Correcting cupped pieces?

[Tom Blank]

I’m working on a couple of small boxes (9 x 6 x 4) for granddaughters. They wanted them dyed, so I made them out of Poplar (sides, ends, and top) with ¼” BB bottoms. The box corners are splined miters and the top and bottom are in dados. The top floats and the bottom will be glued. My “workshop” is in an unheated garage. I started this project late last Fall, resawing and truing the stock. Everything was coming along nicely. I dry fit the sides with the splines, top, and bottom in their dados and brought the boxes up to the office to take pictures to send to the girls. No problems, girls are happy.

The next day, I started to take the boxes back apart and they were stuck tight. I finally got them apart and found all four sides and the top had cupped while sitting overnight in the office. Different temp and humidity than in the garage where I produced the parts.

I found that if I dampen the concave face of each piece and lay them damp side down on the desk, so the moisture soaks in instead of just evaporating, the pieces will flatten out. I took the pieces back to the garage and they got close to flat, but did not flatten all the way.

I’m feeling a bit stupid right at the moment. I’m thinking the pieces need to be flat in the environment closest to what they will live in, the office not the garage. The question: If I dampen the pieces, when the pieces get flat (and dry on all surfaces) can I put on a sealer coat (shellac or lacquer) and capture the “correct” moisture content and will the pieces stay flat?

I suppose on future projects, I’ll need to work only with sheet goods in the Winter, or keep solid stock in house, run it downstairs to the garage to work it, and return it to the house to keep it acclimated. What a pain.

Thanks,

Tom

[Lee Schierer]

You won't capture the moisture, but you will retard the change. If you coat the inside and outside of the boxes with equal amounts and types of finish the wood should stay flat. However, since these are boxes with lids, The lids will tend to make the environment inside different than that out side. Once you get them finished and bring them in the house, keep the lid open a few days so the moisture can equalize. Once they are glued up the splines and glue will also tend to hold them flat.

[John TenEyck]

There's no need to go through gyrations of storing the pieces in your house, etc., nor is there any need to use plywood instead of solid wood in the Winter, or Summer, or any other season. Wood constantly absorbs or releases moisture based on the RH it is exposed to and its MC. The key is for the wood to be in equilibrium with your shop when you work it. Future movement can't be stopped, but you can design for it with respect to how panels must be sized in a door frame, etc., and in how you build. In your case, once the boxes are glued up there won't be much cupping because each piece is supported by the two adjacent ones. The lid is a different story, but there's nothing you can do about that other than to finish it equally on both sides.

Just take the pieces back to your shop and leave them there a few days. They should flatten out again, or pretty close to it. Now build your boxes and finish them however you like, preferably equally inside and out.

John

[Mike Heidrick]

Did you buy the poplar from a box store?

[Tom Blank]

I realize wood changes with it's environment. Seems like it would be advantageous to true and dimension the pieces in an environment that is relatively close to the conditions where the finished project will live. I was so happy with the fit during the dry assembly, I was thinking I'd just use masking tape and roll the four sided around the top and bottom for glue up. May not be able to get away with it that simple now.

The Poplar came from a real wood store not a BORG. The individual pieces came out of two 48" long S2S pieces just under 1" thick and 6" wide. I jointed, resawed, and planed to final dimensions.

TB