Assistance needed from a Guitar guru please.

[xy mosian]

Hi all,
A friend has a Classical guitar roughly 50 years old, could be more. Possibly because of wrong string choice the bridge has come off. I have been asked if I can reattach it.
Photos are included


Clearly the original manufacturer did not do a good job of glueing the bridge in the first place.
Now my plan is dependant on the adhesive used. If it is animal based then I see few problems, apart from clamping which i will do with a U shaped piece of thick ply and one or two wedges.
If the adhesive is PVA, then that gives different problem. I would start by cleaning the bridge and then the soundboard.
Herein lies my problem, and questions.
If I scrape the soundboard, how much glue will have soaked into the wood, and will I be able to detect when the wood is clean?
Once I get the bridge, and soundboard, clean of pva will that be clean enough to use animal glue?
Thanks in advance,
Geoff

 

[MickCheese]

Search for this youtube channel.

https://www.youtube.com/channel/UC8wIqZCt9h6uJbOBCQVuUmg
He seems to fix a bridge every few weeks.

Mick

 

[novocaine]

Its 50 years old,its done very well, give it a break. A light sand of the bridge and a quick scrape of the soundboard. Clamp through the sound hole. Id supsect PVA but hide glue was used by some makers still around that time.

 

[profchris]

A mass market guitar probably used a synthetic glue 50 years ago. A luthier build would be hide glue.

Hide glue is what I always use for bridges. PVA or aliphatic can suffer from cold creep, when the bridge slowly moves closer to the nut. It's still attached, but the intonation suffers.

For either glue the process is the same. Scrape the bridge base clean, ditto the footprint on the top. Check for a fit with no gaps, correct with localised scraping. Glue, clamp 24 hrs, clean up, play.

Clamping is the challenge, as the top is flexible and the bridge wings will try to curl up when glue is applied. Make cauls for inside and out, and at minimum clamp each wing and in the centre. One way to deal with this is to make a block which fits over the top caul, and has threaded inserts with machine screws to put pressure on the wings. Clamp the middle, screw down over the wings.

I see a thin layer of spruce came up on one side. It looks like the builder scored or routed through the finish too deeply, so the fibre could just peel up. That weakness, plus 50 years of the top moving with humidity, explains why it let go. A good job, if steel strings completed the process, as keeping steel strings on it for some time could have produced a banana neck!

 

[profchris]

PS there will be fan braces under the bridge footprint, just to make clamping harder. Your inside caul needs to cope with them.

 

[novocaine]

Chris to the rescue. I'm to lazy to type in full (phone, make to many typos) but someone like Chris will give a full and much, MUCH better answer.
I haven't scrapped a bridge before, but it's a really good approach and I'm not sure why I haven't, I've always placed it on a bit of sandpaper on the sound board (just in front of where it's fitted) and sanded it that way but I like to idea of scrapping it.

 

[profchris]

Sanding is good to fit the contour of the top, but scraping can deal with final gaps.

Also, a light scrape of both surfaces just before gluing gives the strongest joint.

 

[novocaine]

Sanding is good to fit the contour of the top, but scraping can deal with final gaps.

Also, a light scrape of both surfaces just before gluing gives the strongest joint.

noted, thanks for the incite. I shall add it to my knowledge.

 

[xy mosian]

Thank you everyone for your replies. Especially profchris.

My main concern is with any potential problems glueing to the, likely, pva. Possibly to itself. Hence my reason for asking how much to scrape off and if I will notice when I have removed all of it. From replies I get the impression that a general removal of visible adhesive will be sufficient.

From the photo of the bridge is does look like pva, to me. Most hide failed glue joints I have seen have a different edge to the 'islands' of glue. Almost like cracked, dried mud.

I understand that there is fan barring and I had planned to take account of that, probably with some sort of suitable wooden girder, in the clamping.
The caul for the top will be fun to make but I am not averse to using small local wedges.
I will re-read several times, the advice given and hopefully something will stick.

As a matter of interest. After preparing for glueing, as above, will the surfaces be clean enough for hide glue? I would prefer to use that.

Again thank you all for your responses.

Geoff

 

[Ttrees]

You will need 315 type what gels quicker.
Better do some dry runs!

 

[Exluthier]

PVA or aliphatic can suffer from cold creep, when the bridge slowly moves closer to the nut. It's still attached, but the intonation suffers

Just to be clear, there is no such alternative as ‘PVA or aliphatic’. Aliphatic glue is a PVA glue which—because of the different molecular structure from which it gets its name—doesn’t creep. The one which does creep (under increasing glueing pressure) is also a PVA glue, but with an aromatic bond, rather than aliphatic. The molecules in the aromatic bond are round in shape (aliphatic ones are longer, and resist tearing ‘ breaking) and pull apart under increasing pressure. The early aliphatic glues were often yellow coloured, and the standard (I.e. aromatic) ones were white. That seems to have changed recently.














For either glue the process is the same. Scrape the bridge base clean, ditto the footprint on the top. Check for a fit

Clamping is the challenge, as the top is flexible and the bridge wings will try to curl up when glue is applied. Make cauls for inside and out, and at minimum clamp each wing and in the centre. One way to deal with this is to make a block which fits over the top caul, and has threaded inserts with machine screws to put pressure on the wings. Clamp the middle, screw down over the wings.

I see a thin layer of spruce came up on one side. It looks like the builder scored or routed through the finish too deeply, so the fibre could just peel up. That weakness, plus 50 years of the top moving with humidity, explains why it

 

[thetyreman]

I would also use hide glue, if you clean up under the bridge, be extra careful because you might take off too much and then it's higher on one side slightly, even a fraction of a millimetre will be noticible to a guitarist, I would just leave it as it, glue it in place and wax around the area so that the glue doesn't stick then clean it up.

You might need to make or aquire some specialist cam clamps to hold down the bridge during glue up, luthiers often make their own out of mild steel and hardwood, classical guitar luthiers suppliers should sell them. Also in future do not put anything other than nylon strings on it! they aren't designed to take the higher tension of steel strings.

 

[xy mosian]

Ttrees, I am not familiar with '315', it sounds like a variety of so called super glue. That's not really what I am after thanks all the same.

Exluthier. Thank you very much for your input. You have answered a question which has occassionally tickled my brain. In the late 70's I made a couple of Lutes and used a Yellow Aliphatic Resin for the back, ribs, neck etc.. Wonderful stuff. Sadly I cannot remember the brand of course. Animal glue for the rest.
I have been wondering how 'PVA' had suddenly all seemingly become a 'Aliphatic Resin'. I have not come across a PVA with very similar properties.

thetyreman. Fortunately the saddle is apparently a push fit into a slot on the bridge. Perhaps in need a packing shim could be added?
You suggest leaving the soundboard as it is. Surely that prevents the use of animal based glue, not soaking into the grain etc., and reduces the strength of a pva bond as that is reputed not to stick to itself.
As this is, hopefully a one off job, I/we are reluctant to purchase special clamps. One of the advantages of using animal glue is that it needs less pressure than pva. That being the case I can some fudging of some clamping arrangement

As for steel strings, of course we don't know anything of past events so cannot really comment. However there seems to be a large area of the bridge which has not been involved with being attached to the soundboard at all. That cannot have helped at all.

Thanks again all,
Geoff

 

[Ttrees]

The 315 refers to gram strength, and is the normal choice of hhg ( hot hide glue.)
The yellow glue is Elmer's carpenters glue, which I believe is an aliphatic resin like titebond.

 

[profchris]

If you scrape the soundboard footprint until you see no traces of the old glue, animal glue (HHG) will work well. The important thing is a good fit, no gaps.

I've read of some luthiers using 315 gram strength HHG for bridges, but it gels really fast and you risk a poor joint. My bridges are glued on with bog standard pearl glue, made from bones, hooves, ears, tails and who knows what, but they generally stay on - mind you, I'm an amateur builder, not making for sale.

This isn't a special instrument, so I'd use whatever animal glue is to hand on the basis that it will be good enough - the worst case is it comes off again and you have to re-glue it.

Some luthiers just use a rubbed joint, clamping with fingers only. I suspect they are very good at getting a close fit.

One clamping option is to work out a way to brace up the inside between back and top, maybe using blocks and wedges (and being careful not to push the top and back apart) and then you can clamp downwards - you don't need a lot of clamping force. You could even place the guitar on a board, place a piece of wood over the bridge and resting on the fretboard, then wrap rope around (maybe pad it where it touches the body) and use wedges to apply clamping pressure. Something like this (though I'd wrap the string/rope nearer the bridge):

Finally, don't worry too much about taking a little off the bottom of the bridge. I'd be amazed if the saddle was the right height in the first place, and after 50 years the geometry of the guitar will have changed. You'll need to adjust the saddle, and I'd bet you have to reduce it by sanding off the bottom, not make it taller.

 

[xy mosian]

The 315 refers to gram strength, and is the normal choice of hhg ( hot hide glue.)
The yellow glue is Elmer's carpenters glue, which I believe is an aliphatic resin like titebond.

Thank you for explaining that, glue strength is something I have never really got into. I am an amateur woodworker and have not used animal glue enough to come across problems.
Elmer's carpenters glue is a name that rings a bell. although I think I have used it since in a dampish environment in France.

profchris. The great information continues to pore in. Your clamping mechanism, device, thing is something I had not considered. It looks as if it would be particularly useful if the soundhole is a fair distance from the bridge. I think a lot is now dependant upon my getting my hands on this instrument, as in 'Let the dog see the rabbit'.

By the way I believe I have figured out how to check for a pva free soundboard. If a, small, drop of water soaks in, then the surface is suitable for animal glue.

Thanks again both of you.
Geoff