Friday, November 04, 2011

Brendan’s Mandolin Part III

The Back

Below you can see the beautiful cocobolo that I’ve got for the back of Brendan’s twin-pointer; he’s opted to go with the sapwood showing (good choice!). You won’t see better wood than this and as you can see there is enough here for two backs (the next one could be yours!).

The back is joined the same way as the soundboard, however to get the book matching just right, on such pronounced grain, you have to be really careful when you thickness it.

Next, is the joint’s reinforcement strip, I always have the grain running along the length of the back rather than at 90 degrees as many do. Why? Well- when the moisture content of wood changes, the wood will shrink or expand across the grain and not along its length. Also, the majority of moisture is lost from the end grain and cross banding is nearly all end grain! So, hopefully you can see that a back reinforcement strip with the grain running lengthways, should be much more stable and less likely to distort the back with changes of humidity. The strip is glued on in a curved jig.

Next is the X-bracing. I feel that the surface of the back should be part of the surface of a large sphere and as the mandolin’s shape is almost circular, an X-brace works incredibly well to the give the desired curve. I’ve always felt that the back should be a reflector of sound and by using braces to induce a curve, you put the back under tension so that it doesn’t flap or absorb sound energy. The rim of the mandolin will have to be shaped to fit the back.

The Soundboard

Before deciding on the final dimension for the soundboard, I decided to do a few experiments to gauge the stiffness of the red spruce. I find the easiest comparative test for me to carry out is to measure the deflection of the soundboard when a mass is placed on it.

I also did something similar for the bracing. Conclusion? The red spruce is stiff! Therefore I can make the bracing light.

One other concern is that I’ll have to drill a hole through the soundboard to take a wire for the pick-up. To reduce the chance of the ‘board ever splitting at that hole, it’s sensible to have a patch where the hole will be with its grain running at 90 degrees. How thick should the patch be? Thinner than you’d think! I glued some 0.6mm sycamore veneer to a piece of scrap red spruce and tried to break it. You can see the results below!

Well, with my experiments carried out I can confidently work on the soundboard and its private!

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