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Before I sand these braces off and let my student start again, I wanted to get another opinion or two.

The story: So, if you've ever worked with relatively inexperienced woodworking and/or guitar-building students, you've probably had the experience of turning your back from Student 1 in order to work with Student 2, only to come back to Student 1 and find that (s)he has completely misunderstood or not heard key instruction, such as "Go slowly, take off small amounts, no more than 1/8" for now".

Now, after the event, looking at an X-brace that is less than 1/4" high at the bridge, I'm trying to imagine this guitar top holding up over time, and being adequately braced... and I"m having trouble imagining it. In other words, I think we need to start over again.

It's Adi, at around .110" thickness, relatively stiff, for an OM-sized guitar.

What do you fellow builders think?

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Ken Mitchell
Durham, NC

I don't like to go below 5/16 . you are dealing with a main structural brace and the weakness is at the point of some of the highest rotational stress . If you were behind the bridge at the point where you have more tensional stress you may be safer.
Do the student a favor and have him redo it.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

In my very limited experience, I would say it looks pretty good. I've only built two guitars (both OM's), but they may have looked even more aggressively shaped than the one you have pictured. The first is still holding up fine after five years with a happy little belly and sounds great. The second hasn't been strung yet, so I'll have to wait and see on that one. You didn't mention the scale length or intended string gauges. What you want the guitar to do may affect your decision, as well. I think structurally it'll be fine. If it were me, I'd go ahead with the build.

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sweat the small stuff.

I scallop mine that much....I'm no expert but haven't had a failure yet

I am not an expert by any means but I do have a good grasp on physics.

It appears to me that you will not have much of an issue since the rotation would be in front of and behind the bridge plate. The bridge and bridge plate will not really bow and that is where the deepest scallop is so you have the added strength of both the bridge and bridge plate in that area. If it were very shallow at the high point of the scallop then you may have some concern since that is where the bow will happen.

Again, I am not an expert but I have a pretty good understanding of how the parts of a guitar work. I would personally leave it but then someone with more experience may have a different opinion.

Cheers,
Bob

I've done some that low before, no problems.


Sent from my iPhone using Tapatalk

Ken, if the spruce used on the bracing is fairly stiff, you should be fine, especially if using light or medium light strings.

Ken

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http://www.casperguitar.com

I routinely scallop OM sized guitars down to 1/4" at the bridge plate as well as the tone bars. If he is planning on leaving the top at that thickness IMHO you could even go a little lighter on the bracing, especially if the player he's building for has a lighter touch. Won't be a problem.

Cheers,
Danny

I'm not a scalloper, but since it sounds like the consensus is that it will probably survive, then I would recommend that you and all the students pass that top around and flex it to feel the stiffness That's the kind of information that makes a class really valuable, because it can't be learned any way but by direct experience, and is great if you have one like that to feel where is "almost too far". And then study the deflection under string tension when it's done so everyone can correlate that with the feel of the free plate.

Kind of interesting that so far we've got one real expert (John) who is suggesting a redo, and everyone else (with much less experience) suggesting that it looks okay.

The other option that lies in between just redoing or leaving it is to finish the guitar, keep a close eye on it in the first few months, and retrofit a bridge doctor if the top should start to distort.

I remember being freaked out as I built my first guitar - thinking about 160lbs of pull and seeing how little wood was left to fight all that tension. Because I didn't want to overbuild my first guitar and be disappointed with dull sound I had the bridge doctor in mind the whole time - thinking it could save my bacon if I really had shaved away too much. It would obviously change the sound a lot. But at least the guitar would be in tact. That was 10 years ago and still no doctor!

Phil

I am with John with this. It looks to be at 1/4" where the saddle and the bridge pins are. You do not want more than 2 degrees of rotation at the saddle. The weakest area of the brace is right where the bridge wants to rotate.

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http://www.Harvestmoonguitars.com

OK I would like to ask a question to the "experts".

It seems to me that where the brace is the thinnest is exactly where the bridge and bridge plate are. With the sandwich of Bridge Plate/ Soundboard/Bridge there will be nearly Zero flex in that area. It will be nearly impossible to bend the sandwich under even extreme circumstances. So the braces do not flex in that area. It is only in front of and behind the "sandwich that will flex and that area will rotate forward but not bend. That is just plane physics and I have repaired enough older guitars to know that even after taking a bridge off of a guitar that has a large belly and a sunken area just in front of the bridge that the area of the bridge itself is usually perfectly flat and has no bend or flex in it.

Explain to me how that area can possibly flex under normal string tension especially relating to a guitar that has a large belly. I agree that it will rotate but it will not flex.

As I said, I am not an expert but I do know how a guitar works and I know how physics work but maybe I am missing something here.

Bob

I am no expert either. But it looks like the brace is still thin where the bridge ends. What about the sandwich keep the back of the bridge from being pulled up? I am sure you have seen a lot of guitars. I have seen guitars with a bulge just behind the bridge and the saddle rotated toward the nut. I thought preventing that is one of the reasons that we use xbraces in steel string guitars.

At this point it takes about an hour to remove the brace and glue on another. Two years from now when the bridge is showing more and more rotation ...

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http://www.Harvestmoonguitars.com

My X's never go below 0.35 at the low point, and my top thickness for an OM is usually .114-.118 for Sitka or Lutz. Have not used Adi on an OM.

How thick and wide is the bridge plate?

There was a good write up of a panel discussion on through the soundhole brace shaping in the GAL journal a number of years ago. Dana Bourgeois was one of the participants.
They recommended never going below .35 on the X as I recall. Might want to look that up. Should be in one of the Red Books.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I have seen many successful 1930's Martins that were scalloped to 0.31" (5/16). Personally, I would not go below 0.28".....unless the top was way too stiff.

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John

In order for the bridge to rotate, the top must flex a little. The X-braces resist the rotational flexing so if the braces are not stiff enough they won't resist the torsioning no matter how thick the bridge is.

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Old growth, shmold growth!

The top is a wonderful thing , it takes a load of stress from the strings and turns it into work . This is what we hear. The top is not a floor so the braces are not a joist beam . When you look at the stress and the way the loads are applied , we have a series of forces.
There is a coupled force on the bridge , the string ends are pulling up and the string load at the saddle is pushing down , this applied force will rotated the bridge. Also the neck is rotating. Then to add to that there is a compressive force between the neck block at the bridge. We often see the top sink in this area because of this. Next there is a tensional load from the bridge to the tail. Another force is a variable force from the wood shrinking and expanding across the top. The braces hold the top back from this expansion , this causes the top to rise and fall.
As John Arnold points out , Martin never went below 5/16. . You can control these forces by the shape and the HEIGHT of the brace. The higher the brace the stiffer. You can take a top from great to crap real fast. I only build over 180 so I am still learning but YES YOU CAN GO TOO THIN. The sad part is it may take a few years to show its head.
The bridge rotates , and once it gets to a certain point the top pulls off the bridge. The saddle will rotate forward and you will lose your intonation and the strings go sharp. Tite bond will allow creep plus wood can also cold creep. I have many examples of failed tops. A few years ago , there was a post not too different from this. Many convinced the builder it was ok , 2 years later he was replacing that top. I wouldn't have said to redo if I didn't think it wouldn't matter but long term this may be an issue.
Of the builders that have gone this low , how old are your guitars ? What is the intonation compared to day 1 , how much is the top raised behind the bridge ? What glue was used? How think was the top and what are the deflection numbers ? What size was the bridge plate and how thin?
When I scallop , I am careful of the shape. Martin scalloped but didn't peak until it was behind the bridge. Early in my building I did this , and in 2 years the top came off and was replaced. I am not suggesting things to make more work , but if you are putting things out there and learning how to do it , there are lines that you have to be careful not to cross.
Braces do a few jobs ,
Structure
The Transverse or truss rod brace helps to carry the load from the fretboard extension.
X braces , also are structural in that they help to control and disperse load from applied and coupled forces.

Tonal
X braces are both structural and tonal , controlling the top movement to help transform work to pressure waves in the box.
Tone bars are obviously for tone disbursement
Anti Split
While this may technically be a structure issue , these braces are more to support the top from splitting
Popsicle Brace
This is supposed to help to control the crack at the extension. I agree with John Arnold in that this area would be better served with a more Tapazoidal shape than the cross grain brace we have seen and is still used. Most of these cracks from from a mismatch of expansion of the top and fretboard.
Finger braces
they are more to control the cracks in the top. They do add some mass and would be a slight tonal issue and help with some rotation but very slightly. Of the few guitars I have seen without them the inevitable pick guitar cracks were evident on both sides.
Sound hole supports
These just help the top under the rosette and rosette also helps to keep a crack from forming as the hole has exposed end grain . Rosettes , while decorative are also helping the top to keep this area from cracking.

this is just the top , and then to add to this , we can look at the top and how it is shaped. Some people dome the entire guitar , I use a different approach , and in the shape of my sides and top I create a 3 facet top , angle from the neck block to the top of the sound hole , flat from the top of the sound hole to behind the bridge and domed behind the bridge.
Adding a shape can influence the top and how the load is shared. So as you can see , there is more to a top than what the scallop is. Also there are some very good sounding guitars that have no scalloping. You don't have to scallop to make a good guitar.

The thing we have to think about , is that building a guitar is like a recipe , we all have an end result we are looking for. This is what makes our guitars different. In the end the structure of the guitar will determine the strength , how you weaken this will determine the tonal balance. I like my guitars to be balanced in the tonal regions . Not treble or bass heavy.

Another point is the creep of the glue and wood , as they will move over time . This is why I use fish glue and hot hide glue on the tonal areas. Tite bond is a fine glue but I do prefer the protein glues.
hope this helps and I am sure Mr Arnold may chime in to add to this discussion.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I'd cap it with hardwood and taper the cap off at both ends.

Factory guitars are fairly overbraced and that looks to be about the same as most factory guitars... I have a jumbo with about 1/4" of materials around that area and there has been no issue. The guitar is very responsive too. With OM you probably want smaller overall braces anyways because you don't have a whole lot of body area to deal with...

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

What!!!!!! Are you guys planning on living forever. Live for the moment.

I always appreciate the advice and thoughts of people that have way more experience than me. Thank you. The oldest one I can get my hands on to look at is an OM I built for one of my kids about 5 years ago. I just had a look at it the and bridge rotation , if any, is pretty minimal. Possibly a degree. It would appear to, like me, have just a nice amount of belly. It does however appear to be possibly showing signs of cold creep (Titebond). Maybe time to dig out the Hold Heet I've never used. The top is a fairly stiff piece of Lutz that finished out at .104 - .107. If I get a chance today I'll check the intonation. I guess that would be the be the best indicator?

This is a good thread. it would be nice to hear from some more of the old timers. Hopefully some in my camp. Yikes.

Happy Saturday,
Danny

I found that article I was thinking of. "Retrovoicing The Flattop Guitar" GAL journal, fall 1996. Actually one of the guys was going down to 0 .25 on the X not 0.35 as I recalled. Looking back on my building logs I have never gone lower than 0.30.

I was definitely guilty of overvoicing some of my earlier guitars. No major structural issues that I know of but I'm going a little tighter these days which among a number of other things seems to have been a positive tonal step as far as feedback from good players is concerned.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

the advice is worth what you pay for it.

Over scalloping will kill the tone of the guitar. What good is a guitar that is all bass ? It is about tonal balance.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

"Over scalloping will kill the tone of the guitar." I couldn't of agreed more. I vote to replace the brace. Clinton

Bob pretty much nailed it.
Since a picture worth 1000 words ( 2000 of mine!) here's what's going on under the static string load. The saddle is the pivot line for the torque. In the image the blue is pretty much stationary and the closer the colors move to red the more it moves.
The magenta arrows represent the string load and all displacements are greatly exaggerated for visualization purposes.

Hope this helps.

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Jim Watts
http://jameswattsguitars.com

Jim,
Thank you for posting the stress plot. I have spent the last hour or so in my cad program drawing out the point I was trying to get across but that explains it better than I could ever do in a static cad drawing.

Cheers,
Bob