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Back radius turned wrong
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=51246		 Page 1 of 1
Author:  jbrckovic [ Mon Nov 19, 2018 2:24 am ]
Post subject:  Back radius turned wrong
I am in the process of making first acoustic bass guitar, which is my first guitar build in general.
Back braces are radiused in a bracing dishes as specified in plans: 2 outer braces are 40'' and 2 inner are 32''
Everything was under control until I've glued backs to the sides. Now my backs are flat from the outer side, and in inside braces are radiused on the opposite side as they normally should be.
Not sure if it has anything to do with this, but humidity level in my garage dropped from 45-50 to 30 these days.
Also, I've made a mistake not gluing all linings for top and back at the same time. Solid linings for top were glued after gluing backs and sides, and without mold.
Did anything similar happened to anybody? Is there anything I could do to fix it?
Thanks in advance,
Josip
Author:  Colin North [ Mon Nov 19, 2018 4:56 am ]
Post subject:  Re: Back radius turned wrong
Yup, humidity drop will do this easily. I had it happen to an unattached back.
I would remove the back from the sides, remove the braces and start again.
Any particular reason for not using the mold when gluing the top linings?
Author:  DennisK [ Mon Nov 19, 2018 7:12 am ]
Post subject:  Re: Back radius turned wrong
Yep, unless you're building enough to justify the cost of climate control, winter is brace gluing season.
Author:  jbrckovic [ Mon Nov 19, 2018 7:35 am ]
Post subject:  Re: Back radius turned wrong
Hi Colin, thanks for your reply. Do you think it will get back once humidity normalizes?

I'm a hobby builder newbie, very interested for this acoustic bass build, so please excuse my ignorance but isn't it risky to glue braces and assemble the body in such low humidity? Or it actually doesn't affect joints that much. I planned to do it in 40-50% relative humidity.

To be honest, once back was glued to the sides, it didn't fit the mold perfectly. I think that the backs weren't trimmed properly, and I thought since sides were sitting good in a mold (weren't skewed) why not glue linings without mold, and sand back/side joint later.

I definitely don't plan to buy climate control for now, even though I would like to build so much that I'd need one of those.
Author:  Colin North [ Mon Nov 19, 2018 8:05 am ]
Post subject:  Re: Back radius turned wrong
jbrckovic wrote:
Hi Colin, thanks for your reply. Do you think it will get back once humidity normalizes?

I'm a hobby builder newbie, very interested for this acoustic bass build, so please excuse my ignorance but isn't it risky to glue braces and assemble the body in such low humidity? Or it actually doesn't affect joints that much. I planned to do it in 40-50% relative humidity.

To be honest, once back was glued to the sides, it didn't fit the mold perfectly. I think that the backs weren't trimmed properly, and I thought since sides were sitting good in a mold (weren't skewed) why not glue linings without mold, and sand back/side joint later.

I definitely don't plan to buy climate control for now, even though I would like to build so much that I'd need one of those.


Back radius is inlikely to return if humidity returned to 45 degrees. Mine, and many others didn't.

Better to brace / glue plates on in lower RH (40-45 or a bit less) than bracing in high humidity and having the humidity drop quite drastically as we have both experienced.

I prepare my assembled rims, profiling back and top to radius, then fit linings in the mould slightly proud, and sanding profiles again, before fitting the braced back and soundboard. All the time in the mould.
Don't see any advantage to doing otherwise, and it can have "unuseful" downsides like sides going out of square, or shape.
Author:  Haans [ Mon Nov 19, 2018 8:16 am ]
Post subject:  Re: Back radius turned wrong
ALWAYS...first thing to get in order to build instruments is HUMIDITY CONTROL...
Author:  DennisK [ Mon Nov 19, 2018 8:27 am ]
Post subject:  Re: Back radius turned wrong
It should return to normal at SOME humidity level, but not necessarily 45-50%. This is due to hysteresis. If wood is acclimated down from 60% to 40% RH, it will be wetter than if you acclimate up from 20% to 40%. So you'll probably have to get it up to 55-60% before it returns to normal.

I'm always harping on people to pay attention to acclimation direction, but everyone else seems content with maintaining a specific RH%, which only masks the problem since then you never see one turn inside out until after it's released into the wild. Usually results in builders demanding that customers maintain unnecessarily strict humidity control.

I recommend always drying the wood out and acclimating UP to the control level. Low humidity kills more guitars than high humidity, so err on the side of low humidity tolerance. But ideally you should adjust bracing humidity individually based on the species of wood. Brittle rosewoods crack from a small humidity drop, but most of them don't actually expand/contract very much dimensionally with humidity (especially if quartersawn), so they can take a large increase before the glue joints are stressed. Flatsawn sugar maple can take a lot of drying stress before it cracks, but dimensional change is huge, so it stresses itself a lot when the humidity changes in either direction.
Author:  jbrckovic [ Mon Nov 19, 2018 5:00 pm ]
Post subject:  Re: Back radius turned wrong
Thanks for your input guys, I appreciate it. Looks like I'll have to concentrate on humidity control
Author:  Haans [ Mon Nov 19, 2018 5:49 pm ]
Post subject:  Re: Back radius turned wrong
Josip, I do not know what your conditions are in Croatia, but likely you will eventually have to get both humidifier and de-humidifier. With wood this thin, it simply must acclimate to a small range of humidity, and it IS up the the ultimate owner to keep the instrument at that humidity. You cannot be responsible for the lack of understanding of an owner that is not willing to care for the instrument that someone has built for him.

You cannot build with wood that has not acclimated to a standard agreed on climate...that being from 40-45% humidity according to most builders. That also includes temperature, but we will assume a player will not take their guitar out in sub-zero weather just as they would not leave the instrument in the trunk of a car when it is 90 degrees outside.

Further, and instruments sides must be kept in the mold until all kerfing, top and back are glued on. This is accepted procedure by most professionals.

Likely, your back will not return to original shape and radius, sorry to say...
Author:  profchris [ Tue Nov 20, 2018 5:15 am ]
Post subject:  Re: Back radius turned wrong
I build without humidity control, though in the UK where humidity swings are less extreme.

My solution is to make a simple humidity meter, which is a 150 mm strip of wood made from two layers, each about 0.7mm thick, with the grain running at 90 degrees to each other. This curves quite noticeably as the humidity changes.

Mount it on a backboard and mark the extremes of movement (low humidity is when it moves to the cross-grain side, high humidity when it moves to the long grain side.

Once you know extreme your humidity swings are, always brace and assemble the body at the low end of the scale.

Really low humidity when building is a long-term problem though, because if you assemble at 20% and then the guitar spends its time living between 45% and 80% (which I'm guessing might be the indoor range in Croatia, both back and top will dome quite appreciably. In that case I'd try to build when it was in the mid-40s and take the guitar indoors if humidity is forecast to drop lower.
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