Grand Auditorium

[Jeremy D]

Hi Guys,

I am building a GA at the moment. I finally got the math for pate thickness (I think). It will be my second guitar with WRC and blackwood B/S. I was wondering if one of you knowledgeable folks could have a look at the results of my spreadsheet and tell me if you think this is a suitable piece of wood. After sanding out the saw marks I only have about .8 of a mm to work with. Do you think this will be thick enough? Also I noticed that the cross grain modulus is lower than the average example in the book. Will this have any significant negative outcomes, bearing in mind that a GA is a fairly large guitar? BTW I used iStrobotuner for iPad and it seemed to work very well.

Anyway here is the spreadsheet and thanks for looking.. I hope I can get my head around the rest of the book

[Trevor Gore]

That looks to be in the ball-park, Jeremy. The check, of course, is to verify that you get the same results (or very close) as I do in the published table.

If the wood is off-quarter to any degree, the cross grain stiffness of WRC rapidly diminishes, which may explain your low Ecross result. The wood is plenty thick enough to use. I normally plane very close to target and then avoid dings etc. after that. If you have a coarse sanded finish, that may well give you lower stiffness results than planed wood, due to the top layers of wood (which take the majority of the stress) being "mashed" by the grit of the sandpaper, so having very little stiffness, but still retaining its mass.

A good assessment of the "quality" of the wood is the mass of the soundboard. You can compare the wood for different sizes of guitar by having, for comparative purposes, a standard sized guitar (one for SS, one for CL). The lowest mass soundboard (target thickness x area of standard guitar top x density) is the best. Don't worry too much about the thickness. It is what it is for that piece of wood and if it happens to be low density and thick it will likely be lower mass than high density and stiff due to the cube rule effect.

[Jeremy D]

Thanks for the reply Trevor.

Yes I am getting close to the results in the book, to within .015mm at least. I sanded it with 120grit on a drum sander. One edge is straight but still at little rough. The other three are straight from the bandsaw. Would it make much difference if I planed all the edges smooth?

[Trevor Gore]

Would it make much difference if I planed all the edges smooth?

Very little.

I reckon you'd see a small increase in stiffness between a planed surface and a 120 grit sanded one, but probably not enough to worry about. It's the 40 and 60 grit surfaces that show significant degradation of stiffness.

[Jeremy D]

Thanks a lot Trevor,

I will go with this and see how she turns out. I am very much enjoying your books, great work.