Hi all,
I’m working on a table top and wanted to get some advice on whether any horizontal bracing would be necessary to prevent future warping.
The table top will be about 78″ x 40″ and made from walnut boards, which range 6″ to 8″ wide and have been planed to 1.75″ thick. I’ll plan to glue them up, with biscuits used for alignment.
I don’t think breadboard ends would work for the style of table I’m going for, but I was thinking about routing a space underneath to insert a steel C-channel, which would be screwed into the table at different points (only fully tightened in the middle to allow for movement).
The question – is the added brace needed to keep the table flat or is that overkill that I could avoid?
Thanks
Replies
Depends on the table structure and how stable the boards are. If the table is regular apron and leg construction the aprons might do the job. You could glue up your top and wait to see how it behaves before going to extremes. If it stays flat you can call it good.
If the table is a pedestal or otherwise not fully supported it may require the extra help.
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Also, to clarify, I'm going to buy metal legs that will be connected directly to the underside of the table top, so there won't be aprons or other cross supports.
Hello, I recently completed a walnut top for my kitchen island. It was about 48” x 96”x 2-1/2” thick. I definitely would do something to keep it flat. You do not want $1500 worth of walnut to end up bowed. Plus With the legs directly attached, they will never rest flat on the floor. I skipped the biscuits as i found it was unnecessary for alignment and definitely not needed for strength. Personally I like the look of breadboard ends. I M&T’d the ends and pegged them with walnut. I decorated just the corners with iron straps. Your c channel idea should work but the ends have to be tight enough so the steel keeps the entire surface flat yet not so tight that it restricts seasonal wood movement. I guess you will need to hide the ends of your channel too. Don’t forget to finish both sides.
If you use a screwed in steel-to-wood batten under the top to keep it flat, having some screws not fully tightened won't necessarily deal with any wood movement. The wood will still move the screw hole in the wood away from the screw hole in the metal. You'd need a slot in the metal rather than a hole, so the whole screw could move sideways with the wood if it expands/contracts significantly with moisture changes.
In addition, if the boards did try to cup or otherwise warp, this might effectively tighten the screw so it couldn't move, even in a slot in the metal baton.
Personally I would route a dovetail channels underneath the top into which matching dovetailed batons would go. The interface would need only a couple of screws in the centre of the batons, with the rest of the dovetail interface waxed so it could still slide even it it was tightened by the top trying to cup or otherwise warp.
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The amount of any potential warp (and wood expansion/contraction movement) would be lessened if the boards of the table top were quarter sawn rather than plain-sawn. But quarter-sawn boards, especially thicker ones, are expensive.
Lataxe
Yes, slotted holes goes without saying. My point was that I have seen lag screws in slotted holes tightened so much that they restrict wood movement on tops this large.
Andrew Hunter uses sliding dovetails in a solid wood panel surrounded by a frame. There is a video somewhere with Strano on this subject. Lataxe has the right idea if you don't mind a frame. Consider them breadboards all the way around.
https://www.finewoodworking.com/2017/05/31/lighter-stronger-frame-panels
Here is that video -forrestb is referring to. https://www.finewoodworking.com/2017/05/31/interlocking-chinese-joinery
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Yes, the C channel is an excellent idea. I would be sure to have one within a few inches of the ends.
If you think you can anticipate the direction of cup, if any, attach the channel to the concave side to keep the glue line on top compressed.
The only issue I see with dovetailed batons is the work involved in doing a tapered DT, on a top that thick it may not prevent cupping as well as steel, and finally, it will project below the table, which may not be desirable without an apron.
I will go with the brace.
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