Official Luthiers Forum!

I've started doing something different on the last couple of guitars. I like the idea of tying the braces together to work as a unit, but I was bothered by cutting lots of notches in the x-braces since I worried about the loss of stiffness this might cause. So I came up with a pinned joint that seems to work fairly well. I butt the end of the tone bar up to the x-brace and drill a 5/64" diameter hole through the back of the x-brace that extends into the tone bar about 1/4". I then insert a wood toothpick into the hole when gluing the brace into place. The pin adds negligible weight. And the small hole in the x-brace should not cause any loss of stiffness. The only complication comes from finding a way to drill a hole parallel to and about 1/8" above the top.

Barry, why does notching the x-brace close to the soundboard cause loss of stiffness, as long as the edge facing the inside of the box (where the shear force is greatest) remains intact? And how do you drill those holes...?

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Rian Gitar og Mandolin

Arnt, Notching removes nearly half the x-brace so I worry what that might do. I have no data to support this, however. I know the joint will be stiffened somewhat when the brace is glued in, but there is no way to glue a splint over the one-sided joint like we do at the x-brace joint.

I drill the hole by making up a drill bit holder. I take a 1/4" wood dowel and drill a centered hole in the end and glue the drill bit into this. Then I chuck the dowel into the drill and hold the dowel against the top while I drill the hole. This ensures that the center of the hole is 1/8" above the top, which seems to be a good location for the size of my bracing.

So far, I have only done this pinned joint on my tone bars. It seems like it might be overkill for the finger braces, but I might eventually try it there too.

I don't know why you would want to remove nearly half of the X brace to cut a mortice for a tone or finger brace. My notches in the X are about 2mm high by 3mm deep and the width of the brace usually 5 or 6mm, and when the brace is inlet into them and glued with HHG, the resulting effect on the X would be virtually none.

Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

You may be right although I would hesitate to say that there is virtually no effect. Cutting four of these notches into the treble x-brace would have some effect, wouldn't it?

On the lower arm of the X on my OOO bracing the total area removed from the gluing surface of the brace is 4.25% of its gluing surface, which is replaced with a similar wood and HHG. In terms of the lower leg's volume it is only about 1.4% of the brace, again this is replaced by similar wood and HHG. So no I don't believe it has any measurable effect on the integrity of the X brace. Remember these 4 mortices are only 2mmx3mmx5mm each on a brace that is 235mmx6mmx Av6mm.

Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

The point isn't how much wood is removed, it is where it is removed from. The brace / top assembly viewed as a beam, where one edge is the guitar top and the other is the innermost edge of the brace, will have the shear stress (stress applied parallel or tangential to a face) concentrated along these edges; the middle part can to a large degree be removed without affecting the strength of the beam. Since we are talking about wood, care must be taken so the size or shape of the cutout part doesn't serve as a starting point for splits or cracks of course, but for all practical purposes is will remain identical structurally. Acoustically, on the other hand...

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Rian Gitar og Mandolin

Cutting notches won't affect the stiffness of the X-brace at all unless they get too close to the top of the brace. Think of a truss used in a building- the load is carried at the extremes of the brace. The soundboard is one extreme and the back of the brace is the other. So long as there is enough "web" to transfer the shear force you can fill the middle part of the brace full of holes with no effect.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I change my mind a little bit: because spruce is so orthotropic (anisotropic - whatever) my above statement would be true with braces exactly parallel to the grain. Because X-braces are not parallel the stiffness of the top along that axis will be less than that of the brace so there could be some less stiffness overall with a notch cut along the brace edge which meets the top. On the other hand it may be made up for by the width of the top. It's probably close anyway. One would have to actually perform the calculation to know for sure and I'm just too out of practice.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Thanks Hesh

I didn't expect so many responses. My question was answered, and then some. Thanks to all.

Ron M.

So in your opinion a Rolls Royce is immortal unless some stupid person mucks it up, and ditto for your own instruments. There are probably fewer dummies on your side of the pond, so you can sleep well. I sincerely hope that you never encounter a more negative and depressing situation than the thought of building an repairable guitar.

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It's not the miles ahead, it's the stone in your shoe


In Markham,Virginia

Interesting discussion. I never even thought to but joint or inset these braces but then I always feather them down paper thin. I always thought of braces more as individuals rather then as a unit except for the functionality of the X brace.