Bowed Back board

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TallDad71
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Bowed Back board

Post by TallDad71 » Sat Feb 10, 2018 3:18 am

I was attempting to make a 'Live Back' All going well fully thicknessed according to guidelines and braced.

One week later the 9m radius has inverted as the boards have bowed (concave rather than convex), it still sounds lovely but looks ridiculous. What am I to do to get the curve on the outside of the guitar again?

I haven't attached the board to the sides yet so I have lots of options. I dont really want to remove the braces and put them on the back as the symmetry isn't half as good on the inside.

I was wondering what the effect would be of sanding the boards down from 2.6mm to 2mm. Would this be enough for the braces to hold the radius or simply make the back flat?

Any suggestions gratefully received.
Alan
Peregrine Guitars

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kiwigeo
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Re: Bowed Back board

Post by kiwigeo » Sat Feb 10, 2018 6:15 pm

What's the bracing pattern? Are you using only cross braces or a Gore style bracing pattern with radial bracing over the lower bout?
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Re: Bowed Back board

Post by Mitch Lees » Sat Feb 10, 2018 7:07 pm

Did you by any chance take a note of the humidity level when the braces were glued and what it was when you noticed the radius reversal?

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TallDad71
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Re: Bowed Back board

Post by TallDad71 » Sun Feb 11, 2018 4:22 am

Humidity is definitely relevant. The boards came from Spain, dry and arrid and arrived in England, home of the raindrop! They’d had three months in the house to acclimatise and were very flat afterwards. Sanding down to 2.5mm showed up slight curvature, I braced against it with pattern as above.

The back is currently sat in a jig which is bending the convex pattern back in. This won’t help long term I know but will give me a chance to think about the issue
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Allen
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Re: Bowed Back board

Post by Allen » Sun Feb 11, 2018 6:45 am

What was the RH when you braced it up, and what is it now? That will tell you what needs to be done.

I suspect your only option is to remove the 3 transverse braces at the very least (you might get away with the 4 radial ones) and redo them with RH at a more appropriate level.
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TallDad71
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Re: Bowed Back board

Post by TallDad71 » Sun Feb 11, 2018 7:39 am

Unfortunately RH was constant throughout as braces and backs were moved from house to workshop and back again in 2 hours. RH was about 2° different to house around 55% here in U.K.
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Re: Bowed Back board

Post by kiwigeo » Sun Feb 11, 2018 8:58 am

Allen wrote:
Sun Feb 11, 2018 6:45 am
What was the RH when you braced it up, and what is it now? That will tell you what needs to be done.

I suspect your only option is to remove the 3 transverse braces at the very least (you might get away with the 4 radial ones) and redo them with RH at a more appropriate level.
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Trevor Gore
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Re: Bowed Back board

Post by Trevor Gore » Sun Feb 11, 2018 12:44 pm

TD 71,

I think you have the book. Check out Build, Section 4.1. If you don't want to be plagued with perennial problems, both in the near term and down the track, some form of humidity control is essential. Any cross-grain bracing scheme will attempt to invert the panel curvature once the RH drops below the RH the bracing was glued on at. A 10% drop is enough to cause inversion.

BTW, I normally brace backs with a 9 foot radius (3m) rather than 9m (~30 feet). I do tops at ~ 9m. Not sure that when you mentioned this, it was what you actually did, or just a typo.

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TallDad71
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Re: Bowed Back board

Post by TallDad71 » Mon Feb 12, 2018 4:14 am

I don’t think that the RH is the issue. Or at least if it is I suspect it is the difference between here and Spain. My dehumidifier is always pumping out water from the workshop and the reading between house and shop are similar.

I suspect the issue is more to do with the curvature of the wooden braces not being able to keep the natural shape of the billets in place.

Trevor, I will reexam my radius dish, it wasn’t a typo as I have been using a 9m dish not 9 foot. I will explore the option of re bracing with a 3m dish.

Thanks for all your help folks.
Alan
Peregrine Guitars

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Re: Bowed Back board

Post by simonm » Thu Feb 15, 2018 7:37 am

For the next one, two thoughts.
1) If the wood seems to have any tendency to warp, use that to your advantage. Brace to take advantage of any "natural" tendency.
2) Don't brace either top or back until you are ready to close up the box. I aim to get the box closed within 36 hours and quicker if I can. That makes humidity control easier. I generally brace the top first and do the back while I am waiting to do other stuff on the back. I do not start any bracing unless I am sure I have the time to do it in two days.

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Re: Bowed Back board

Post by jeffhigh » Tue Feb 20, 2018 5:55 am

I would very much doubt that the sourcing from spain has anything to do with it. You are doing yourself a disservice if you put the problem down to this.
At 2-3mm timber acclimatises very quickly, 3 months is plenty
55% RH is pretty much the top of the range for bracing, most are aiming for 45 or lower.
Also consider that hygrometers are not always accurate

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TallDad71
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Re: Bowed Back board

Post by TallDad71 » Mon Feb 26, 2018 10:46 am

Cracked the issue. It was a humidity problem! Not the back board itself but the bracing, I had mistakenly used spruce kept in the workshop overnight rather than fresh from the house. Once removed the board sprang back to shape the the bracings turned to bananas.

Thanks for all you help guys.
Alan
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kiwigeo
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Re: Bowed Back board

Post by kiwigeo » Tue Feb 27, 2018 2:29 pm

TallDad71 wrote:
Mon Feb 26, 2018 10:46 am
Cracked the issue. It was a humidity problem! Not the back board itself but the bracing, I had mistakenly used spruce kept in the workshop overnight rather than fresh from the house. Once removed the board sprang back to shape the the bracings turned to bananas.

Thanks for all you help guys.
Glad you got it sorted :)
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Re: Bowed Back board

Post by jeffhigh » Wed Feb 28, 2018 6:01 am

TallDad71 wrote:
Mon Feb 26, 2018 10:46 am
Cracked the issue. It was a humidity problem! Not the back board itself but the bracing, I had mistakenly used spruce kept in the workshop overnight rather than fresh from the house. Once removed the board sprang back to shape the the bracings turned to bananas.

Thanks for all you help guys.
Unlikely
Unless the bracing was bone dry when glued, it is not going to lengthen from regaining moisture and cause backbow.
If the bracing was at high MC at gluing time and shrinks it will increase the radius rather than causing backbow.
Longitudinal movement is much much less than the crossgrain movement in the back board

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