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My mahagony dread build calls for an arched back, wide at the waist and thins at the bottom slightly and about 1" at the top. My question is do I need to "pre-bend" the back or just glue and clamp and expect the glue to hold all of that bending force? Are there any tricks to clamp the back and then wet it to allow it to relax over time?

Rob: Not quite sure what you are asking,but rest assured that most backs are radiused both crosswise and lengthwise and the glue holds it on without any preforming other then what the braces themselves provide.

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Tom, Thanks. This back is only radiused lengthwise and therefore the cross braces don't have any influence on the lengthwise bend. There is plenty of surface area at the head and tail block and purfling along the edge. I figured it would be better to ask than make a box spring!

Rob, are you absolutely sure there's no transverse arch? I don't know much about dreads, but that doesn't sound right to me. In fact, I'm not sure I've ever seen an acoustic guitar in my lifetime that didn't have at least some degree of transverse arch in the back. Surely, more knowledgable people will chime in. Regardless, I don't think you need to pre-bend the back in either axis. The bracing pattern and the taper of the sides will give it its shape.

The plans called for a slight arch across but to simplify my first build I am only doing the vertical. I think the thickness I have for the back is adding to the issue as I measured it to be .13 thick. I will plane that down to around .10 and that 30% reduction will most likely reduce some of the spring I am seeing (feeling).

Thanks

Transverse arch is more important than longitudinal. It is what keeps the back from cracking when the humidity is low.
A flat back can actually go concave when it is dry, but one like you describe is prevented from doing that. As a result, I suspect that it is probably the worst design for durability.

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John

+1
I would skip the longitudinal arch if you've already got it braced flat. Binding will be a lot easier that way too, since you won't have to deal with this problem.

Or peel the braces off and redo with transverse arch. I clamp arched braces one at a time with cam clamps and a flexible backing slat, as described in the Cumpiano&Natelson book. Works well.

Rob: Saw the picture of your back on the MIMF and wonder what is the thickness...?

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I've built a number of guitars with cylindrical backs - it works fine. I showed them to a few builders and nobody ever noticed that the back was only radiused in the transverse direction (18' radius) until I mentioned it...!

Trev

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Thanks for the replys. Plan forward is to plane off the braces, thin the back to around .10 (currently .13) and install an arch. A little rework but sounds like this willcover my A&% (Back).

I took Charles Fox's 6 week class in Vermont in 1980. We thicknessed our back, glued on a center strip, and proceeded to glue on our previously arched braces. Lining was glued onto the sides which had already been glued to the top and neck. Essentially we were following a model of how classical guitars were made back then.

Using an arched sanding block longer than the width of the lower bout, we sanded the same radius onto the lining/sides the length of the body. This was a sanded in a straight line from end block to neck block along the sides which had been tapered with a coping saw.

The back fit on great and the guitar has been free of defects since. I've done about 10 more acoustic guitars using the same method with the same satisfactory results. I guess that was the way things were done back then when guitarmaking was not as common as it is today.

Over the years the radius dish seems to be the norm, doing away with some of the more traditional techniques. I've not tried one yet myself but expect that someday I will.

Thanks Marty. I spent the weekend "fixing" the back and I am happy with the results. Now I need to make up the sanding board and start that process. Any thoughts on using a hand plane to start the angling on the sides before going right to the sanding?

Rob