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Classical Bridge re-glue
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10137&t=50799		 Page 1 of 1
Author:  Conor_Searl [ Mon Jul 09, 2018 1:17 pm ]
Post subject:  Classical Bridge re-glue
I've got this classical guitar that needs a bridge re-glue. It seems like a pretty standard repair that I've done before on steel strings, but I'm a little confused by this particular makers process in gluing the bridge on in the first place. It looks like they finished the top and then routed the finish off the top under the bridge footprint. But the rout is smaller than the actual bridge. Wouldn't this leave the bridge sitting on a bit of a shelf all the way around its perimeter, which in turn would prevent a good solid glue joint?

Anyway in fixing it my plan is to clean up the bottom of the bridge back to bare rosewood, and then remove that small shelf of finish around the perimeter of the bridge footprint, scrape the cedar flat, clamp and glue. Is it worth my time to try and get the cedar that's torn off and stuck to the bottom of the bridge off and put back in place?

Or, the bridge fits back on pretty cleanly, the tear out seems to mate well, should I just glue as is and clamp?
Author:  DanKirkland [ Mon Jul 09, 2018 2:10 pm ]
Post subject:  Re: Classical Bridge re-glue
Conor_Searl wrote:
I've got this classical guitar that needs a bridge re-glue. It seems like a pretty standard repair that I've done before on steel strings, but I'm a little confused by this particular makers process in gluing the bridge on in the first place. It looks like they finished the top and then routed the finish off the top under the bridge footprint. But the rout is smaller than the actual bridge. Wouldn't this leave the bridge sitting on a bit of a shelf all the way around its perimeter, which in turn would prevent a good solid glue joint?


Many many many makers (Martin included) do this to make the line around the bridge look nice and sharp. It's common practice.

Conor_Searl wrote:
Anyway in fixing it my plan is to clean up the bottom of the bridge back to bare rosewood, and then remove that small shelf of finish around the perimeter of the bridge footprint, scrape the cedar flat, clamp and glue. Is it worth my time to try and get the cedar that's torn off and stuck to the bottom of the bridge off and put back in place?


That method will work. It'll cost you a little more time so you should charge appropriately. I see lots of gaps in the original gluing so this would be a great choice for the repair.

Conor_Searl wrote:
Or, the bridge fits back on pretty cleanly, the tear out seems to mate well, should I just glue as is and clamp?


This method will also work. But if you're planning on using something like hide glue you should scrape both parts clean and then glue. If you're planning on using something like titebond I you can get away with little gaps.

Since you asked, I'd go for the scrape both surfaces clean option. You'll gain some glue surface by scraping the finish back and thus you'll get a better glue joint. You do run the risk of it looking like it's been reglued though so keep that in mind. If you don't care about a little wood showing I'd go for this method.
Author:  Conor_Searl [ Mon Jul 09, 2018 4:04 pm ]
Post subject:  Re: Classical Bridge re-glue
DanKirkland wrote:
Many many many makers (Martin included) do this to make the line around the bridge look nice and sharp. It's common practice.


I figured that was probably the case. Does it not leave a thin void under the bridge that the glue needs to then fill, and is that not a potential problem? Just wondering for future reference...
Author:  Conor_Searl [ Mon Jul 09, 2018 4:07 pm ]
Post subject:  Re: Classical Bridge re-glue
Is there an obvious reason for a fail like this? It doesn't seem like the guitar got too hot, since it wasn't really the glue that gave but the actual wood on the top. The owner did have a steel string on their D, but it seems like it would be a bit of a stretch to think that one string added the fatal amount of tension.
Author:  bionta [ Mon Jul 09, 2018 4:16 pm ]
Post subject:  Re: Classical Bridge re-glue
You didn’t mention a rabbet and I can’t see it in the picture. Not an expert here but what I’ve seen and done is to cut a tiny rabbet around the perimeter of the bridge so that the glued surface drops down inside of the ridge of finish you pointed out. That eliminates the problem of the gap that would otherwise be filled with glue and weaken the joint. The rabbetted edge of the bridge hides the edge of the finish.

Search the forum for this topic and mentions of David Collins and Hesh for much better explanations.
Author:  DanKirkland [ Mon Jul 09, 2018 4:36 pm ]
Post subject:  Re: Classical Bridge re-glue
Conor_Searl wrote:
DanKirkland wrote:
Many many many makers (Martin included) do this to make the line around the bridge look nice and sharp. It's common practice.


I figured that was probably the case. Does it not leave a thin void under the bridge that the glue needs to then fill, and is that not a potential problem? Just wondering for future reference...


It does, there's not really a void, it's just that the glue doesn't stick to the finish. There's enough glue area of the bare wood even with the tension of the strings the bridge can be attached fairly well.

Regarding a rabbet, Guild does this and it is infuriating to fix. It's easier for everyone involved if you just have a flat bottomed bridge with a flat area of the finish.

You can cut a rabbet if you want to hide the edge. Most of the time it's not worth the time and effort depending on the guitar.
Author:  DanKirkland [ Mon Jul 09, 2018 4:47 pm ]
Post subject:  Re: Classical Bridge re-glue
Conor_Searl wrote:
Is there an obvious reason for a fail like this? It doesn't seem like the guitar got too hot, since it wasn't really the glue that gave but the actual wood on the top. The owner did have a steel string on their D, but it seems like it would be a bit of a stretch to think that one string added the fatal amount of tension.


Typically the way they remove the finish at the factory is with a knife. They score the finish heavily and then remove it. What likely happened was that they scored just a bit too deep through the finish and they cut the top layer of fibers of the wood. Therefore when the time came that top layer of wood was weakened by the cut and it gave way.
Author:  Freeman [ Mon Jul 09, 2018 6:15 pm ]
Post subject:  Re: Classical Bridge re-glue
What is a little bit surprising is that they didn't finish over the bridge like most classicals.

Why did it fail? The bridge on any fixed bridge guitar is the most highly stressed joint. Sure the tension is a little less on a classical but you have the same shear and rotational forces, in fact I will argue that pinned bridges are stronger than tied because the ball ends of the strings are pulling up on the bridge plate, not on the back of the bridge like a tie block.

Remember that when you get ready to glue it back on to make a caul that fits over the fan braces - they are very delicate. With the amount of top wood that came up with the bridge I would be very tempted to not try to sand the top and bridge but to refit the torn wood back into the top. That was not a glue failure, it was failure of the wood itself.
Author:  Conor_Searl [ Mon Jul 09, 2018 10:49 pm ]
Post subject:  Re: Classical Bridge re-glue
Freeman wrote:
Remember that when you get ready to glue it back on to make a caul that fits over the fan braces - they are very delicate. With the amount of top wood that came up with the bridge I would be very tempted to not try to sand the top and bridge but to refit the torn wood back into the top. That was not a glue failure, it was failure of the wood itself.


This guitar is a small La Patrie (seagull family) nylon string guitar, as such I couldn't get any of my longer clamps through the smallish soundhole to catch the ends of the bridge. And even if I could get around the length somehow, I'm not sure that I'd be able to fit 3 in appropriate places. So I ordered the Fox classical bridge clamp from LMII, with that I should be able to get away with one cam clamp in the center. For the caul inside should I make one that arches over the center fan brace and rests directly on the soundboard, or would a longer piece of plexi glass sitting across several of the braces be a better way to go?
Author:  DanKirkland [ Tue Jul 10, 2018 9:07 am ]
Post subject:  Re: Classical Bridge re-glue
Conor_Searl wrote:

This guitar is a small La Patrie (seagull family) nylon string guitar, as such I couldn't get any of my longer clamps through the smallish soundhole to catch the ends of the bridge. And even if I could get around the length somehow, I'm not sure that I'd be able to fit 3 in appropriate places. So I ordered the Fox classical bridge clamp from LMII, with that I should be able to get away with one cam clamp in the center. For the caul inside should I make one that arches over the center fan brace and rests directly on the soundboard, or would a longer piece of plexi glass sitting across several of the braces be a better way to go?
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That clamp from LMII is a handy one. I'd go with your idea for a caul that rests directly on the soundboard and avoids the braces entirely. You'll get better clamping pressure that way too.
Author:  Freeman [ Tue Jul 10, 2018 10:57 am ]
Post subject:  Re: Classical Bridge re-glue
Do you know what kind of bracing it has? Some of the more modern nylon string guitars actually are X braced, however the classic classical bracing is 5 or 7 fan braces. You can put a light bulb inside the guitar and trace the shadows of the braces on the top - use that to make your inside caul. For any bridge repair I like to clamp directly against the top and bridge plate if it has one. I tend to put a lot of clamping pressure on - usually three ibex style clamps.

I built a classical a long time ago with 7 fan Hauser bracing. When the back was off I made a caul that fit over the fans. Basically I took a piece of 1 inch MDF and cut slots in it with a table saw where each of the fans were located - I could reach inside and hold it over the bridge plate while I set the clamps. I don't have a picture of the caul but here is the insides of my guitar - that will give you an idea of how it was done.

Image
Author:  Conor_Searl [ Tue Jul 10, 2018 11:18 am ]
Post subject:  Re: Classical Bridge re-glue
Freeman wrote:
Do you know what kind of bracing it has? Some of the more modern nylon string guitars actually are X braced, however the classic classical bracing is 5 or 7 fan braces. You can put a light bulb inside the guitar and trace the shadows of the braces on the top - use that to make your inside caul.


Definitely fan bracing. I've heard several people say you can put a light inside and see through the top, but I haven't had any luck with any of the flashlights I have kicking around. What kind of light do people use? Having said that stew mac had a cool video a few weeks ago about how to trace a guitar and the guy used magnets to locate the braces and then put the marks down on a piece of vellum, I suppose something like that would work for this too.
Author:  Freeman [ Tue Jul 10, 2018 11:47 am ]
Post subject:  Re: Classical Bridge re-glue
Try a small standard light bulb - CF or one of the new "cool" energy efficient bulbs. Just a socket on the end of a zip chord with a small bulb. Lay some foil or something on the inside of the back to protect it from the head and don't leave it on very long.
Author:  SteveSmith [ Sun Jul 22, 2018 12:20 pm ]
Post subject:  Re: Classical Bridge re-glue
I use the magnet trick all the time to locate bridge plates and braces. I just put masking tape on the top to make the marks on.
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