Bridge repair?

Construction and repair of Classical Guitar and related instruments
vesa
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Bridge repair?

Post by vesa » Wed Dec 20, 2017 8:54 pm

Repair tips needed.

A bridge came off, glued with Titebond,
did very little damage and can be reused.
The few spruce chips that had come off with the bridge
are very thin(maybe 0.1 - 0.2 mm).

1. How to clean up Titebond, any solvent that can be used instead of
or together with scraping?
2. Glue it back with Titebond/slow setting Araldite/
or switch to Hide glue?
3. As seen it came off only from the ¨rosewood side¨ and the contact with the top is perfect. Should I crosshatch the bridge to be more secure?
DSCN1767.jpg
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Vesa Kuokkanen

Antonio Marin nr. 813 1995 (Bouchet)
Vesa Kuokkanen 2016

Marshall Dixon
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Re: Bridge repair?

Post by Marshall Dixon » Thu Dec 21, 2017 4:55 am

Hello vesa,

If you search the net for "De Glue Goo" it's a product to remove this type of glue. I've never used it and my big question is how well does it clean the glue that has seeped into the wood fibers?

That looks like a real clean break and it looks to be some rosewood coloration on the bridge area of the top. Do you suppose there was some oily residue in the rosewood? I've read about people having problems with that even though acetone was used to clean the rosewood surface.

Good luck with that fix. Please let us know how it turns out.

Euan Hannah
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Re: Bridge repair?

Post by Euan Hannah » Thu Dec 21, 2017 8:09 am

Moisture and heat will remove hot hide and normal titebond but work slowly and carefully. Most chemicals such as acetone will cause havoc with the surrounding finish unless extreme care is taken. Personally I would carefully scrape both surfaces clean without any chemicals or moisture and I would reglue with slow setting exrta strong epoxy. While not every luthier's favourite glue, it very rarely fails in my experience and I have done a lot of such repair work. Make sure humidity of the components and surrounding working area are stable and within acceptable levels when regluing. Cross hatching the bridge within reason is OK but not essential, more important is making sure the bridge surface is true.

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Michael.N.
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Re: Bridge repair?

Post by Michael.N. » Thu Dec 21, 2017 8:24 am

Learn to use hide. Providing the mating surfaces are good it would just be a simple matter of applying more glue. If anything the joint will be stronger, the surfaces are already sized.
Historicalguitars.

vesa
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Re: Bridge repair?

Post by vesa » Thu Dec 21, 2017 2:23 pm

Thanks for the help.
Marshall Dixon wrote:
That looks like a real clean break and it looks to be some rosewood coloration on the bridge area of the top. Do you suppose there was some oily residue in the rosewood?
Yes if sh... happens it should be like this. Very neat and clean cut.
Must be the oily EIRW because the glue was not old
and there was enough of it,
the joint was good (no way I could stick a cigarette paper between the parts),
both parts had been longer than 6 months 45 % humidity and the strings came on 2 months after glueing.
"De Glue Goo" looks interesting but could not find it on sale anywhere in Europe, but thanks for the tip MD.
Euan Hannah wrote:
such as acetone will cause havoc with the surrounding finish unless extreme care is taken. Personally I would carefully scrape both surfaces clean without any chemicals or moisture and I would reglue with slow setting exrta strong epoxy.
Euan, what are the epoxies you have used. I have found Araldite best of those that available in local stores, never used it to joining though.
I think I might end up with scraping but will I see if some chemicals will do part of the job.
Michael.N. wrote:
Learn to use hide.
My next bridge will be glued with HHG. :kap:
Vesa Kuokkanen

Antonio Marin nr. 813 1995 (Bouchet)
Vesa Kuokkanen 2016

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singularity
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Re: Bridge repair?

Post by singularity » Thu Dec 21, 2017 2:51 pm

Vesa, here's my $2*10^-2 with a huge grain of salt. Based only on the above picture it looks to me like this is a laminated top with spruce veneer partially removed for bridge to inset and fit into the top. This may be reason why your paper test returns false positive. Few spruce fiber leftovers are probably not allowing the bridge to fit tightly to the top as it shows in your picture. OTOH, not seeing it in person I may be completely wrong, but either way, to fix it I would make sure that the top/ bridge area is completely flat and clean before gluing bridge to the top.

SteveL123
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Re: Bridge repair?

Post by SteveL123 » Thu Dec 21, 2017 2:56 pm

This is an interesting subject to me since I'm trying to acquire the skills to do bridge repair (remove then re-glue).

Vesa, Why do you think the bridge failed? Was it due to the glue used (Titebond), surface prep not adequate, or something else?

Marshall Dixon
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Re: Bridge repair?

Post by Marshall Dixon » Thu Dec 21, 2017 3:19 pm

vesa wrote:
Thu Dec 21, 2017 2:23 pm

"De Glue Goo" looks interesting but could not find it on sale anywhere in Europe, but thanks for the tip MD.
Here is some info that @simonm posted as a reference in another recent post about glues.
Alan Carruth wrote:
Sun Oct 31, 2010 2:25 am
Titebond, and that ilk, will break down chemically if you apply acetic acid. It seems to work faster where the glue line is thicker. I once got a very fragile upright bass top off using hot photographer's stop bath (28% acetic, iirc) mixed with methyl cellulose (to form a gel). The top had been thinned down (with a disk sander) to 3mm or less all around the edge, and then glued with Titebond. It took five hours to remove, but suffered no further damage. Be warned that the use of a plain steel knife or spatula to do this will permanently stain the wood black: you will need a stainless steel knife.

The person who gave me the methyl cellulose got it from the restoration department at the Museum of Fine Arts in Boston, where they ues it all the time in a similar manner. She said that they begin every day with two anathemas: one on people who 'repair' antiques with wood screws, and another on those who do the same with Titebond. Once you have Titebond in the joint, the only way to remove it all and get a good joint is the scrape off wood, and that's verboten in antiques restoration. With HHG all you need is warm water.

I do use Ttiebond in new construction, but less and less as time goes by. I too have repaired my share of instruments that were put together with HHG and subsequently came apart. In general, I find these objects were not properly stored: HHG is perfectly durable so long as it remains dry. High humidity and heat are always factors when I see the sort of 'crystalization' of the glue line that leads to total failure. I usually see it as a mercy: the glue line fails rather than the wood, and reassembly is generally possible, althoujgh not always easy.

vesa
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Re: Bridge repair?

Post by vesa » Thu Dec 21, 2017 3:44 pm

singularity wrote: Based only on the above picture it looks to me like this is a laminated top with spruce veneer partially removed for bridge to inset and fit into the top.
Sorry to disappoint you, but it is solid german spruce top - I made it.
It is my mistake we are discussing (and that disapoints me). :)
SteveL123 wrote: Vesa, Why do you think the bridge failed? Was it due to the glue used (Titebond), surface prep not adequate, or something else?
Titebond I used was fresh I have glued bridges on similar way in my earlier builds also using Titebond with no problems.
Humidity was just right and so was the clamping.
Surface prep and joint ok too, but I think rosewood was more oily than in my other builds and I should have cleaned EIRW surface with acetone before glueing as Marshall Dixon points out but I did not.
And then there is the ¨something else¨.
What you see in the pic (the top) is rosewood oil in Titebond
not rosewood wood in Titebond,
the bridge surface is just as nice and clean as it was before I glued it on
(no Titebond residue on it).
Marshall Dixon wrote: Here is some info that @simonm posted as a reference in another recent post about glues.
Thanks MD, I might also do a small scale testing of different acids, solvents etc. that I have left over from my motor bike renovations.
I'll post pics when job done.
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Antonio Marin nr. 813 1995 (Bouchet)
Vesa Kuokkanen 2016

Marshall Dixon
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Re: Bridge repair?

Post by Marshall Dixon » Thu Dec 21, 2017 4:02 pm

I built a guitar with near total use of HHG about a year ago (not used on headstock veneer, neck scarf and tailblock). The biggest issues were the top and back to linings. Instead of doing them in a one step process, I glued up the upper and lower bouts in quadrants. This worked really well. Plus... and this is a big thing in my estimation, any gaps in the mating surfaces could be reactivated with heat (from a clothes iron pressed around the rim) and a little moisture worked into any gaps (there were a few). After cutting the binding channels the mating surfaces can be assessed again.

@Euan Hannah: The acetone would be used (in theory) to remove the residual oils in rosewood before glue-up. This was taken to extremes in several older books on lutherie (Sloan, Overholtzer) where alot of instruction was dedicated to making an acetone bathtub!

LMI, I believe it is, has a blurb about this (oils in rosewood) in their product info section for the epoxy that they market.

SteveL123
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Re: Bridge repair?

Post by SteveL123 » Thu Dec 21, 2017 4:40 pm

So it was due to oil left on the Rosewood surface? That's part of surface prep? I wonder if you had used HHG on the same bridge would it have held? What did Torres use to de-grease Rosewood before Acetone and other solvents were invented?

Alan Carruth
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Re: Bridge repair?

Post by Alan Carruth » Thu Dec 21, 2017 5:35 pm

Did you scrape the under surface of the bridge just before gluing it? Tests done at the Forest Products Lab back in the '40s found that they got better glue joints on surfaces worked within 15 minutes of applying glue. The reason is 'surface energy', which sounds sort of 'new age' but isn't.

When you remove material from a surface you're breaking molecular bonds; weak ones, to be sure, but still... It can take a while for those open bond sites to find something to latch onto, and if you put glue on the surface during that time it forms a stronger bond. The test for surface energy is to spritz the surface with a mist of water. Water is a polar molecule: there's a slight electrical charge on the two ends, so it's attracted to open bond sites. If the water beads up the surface energy is low; in fact, they determine the exact surface energy by measuring the angle of the edge of the drop to the surface. If the water spreads out into a film the surface energy is high.

Some materials seem to naturally have lower surface energy than others; they're just harder to glue. I once used a fossil mammoth ivory bridge on a guitar, and the literature that came with it warned that it could be hard to glue. I did the spritz test, and the water beaded right up. It makes sense; the stuff has been in the ground for a few thousand years, and there's not much chemistry left that hasn't been done. Scraping it re-activated the surface, but not for long. When I put it down I had everything ready and warm, scraped the surface of the bridge lightly, slapped some hide glue on the top and the bridge, and clamped it. No problems.

Another thing that can happen is that the wood of the bridge absorbs moisture from the glue, and the surface swells a little. This makes the flat surface become a little convex; the edges curl up. It might not be much, but it's enough to give a thicker glue line along the edges. Since most of the strength of the glue line is chemical rather than mechanical, the thicker glue line is weaker. From what I can see on the photo, it looks as though the bridge peeled up more or less cleanly along the back edge, which would be consistent with this. Once it starts to peel there's less glue surface to carry the load, and the stress at the edge rises, so it peels faster.

The solution for this is two fold. One is to be careful with the grain direction on the bridge. Wood swells more tangentially than radially, so a flat cut bridge will be more prone to this than a quartered one. Actually, IMO, the best cut for bridge wood is skew, since it as the highest splitting resistance, but doesn't tend to incur the penalty of flat cut. Particularly avoid flat cut with the with the ring lines cupping downward.

The other is to make the gluing surface slightly concave from front to back. holding a straight edge on it you should just see a bit of light in the middle, and, of course, you must avoid having the surface fall away at the edges, particularly the back edge.

'Toothing' the surface is more likely to make the bond weaker than stronger. Again, most of the strength of he bond seems to be chemical, and a thicker line tends to be weaker. Martin came up with the 'belly' bridge when they started to get a lot of warranty calls on the narrower 'bar' bridges after switching to steel strings. Recently, luthier Mario Proulx has pointed out that he's had no problems gluing bridges like the early Martin ones down with hide glue, but he doesn't tooth them the way Martin did. The old ones I've seen at toothed to the point where they only have about half the effective glue area of a smooth surface, so it's no wonder they let go.

I've never had any problems gluing down IRW, or anything else, that I could attribute to oils on the surface. When I have had problems it's always been from the stuff I mention above.

You probably know most or all of this, and I hope you won't take the reminder as criticism. This is probably the most highly stressed glue line on the guitar, and it's worth some effort to get it just right.

vesa
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Re: Bridge repair?

Post by vesa » Thu Dec 21, 2017 5:43 pm

SteveL123 wrote: I wonder if you had used HHG on the same bridge would it have held?
We'll newer know, but I have a feeling that it would have held because,
and this will sound very weird, the joint was too ¨good¨
(too tight, a perfect touch of two surfaces) for Titebond that bonds mechanically but would have been perfect for HHG that bonds organically.
And this is now pure speculation.
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Antonio Marin nr. 813 1995 (Bouchet)
Vesa Kuokkanen 2016

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SeanWinkler
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Re: Bridge repair?

Post by SeanWinkler » Thu Dec 21, 2017 5:47 pm

Just curious--is it an optical illusion, or are the corners of the bridge not square? The footprint on the top looks square, but the two edges of the bridge closest to the footprint in your photo (presumable the back edge of the bridge since it's upside-down), look like the corners are snipped off a bit.
Remember Anthony Weller, please help. Contact myself or Aaron Green for details.

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Steve Ganz
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Re: Bridge repair?

Post by Steve Ganz » Thu Dec 21, 2017 7:05 pm

singularity wrote:
Thu Dec 21, 2017 2:51 pm
Vesa, here's my $2*10^-2 with a huge grain of salt. Based only on the above picture it looks to me like this is a laminated top with spruce veneer partially removed for bridge to inset and fit into the top. This may be reason why your paper test returns false positive. Few spruce fiber leftovers are probably not allowing the bridge to fit tightly to the top as it shows in your picture. OTOH, not seeing it in person I may be completely wrong, but either way, to fix it I would make sure that the top/ bridge area is completely flat and clean before gluing bridge to the top.
I agree that it does look laminated not solid. Why does it appear that the grain under the bridge is ninety degrees from the grain of the spruce. And...why are there traces, prsumably of spruce, on both the bridge and the top?
I have never seen a solid top that would give that appearance. Please educate me and singularity if you feel inclined to do so.
Steve

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