First up, I finally completed guitar #1 after all your good advice on side bending. I'm very happy with both the look and the tone of it. So happy in fact that I am building three more.
Some advice please.
I have built the three backs. Two Indian Rosewood and one Sapele (like build #1).
They have been hanging in the workshop for a couple of weeks awaiting the other bits for final assembly. The Sapele one has gone rogue on me! It has become concave rather than the 15 foot convex radius that I built it to. And it rigidly refuses to do anything different.
Is there a cure that I can use to muscle it back to convex?
Only to remove the braces and rebrace at a lower Relative Humidity.
What is the RH now and what was it when you glued on the braces?
If you don't know, then this back is teaching you a lesson and better now than when it is glued on to the sides.
It was either 60% or 45%.
Can't honestly remember and I guess that underscores your point! The lesson is well learned. If it WAS 60% then there's the issue huh?
It's 41% now. Is there enough difference between that and 60% or 45% to cause this rogue behaviour?
Bloody hell - this is Witchcraft!
Unfortunately a glued up back which becomes concave is telling you it's history, that it was glued up at a much higher moisture content than it is in now and you just cant argue with it.
If you have a problem now, imagine what will happen when you get down to 35
But look on the bright side, you can pull the braces now and fix it and not have a closed box with a concave cracked back
seems to me you could reverse back braced side and follow the wood since you now know how it wants to be...flip and redo in other words...? But wait for others op's.......
It's not so much that the back plate wants to go into a particular curvature, it's that it is shrinking across the grain but being restrained by the bracing and pulling out the curvature.
Jeff pretty much sums it up as it happens (and it's happened to most of us) - another issue that you may or may not be across is that in flat sawn (as opposed to quartersawn) timber there is a direction of "cup" depending how the wood came out of the tree and the orientation of the growth rings. I'm on the fly at the moment so google "timber cupping" or "flat sawn cupping direction" or something like that and it will give you a pointer as to which way the grain should go relative to the side you put your braces on.
Yes it certainly has happened to most of us.
On an acoustic I built about 5 years ago, I braced at low RH, but forgot to do that when it came to gluing the back to the sides,
When we got a 15% day the back cracked in two spots, between the braces.
It's a pretty strong lesson!
Interestingly, I was thinking about all your good sense, and recalled that I bought that wood in Auckland where it had been sitting in a plank for the last 20 years. The end of the plank wasn't sealed.
Auckland is far, far more humid than Christchurch where I live so I guess that wouldn't have helped my cause. It never occurred to me that it might need drying.
Thanks all for your advice. A big lesson learned and I now have a very prominent RH gauge in the workshop!
Interestingly, the Rosewood backs are, at best, flat so I have obviously stirred up a hornet's nest. Or maybe it's contagious. I have removed the braces from the Sapele back (ugly business going backwards isn't it?). It also split down the join in the process which caused some grumpiness but I now think I have it under control. Just waiting for a 40% RH day to re-brace it.
Over and out.
I just got it - "Ill tempered backs" .... a Kiwi, an Aotearoa Bro....The mighty All Blacks.......yep, when the forwards are slow to the breakdown and just hang around the ruck, you'll get "Ill tempered backs"..........Which is what you are going to see a lot of this year as the Brumbies dominate the Super Rugby!