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Hi Folks, I heard a snap and the action went bad real quick so I took the strings off and the bridge literaly came off in my hand. I used tightbond an ebony bridge on a red spruce top. Almost all the glue seems to be left on the spruce. Do I need to clean the ebony with acetone? should I rough it up before I reglue? Thanks, Rob

I had my first bridge lift up too, although it was only the back edge so it took some effort to unglue the rest. From what I could tell, I had done two things wrong:

1. I had sanded the bridge to match the radius of the top, but didn't do that great a job of it and had a bit of a hump front-to-back on the glue side of the bridge. That's why only the edge lifted, because the center had good contact with the soundboard, but the front and back edges had a gap that left a thick and brittle glue line.

2. When gluing the bridge the first time, with hide glue, I got it all nice and suctioned down in almost the right place, but then I couldn't find the position pin holes in the soundboard and the glue got too set up to move it anymore. I just left it as a rub joint. This may not have actually been a problem though, since the center held on fine.

So to solve #1, I took a scraper to it, pulling in the side-to-side direction, to make sure it was perfectly flat in the front-to-back direction. Next time I'll skip the sanding and just scrape and check until it's a close match.

To solve number 2, I drilled out one of the pin holes one size larger. Then when positioning the bridge, I used the smaller drill bit to find the larger pin hole. Once I knew where it was, it was easy to get the larger pin into it, and subsequently locate the other pin hole. Then I put 3 cam clamps on it, with fairly light pressure. No sign of trouble now a month later, with mediums on it (it had lights when it lifted).

So, make sure the bridge doesn't have a front-to-back hump to it, and that both surfaces are freshly scraped or sanded. You don't need to rough up surfaces for a wood-to-wood joint. I'd also recommend a more creep-resistant glue, like hot hide (not the bottled liquid stuff) or fish glue or LMI white, although as long as the guitar doesn't get left in a hot car or anything, titebond should be fine.

I'm assuming you built the guitar, from your comment. A picture might be useful, but sounds like the fit between bridge and top wasn't very good when you glued it - "all of the glue seemed to be left on the spruce" - if there was significant glue there, the fit likely wasn't good enough. The bridge ought to fit like a glove, without gaps, on the spruce top (and on the bare spruce - of you've got finish between the glue and the spruce, it won't hold.) I'd suggest taping the top so you don't get errant scratches. remove the glue from the top, scrape any remaining finish on the top where the bridge will be glued, and glue it again. No need to "rough it up" before gluing, but a pass with a scraper or sandpaper to remove residual glue or other contaminants is a good idea - the bond will be stronger if its a smooth, tight fit. Titebond is fine.

Thanks, The fit was very good, I made sure there was no light coming through at all. The glue remaining is very even and consistent. I feel that the glue failed against the ebony, perhaps I didn't use enough. I have some photos but I can't seem to figure out how to post them. Rob

You'll have to tell me how the light coming through works.
I use the thinnest feeler gauge, testing all around the periphery. Of course it tells me nothing about what's happening in the middle.
Did you coat the Ebony?

I propped a strong flashlight in the soundhole and sighted along the top. It was pretty evident where adjustments were needed. I didn't coat the bridge, should I have? it seems the ebony was somehow contaminated, maybe finger oils, maybe natural oils in the wood. The glue all failed at once with very little glue left on the bridge.

Could someone please explain how to post photos? I must be missing something. Thanks, Rob

Huh, sounds familiar...

http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=30873

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Mountain Song Guitars www.mountainsongguitars.com

I dont realy like your light test,I think if the leading edge ( or back for that matter ) was tight , it may not allow light through. I have had success by putting some light pencil lines about 1/4 inch apart running parallell witht the top grain , on the glue surface of the bridge ,I thne place a piece of adhesive sandpaper , on the top , where the bridge will be glued, sand until all the pencil lines are gone and you have nice mated surfaces for glueing. jody

If all the glue stayed off the ebony, then there's a good bet that it was a contaminated surface. You need to scrape or sand all surfaces shortly before gluing to up their surface energy. The longer a piece has sat since the last time it was cut until gluing, the less likely it is to bond well. Don't bother with acetone, it's more likely to hurt than help.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

FWIW, I had pretty much the same thing happen on my #2. I traced it to shellac. Apparently I never scraped the bridge footprint back to wood before glueing the bridge . Did you coat the top of yours??

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Rob