Official Luthiers Forum!

I've built about 25 guitars in the past 3 or so years and have had a few bridges come off, more than I think should. I attached bridges with LMI instrument glue and epoxy. With the white glue I scrape the finish off in the bridge area, and with the epoxy I scuff the finish and glue right to it. The epoxy is system3 5-minute. I've had them come off both ways. I do try to control humidity in the shop at about 45. Is this normal or am I doing something really wrong? Any thoughts?

Not enough information to give a definitive answer. You don't say if these are steel string or classical, pin or pinless, or what the finish is. I have no experience with epoxying bridges on top of finish (nor do I intend to), and especially not with 5-minute epoxy, so can't speak to that at all, except to suggest that it's not the generally accepted method.

If you've had a bunch of failures, I'd look at the basics: are the bridges well-fitted to the soundboard, are the surfaces fresh and smooth, are you clamping evenly and strongly, do you allow the joint enough cure time before stringing up? All of those are pretty necessary. The easiest one to screw up, in my experience, is clamping. Unless you made a good caul that supports the entire bridge, including the wings, it's difficult to get even pressure and good results.

Hope something there is useful!

Rick Davis
Running Dog Guitars

Glueing right on top of the finish is generally not a good idea. What type of finish?

As for the LMI white, it's generally user error for glue joints letting go. Either you don't have enough glue in the joint, surface coverage of the glue was poor, didn't have adaquate clamping pressure or the surfaces were not prepared well or a combination of all of the above.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

What about too much clamping pressure? squeezing all the glue out.

Highly unlikly to squeze all the glue out from a joint. You'd need a massive amount of pressure to do that. The only way to starve a glue joint is with an insufficient amount of glue.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

A basic question...

Do you sand the gluing gluing surface of the bridge to match the exact profile of the top? If this is not done, you'll have lots of tension on the wings trying to detach it from the top. This joint must be perfect IMO.

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

I haven't use LMII white glue, but wasn't there a post recently that this glue has a short shelf life?

LMI glue, in my experience, has about the same shelf life as Titebond. I use it within 6 months, preferably less, and haven't had any issues (constant use over more than 10 years). One batch I received was thicker than usual; I wasn't comfortable with it and LMI replaced it without question.

I doubt that glue quality or age is the problem here. It sounds like some on-going problem with humidity or basic woodworking procedures -- but again, we don't have enough information to make an informed opinion.

Rick

Thanks for the tips guys. A little more info The guitars are steel strings with pin bridges. I do try to get the same radious on the bridge as the top but are not perfect. my wings do get pulled down when clamped. the wings themselves are.o8-.09 thick. I do have acouple of bridge styles I use, the martin style and one similar to the kasha where the bass wing is wider than the trebile. Lights are the max string gauge often extra light. I would say the top arch is 1/8-3/16 at an 18" span. The tops are pretty lightly built with a thickness at the bridge .09-.095 regardless of body size. Lately my X braces and all braces have been 1/4" wide with the X.55" tall at its tallest. When the bridges come off it is a pretty clean seperation with hardly any wood tear out. Thanks again for the feed back.

Brent

pinless bridges see more shear loading than pined bridges because the ball ends are pulls directly on the rear of the bridge as where on a pinned bridges balls pull the top and bridge plate into cpmpression loading. On the epoxy to finish glue joint I would never do this. You are expecting the finish to hold the load.

If you are seeing clean separation, with no tearout, you're not getting good adhesion on one side of the joint. For instance, using any of the oily woods (many of which are used for bridges), most comments I've seen say first give the back of the bridge a light sanding, then wipe with solvent just before glue-up. You should get some hint if you can remember which side of the joint the glue was left on. If the glue was left on the soundboard, then the bridge side needs different preparation. If the glue was left on the bridge, then the soundboard should be readied differently. I have to agree with the people above about not gluing directly on the finish- I always want fresh wood on both sides of the joint, with the adhesive the only other component in the joint.

I'd also worry about not getting the bridges to fit as well as possible to the arch of the top. If the outer edge of one of the wings starts to lift off, that is a huge stress riser in the joint, making it a lot likelier to fail. The best way to get that fit, in my opinion, is to tape sandpaper directly across the top, then sand the bridge to fit. I only go to 180 grit on the bottom of the bridge also.

_________________
Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

Possibly a contributing factor: when a flexible plate is bonded to a rigid plate, there are stress concentrations around the perimeter of the bond. More flexible tops will have higher stress concentrations, unless the bridge plate is stiff and bigger than the bridge all around it.

Still, the stress on bridge glue is pretty small -- 180 lb over 8 square inches is 23 psi shear stress, and the moment should produce (very roughly) 50psi-100psi tensile stress at the back edge. A stress concentration could bring that into the hundreds of psi. But Titebond is rated to 3500psi, and even 5 minute epoxy is 1500 psi. So that suggests something like a surface prep issue, like Gene said.

_________________
David Malicky

How does joint look after the separation?

Is there any wood torn away from either the top or the bridge?

What about the glue? Does the majority of the glue seem to remain on the top? Or on the bridge?

_________________
Jim Hansen

Brent: Think your fit of the bottom of the bridge may be at least part of the problem. When I fit a bridge I hold it loosely in place with light finger pressure and check all around the perimeter with .002 feeler gauge to make sure the feeler will not enter anywhere.If the gauge goes in anywhere,more fitting is required.
Tom

_________________
A person who has never made a mistake has never made anything!!!

Let me back up a bit. Like I said I was gluing with LMI's glue. I would finish the guitar then lay out the bridge and scrape off the finish. the problem here was geting a clean and smooth surface. Also I would get a line around the bridge where i scraped the finish would be chiped and looked bad. That is partly why i went to epoxy. I have not tryed tapping off the bridge area b efore finishing because I thought I would get the same results after I pulled up the tape. Maybe that is something I should try. Like I said I've had such a hard time getting that clean line without chip out around the bridge.

Do you score the finish around the perimeter of the bridge with a fresh exato blade before scraping? This should eliminate your chip out of the finish.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Brent...my advice would be to start from scratch and take a best practices approach on all of the steps for gluing a bridge to a top. I would recommend the following:

1) Tape on 120 grit sandpaper to the finished top
2) Sand the bridge back and forth in the area where the bridge will eventually locate. Don't stop until and unless the bottom achieves the exact contour of the top.
3) Locate and center the bridge in its exact position and tape in place.
4) Drill two 1/16"location holes through the bridge and bridgeplate where pins will hold it in place while clamping
5) Pin the bridge securely to its location and lightly scribe the lacquer with a sharp scalpel blade. Keep going over the scribe line until you reach wood.
6) Remove the bridge and scrape away the finish down to bare wood.
7) clean both surfaces and HHG the pinned bridge to the top with a Stewmac bridge clamp and proper bridgeplate caul.
8) Remove the clamp in the morning and let us know how it worked.

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com