domed back goes flat

Construction and repair of Classical Guitar and related instruments
Ryeman
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Joined: Sat Oct 22, 2016 4:31 pm

domed back goes flat

Post by Ryeman » Sat Mar 03, 2018 10:59 am

A week ago I glued three bars on a guitar back and achieved a nice dome across the back of about 3mm. Yesterday I noticed that the lower bout area of the back had gone almost flat, and the waist and upper bout areas were completely flat.
Not the end of the world; the back hadn't been glued in place. I can plane the bars off and start again. But why has it happened? Will it happen again? And what can be learned from what has happened?
A bit more information..
My workshop is a "posh" shed in the garden. I don't keep wood in there. All guitar wood and work in progress is kept in my dry central-heated house.
The back was 40 year old Indian Rosewood, 2 piece, cut dead on the quarter. It has stayed dead flat ever since I bought it. The bars were Honduras Mahogany which I have had for more than 20 years. 5 years ago it was cut into 10 x 20mm strips and these have stayed dead straight.
Six months ago I joined the back, cut it into a guitar shape, but left it 5mm thick, storing it in the house, flat against a board. It stayed flat throughout.
A week ago I planed it down to thickness - about 2.7mm. I noticed that it was very stiff across the grain. My background is in making lutes and early guitars, and I don't have much experience with Rosewood. It was certainly a lot stiffer across the grain than Sycamore, and other European hardwoods that I am more used to working with.
After planing it to thickness I glued on a central re-enforcing strip of cross-grain Mahogany, using a caul to try for an end-to -end-dome of about 3mm. Not sure if this is normal in Classical guitar construction, but I wanted to avoid the back coming out hollow from end to end. When the glue was drying I planed the back bars down to 8mm thick, planed a 3mm dome on the longest, then used this as a template to mark out the other two.
Then I glued the bars in place, using cam cramps, and a thin flexible softwood caul behind each bar.
Next day the back was still domed, but the end to end dome had gone flat. Now, a week later, the end to end dome is back, but the dome across the back had almost disappeared across the lower bout, and completely gone at the waist and upper bout.
The back has been in the house for the whole of the week. I left it flat for a day, and when all seemed ok I stored it upright, on end, snug in a cardboard box, and resting against a flat surface.

I look forward to any comments that might shed light on the problem.

Alan

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Michael.N.
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Location: UK

Re: domed back goes flat

Post by Michael.N. » Sat Mar 03, 2018 11:11 am

This is a humidity problem, it's almost certainly changed since you glued the back bars and the recent cold weather will have taken it's toll.
If I make a flat back (as in a vihuela or baroque guitar) and leave that back lying around for a month or so they nearly always go slightly concave.
The answer is to control the humidity and to glue the back bars, shape them the same day and glue the back on to the instrument the next day.
Historicalguitars.

Ryeman
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Joined: Sat Oct 22, 2016 4:31 pm

Re: domed back goes flat

Post by Ryeman » Sat Mar 03, 2018 11:43 am

Michael, thank you for your prompt reply. What you are saying does make sense. The recent cold dry weather has dropped the humidity since the bars were glued on; the back has tried to shrink, and has pulled flat - trying to cup. When I made lutes I always artificially dried the front before gluing the front cross braces on. Maybe I should have done this with the Rosewood guitar back- five minutes in front of a fan-heater....
The cold weather actually stopped me getting on with things, or I would have got the back glued on , as you suggest I should have. But would the back still have pulled flat even after being glued on, if it was going to try to shrink when the humidity dropped? Do domed backs become flat over time?

Alan

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Michael.N.
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Location: UK

Re: domed back goes flat

Post by Michael.N. » Sat Mar 03, 2018 5:41 pm

Not in my experience. They may go slightly flatter but it's not something that is very noticeable.
Historicalguitars.

Ryeman
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Joined: Sat Oct 22, 2016 4:31 pm

Re: domed back goes flat

Post by Ryeman » Sat Mar 03, 2018 5:52 pm

So, presumably, some structural element helps prevent a back that is glued to the sides from flattening if there is a drop in the humidity level. Interesting.

Alan

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Brian McCombs
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Location: Union City, Michigan

Re: domed back goes flat

Post by Brian McCombs » Sat Mar 03, 2018 8:03 pm

Ryeman wrote:
Sat Mar 03, 2018 5:52 pm
So, presumably, some structural element helps prevent a back that is glued to the sides from flattening if there is a drop in the humidity level. Interesting.

Alan
Yes, the structural element is the brace and the radius you employed.

Once it is rigidly attached to the sides....it really cannot go much flatter unless the sides should splay out, which would be catastrophic. There have been times when I braced a top or back early, allowed them to sit, only to find them to have moved, some significantly. I had a top curl up in the most amazing demonstration of how braces can just up and curve the wrong way. I wish I had taken a picture of it. It was accidentally humidified on one side....long story. I don't brace anything until I'm ready and waiting to install it. Even without humidity fluctuations, wood will creep if allowed.

Ryeman
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Joined: Sat Oct 22, 2016 4:31 pm

Re: domed back goes flat

Post by Ryeman » Sun Mar 04, 2018 9:33 am

Brian, thanks for your comments.

Over the last 24 hours my guitar back has gradually assumed its original domed shape. No doubt this is due to a change in humidity. It has gone from being very dry outside to quite damp, and these changes have no doubt affected the humidity level in the house.

Alan

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