back gone flat

[Tim Detzer]

Morning!
Last night I glued on the back (15 ft radius) to a soprano uke I'm making. When I removed it from the clamps this morning there was no radius at all-completely flat. In fact in one small area in the lower bout it actually was very, very slighty concave. I checked the braces and they are still attached. I guess I clamped too hard and the brace strength was insufficient.

The question is leave it or redo it?

Thanks!

[Steve Senseney]

I have a couple of Uke's like that!!

The problem is not the clamping, but humidity issues on the back and bracing.

Your choices are to live with it or take the back off.

If you cannot live with it you can take the back off, and redo the bracing and then reglue the back.

What kind of glue did you use?

[Tim Detzer]

I can definitely live with it, but if I had to fix it I'd rather do it now before its finished, you know.

I used regular Titebond.

As far as the humidity goes mt basement is pretty stable BUT, the rel humidity has gone up by a few %'s since I braced it. Could a change from 40 to 45% flatten it out like that?

[Steve Senseney]

Evidently yes.

The tight bond can be softened with heat. If you can use a clothes iron to heat the joint sufficiently, it might release.

It can (possibly) be released with some addition of and vinegar.

[Chuck Tweedy]

However, normally if the humidity goes DOWN compared to the time of glue-up is when a braced plate will go concave.

[Steve Senseney]

But it could be that the back was a little higher humidity than the brace, or that the differential of expansion longitudinally, as compared to radially induced the change.

[Chuck Tweedy]

I suppose anything is possible. Over-clamping, etc. etc.
It would be very easy to remove - even with just a hobby iron.
Tim, this might be a good opportunity to learn a very valuable luthery skill!!

[Chuck Tweedy]

After reading the original post, I gota' ask...

How did you clamp it Tim??

[Tim Detzer]

Silly, should have checked it before I glued it on. I think I'll leave it if it won't be an issue.

Thanks for your advice!

[Tim Detzer]

I clamped it with cam clamps. At the time I didn't think I was using too much pressure.

[Chuck Tweedy]

Was the rim ground to fit the back in a spherical sanding dish??

[Tim Detzer]

Chuck,
I missed your first reply. I've actually had the pleasure of removing a back before. Glued the back on while the uke was clamped to the workboard throught the soundhole. Ooooops.

No, I don't have a dish. The rim was flat. I've never tapered the rim before-should I be?

[Chuck Tweedy]

Yes, I think with a small back like a soprano uke, the flat rim could pull the whole back in like that.

If you do not have a dish, you can sand with a circular-arc sanding stick to establish a reasonably arched rim.

[Tim Detzer]

Well, if that turned out to be the problem and not the humidity should it be fixed or will it remain happy for a while?

[Chuck Tweedy]

I don't really understand your question Tim, but if you want to just leave it - it will probably be fine.

[Tim Detzer]

Thanks Chuck, you just answered it!

[Michael Lewis]

To help prevent the plates from flattening out after gluing, you can use a hair drier on them immediately BEFORE gluing. This tends to shrink them a bit as it drops their moisture content temporarily, which returns after a while and the radius comes back. You can also help avoid over clamping with a clamping caul that fits the entire plate and contacts just around the edge, and padded with heater hose, fuel line, weather stripping foam tape, or some other fairly soft surface. Little things can add up to big differences.