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Forum topic by amdepalma posted 05-13-2016 09:18 PM 322 views 0 times favorited 2 replies Add to Favorites Watch
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amdepalma

3 posts in 215 days


05-13-2016 09:18 PM

Hey All,

Designing a new table and am wondering if I am going to have a wood movement issue here and would love some thoughts. First let me say that I feel like I know enough to be paranoid about wood movement after reading Bruce Hoadley’s “understanding wood” book so thought I would get a couple other eyes on it.
My primary concern with this design is the beam running through the arches. It is going to be 4 inches and tangential expansion and contraction will be happening on the vertical/y axis. 3 questions:

Will the movement across the 4 inch beam be enough to cause warping or cracking in either the arches or the table top?
If I were going to make the beam “float” in the channel carved out of the arches (to avoid any wood movement issues on the arches) to give the beam room to expand with 4) 3/4” dowels running through the arches on each side, would that be enough support to keep the table stable or is that just a bad design idea?

If you need me to illustrate my idea in question 2 I can do that and repost. Just let me know. Thanks for the help!


2 replies so far

View MadMark's profile

MadMark

976 posts in 913 days


#1 posted 05-14-2016 05:10 AM

Download a free copy of the USDA “wood handbook: wood as an engineering material”. It has charts on wood movement on every commercially significant specie of wood. Its a great reference for any woodworker.

M

-- Madmark - Madmark2150@yahoo.com Wiretreefarm.com

View JBrow's profile

JBrow

814 posts in 380 days


#2 posted 05-15-2016 02:07 AM

amdepalma,

From your description, I picture an arch and the beam assembly that supports and/or contacts the top. I am guessing there are two arches connected by the beam running perpendicular to the arches, perhaps centered on the arches. The faces of the beam and the arches are perpendicular to the table top surface and the edges of the arches and beam support and/or contact the top.

If I have this pictured correctly, and I am not sure I do, the grain direction of the arches and the beam are parallel to the top. If the same wood cut from the same area of the log (e.g. quarter sawn) are used, wood movement would occur but mostly equally in the arches and the beam.

In my picture, if the beam is especially thick, then it could also expand and contract perpendicular to the grain direction of the arches and could be a joint of concern. If the beam is especially thick, a mortise and tenon or tongue and groove joint where the beam joins the arch would reduce the amount of wood in the beam. This is because the portion of the beam entering the joint is thinner thereby reducing or eliminating the concern of the arch cracking.

I am not sure the dowel idea would solve the problem since the dowels could lock the beam to the arch. If the beam expands but the arch does not move, and the beam takes the dowels with it during expansion, the arch could crack. One way to glue a joint while allowing for some wood movement is to apply glue only in the center region of the joint. This allows expansion and contraction to occur in the un-glued areas of the joint away from the glued area in both directions. Alternatively, applying glue only in the region at one end of the joint directs expansion and contraction of the un-glued wood in one direct, away from the glued area of the joint. Also polyvinyl acetate (PVA) glues, as I understand glues, offers a little bit of elasticity, but not enough to prevent wood movement joint failure. But it is probably a good choice for either of these gluing techniques.

If I have this pictured wrong, an illustration of the assembly would help a lot. Also if this assembly contacts the top, the method of fastening the top to the base could be helpful.

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