Official Luthiers Forum!

Why do this? Seems like it's only purpose is to keep the part in place if the glue lets go. Top braces feather down to nothing so they're not holding anything if the glue lets go. I realize we want support as close to the edge as possible but once you feather down to nothing what's the difference? I'm sure there's more to this that I just don't know.

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Hutch

Get the heck off the couch and go build a guitar!!!!

Any brace that I taper down to nothing is behind the bridge, where the stress is upward, and is not tucked. Any brace ends that intersects the kerfed lining in front of the bridge area, where the stress is downward, are a minimum of .100" thick and are tucked.

By tucking you're expecting the glue to fail and will rely on the lining to hold it in place. Or maybe because the brace is trapped in a tight joint there's less stress on the glue and less chance of failure. I think that makes sense. OK, I'm on board with it, just needed to discuss it to get my brain wrapped around it. Thanks James

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Hutch

Get the heck off the couch and go build a guitar!!!!

Feathering actually makes the brace more resistant to coming loose from the top than it would if you left it thick (un-tucked). It eliminates the stress riser at the end of the brace. If the top gets an external weight load, the feathered end can flex with the top, whereas a thick end will just pop loose.

The problem with tucking is that it makes the top too stiff where you need the most flexibility around the perimeter of the top behind the bridge.

I think we may wish to differentiate between the historical Martin factory practice of tucking brace ends or bridge plate into the X brace and the common practice of pocketing the UTB or upper arms of the X-brace into the lining to more effectively transfer loads to the sides. The former dramatically escalates the difficulty of what should be a simple repair (e.g., regluing the end of a finger or tone bar), while the later avoids making the top carry the entire load when transferring downward/in-to-body loads from the UTB and upper X arms to the sides.

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For the times they are a changin'

- Bob Dylan

The problem with brace ends that are not properly tucked in is due the to low peel strength of most glues. Once the end come loose the whole brace can simply peel up fairly quickly.

Gibson used to use the nose of a belt sander to take the brace ends down to almost nothing. They would not bother to cut notches in the liners, but simply rely on clamping pressure to compress the liner wood. The problem is that they were using spruce for both the liners and the braces, and sometimes the liner was hard enough to compress the brace, rather than the other way around. The brace would fracture at the crush line at the first provocation, and peel up. Back when I did repairs I had several old Gibbys come in with the neck shifted inward, and no UTB. When I asked the customers that's admit that there had been a stick rattling around in the box, which they tossed as soon as they could fish it out. Pulling everything into line and replacing the UTB with one that had some height at the ends, and brackets to hold things in place resulted in a very nice little guitar that was stable.