Official Luthiers Forum!

Hi folks. I am in the process of thickness sanding an EIR back and sides for a 12 string dreadnought. Can someone advise me on the thickness for the back and sides. The top is Sitka Spruce. Thanks.

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Under Compensated Nut!

I've successfully used .090" on the EIR back and about .085" on the sides. 3 years and counting...it's still as stable as the day it was born.

The top is another matter...I used about 10% greater thickness and 25% higher bracing than I would have used on a comparable 6er using light strings.

Al Carruth was a great source of information for me. Perhaps he'll chime in.

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JJ
Napa, CA
http://www.DonohueGuitars.com

I've built two 12 strings, and like JJ, just make the back and sides whatever works well for bending that wood. I used EIR on an X braced OM and mahogany on a ladder braced 000 - both were about 0.080 on the sides to bend well and 90 on the backs. Pretty normal bracing on the backs and I put three little stiffners on the sides.

The tops are a different matter - my Martin D12-28 has exactly the same thickness and bracing as an uscalloped D28 (but over the years developed a bit of a belly), Taylor uses thicker top plates and an extra tone bar (which I did on the OM). The ladder braced one was thicker still (0.150) and of course the braces are completely different. fwiw - I also down tune all of my 12's - D for the OM and usually C for the ladder braced one.

I'm building a 12 string in class with Alan Carruth right now. It's SJ sized and will be white oak b&s and lutz top. We thicknessed the sides to 2mm and the back to 2.5mm, which is exactly the same as we did for both 6 strings I built in class. Those were an EIR OM and an Oregon myrtle SJ. He had me thickness the top 25% more than we would have for a 6 string using the stiffness numbers for this top. It tapers in thickness from the neck all the way through the lower bout with the bridge area being the target thickness. IIRC, Alan said we will make the braces 25% taller and slightly wider than we did for a 6 string, as well. It's also going to be a 12 fret with a 24.5" scale length.

Matt