Keeping Table Tops Flat


Econdron
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Title may be a little mis-leading, but I manufacture metal bases for table tops, I don't do any of the woodworking myself. I get questions quite a bit asking things like: Are your bases sturdy enough to keep my top flat? or other questions along those same lines. Hoping you guys can clarify some things for me, and hopefully I haven't been giving people ignorant answers. But if I'm not mistaken, tops should be made in a way where they don't need to rely on the base to keep them flat, correct? We put slots for the mounting holes so the wood can freely expand and contract over the bases. Sometimes I'll have people tell me they have a top that cupped a lot, like a couple inches over a 48" width and they want me to make a base to pull it flat. Is this a reasonable solution? There's a lot to this wood movement issue that I don't know much about, if a top warps, does that mean it was made improperly? I had a customer who told me when they started making tops, they used to finish 3 sides, but not the bottom to save time, and all their tables cupped, but they don't really have that issue anymore once they started finishing all 4 sides. Thanks for any help you can give me!

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Yes, and no. A slab not staying flat is a slab. There is not necessarily a problem with base or build. Wood moves, and the bigger the piece, the bigger the movement. If the slab is made of a bunch of smaller pieces and it does not stay flat, there may be an issue with construction or the balance of finishing. Ultimately, your base should not be expected to do much. If you bolt that slab off to a substantial base and it goes to move, you are encouraging checks and cracks. 

My two cents. 

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To an extent the base of a table will help keep the top flat. The better way to keep the flat is good construction practices but like said above that is not a guarantee. Having lumber that is properly dried and at EMC can go a long way in making things better.

I don't know if it would cause it but with dining room tables if the cup is downward with the center lifting up washing the table and leaving it wet with water could cause that. Finishes don't make wood waterproof especially those new trendy finishes that are hitting the market.

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43 minutes ago, Econdron said:

Sometimes I'll have people tell me they have a top that cupped a lot, like a couple inches over a 48" width and they want me to make a base to pull it flat. Is this a reasonable solution?

Nope.  Metal is strong but wood is going to move.  If it is already cupped you may be able to wrench it down flat but it will proceed to crack in the future.  It might do it right away or you may hear a load bang in the middle of the night. 

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33 minutes ago, Econdron said:

Ok good, that's pretty much what I've been telling people. So if you make a top that's pretty seriously cupped, it needs to be scrapped and re-made? I'm glad I don't do woodworking. You guys must have a lot of patience!

There are ways to salvage it but they make most people cringe. Putting kerf cuts in the bottom with a circular saw pulling the top flat and then filling said cuts with epoxy is a great way to fix it. It's not 100% on stopping it from happening again.

My favorite is to cut the trendy slabs in to lumber and go with a more traditional approach.

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Everything above is correct, but your metal bases should still be strong enough to counteract a top warping. In an all wood piece of furniture, the base is there to properly support the table top. Otherwise you would just glue and screw 3-4 legs to the table top and be done with it. In your situation, i assume you are making metal bases for predominately live edge slab tops? In that case, yes, i would expect to have a base capable of pulling 1/4"+/- of cup out of a 36" wide slab and keeping it flat for the life of the table. Just like i expect the base of my tables to keep my table top's relatively flat. If your customers need to mitigate over 2" of cup across 3-4' then they need to look to themselves to correct that. 

 

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18 hours ago, Pwk5017 said:

i assume you are making metal bases for predominately live edge slab tops? In that case, yes, i would expect to have a base capable of pulling 1/4"+/- of cup out of a 36" wide slab and keeping it flat for the life of the table.

 

Live edge slabs actually aren't that popular any more, at least not in my industry. It used to be everyone wanted one, that fad I feel like only lasted maybe a year. People still think they're cool and some are still buying them, but I've seen that business almost drop off completely. The bases we make go to all types of applications, reclaimed wood, "farm house" tables, slabs, even stone and glass tops. 

 

I've got a few tops here a local company gave to me as a trade for some bases, most of them are 1-1/4" thick, and our standard bases easily pull those flat (they all are slightly cupped, I don't trust the quality of this company), but I have a top that's 2-1/2" thick and another book-matched double live edge top that is also about 2-1/2" thick, this one isn't so much cupped, more like "v'd", like each slab is straight, but they both are angled at the seam where they are joined. These thicker slabs I can't get those pulled flat, if I put a very stout base under them, the screws just strip the wood, or snap. Guess I could use lag bolts, haven't tried that yet. They're not bad, maybe 3/8" from outer edge to center over 36" or so. I just can't get them to budge.

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It's very unusual for a dry, stable 2-1/2" slab to cup that much. It was either dried incorrectly, not dry enough, or never allowed to stabilize at ambient MC, or has the tree's pith right down the middle. Allowing movement and preventing cupping is something your bases can do to an extent, but if I'm assuming correctly, even if you deal with cupping, the slab will try to twist, or warp, and if you handle that it will crack.

23 hours ago, Chestnut said:

. Putting kerf cuts in the bottom with a circular saw pulling the top flat and then filling said cuts with epoxy

This works for wood that wasn't dried properly and had internal stress. I'm guessing not so much for wood that wasn't dried enough, as that one would cup backwards instead.

If it has the pith, well... you have to remove it. if possible, a V cut from the bottom that doesn't cut the top can be filled with epoxy to replace the pith without affecting the top appearance. Not the most elegant but could work.

if it's not dry enough, you'd have to sticker it under some weight and let it rest until it's ready to be used.

 

If the screws are stripping the wood or snapping, the base is not the problem. The base would be the problem if it bent or broke trying to hold down the top, or if it didn't allow for expansion/contraction, other than that, it's all about the wood.

 

 

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