Uke top bowed after bracing

[HogTime]

Building a StewMac soprano uke. Was getting ready to glue on the top and noticed it was noticeably bowed at the widest part. The gap between a flat surface and the outside of the top is about 1/8", inline with the bridge plate center.

I don't think it was that way after I braced it a a couple of weeks, but it could have been. Maybe happened because of the way I clamped it when gluing the braces? I wet down the bowed area on the inside and clamped it to a flat surface overnight. Not much, if any, improvement.

My main question is should I worry about it or just go ahead and glue it? If it's a worry, how should I correct it?

Thanks,

[sunburst]

A lot of us learn this lesson the same way. Brace your plates right before you glue them to the rim or they will likely cup (warp up or down at the side edges). If the relative humidity changes even slightly in your shop that will happen, so gluing to the rim immediately after bracing avoids that. You need to reestablish the exact same relative humidity in the shop as when the braces were glued to flatten it. Am I understanding you correctly that the plate is concave on the side with the braces? If so, and you want to be proactive rather than passive waiting for the humidity to return, you need to dry the wood, not wet it. Try a little careful work with a hair drier. You can probably watch the wood move. You can have a spritz bottle in one hand and a hair drier in the other and make thin pieces of wood move so much it can astound you!

[HogTime]

Thanks very much! Yes, it's concave. My "shop" is an area in my small house, so little control over humidity. We did just have some long overdue rain. Not sure when the curve occurred, though. I'll try the hair drier.

[Wilbur Tabacsko]

If the top is straight from head to tail and the area of the bridge plate is straight, with just the outside edges being curled, I would glue it and never look back. Would like to see it finished.

[Graham McDonald]

I will disagree with John (which I rarely do) and suggest that it doesn't really matter when you glue the braces on, as long as the humidity is at the correct percentage when you do it. The braced plate can set around for weeks as long as the braces were glued on at around 35-40% relative humidity. It is better to glue the top or back to the sides at about the same humidity levels, but you can usually get away with it if it isn't. Wetting it and clamping isn't going to change the shape of it. All you can do with your uke top and back is chisel the braces off, get some more bits of spruce and re-brace when the humidity is right. I suspect that it was too damp when you glued originally and when the weather returned to normal California dryness the outside went concave. If it is the other way around and the outside surface is convex, don't worry about it, but you may have to adjust the neck angle on the uke once the body is all glued up.

[sunburst]

Graham, that's not really a disagreement, you are correct that plate and brace wood that is at equilibrium moisture content with the shop air can be glued indefinitely in advance of gluing the plate to the rim, as long as the relative humidity is maintained at the same level throughout the process, but for many builders it is the exception rather than the rule to have stable wood in a stable environment. That is the reasoning behind gluing braces to rims, then gluing the plate to the rim immediately. It gives us a chance to get things lined up before they start to move rather than having to always use EMC wood in a stable environment. It can be a difficult thing for many builders, especially those without big budgets for shop environmental controls in many parts of the world, to maintain even relative humidity levels, so they can be faced with two choices: glue it up quickly before the conditions change or deal with warped plates.

[Graham McDonald]

What I was getting at was that even if you glue the braces and then glue the top/back to the sides in a high humidity environment, it will still go concave once the humidity drops. And then it is trickier to replace the bracing The critical thing is to only glue braces at the right humidity.

Graham, that's not really a disagreement, you are correct that plate and brace wood that is at equilibrium moisture content with the shop air can be glued indefinitely in advance of gluing the plate to the rim, as long as the relative humidity is maintained at the same level throughout the process, but for many builders it is the exception rather than the rule to have stable wood in a stable environment. That is the reasoning behind gluing braces to rims, then gluing the plate to the rim immediately. It gives us a chance to get things lined up before they start to move rather than having to always use EMC wood in a stable environment. It can be a difficult thing for many builders, especially those without big budgets for shop environmental controls in many parts of the world, to maintain even relative humidity levels, so they can be faced with two choices: glue it up quickly before the conditions change or deal with warped plates.

[John Arnold]

Concave on the brace side means that the top has gained moisture since the braces were glued on it. Once it dries out, it should return to a flatter shape.
Wood swells when the moisture content gets higher. The braces don't get longer, so the top responds by curving.

[HogTime]

No hair dryer available, but still success. Nice sunny day, so I laid the top on a table outside in full sun. Result: Much flatter. Not perfect, but good enough.

MC solves another problem!