A request for help fixing a mistake

[Adam Savage]

Hello all,
I have finally got around to starting work on my first home-built acoustic (my take on a sloped-shoulder dreadnought), indian rosewood and a spruce top, the variety of which Im not sure, but its very pale.
However, in an attempt to remove what I thought was excessive cupping of the back plates, I have taken the back thickness down a little too far.
The thickness I have measured is 0.080", and the back feels to my very inexperienced hands, a little 'bendy'.
Now, my questions are :
1) Is this a problem?
2) If so, is it salvageable?
3) If so, how? Making a smaller bodied acoustic? Adding an extra parallel back brace? Adding a further brace parallel to the centre seam on each side?
4) Chalk this one up to (in)experience and not paying enough attention, and use it as binding material?

I would appreciate any thoughts you may offer up,
Thanks,
Adam

[Kevin Bryden]

80 thou, thats about right iirc. that is 2mm, some people go to 1.8 i think. should be ok.

[Adam Savage]

Kevin - thanks for that. The reason I had posted is that I'm working from the Cumpiano/Natelson book, and they suggest somewhere between 0.100 and 0.110" I think, with 0.085" for the sides.

Cheers,
Adam

[Rodger Knox]

That should be fine without any extra bracing. I'm working on one now that has a .070" thick back, and I did that on purpose. It's mesquite.
You may want to consider going a little thinner than C&N recommends for the sides as well, it will make bending easier. I usually shoot for .060" for the sides, depending on the wood. Generally speaking, lighter is better and a thinner back and sides is a reasonable way to reduce weight. The top, on ther other hand, is not a reasonable place to reduce weight. Lighter is still better, but there's a minimum stiffness required from the top. Stiffness is proportional to the square of the thickness, so going thin on the top is usually not a good idea.

[Adam Savage]

Rodger - many thanks for your reply also. I think my concerns have been quelled for the time being at least - I really thought I had messed up the first part of my first build... But hopefully that isnt the case.
I am, at least mentally, happy with just about every process for the rest of the build (with the exception of getting the neck join correct and making a good job of hand finishing - no spray equipment unfortunately), I just need to get going with it

Cheers,
Adam

[Chuck Tweedy]

I concur.

The point at which a plate begins to feel "bendy" is exactly where you want to be.

[Mario Proulx]

.080" is a good starting point.... <bg>

[Clay Schaeffer]

C&N's recommendation is good for a first guitar, and leaves enough wood for sanding out ripples and imperfections. Building "light" can make a more responsive guitar, but doesn't guarantee that it will be, and does make a more delicate instrument.
At 80 thousandths for Indian rosewood I think the back will be fine. I would use that same thickness for the sides.

[Adam Savage]

Many thanks to everyone who has replied - you have given me a good bit of confidence that I am at least heading in the right direction. I have the good fortune to own a thickness sander, so at least until side bending starts, there shouldnt be too many ripples to sand out. And the 80 thou thickness is after sanding to 150 grit both sides, so there shouldn't be too much more to come off in the finishing stages.
As this is currently a wee bit thinner than C&N recommend (even though it's a quite acceptable thickness), is there anything I should be looking out for/feeling/listening to to help ensure a nicely playable instrument, with regards to the back and side/soundbox construction?

Many thanks, it is much appreciated indeed.

Adam