-02-2019, 10:09 PM
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Interesting approach. I will be very interested in knowing the result, in time. Thanks for the update.
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post #3 of 3 Old 11-02-2019, 10:40 PM
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Expansion at the outer circumference ....

What ever number that is, from 30. 2" to 30.7" it won't be the same amount at the center hole. As the width of the wedge/triangle decreases the amount of expansion will also decrease virtually to zero at the center, because there just isn't any material left to expand. It will also be uniformly distributed along the lengths if all the members are the same material, Cherry. Lengths won't change, just widths. If the expansion of the pieces were at different rates, then you would have an issue.

Let's assume the we have a circular shape of 30" made of steel with a 1" center hole. We can heat the shape uniformly, let it cool, and reheat it all day long and it won't self destruct. It will certainly expand and contract, but will do so uniformly, so no issues. OK, steel isn't wood, but the material is homogeneous either way.

Unless someone builds this very difficult to construct shape, we may never know the "for certain" result, but I'll go out on a limb and say it won't self destruct.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 11-02-2019 at 10:44 PM.
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