Official Luthiers Forum!

So - Here's one of those questions that I just don't yet have a feel for - having only built four guitars.

Building a OOO Koa - 14 fret. Going well ,so far (for me that means I've not yet done irreparable damage to the guitar with a powertool). About to glue the back on (using HHG) (top went on very nicely - thank's all for guidance on accurate fitting of the braces to the sides Chris Paulick's video very helpful - thanks!)

So, I'm fitting the back to the soundbox and realizing that it has a bit of a reverse arch in it (e.g. I'm going to have to apply quite a bit of pressure while gluing to have the back follow the arch of the sides as they taper to 4 1/4" at the neck. Normal? Concern? I don't have a concern that I'll get a good tight fit - my go bar setup is sufficient to have a tight fit while the glue is settign, but I wonder about that tension over time.

Thanks!

Reverse arch? You mean the back curves in the other direction? It is normal to have to pull the back down at the headblock because the back tends to flatten out in that direction. And this is not a problem because the rest of the box is sufficiently rigid to resist the slight amount of tension this adds. But if you back is reverse curved, you probably have a humidity problem, and may need to carve off the braces and start again.

Hmm - yes - that is indeed what I mean - Humidity's always a challenge in my shop - with humidifiesr going full blast in winter I have trouble keepoing it about 35% RH and summer goes to 50 with air conditioning pulling wmoisture out. With no tenstion on it at all, it arches (reverse) only about 1/8" in the wrong direction. You recommend starting again on braces?

I usually have a little difference between the side contour and the back arch because there are no braces running longitudinally to hold that arch, but it shouldn't take much pressure to make it fit. It might depend on the size of your radius but I agree with Barry that you should redo the braces.

_________________
Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

Corky bro the confusing thing here is what Barry and Ken are pointing out. A ladder braced back won't fit the rim in the head block area without a slight bit of encouragement and this is perfectly normal.

If what you are seeing is that the back when say sitting brace side down on your bench has a slight depression in the middle of the lower bout then this is indeed a RH problem and the braces need to come off and be redone under stable RH conditions with well acclimated bracing stock.

Many builders try to maintain something like 42-48% RH in their shops at all times and especially before and during bracing for exactly the reasons of what you may be seeing with your back. I have taken a braced top with a 25'ish dome built into it out of my shop onto my deck to sand (trying to keep up with Lance..... ) and watched it flatten out in 20 minutes time.

For builders where maintaining proper RH is difficult it's a good idea to have the rim prepared to accept the top and back in every way possible except of course the pockets prior to bracing so that once the top and back are braced they can be attached in short order to the rim. Once the back and top are attached to the rim some RH fluctuations are not as potentially painful as when the plates are unattached.

If your shop has decent RH management with constant RH in an acceptable range braced plates can hang around forever awaiting attachment to the rim.

Thanks Hesh, Ken, Barry -
I didn't like the bracing job I did on that back anyway I'll take them off and go again (I think I'll wait a week - to let the bracing stock get used to the 50% humidity which will be the rule in my shop until I turn the heat back on in October.
I think I've got to focus on a better humidification system. My house gets crazy dry in winter (steam heat - counterintuitive) and I find it hard to live in anyway. The spot humidifiers I use in the shop aren't making a dent in it.

Thanks!