Mahogany top thickness?

[Bob Gramann]

I’m building what for me is a weird OM sized guitar: Honduran mahogany top, back, and sides. My intention is to thickness the mahogany top so that it deflects under pressure about the same amount as my spruce tops and then brace as I normally do. I haven’t taken it to final thickness, but I’m guessing it will come out in the .110 to .115” range, or maybe just a hair thinner. Does anyone who’s built a mahogany-topped guitar have any reasons not to do that? What worked for you? Thanks.

[Barry Daniels]

I recently built an 00 sized mahogany and the top was .112" It worked out fine.

[Freeman Keller]

Some years ago I built an all mahogany double ought with the top at 0.120 and fairly deeply scalloped braces. It remains one of may favorite guitars.

[Alan Carruth]

The mahogany I've tested tends to have a Young's modulus along the grain (E-long) that's similar to some of the spruces, but it's denser. E-long predicts stiffness at a given thickness, so you really can't usually make a mahogany top much thinner than spruce, but it will weigh more. That affects the sound.

Years back, when I did repair work, I got two mahogany topped Martins in for the same reason at different times: the bridges had come off. In the process the tops had distorted to the point where there was a crease in the upper surface at the front edge of the bridge, and a break on the inside. This also often happens when bending mahogany, particularly the softer pieces; it crushes on the inside of the bend and then breaks. On those Martins the bridge plate edges were right on a line with the front and back edges of the bridge, so there was a really big stress riser. Still I can't recall ever seeing a spruce top do that. Ever since then I've been extending my bridge plates out in front of and behind the bridge, and tapering them down to as close to a feather edge as I can on the front and back.

[Bob Gramann]

Thank you to all. It sounds like I’m going to get away with what I planned to do. I put the rosette in with the top at .130” . After it dries for a few days, I’ll thin it to where it flexes like my spruce tops and go from there. Yes, it will sound different from spruce—that’s part of the reason for doing it. (I have plenty of spruce). Alan, thank you for the tip on the bridge plate. I usually use Osage Orange for my bridge plates and make them a bit oversize. I like to have enough shelf on the front of the plate that, if someone wants a pickup, I can install a K&K without crowding the string balls. On this one, I might leave even a little more so I have room for the suggested taper.

[Bob Gramann]

Followup: I thicknessed the top today. It landed at .117” when the deflection was right. Your inputs helped me be comfortable with the result. It will still be two or three months until I get to hear how it sounds. Thank you for your help.

[Alan Carruth]

Sounds about right. Did it have a strong stripe figure? Note that the cross stiffness will tend to be higher than most softwoods.

[Bob Gramann]

No stripes. It’s pretty generic looking mahogany. I did note the nice cross stiffness.

I have some nice, ribboned sapele planks that are crying out to be a guitar. So far, I haven’t been listening, but they may get their chance someday.