Top and back graduations

[Steve Sorensen]

Anybody willing to give me some feedback on your opinion of the top and back graduations in the Siminoff book?

After ending up bending sides that are about 1/2 as thick as the book recommends, I'm wondering if the back and top thicknesses are also super-sized?

I ended up using an ad hoc blend of Siminoff's graduations and Adrian Minarovic's measurements for my first one. . . almost ready to hear how that went.

Thanks,
Steve

[Rolfe]

I don't have the book in front of me, but---depending on the stiffness and density of the wood---I like 2.5-2.6mm in the recurve moving fairly quickly to 4.4-4.6mm in the center. With the backs I go down to 2.1-2.2mm in the recurve. Most go thicker there.

[Mark Franzke]

Roger recommends those measurements as a starting point for your final graduations and tap tuning. You should end up thinner than that after final shaping.

[Dennis Davis]

I believe the thickness Roger calls out for sides is correct. I don't have his book with me but he wants to end up with .090 thickness after sanding so he bends somewhere around .112 to allow for sanding. The early Gibson instruments had sides that matched the binding thickness (.090)

I don't use his drawings for tops and backs but if I want to know anything about Gibson history I call Roger. He has a wealth of information in his head and in his notes. He also has owned 3 Loar signed instruments and probably still has them, I haven't inquired.

I realize that there are great builders using sides a thin as .065 and believe me I am not trying to take anything away from that but it is possible to bend at greater thicknesses the way the Gibson boys did but I believe you need to use steam to do it, at least I do.

Dennis Davis
Neild Mandolins

[Lefty Luthier]

For a 16+ ring per inch Spruce soundboard, assuming you adhere to a true curtate cycloid contour, you should find that a thickness of 0.110 at the rim and 0.180 at the center should tap out about 200 Hz or just below a G# when secured to a rigid rim, before tone bars are added. That should permit adjusting the tone bars to about 233 Hz treble and 208 Hz bass without too much difficulty. Stiffer woods such as Alaskan Yellow Cedar and Russian Spruce may have to be a bit thinner.