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Putting this in the Guitar builder forum because I'm hoping to have a positive impact on folks installing new bridges so that perhaps, perhaps your creations never need to come visit me in repair land.

Did this one in the wee hours this AM and thought it was a great example of what NOT to do..... This is also typical and we frequently see lifting bridge on instruments where the bridge patch was not cleared nearly enough of the finish and subsequently the bridge lifts.

The back edge of the bridge to body joint is the money shot or where all the heavy lifting is done. Here we see the finish left in place not permitting a huge amount of bridge patch to be glued to the bridge. We also see the pin holes that already compromise this glue joint near the back edge by removing gluing surface.

So I cleared (and forgot to take a pic) to within maybe .010" of the perimeter, rabbited the bridge edges as Collings and Taylor do and reglued it after 12 seconds in my microwave with HHG. Set the clamps and will finish this one up tomorrow morning.

Looking at how little finish the maker cleared this is really what I would call a manufacturing defect in that the bridge was never on there very well with this skimpy little glue patch. I would estimate that 30 - 40% of potential gluing surface including that all important back edge of the bridge was not used my the maker and that's why this bridge lifted very early on in this guitar's life.

Guildy as charged.

I always take a sharp scraper and create a small chamfer around the bridge in order to make sure the bridge is avoiding any leftover finish. Just take a minute or two.

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Jim Watts
http://jameswattsguitars.com

Almost did that recently and caught my fool self at the last minute. It was late I was tired and distracted.

Hilarious.


Sent from my iPhone using Tapatalk Pro

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That's the method that Cordoba uses too.

Did the builder rout a corresponding ledge on the bridge Hesh?

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

Trying to learn something here...If this bridge's pin holes were slotted, wouldn't the string tension load be at the underside of the soundboard, leaving little load on the bridge proper? The bridge would be offering a location for the saddle, and a path for string tension to get to the soundboard, neither of which involve force to peel the bridge off the soundboard.

I can't remember ever seeing a discussion about slotted bridges and forces acting on them.

Am I misunderstanding this?

Thanks!

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Peter Havriluk

Good one Ed!

Another good one!

No ledge, just slap it down on too little glue and call it good. It wasn't.

To some extent but not totally. The height of the saddle crates torque on the bridge and saddle is the saddle is well fit..... that causes the bridge to rock forward and aft and this stresses the back edge of the glue joint. The front edge not so much. The higher the saddle the more torque and that is why an acoustic with higher action has more volume if the strings are higher over the top in front of the saddle that an acoustic with a lower saddle all other things being the same and they..... never..... are.

Additional contributors here I'm sure are the guitar was a little dry, not a lot but a little and has sharp fret ends. A dry guitar will shed a bridge too as the dimensions of the top change under a bridge who's dimensions are not changing to the same degree.

But the major cause of death for this bridge was poor prep for gluing, too little glue with an dishonorable mention to being dry.

Peter one last example to help this be more clear. You know about people who tilt their saddles around 7 degrees aft? The entire purpose of the tilt is to convert more of the rocking resulting from torque to downward pressure. There is still the tendency to rock the saddle/bridge but to a lesser degree.

I think you missed a few, one may have showed stress patterns for pinned and pinless bridges (might have been another forum)
Here's a primer about the advantages of slotting from another forum, scroll down to Dave Collins post https://umgf.com/bridge-pin-dilemma-setup-t69377-s20.html#p935172

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Driving in in the light snow to finish this one up and jump on a few others.

Next will be removing the clamps. Hot water clean-up around the bridge (works great for HHG). Then it's reream the pin holes, 5 degrees while fitting the pins and clearing any string notches of glue. Then refit the saddle. Bridges may get a bit distorted from the heat and trauma and saddles don't always fit exactly as they once did.

Then it's set it up, write a polite.... note on tag about acoustic guitars needing humidification in Michigan winters and..... next

If the guitars top was perfectly solid then the slotted and locked string ball ends would indeed have much less effect on pealing the bridge off however; it's not, the top is flexible and creates a dome and as mentioned rocks back and forth when the strings vibrate. So eventually it will work a weak glue joint free.

Someone on the AG forum did a cool little experiment showing that an unglued bridge on a solid body guitar doesn't even need glue at full string tension. The strings going trough the body.

Much obliged for the replies to my questions. Thanks!

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Peter Havriluk

Finished up the Guild bridge reglue and just got home. I work nights these days and like it a lot. No phones ringing and nice and quiet.

So here is the bridge reglued with HHG and you can see what you should see on a proper bridge reglue, nothing. There should be no tell tale that the bridge was ever off or reglued.

Of course there are exceptions such as poor prior work which can include clearing the bridge patch to the perimeter where you can see that the bridge was off and back on. Or the wrong glues used prior, etc.

But for a first time lifted bridge the standard is an invisible repair and in this case there is 30 - 40% more gluing area being exploited and I fit the bridge to the top dome so that only minor finger pressure is required to seat the bridge in all places including the wings.

You can see how it looks like the bridge is sitting on the finish with no gaps. There is a very small rabbit around the perimeter of this bridge that sits on the finish ledge resulting in a very clean, f*ctory original fit and look. I also used HHG an upgrade from the BS used in the f*ctory where this one was produced.

Thanks for looking. Bridge reglues, fret dresses, set-ups are the bread and butter of my world these days and I completely enjoy it all.

We did a bit of a deep dive on this topic a few years back, which included some modeling and visualization in one of the boy's CAD/CAM/analysis packages. It turns out that the load on the glue joint is both a shear load across the bridge patch and a tensile load due to bridge rotation on the rear edge. For the shear load, there's more than enough area even if the bridge rabbet were 1/4" wide ( we routinely kept 7/8" - 15/16" wide bridges on somewhat compromised tops). The tension load is different, because the closer the rear edge of the bridge is from where the load is applied (the saddle slot) the higher the tensile load is along that edge.

That said, the tensile loads seen even in narrower pyramid or rectangular bridges were well below even conservative tensile limits of any of the common luthiery glues PROVIDED THE GLUELINE WAS THIN AND WELL MADE. Once the glue lines get thick, ~ 0.003"-.005" if memory serves, an AR/PVA or hide glue line may see failure in the actual adhesive.

So in summary, my experience was that the quality of the glue line is much more important that retaining maximum gluing area or even the type of glue used. Yes - AR/PVA is a poor adhesive choice for bridges due to cold creep and resistance to heat, but even Titebond will handle truly ridiculous string tension, badly overset necks, and destructive RH cycling without complaint if the glue line is a) thin (<0.001") and b) well-made. We performed hundreds of bridge reglues on a wide range of guitars while I was at Greenridge, and the clear majority of post mortem inspections of the failed joint showed thick, weak glue lines that were less able to handle routine string loads and RH variations.

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For the times they are a changin'

- Bob Dylan

Neither do floating bridges on tail-piece acoustics.