Wood Movement Question

[Jeff Craven]

Will screwing this brace to the lid do anything to prevent the lid from cupping?

The lid is 20" wide and the brace is 3/4" x 1".

[Jerry Miner]

How thick is that top? The brace looks a little wimpy compared to the top, but maybe it's a trick of the camera angle. (I am not an engineer, but I think a 1 1/2" brace is at least twice as resistant to bending as a 1" brace)

The brace will help, even if it's undersized---but the IMPORTANT thing is to screw it on dry, no glue, through slotted holes, as the top will expand and contract perpendicular to the brace. If it's not allowed to move, something bad will happen (cupping, splitting).

[Robert Payne]

I built a Toy Chest (really a hope/blanket chest when he is grown up) for my grandson about 6 years ago adapted from a plan in American Woodworker that has a 19" x 48" lid made of glued up 4/4 white oak milled to about 7/8". I added three cleats -- shown in the picture below -- that are about 2-1/2" wide and are fastened with slotted holes (plugs over the screw heads). The top has never cupped at all thanks to those cleats. As an aside, I'm also shown a picture of the two-masted schooner carved into the top that I protected with a 1/8" sheet of Lexan secured with mirror roses at the corners. My grandson turned eight two days ago and the chest still looks great.

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[Art Mann]

If you choose to use the braces, I suggest you fasten them on with wood screws rather than glue. This will allow a small amount of movement between the top and the braces. Otherwise, you might be doing more harm than good. Wood expands and contracts at different rates, depending on whether it is with the grain or cross grain. This property could actually induce warping under extreme changes in moisture and temperature if the parts are too tightly fixed together. I have seen a number of requests for help on this forum from people who experienced just such a problem.

[johnny means]

Gluing your brace on will be the cause of cupping.

[Jim Matthews]

FWW showed a similar design executed by Andrew Hunter
with the cleats inset in a sliding dovetail.

The top is free to expand along the length of the cleat,
but there's a continuous mechanical connection.

The cleats are more robust than those you've shown above.
He used quartersawn oak for the cleats, for dimensional stability.

http://www.taunton.com/promotions/FW_Blanket_Chest.pdf

[Jeff Craven]

Thanks for all of the tips. The dovetailed cleat is something interesting that I've never seen before.

My lid is only 3/4" thick, but the wide bevel makes that picture a little deceiving. It cupped quite a bit over the winter when it was dry in the basement. Now it is getting more humid, it has flattened back out. I wish I just made breadboard ends from the beginning, but didn't really have the tools to do that.

[glenn bradley]

FWW showed a similar design executed by Andrew Hunter
with the cleats inset in a sliding dovetail.

The top is free to expand along the length of the cleat,
but there's a continuous mechanical connection.

The cleats are more robust than those you've shown above.
He used quartersawn oak for the cleats, for dimensional stability.

http://www.taunton.com/promotions/FW_Blanket_Chest.pdf

Cool solution. I had seen that article in passing but, missed that detail.

[Andy Pratt]

Brace looks a little too small for the thickness/size of the lid, but certainly better than nothing. The brace is obviously going to fight against lid movement as others have mentioned so you can't glue it and can only screw it if you do slotted screw holes. People get away with just screwing/nailing them in place but those people's designs don't hold up unless their wood is perfect (quartersawn) and they get lucky, the dovetail idea is better.

If you are stuck with an existing project that is cupping then do multiple braces as per your picture or thicker with slotted screwholes and you will get a satisfactory result with the minimum amount of modification.

[Rick Thom]

One of the best solutions to combat seasonal movement is to finish the board the same on the bottom side as the top. This won't eliminate cupping but will lessen moisture being trapped and absorbed by the wood fibbers.. the cause of the cupping and expansion/contraction.
I also agree with Jim's suggestion (post #6) re: sliding dovetails. Although using QS material isn't mandatory, it will be less susceptible to bending.

[Kent A Bathurst]

1. Robert is correct - use slotted holes in the cleats.
2. Johnny is correct - using glue will cause problems.
3. Sliding dovetail is cool. Kind of over-the-top compared to the usual solution, but if you have that kind of time for a project like this - tee it up.
4. Rick is correct to a point: You should use basically equivalent finishing schedules on top and underside surfaces, so that moisture movement is equal. Will not stop cupping. But - if you don't do it, you will make it worse.
5. Last observation -

Those strap hinges are not going to allow for seasonal movement. The lid is going to expand, with the hinged joint as the reference point. That steel will not move. Period.

I don't know the species. Let me guess at maple. I betcha the entire 20" lid will expand and contract by nearly 1/4". The hinges look like maybe 15", so from the hinge to that last screw will expand and contract by 3/16" or so. All guesses on the dimensions, but you get the point.

Life is going to be interesting in a season or two.

[Jim Matthews]

Cool solution. I had seen that article in passing but, missed that detail.

There's a companion video, showing the low tech method of cutting the cleat and channel with the same guide.
He drives it home with a mallet and fixes it at the back so that the clearance angle is constant on the lid.

It's as simple as possible, but no simpler.

As with most of the clever things I've mentioned, I wish I had thought of it...