detached bridge

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JurgenV
Myrtle
Posts: 74
Joined: Mon Dec 10, 2018 6:58 am
Location: Bavaria, Germany

detached bridge

Post by JurgenV » Wed Apr 14, 2021 2:49 pm

I finally sold one of my guitars instead of just giving them away and after 3 month with the new owner I received bad news today. The bridge of the first classical I built with falcate bracing came off during the night. I received some pictures and it looks like there is no big tear out. As I never had a case like in around 15 guitars I built I find it quite annoying for this to happen with the one guitar I sold. So I would like to know what happened here.
The guitar had been already half a year strung up in my flat and didn´t show any tendency of the bridge to start lifting. The owner admitted that the humidity in the rooms was quite low with around 36% at 22°C. As my flat is more around 45-50% relative humidity: from your experience could that be a reason for the problem? Or would you judge from the pictures that something with the gluing went wrong? The detail picture of the bride looks a bit suspicious to me.
Your opinions would be appreciated
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seeaxe
Blackwood
Posts: 768
Joined: Thu Dec 18, 2008 7:20 pm
Location: Auckland NZ

Re: detached bridge

Post by seeaxe » Wed Apr 14, 2021 4:55 pm

Hi Jurgen

Sorry to hear about the bridge.
Q1. How did you prepare the soundboard and bridge for glueing? Do you mask out the soundboard before finishing the top or do you scrape back the finish to bare wood afterwards?

Q2. Are those wood fibres stuck to the underside of the bridge? Strange that they are only at one end of the bridge.

Q3. What glue did you use?

The fact that there is glue all over the bottom of the bridge indicates there was nothing wrong with the glue to bridge interface, rather something is amiss with the soundboard to glue interface. Hence q1. Is it possible there were traces of finish left at one side of the bridge that meant it didn't adhere as well as it should?

Personally I don't think the reduced humidity is the issue. I would expect that to cause some cracking in the soundboard rather than cause the bridge to fall off.

Let's hope others will chip in. At least it's a pretty clean break and you can put it back on.
Richard

JurgenV
Myrtle
Posts: 74
Joined: Mon Dec 10, 2018 6:58 am
Location: Bavaria, Germany

Re: detached bridge

Post by JurgenV » Wed Apr 14, 2021 8:19 pm

Hi Richard,

Q1: i scrape back the finish
Q2: I suppose. It looks to me that way, but the guitar is now around 1100km far from me
Q3: Titebond classic

And at the moment I don´t have a clue on how to get the guitar back to glue the bridge again

seeaxe
Blackwood
Posts: 768
Joined: Thu Dec 18, 2008 7:20 pm
Location: Auckland NZ

Re: detached bridge

Post by seeaxe » Fri Apr 16, 2021 10:13 am

I was chatting to a luthier friend today (who sells lots of guitars) and mentioned your problem. He advised finding a luthier local to where the guitar is and getting him/her to fix it. Cheaper than cartage back and forth, most likely. Not ideal but more economical.

For what its worth (amateur builder here) my thoughts on why it happened are that you possibly didn't scrape back the finish enough on one side, (the side with no spruce fibres attached). And possibly too much glue and/or not enough clamping pressure, but I'm guessing there.

As I'm sure you know, Titebond is pretty reliable stuff, I've never used anything else for bridges and have always been able to string up next morning with no problems.

I hope you get it sorted out.

All the best
Richard
Richard

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Mark McLean
Blackwood
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Joined: Thu Apr 10, 2008 2:03 pm
Location: Sydney

Re: detached bridge

Post by Mark McLean » Tue Apr 20, 2021 8:52 am

Jurgen
It is great that you got to sell one of your guitars, and a bummer that this happened to it. But I am sure you will find a solution. I agree completely with Richard’s assessment. It is probably not a humidity issue. It seems like adhesion failed across a significant area, and incomplete removal of the finish is very likely a cause. But two other thoughts come to my mind:
1. Is there much arch to the soundboard, and was the underside of the bridge shaped to match? Poor contact can result if it is not, especially with a long classical bridge like that one.
2. Titebond is good, and what I always use now. The only bridge failure I have has was with hide glue. However, Titebond can be less effective when it is old, and you don’t know how old it was even when you bought it. There is a manufacture date on the bottle and it is recommended to replace it after 2 years. Here is the manufacturer’s explanation of the date code - “ The first letter is A for made in America, the first two digits after the A is the last two digits of the year of manufacture, the fourth and fifth digits represent the month, the sixth and seventh digits represent the day of the month and the last three digits represent the batch number for that day.“

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Trevor Gore
Blackwood
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Joined: Mon Jun 20, 2011 8:11 pm

Re: detached bridge

Post by Trevor Gore » Tue Apr 20, 2021 1:02 pm

This may not be the only problem, but certainly looks like one of them...

Looking at the thickness of the residual glue on the tail side of the detached bridge, that glue line looks way too thick, to the extent that the the joint looks like it wasn't properly closed. There could be numerous reasons for that, ranging from an inadequate clamping arrangement, too slow setting the clamps, clamps moving after setting them, or the bridge itself warping due to water absorption so the bridge base turns convex whilst the glue dries, thus lifting off the joint on the edge that is most loosely clamped. If that happens to be the tail side, the joint is going to fail. The amount a bridge can potentially curl depends on the wood species and how well quartered it was cut.

There seems relatively little fibre adhesion to the base of the bridge which also suggests some sort of contamination problem, either uncleared finish, or contamination whilst cleaning off the scrapings.

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