Reattaching a Bridge - Luthier Tips du Jour Mailbag
In this episode, Robbie O’Brien shows how to re-attach a bridge that has come un-glued.
Video Transcription
[on-screen text reads: Luthier Tips du Jour Mailbag]
Mailbag question: My guitar bridge has come off, whats the best method for re-attaching it?
Robert O'Brien: Today's Tips du Jour Mailbag question comes to us from Brazil. "Dear Robert, the bridge on my guitar has come off. Can you offer some advice on how to best reattach it? Thanks for your videos." Fernando in Brazil.
What causes a bridge to come off
First of all, Fernando, let's talk about what causes the bridge to actually come off. The most common reason I see in my shop for a bridge coming off is leaving it in a hot car. Under the string tension and the heat, the glue will soften and the bridge comes off. Now, the technical term for that in English is bad. In Brazil, I believe you would call that [foreign language 00:00:38]. So don't do that. The second most common reason for a bridge coming unglued is drastic or extreme changes in relative humidity. Our modern day glues are good, however, there's a lot of movement in wood with the expansion and contraction of that top. And it's really tough for glue to be able to keep up with that.
Another reason for a bridge coming unglued is a glue failure. You don't see this very often, but when you do, it's usually an operator error and not the glue. In other words, the surface was not prepared correctly, there's gaps in there or perhaps you even used an oily type wood that wasn't prepared properly. But occasionally the glue will fail. And to add to that list, here's one more. Wood failure. Now occasionally I see this in my shop. I have a guitar on my bench right now where the bridge has come off and I believe looking at it that it was actually wood failure. So let's take a look at it. I'll talk about the wood failure and then I'll show you how to reattach the bridge.
The problem
So here's the guitar that I mentioned on my bench where the bridge has come undone. And if you look closely at the bridge, you'll notice there's a lot of wood on the bottom of the bridge. This tells me that the wood failed and not the glue joint.
The guitars that have come into my shop where there's been an actual wood failure and not glue failure, it was because when the bridge location was masked off and finished, the luthier came in and scored the finish and the tape, removed it to expose the area so he could glue the bridge. However, if you score too deeply into the wood, you really weaken that fiber and the bridge will then lift up. And I've seen that on a couple of occasions. So you want to try and avoid that obviously. And cedar is a very strong candidate for having that happen. So since this is a very clean break, I'm just going to glue this bridge right back in the same location. Now the key is the same location. How do you keep it in the same location?
Also, when I got the guitar into the shop, I took a look inside the sound hole. If the bridge came off, perhaps there's some internal damage to braces and things like that. And you want to make sure that there's not some other symptoms you need to take care of. So I've checked all that out, everything's structurally sound. It's just the bridge that needs to go back on. It's very important that the bridge goes back in exactly the same location. So what I've done is drilled a couple of holes into the saddle slot. And now I'm going to use a couple of locating pins that I got from LMI to hold the bridge in place as it's getting glued on. Before I glue any bridge to the top of a guitar, I like to coat it with a very thin coat of wax.
Now that helps protect the bridge, makes it look good. It also helps the glue that's going to squeeze out and get on the side remove a little bit easier. I'm going to use a special lutherie wax that I got from my buddy in Brazil, Luciano Borges. He gave me this in our recent class when I was there. It's a special recipe that he makes and is great for putting on the bridge. I just apply a little bit to the wings here and across the back of the tie block. Make sure that you get it all on there, on the edges that's the most important part because you don't want the glue to get stuck on the edges of the bridge. To reattach the bridge, I'm going to be using LMI yellow glue. And I'm going to apply it liberally here on the bottom of the bridge.
Make sure that all the little wood fibers stay in place. You don't want to remove any of that stuff because that's going to create a gap when it goes to get glued back on. You want all those to go back into place. It looks like it was a pretty clean separation. And if I get a little squeeze out on this, I'm not opposed to that. I want to make sure that I get good coverage.
Attaching the bridge
So I'm going to place the bridge into the correct location. I'm then going to use my locating pins that I got from LMI to put in those holes that I made previously. And since I'm going to be using the LMI vacuum clamp to reattach the bridge, I want to make sure that those pins go way down inside that saddle slot so that there's no possibility of them puncturing the membrane. I'm going to clean out a little bit of the squeeze out right now before I even put the bridge clamp on there.
Removing the glue
I'm now going to turn on my LMI vacuum pump and use the bridge clamping jig to attach the bridge.
After 10 minutes, I like to remove the clamp and clean out the squeeze out. And you can tell I've got quite a bit on here. I was fairly liberal with the glue on this one on purpose. I want to make sure this thing gets glued down properly. To remove the glue, I use this super high-tech glue removal tool that I had NASA designed for me [Robert gently works a small narrow piece of scrap wood that has been cut like a stake with a sharp flat end on it], and I just run it along the edge of the bridge like so. If you think you need a little moisture, a little wet paper towel will help soften the glue just a little bit. This is where the wax comes into play. It's much easier to get it off the edges of the bridge now. Be very careful you don't damage your top because these tops are really soft, especially the cedar. I'm just going to be careful not to damage the top. Once I get all the glue off, I'm going to put the bridge clamp back on for about another 20 or 30 minutes and that's all it takes.
So Fernando in Brazil, thank you very much for your question. And with this information, I believe you can reattach the bridge with no problem. Now remember the reasons why the bridge came off in the first place and hopefully you can avoid that in the future.
[on-screen text reads: More Luthier Tips and online courses available at www.obrienguitars.com. Private and small group guitar building and finishing instruction available.]
