Tabletop Cupping--HELP!!

[Todd Bluth]

Hey everyone,

Newbie here, hoping to get some collective wisdom. I'm a novice, and I'm making a dining table out of hickory. Glued up my boards, got the tabletop milled flat, cut in half (I'm making a leaf as well), set my two sides on a workbench for about 4 days. Finished thickness is 1 3/8". Came out a few days later to see one side had cupped considerably down the middle along the grain, about 1/4" across the width (41"), maybe a bit less.

Did some research here and I think I know what I did wrong, both in glue-up and in laying it flat in my workbench rather than on stickers. Didn't help we had a major cold front swing through and temps/humidity changed drastically.

Long story short, I've got both ends of the table top on stickers now, crown side up, and put weight on the top for about 36 hrs now. Seems to have helped some, the cup is down to about 1/8". My question is, I've read that applying water to the convex side can help flatten the board by evening out the moisture content--should I try that? Should I just keep the weight on longer? How much weight should I be putting on?

Related note, is this an issue with hickory? Made my last table from poplar and didn't run into this problem.

Thanks!

[Steve Eure]

As you are aware, all woods move. Some more than others. You did not mention the type of cut of lumber you are dealing with. Is it rift sawn, plain sawn, quarter sawn, etc.? If it is anything other than quarter sawn, than you may need to alternate the grain pattern, during glue up.
In my experience, even with weight added, the cup may return with seasonal changes. My advise would be to wait to see if it cups when you remove the weight. If it does, I would re-cut it along your glue seam, flip every other board and re-glue. That may be your only option.
Keep us posted on the outcome. You are sure to get more and possibly better advise from others. Good luck.

[Jim Dwight]

I don't understand why 1/8 of warp over the width of the table is an issue. It you fasten it to a base, that should pull out fairly easily (but Hickory is hard and and strong). If you can clamp if down flat you should be able to pull it down flat with your favorite way of fastening the top.

I am also not a fan of alternating growth rings. I've done it but it didn't seem to change anything. I orient the wood by appearance. It works out fine.

I just don't think it's a reasonable expectation that large wood panels will stay flat on their own. That is why table tops get fastened to a base. Cabinet tops get fastened to the cabinet. The structure holds the panels reasonably flat. A little movement is inherent in wood. Only way to eliminate it is to use a different material. If it is totally flat today, that doesn't mean it will stay that way.

[Dan Hahr]

Leave it on stickers and wait. When the moisture content equalizes. It will go back to flat. Don’t rush it.

Dan

[Todd Bluth]

Thanks everyone for the input! Jim, I think ordinarily fastening the table down would be just fine, but this is actually a floating tabletop on runners fastened inside the aprons (to make space for the leaf) of a four leg base, and the crown would point downward anyway, so I don't think that particular solution will work here...although that would certainly be easiest. If this were a trestle table without a leaf, I would absolutely do it that way.

Dan, approx how long do you think that would take? I know it depends on humidity and MC and such but...maybe a ballpark?

I had a eureka moment today. I realized I could flatten the tabletop sides with my body weight (185). So it wasn't a ton of pressure at all. So I put one half of my tabletop on tall blocks crown side up, put a huge block on top as a fulcrum, then put the other half of the tabletop on top, crown side down. Put pipe clamps on each corner, then tightened the sections gently against each other until everything was dead flat. Didn't require much pressure at all. I'll put 2" hickory breadboards on each end using M&T, and if it hasn't already flattened out on its own like this in a couple weeks, the breadboards ought to keep it acceptably flat as well...right?

[Jim Dwight]

Seems like a good plan to me.

[Prashun Patel]

The thing you have going for you is that you can press it flat pretty easily. This means you should be able to mechanically fasten it reasonable flat with a breadboard or screws strategically placed underneath.

I wouldn’t put faith in water or weights as anything more than a temp fix.

The best mechanical solution imho is hidden battens. You can dovetail wood battens but they would normally be thick enough to be visible at low angle viewing. If that it not compatible with your design you can recess u channel into the bottom, fastened with elongated holes and lag screws. If your top is thin, drill the screw holes into the corner of the u channel so it goes in at 45 deg to give your screws a little more purchase (I just saw this trick on a Stickley branded table my parents bought.)

[Lee Schierer]

I'll put 2" hickory breadboards on each end using M&T, and if it hasn't already flattened out on its own like this in a couple weeks, the breadboards ought to keep it acceptably flat as well...right?

Just be sure that you allow the wood to expand and contract with seasonal changes of moisture. Also I highly recommend that you apply equal amounts of finish to all sides of your pieces to help equalize future moisture changes. I've built several pieces of furniture from hickory. Two of the first ones, were dressers and I glued the drawer runners to the solid wood side panels. About a year later I heard this loud pop one night. It took me a few days to figure it out, then I noticed that two of the side panels had split top to bottom because the wood couldn't move. All the pieces I've built since then have not had restrictions on panel expansion and contraction seasonally and I've had no additional problems. This desk top is 30 x 72 and is attached with figure 8 fasteners to the base. It is still flat.

[Todd Bluth]

Thanks, good call on the finish. I was planning on cutting the outer mortises about 1/8" wider than the tenon to account for movement.

[Richard Coers]

Thanks, good call on the finish. I was planning on cutting the outer mortises about 1/8" wider than the tenon to account for movement.

1/8" isn't much for a 41" wide top. I usually build in a 1% allowance for movement. That works better if the top is restrained in the middle and both ends move.

[Todd Bluth]

So roughly 1/4" if my math is right?

[Dan Hahr]

Put it in your house with both sides open to air movement. Give it a couple weeks. It didn’t take any physical force to warp it. It won’t take any to flatten it. You have to allow enough time to undo the unequal moisture transfer you caused.

Dan

[Lee Schierer]

Thanks, good call on the finish. I was planning on cutting the outer mortises about 1/8" wider than the tenon to account for movement.

Probably not enough. This is where a decent moisture meter can be your friend. If you know where your lumber moisture level is you can determine from various tables where it is likely to go in the location where it will be used. I built a soft maple dresser for my son in Erie Pa and shipped it to Tucson AZ. After a short time there the top had shrunk nearly 1/4 inch. After returning east a few years later the dresser top returned to the original size.

This example will show how much a hickory floor (.00369) and a pine floor (.00263) will expand when the moisture content changes from 6 to 9 percent across a 15 foot span of hardwood.

Hickory: 3 x .00411 x 180 inches = 2.21 inches.
Pine: 3 x .00263 x 180 inches = 1.42 inches.

In your case 3 x .00411 x 41 inches = .50553 inches

[Mel Fulks]

Hickory is not gonna behave like fine mahogany. Mahogany is great ,but It's more useful to the craftsman than the
buyer !