Bridge repair?

Construction and repair of Classical Guitar and related instruments
simonm
Moderator
Moderator
Posts: 6789
Joined: Sat Jun 27, 2009 10:22 am
Location: Germany, Würzburg. Spain, IB

Re: Bridge repair?

Post by simonm » Thu Dec 21, 2017 8:12 pm

Steve Ganz wrote:
Thu Dec 21, 2017 7:05 pm

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? ….

This puzzled me for a good while too … Eventually I figured it out. :-)

What looks like mahogany is the glue layer having taken on some colour from the Rosewood. The spruce on the bottom of the bridge corresponds to small pieces from the top where the glue did not let go. The "white spots" in the "red glue that looks like mahogany" are the holes left in the spruce in the few spots where it failed rather than the glue. Once you figure out what you are looking at it matches up perfectly.

PS
Alan Carruth wrote:
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. (…)

I don't know when I picked up on this but I have done it on most of my guitars. Once I have everything ready, just before I glue the the bridge on, I scrape it and the guitar top briefly using a stiff razor blade as a scraper.


and a PPS

I like the MIchael Thames videos on you tube about gluing bridges.

vesa
Posts: 476
Joined: Tue Mar 01, 2016 12:52 pm
Location: Sulva, Finland

Re: Bridge repair?

Post by vesa » Thu Dec 21, 2017 9:48 pm

Alan Carruth wrote: Did you scrape the under surface of the bridge just before gluing it?
That might be that ¨something else¨ I mentioned in my speculations about why??
I did that last may and as far as I remember I did not glue the bridge on the same day so I might not have done it.
Alan Carruth wrote:The other is to make the gluing surface slightly concave from front to back

That is how I make them, also this one.
Alan Carruth wrote:You probably know most or all of this, and I hope you won't take the reminder as criticism.
Some it was and some was not but your explanation certainly made it more clear for me and I am very grateful for your help.
Alan do you think cleaning the surface with acetone before glueing is unnecessary?
Steve Ganz wrote: I agree that it does look laminated not solid.
Titebond on the top is colored by the rosewood.
Simon explains it here
Simonm wrote:
This puzzled me for a good while too … Eventually I figured it out. :-)

What looks like mahogany is the glue layer having taken on some colour from the Rosewood. The spruce on the bottom of the bridge corresponds to small pieces from the top where the glue did not let go. The "white spots" in the "red glue that looks like mahogany" are the holes left in the spruce in the few spots where it failed rather than the glue. Once you figure out what you are looking at it matches up perfectly.
Sean Winkler wrote: look like the corners are snipped off a bit.
That's right. The guitar had been in a case when it happened so the wings have hit the case when it flew off and small pieces of RW about 0.5 mm in width has torn off from the rear side if the bridge in both ends.
Vesa Kuokkanen

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

User avatar
Michael.N.
Posts: 6703
Joined: Sun Mar 25, 2007 7:28 am
Location: UK

Re: Bridge repair?

Post by Michael.N. » Fri Dec 22, 2017 9:31 am

Try to retain the spruce fibres although the individual torn small fibres may not be worth the trouble. The large cluster at the foreground of the picture/bridge should be saved. With hide you may well be able to glue the bridge without doing anything or rather very little to the surface. If the bridge gluing surface needs a little scraping you should remove that spruce cluster and transfer them to the soundboard.
Try white (clear) vinegar on the Titebond.
Historicalguitars.

Alan Carruth
Luthier
Posts: 2722
Joined: Mon Dec 24, 2007 6:56 pm

Re: Bridge repair?

Post by Alan Carruth » Fri Dec 22, 2017 5:34 pm

I can't remember that I've ever wiped down with a solvent before gluing, so I can't say from my own experience whether it would work well or not. Some folks say it helps in gluing oily woods, others say it just pulls oils to the surface and smears them around. I rather incline to the latter interpretation. Certainly when I have tried solvent cleaning, in connection with finishing, I've found that a simple wipe down doesn't do much. If you really want to get all the oil you have to keep at it, with repeated wipes using clean rags or paper towels, until there's no more color coming off. Even then it doesn't seem to get all the oil. I've never had problems gluing the usual suspects, but I've never worked with teak or anything else that oily.

As for 'too good a fit', I've never run into that one, particularly with Titebond. My understanding is that most of the strength of a glue bond is chemical rather than mechanical, so you want to have the minimum amount of actual glue in the joint. I've had starved joints with HHG from a combination of too much heat and clamping, and maybe thin glue, but never with Titebond, even when I've thinned it out 10% or so to extend the work time for large laminates.

As simonm wrote, what looks superficially like a cross laminate is just color in the glue from the rosewood. It suggests that there was not a good chemical bond between the bridge and the glue layer, which is what prompted me to think of low surface energy.

SteveL123
Posts: 548
Joined: Thu Feb 09, 2017 5:05 pm

Re: Bridge repair?

Post by SteveL123 » Sun Dec 24, 2017 9:39 pm

Per MSDS http://go.rockler.com/msds/de-glue-goo- ... r-msds.pdf De-glue goo is Methyl cellulose + Acetic Acid diluted to 5%. I read somewhere else that it's higher than that, more like 25% but I can't find that link. Since I do not have any Methyl cellulose thickener and want to do an experiment, I used folder strips of paper towel and white vinegar (4% acetic acid per the label) on this bridge and harmonic bar (steamed off of a junk guitar for practice on an Espresso steamer I just made). The 2 pieces was glued together with Titebond Original last night. The Vinegar has been on for about 45 minutes now. I'll keep checking on it every hour, apply more vinegar if it dries up and see how long it takes to release or if it will release.
You do not have the required permissions to view the files attached to this post.

vesa
Posts: 476
Joined: Tue Mar 01, 2016 12:52 pm
Location: Sulva, Finland

Re: Bridge repair?

Post by vesa » Thu Jan 04, 2018 6:59 pm

Thanks for the help fellas.
Job done.
To begin with I softened a small part of the Titebond on the top
with household vinegar.
It helped a bit, but as I assumed I ended up with scraping it because vinegar did not help with the last part of the Titebond layer.
So I scraped it off very carefully with a goose neck cabinet scraper
and it took me about 2 hours to do it (just the top, not the bridge).
I tried to scrape with a razor blade to begin with
but hardened Titebond is too much for that tool.
Then I scraped the bridge and checked
that the shape of the glueing surface was ok.
Next was cleaning the surface with acetone which took 10 minutes
and after that I let it gas out.
Then the final scraping and then I glued it on with slow setting Araldite.
The guitar is back to the customer and the best part of the story is that
the guitar sounds as good as it sounded before the bridge popped off.
Mistakes made and the lesson learned:
1) The major mistake must have been not to scrape the surfaces just before glueing (which I cannot remember if I did or did not but I assume not).
2) Oily rosewood might have ¨helped¨ a bit and this is only IMO.
All kind of dirt: oil, coffee, dust, paint what so ever on the surfaces to be glued together makes joining everything from difficult to impossible,
so why not oils in tropical trees.
DSCN1773.jpg
DSCN1778.jpg
You do not have the required permissions to view the files attached to this post.
Vesa Kuokkanen

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

Marshall Dixon
Posts: 548
Joined: Fri May 15, 2015 5:06 pm
Location: SW Oregon

Re: Bridge repair?

Post by Marshall Dixon » Fri Jan 05, 2018 4:25 pm

Nice job! I think the only way to explain it is an oily residue in the wood. When bending rosewood I've noticed some pieces exude more oils than others. The break was so clean it looked like a sub-layer of plywood visible on the top and with a nearly clean bridge surface. I've read of luthiers who warn of contaminating the surface even with the oils on our hands.

Several days ago I found a copy of Arthur Overholtzeres book, 'Classical Guitar Making,' in a local used book store. He made alot of effort to eliminate the oil in the back and sides of rosewood and writes of standing the planks upright on salt, boiling in Spic n' Span detergent or heated acetone bath, etc.

Thanks for sharing this. It's going to make me pay more attention!

SteveL123
Posts: 548
Joined: Thu Feb 09, 2017 5:05 pm

Re: Bridge repair?

Post by SteveL123 » Sun Jan 14, 2018 3:11 pm

Alan Carruth wrote:
Thu Dec 21, 2017 5:35 pm
...............

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.
Al, this is very interesting stuff about glues and make a lot of sense. May I ask if the chemical bond nature of glue applies to both Titebond and HHG? What about other glues such as Epoxy, CA, Gorilla (Polyurethane) etc?

Return to “Luthiers”