Official Luthiers Forum!

    I built a few dozen guitars with the braces tucked into the X legs as well as the bridge plate tucked in. I changed after i did a few more repairs on guitars from other builders with them tucked in. When a brace is tucked into the X braces and then tucked into the kerfed ribbon lining at the perimeter of the guitar, it is very difficult to replace if they are broken or even if they have been simly displaced by an impact to the top. They will pop right ut of those small relieved sections that they are tucked into with sufficient force in an impact and can do damage to the brace that they are tucked into on the way out.

    I've also replaced several bridge plates that were buried under the X brace lower leg edges. A bridge plate replacement made necessary by damage or excessive distortion or extreme wear at the pin holes is a daunting task anyway simply because of the large glue surface and area covered by the piece. Extreme care must be taken to not overheat either the plate or the top during removal and the reversed chisels that are necessary are tools that take some time and experience to become familiar and skillful with. It's very easy to damage a top during the removal process. The new tools that Stew Mac sells for making nicely matched plugs and holes to glue them into have made the bridge pin hole damage repair a much easier and less stressful thing for repair people to deal with and can make replacement of the bridge plate an alternative to repairing it in those instances.

   When the edges are tucked under to X braces, the plate must actually be cut first at some point across the width to allow for it to be removed as two pieces to avoid liflting either leg of the X portion of the bracing. Replacing it with an identical piece that is tucked in under the X braces is very difficult since it is difficult to gain enough flex or reief in it to allow the ends to enter the cuts made for the original plate.

    If you have them glued down with a proper match between surfaces and a good quality glue applied under the correct pressure and allowed to dry sufficiently before being put under a load, braces that are placed by just butting them against their adjacent members of the system will provide long life and strength very close to that of braces tucked under one another. You'll never know the difference until it's time to repair either.

   There aren't alot of builders who are tucking them under anymore simply because either method provides dependable integrity and longevity and the material removed to provide space for those little tips to be tucked in is, quite simply, material that could be providing strength and additional glue surface at critical intersection points.

   Over 450 guitars with no braces tucked in so far and only a single brace knocked loose by a baseball blow to the top in a living room is all of the personal experience i can pull conclusion from. That one instance was an easy warranty repair since it wasn't tucked in and could be lifted and glued back in place with little or no trouble and has stayed put for the 7 years since. I've seen as many guitars from builders of all sizes using either approach in need of similar repair attention over the years so the track record across my benches has been one to influence me in my choice.

Just my opinions and experience,
Kevin Gallagher/Omega Guitars

   

Food for thought! Kevin, in you thorough (and eloquent!) post I didn't see you mention whether you taper the braces that butt up against other braces down to nothing like Alan mentions. I do it that way so the brace ends will flex with any movement of the plate and not pop loose. I only inlet the x and upper transverse braces into the linings, all the others are feathered out.

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Rian Gitar og Mandolin

[QUOTE=rich altieri] Tim, any chance you could show a picture closeup of your approach relative to not connecting the braces to the x brace??? [/QUOTE]
Rich:
It's just a simple butt joint. The finger brace ends are left about 1/16" thick and lower tone bars are about 3/32" thick where they butt to the main X brace. My bridge plate is roughly .070" thick. I do put a dab of glue on the ends and butt them up before clamping in the Go-Bar deck.

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tim...
http://www.mcknightguitars.com

arnt,
   I do tuck my X braces and my tone bars into the linings at the perimeter of the guitar even though I don't tuck anything under one another where braces intersect. The only things that don't get inserted into the linings are the finger bracess which run off the lower legs of the X and the top brace which runs across the top above the soundhole and provides the access hole for my truss rod adjuster at that end. The reason for the top brace not being inserted is its greater mass and shorter length than the X braces and the inability to flex it to reinsert it in the event of a repair that would make it necessary. The finger braces are in place to provide rigidity in those critical areas of bridge torque where they typically run off of the X structure without reducing the ability of the top to flex at its perimeter along the sides of the lower bout and effectively transfer vibration. The recurve area around the perimeter of a fine violin or archtop guitar is critical to the efficiency of the top's performance. It provides a minimized thickness that allows the top to pump freely while the mass of the area providing the foundation for the bridge and string pressure load to be dependable and rigid enough to absorb and disburse the vibrations. It is referred to some archtop builders as the "hinge" area of the top. The same is true of a flat top steel stung guitar in that the top must be able to pump in and out as freely as possible with as much direct contact with the linings as possible. It just minimizes my cuts into the linings and allows me to leave as much uninterrupted contact area as I'm able in that critical lower bout.

   The X braces are long enough that in the event of an accident in which one is jarred loose from the top or broken and must ne replaced, I'm able to flex them sufficiently to get both ends under the ribbon linings again to the same extent. Otherwise, they stay put pretty well until a repair is performed to correct a small area that has been knocked loose along the length. The ribbon linings provide additional support against the top during an impact and the subsequent flaxing of the top plate that can occur. That flexing is another reason for my not tucking braces under one another. In the event that a top is damaged or just struck with force enough to flex it dramatically, the braces as a sytem of individual and independent components can flex along with it and not work as levers to lift one another. A single brace lifting or being cracked is much less work to repair than a few that have been able to lift others around them when struck.

    I hope that helps to reveal a little of my reasoning...whether right or wrong or agreed with or not...for developing and adopting my philosophy for building. It all just comes from my own personal experience with my Omega guitars and those of others and I appreciate all that other builders learn from their own experiences and are generous enough to share with us all.

Thanks,
Kevin Gallagher/Omega Guitars Kevin Gallagher38623.4411921296

An alternative to flexing and re-inserting braces is the one I've used a number of times on Gibsons that have come in without the shoulder brace (some still with the strings at tesion!). I simply removed a couple of sections of kerfing, glued in the brace, and then installed a bracket such as is commonly seen on Classicals.

In my scheme there's always one end of the brace that feathers out and is not inletted except on the X and shoulder braces, so all the 'short stuff' can be replaced.

It is certainly true that properly designed and glued braces seldom come loose, but years of repairing ones that weren't properly done got me thinking about how i'd do mine so they'd be better. I hope....

PS
Hot hide glue re-glues much better than the synthetics. Just one more reason to use it on bracing.

HI Alan
    I love this place. I tuck braces into kerfing but I dont tuck braces into braces. I guess I am martinesque.
   The main X and truss rod are the only ones I tuck. We have chatted a few times at ASIA and I do respect your work. Maybe you can enlighten me.
john hall

The basic facts that you need to consider on this are that:
1) softwoods have a Young's modulus from 5 to 12 times higher along the grain than across, and,
2) glues usually have very low peel resistance. Once you start the edge of a glue joint lifting it's usually real easy to peel the part off if there's some flexibility and you're willing to take some time.

Think about a brace about .250" tall glued crosswise on a top that is, say, .125" thick. If that brace just cuts off abrubtly somewhere out in the middle of the top it makes a huge stress concentration. The stiffness of the brace is proportional to the Young's modulus of the material and the cube of the height. The brace is twice at tall as the top is thick, and the Young's modulus along the grain of the brace is (say) eight times as high as that of the top. Thus the brace at it's end is (2x2x2)x8= 64 times as stiff as the top. If there's some load on the top right at the end of the brace (your fat cousin sits on your guitar) the top is far more likely to bend than the brace, and that puts all of the stress on the glue line, which tends to peel apart.

If you shave the end of the brace down to about 1/2 as thick as the top at that point then the brace end is (1/2 x 1/2 x 1/2) x 8 = 1 times as stiff as the top at that point, and can bend along with it to some extent. So long as the brace end comes smoothly down to that height at the end and the taper is reasonably long you've avoided a stress concentration, and reduced the liklihood of the brace peeling loose.

There's a problem with this when there is normally a large static load down on the brace, such as you have with the shoulder brace from the neck, or the upper ends of the X that take up the bridge torque. If the brace is shaved down to nothing then all of that down load will have to be dissipated by the top itself, distributed over a more or less wide area depending on how fast you taper your brace ends. You'd be likely to end up with a pretty good dimple in the top if you did that, so you have to leave some brace ends tall enough to take up that stress. Those braces must, therefore, be inletted into something to support them and keep them from peeling loose.

The bridge patch serves, as far as I can figure out, two structural purposes:
1) it provides a bearing surface for the ball ends of the strings, and
2) it 'balances' the construction of the top at the bridge location, helping to avoid stress from the cross grain of the bridge on the top surface. Bridge +top+plate=plywood.

In addition it adds some stiffness and mass, which has an acoustic effect.

If the bridge patch extends beyond the front edge of the bridge then the deformation of the top due to staic torque will tend to peel it loose if it's stiffer than the top. You are, of course, dealing with the long grain direction of the top in this case, so it's pretty stiff, but hardwoods don't lose as much by being in cross grain bending as softwoods do, and they're still pretty stiff. Thus I like to taper the front edge of the bridge patch down to avoid a stress concentration there, too. I don't see any real need to inlet the bridge patch into the braces, though, as it's not being subjected to a peeling load. As always, you want to either have things fit tightly or else not touch at all, to avoid buzzing.   

Wow, what a great explanation! Thanks Alan. That was in plain enough English that even I could understand it. I have been doing all the things you mentioned, but I couldn't explain it at all. Thanks again for clearing that up, at least for me.

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Dave Rector
Rector Guitars

I think that it is important to note that the stress riser is still there is you butt a tone bar against the X brace. Even though the line between them may be microscopicly thin. If you put glue on the end of the tone bar you still have problems. One, the glue is on end grain which is weak and two, the same stresses that could cause you problems will stress the glue line with a shearing force which many glues, (hide especially) are knows to be weak. Tucking the brace makes the joint mechanically strong. Tapering the brack removes the stress riser. So, as Al said, either tuck or taper.

I think it is also interesting that builders have two perspectives on this issue. Either make is much less likely to peel up or make it easier to repair if/when it does. Both points of view have merit.