Re-gluing Cocobolo Bridge

[Jon Burton]

Hi all,

I wondered whether I could pick your brains about this topic...

A guitar I made recently had the bridge peel off. It was originally glued with titebond original; I have a feeling the reason for the failure was the oily nature of this wood (the fretboard extension had also come unglued).

It's now got to be glued back on. Having done a dry run, clamping the bridge down, I noticed there is a considerable gap under the bridge, even under clamping pressure. I've attached a photo: the gap seems to extend from the back of the main body of the bridge (i.e. excluding the wings) inwards for a distance of about 1cm. I estimate the width to be around 0.2mm (0.008").

I think titebond is out of the question for this job, due to the gap. My thoughts then turned to Hot Hide Glue: I've read that it has reasonable gap-filling properties...

Having not used HHG before, I have a few questions:

Do you think Hide Glue would be an appropriate choice here?

What would be the best way to prep the glueing surfaces (after having removed the old glue)? (I was thinking simply sand the cocobolo to a fresh surface and glue from there...)

I reckon I can get the clamp on and tightened in about 60 sec. Would the open time be long enough, or would I have to work ore quickly?!

(Obviously, once I get my hands on some glue and some kind of gadget to heat it in I'll do a few practice runs on scrap, to get the feel of it).

Any advice or suggestions would be very much appreciated!

[Hesh Breakstone]

Hi Jon:

The bridge came off because it was never fitted well and possibly not clamped well either.

Titebond original and HHG both suck at gap filling applications and of all things bridges need to have very good wood-to-wood contact with as much of the footprint cleaned up and used for gluing area as possible.

Your bridge needs to be very well fitted before even attempting to do dry runs or the actual clamping.  I view clamping as applying appropriate pressure for a decent glue bond and never as a way to force fit parts that do not fit.

 Coco is indeed oily and there are two camps on this one with one believing that you can wipe the thing prior to gluing with acetone and the other camp believing that the acetone makes no difference and additional oils will surface anyway.  The second camp will scrape only, no acetone, less than 15 minutes prior to gluing.  Freshly scraped surfaces do increase the ability of the glue to bond well and a scraped surface is always a better bet with glue than a sanded surface since the abrasion of the sanding action tears up the wood at a molecular level.

Fit the bridge as close to perfectly as possible and you want no gaps that cannot be closed with the most minimal pressure.  Again if you try to clamp the thing into submission you will be getting additional bridge gluing practice at some point in the future when the bridge decides to lift.

On my site lenaweelutherie.com under Luthier info there is a short toot on fitting bridges.  I use sand paper for the flossing but then scrape with a single edge razor blade prior to gluing.

In your case clean up all old glue with a very sharp chisel first.

HHG requires some things to use it and some knowledge as well.  Practice makes perfect but HHG is NOT forgiving if we can't get things in place quickly (15 seconds or less with no preheating and a bit longer with preheating).  Titebond would be my choice here because a bridge is one of the least forgiving things that we will glue and not exactly something to learn HHG chops with IMHO.

If you do use HHG you can preheat the bridge in a microwave.  A 600 watt microwave takes about 15 seconds to get the bridge hot but with Coco it also will help the thing leach oils so perhaps preheating a coco bridge is not a good idea.

With what and how you clamp will come into play as well.  

In addition the gluing surfaces need to be very well cleaned up with all old glue removed and fresh wood exposed.

In short you want all the gluing surface that you can get, excellent fit of the bridge to the top, appropriate clamping pressure with a well fitting caul that distributes the pressure as evenly as possible.  More than one clamp can be used too at times.

By the way what are those metal pins?  Could they be obstructing the fit of the bridge?

And lastly HHG is the very least forgiving with any gap what so ever because the bond is at a molecular level with a very thin glue line.  It's also a great Lutherie glue but in order to realize the benefits you really need to read up on how to mix, how to heat never exceeding 145F, and the very short open time.  Titebond original is an excellent choice as well and in this case what I would recommend since you have not used HHG prior.

Hope this helps, let me know if you have any questions and I'm always very happy to help.

[Jon Burton]

Many thanks indeed Hesh for your very informative post!

OK, so now you have helped me identify the problem: the first thing I'll have to do is read your toot and fit the bridge correctly. 

I'll take your advice and use titebond: I'll leave learning about HHG for the future! (the only reason I was considering it in this case was because I believed it had gap-filling properties...)

Those metal pins are to locate the bridge. They are 5mm diameter (as are the bridge pin holes). I drilled two larger holes in the caul which goes inside the instrument on the bridge plate. The pins protrude just enough through this caul to enable me to pull them out while the piece is glueing (that's the idea, anyway). 

Thanks again Hesh! Now, I'm off to start reading up on the process!

[Glen H]

What Hesh said.  You're wasting your time if you don't get a wood to wood tight tight fit.  No matter what kind of wood it is, esp. at the bridge. FWIW, I've never wiped Coco with anything and I've never treated it differently than any other wood, and it's my favorite wood and I've never (yet) had a seam open up.  And I always use LMI instrument glue. 

[Ned Knepp]

Hi Jon,  Nice looking bridge!

I think that what you want to fill the gap is the bridge. If it' not fitting flat, you need to determine why it's not fitting flat and fix that before you glue the bridge back onto place.  I'm not crazy about using titebond for bridges but it should have held when you did this the first time. I'm guess, and it IS a guess, that you have two issues.

The first is the fit of the bridge to the top. You need it to fit with no gaps before you glue it down. While some glues are designed to "fill"  it's usually a mistake to think that the filled area is structurally sound when under a load. Glues are almost always designed to bound surfaces without gaps between them and the strongest bonds are almost always achieved when the surfaces are flat with a very thin layer of glue remaining after squeeze out.  Gaps are usually considered flaws in that bound even if the glue fills them. Get the bridge/ top fit as flat and clean as you can get it.

I'm guessing that the second issue is that cocobolo is an oily wood and that you didn't wipe the glue surface with something to remove the oil before you glued it up. I use alcohol or naptha to wipe down ebony and rosewood before I glue them up and usually don't have any problems. 

There may be other things that I'm missing so wait for some other members to kick in but this is what I think.

[LARRY KLOSE]

Lynn Dudenbostel uses LMI Luthier's Glue (a white PV glue) to glue bridges--or at least he did in a video he did for one of the craft cable channels.  I haven't tried it but it is billed as not being as "squishy" as Titebond, i.e. it transmits sound better.

YMMV, no personal interest, etc., etc.

Larry

[Jon Burton]

Many thanks for the input Glen, Ned and Larry.

Yes, I think the consensus is the biggest problem is the fit of the joint. This is the first thing I will attempt to sort out. Then I'll sort out the glue... I did have some LMI white glue a while back: it did seem to set in a very clear, almost crystalline way so I wouldn't be surprised if it was less 'squishy' than titebond...

It would be interesting to do some experiments on cocobolo, comparing the strength of the glue bond with different treatments of the gluing surface (wiped with acetone; Scraped; etc) and compare the load neccesary for failure... Would take a bit of setting up but I might have a go one rainy weekend in the future!

Thanks again for everyone's input: I appreciate it.

[Frank Ford]

How about a slightly dissenting opinion?

Offhand, I like the look of both the bottom of the bridge and the gluing surface of the spruce.   What I don't like is the fact that you're not able to dry clamp it down tightly.  I use some pretty  beefy clamps for bridge gluing and wouldn't have a problem getting this one down nice and solid, with a stout clamping caul that puts pressure at the back and front edges, and a heavy caul inside the bridge plate.  So, I'd say the first order of business is to revisit your clamping procedure.

Then, I'd scrape both surfaces as clean and flat as I could without removing much stock, stick the bridge in the microwave to get it hot as the outside of a hot cup of coffee, walk over to the bench and clamp it up right quick with some hot hide glue.  A bridge  can handle a few minutes at 150F, and with it that hot, there would be plenty of time to clamp it before the glue even thinks about starting to gel.

Me, I never use anything but my microwave to heat bridges, and the glue itself.  By the way, it causes no harm to accidentally overheat the glue for  a few moments.  The temperature warnings about hide glue are strictly for those who use a glue pot to keep the stuff ready for use all day - above  145 degrees, it can start to break down after half a day or so. Fools such as myself can cook the small batches  in the microwave with relative impunity, use it right away, and toss the excess.

People often  overthink the use of hide glue.  Most of the old-timers I've known have used it VERY casually, and had lifetimes of good results.  One rule they NEVER break is that of clamping before the glue begins to gel.  Clamping hide glue in the gelled state simply doesn't work, and glue failure is a basic certainty. Hideo Kamimoto is the guy who got me into buying hide glue in 50-lb. sacks, and he told me he never knew there was a rule about overheating it.  He simply had a pan of water on a hot plate and a jar of glue sitting in the water, which sometimes boiled, sometimes simmered.

[Jon Burton]

Thank you very much for the input Frank! A different perspective on the problem...

So,you reckon with the bridge heated in the microwave I'd have enough time before the glue gelled to clamp the bridge down (I could do it in just under 60 seconds. Maybe faster, with practise!)? It's certainly encouraging to know hide glue is more forgiving than would be expected.

I'll have to look into the clamping side of things. I do have a feeling that this stewmac caul doesn't put as much pressure as would be desireable on the back of this bridge (but then, it is a one-size-fits-all caul...). A G clamp (or three) with a deep throat and a caul made to fit the bridge may be a better option. Cranking the clamp down hard does frighten me a bit though, I have to say...

Anyway, the bridge has been sanded to conform to the top profile, as per Hesh's tutorial. I'm going to sleep on it (it's night time in my part of the world) and decide on a course of action tomorrow. There's been a lot of advice offered here and I'm really grateful for everybody for their take on the problem.

[Frank Ford]

Well, I do tend to be fearless when it comes to clamping bridges.  This is my go-to clamp for use with the StewMac caul setup, and the smallest clamp I'd use with it:

  I don't always use that particular screwed-on lower caul, but I do always use something that heavy, and I lean pretty hard on the clamp handle when I squeeze things together.  

The literature for hide glue suggests pressure in the neighborhood of 100 lbs/sq. in., so that means we can put better than 500 lbs. load on a bridge/top joint without any fear of "starving" by squeezing out all the glue.

[Jon Burton]

I remember seeing that clamp on your website, Frank. One day I'll make something like that, or have one made... 

[Hesh Breakstone]

Yeah I want one of those too!

We keep our HHG in a pot all day long but I like the microwave idea and also like the idea of never having to worry about forgetting to pull the plug on the pot when I am half way home....

Thanks for some great ideas Frank!  Does it impact the glue heat if your business partner left a three day old burrito in the microwave.... ;)

[Ned Knepp]

I'd think that a better question is; Would it impact the burrito if your business partner left a three day old cup of glue in the microwave? 

Or how about; Would it impact the bridge glue joint if we mixed a three day old burrito and a cup of glue in the microwave?