joining top plates, one is cupped

[James Leitch]

This is my first attempt at a guitar and I've been having trouble joining the top plates. The wood is Western Red Cedar, quartersawn, but one of the plates is cupped about 1/8 inch or so. Right now I have it sitting, cup facing down, with a Stanley No. 5 plane on top to weigh it down.

Is this a significant factor? When I try to joint the edges should I press the board flat or clamp them together? When I go to glue them will this cause a problem?

Should I plane the cupped board flat before I attempt to join them? The other board is good and flat. If I leave it alone weighed down for a while, will it go flat?

Or is this piece brace wood now?

[Bob Gramann]

Worry about the humidity on one side vs the other. The dry side will shrink, the wet expand. Stand it up on its edge for a couple of days and see if the problem goes away. If not, you can dampen the inside of the curl to get it flat for jointing. After that, the bracing should take care of it. I’m guessing that the warped piece was the one on top of the pile.

Do you have humidity control in your shop? It’s essential and will save you from many problems downstream. Most of us build in the 40 to 50% humidity range. It takes an accurate hygrometer, a sealed up shop, and a dehumidifier.

[James Leitch]

Thanks for responding!

I don't have a workshop so much as a kitchen and a dining room, so humidity control is basically just my central air. The wood has been in the house for about 6 weeks or so, and during that time the RH has been on average 52%, with a total range of 47-57%. We do have a dehumidifier I could run in the kitchen but there really aren't many rooms in the house that can be sealed off besides the bedrooms. Even those doors have some pretty big gaps.

I think I misread your comment and wetted the outside of the curve, so that will be fun to deal with

The good news is that I bought another set of WRC in case I mangle this one. So if I can't get this one glued up I can try again with another set and maybe keep these for when I have a little more experience with such thin stock. The mahogany I bought for the back and sides seems to be behaving itself, at any rate.

[Bob Gramann]

In the relatively short long run, you will be happier if you solve the humidity problem. Setting up one of the bedrooms with the dehumidifier would be worthwhile. If you think the curved wood is bothering you now, you will really be bothered if the humidity changes between the time you start bracing the plates and the time you get the box closed.

The piece you misdampened will recover in time. Don’t worry.

When I first started, I was limited to working in the winter when the humidity was low. I graduated to a bedroom with a dehumidifier, then a section in the basement with plastic sheets separating it from the rest. The fire hazard of the hanging sheets bothered me. I now have a nice, humidity controlled workshop. Do what you have to do, but control the humidity.

[Nick Kitchener]

My first build had a 8ft long log of hard maple acclimating to detect any distortions, sure enough it did but a section perfect for the guitar was stable and perfectly straight.