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What are some various glues I can use to glue on a cocobolo bridge successfully?

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Scrape the bridge bottom then Titebond or HHG/fish glue.

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John Lewis
Wannabe builder owned by 2 crazy dachshunds

Zackly...

Close to how I do it, scrape, wipe with DNA, glue with titebond. And that goes for all rosewood and ebony bridges.

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Joe Beaver
Maker of Sawdust

Is scraping better than sanding? Seems like you want a surface the glue can grab on to. Seems like scraping would make the surface to slick... Also, I have heard that using a solvent can bring out more oil to the surface. Don't know whether that is true of not... Here's the bridge in question.

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Scraping, or otherwise removing material, from the glue surface raises the 'surface energy' and promotes a stronger bond, but you have to do it within 15 minutes of actually gluing. Scraping or planing is best.

I've heard mixed reports about the utility of wiping with a solvent. The object is to remove oils that might interfere with the bond, but some folks say that it just spreads them around on the surface. I suspect it also reduces the surface energy. Basically, I've never used a solvent wipe and have not had problems with any of the oily woods I've used since I learned about surface energy enhancement. Yes, it sounds 'New Age', but it's real and it works.

I'll note that there is some debate over the exact mechanisms by which glue bonds. The two contenders are 'mechanical' and 'chemical'. Mechanical bonding assumes that most of the bond comes from the actual strength of the glue material itself, filling in gaps, which needs some sort of 'tooth' to hold onto. This was the rational behind the use of a 'tooth plane' to scrub the surface of bridges, which is what Martin used to do.

'Chemical' bonding assumes that most of the strength the joint is in the chemical bonds between the wood surface and the glue. Most glues are actually not very strong materials as compared with the substrates, such as the cellulose in wood, that they're bonding to. In that case you'd want to have as thin a glue line as possible, to maximize the area of chemical bonding while reducing the thickness of the weaker glue line.

In practice there has to be some of both things going on, of course. On a microscopic scale wood is mostly air space, and any solid that fills it up must be stronger than air. OTOH, it's generally acknowledged that thinner glue lines work better, even with 'strong' glues such as epoxy. The Martin toothed bridges have a lamentable tendency to come up, and work better when the toothed surface is smoothed off.

If you look at micro photographs of wood surfaces the smoothest surface comes from a sharp plane. Scrapers have a tendency to mash over or burnish the wood, and make a rougher surface. It's still smoother than what you get with any sandpaper below about 220 grit, though. Sanded surfaces also have a lot of torn fibers hanging on, and dust particles that can clog up the surface. It doesn't do much good to make a great bond with loose dust, and the stuff is harder to remove than you might think.

Back in WW II, when they used lot of wood in airplanes, the Forest Product Lab looked into problems of glue joints. They found that freshly worked structures in control surfaces glue better. They also looked at delaminations in propellers, and found that sanding the layers to thickness caused problems, while thicknessing with a planer did not.

My understanding of it has always been that it is a chemical bond. So another consideration is that sanding, or worse toothing, will provide less surface area of wood to wood contact seeing that the squeeze out in the sanding grooves of airspace are not part of the bond or at least a very weakened part of it. In the end though my guess is that this is yet again one of those arguments that a physicist can show you on paper is right or wrong but in the real world the difference is moot. There are still plenty of old Martin toothed bridge out there that have maintained string tension for decades and sanding is probably a common practice in many shops. When building I like to use a plane, one swoop with a plane that takes a thin shaving off and you are done. In repair I often sand then scrape because the bridge has become arched or perhaps was arched from the start. When I first started building I used cocobolo a lot and in those days just sanded, fretboards and bridges, and used either LMI white or Titebond. So far none have come back in 20 plus years and never used a solvent either but have noticed on some occasions that the glue squeeze out comes out with a reddish color so maybe the glue actually dissolves some of the oils too.

Steve, really beautiful piece of wood and great work, as well. No further info to add about glueing cocobolo, unfortunately. I wanted to ask, though, if that black ink line and sap wood give you any trepidation about possible stress cracks in the future. That’s riiiiggghhht where the saddle is going to want to tip forward.

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sweat the small stuff.

Heath, I had that same question and posted it to the forum a while back and folks told me not to worry about it. I hope it works out okay because I love sapwood... Plus, I've already glued on an equally nice matching fingerboard.

Based on the discussion, it appears that it is better to scrape under the bridge, rather than sanding it. I guess I'll sand it to the necessary shape and then scrap it just before gluing...

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

I have heard that titebond has issues with coco.
So I switched to Elmers Carpenters glue. Have used a ton of coco and never had anything let loose.

Sanded, scraped, planed... didn’t seem to matter as long as I used Elmers.
Titebond...not so,good.

Used reg titebond to glue on a coco FB, did not wipe off fb or anything else huge upbow, Used system 3 , 2 part epoxy. I would use either that epoxy or a new product , clear white gorilla poly glue, comes in a 4oz bottle on a coco bridge . I/ve also had trouble gluing coco backs with titebond and switched to sytem 3 , 2 part epoxy..

I have heard some use this with coco parts;

http://www.lmii.com/products/mostly-not ... wood-epoxy

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Guitar Building = Continuous Improvement Process

I would be wary of using epoxy for a bridge... a fingerboard maybe.

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Steve, I recently built a cedar and coco guitar, but the bridge was ebony so I can't comment on that particular part. However I had heard all the horror stories about gluing cocobolo so I did a couple of experiments - I shot two fairly long scraps about the thickness of my plates (a little less than 1/8) and did a butt joint just like the back seam. I used Titebond, the edge was cut with a plane and I wiped the wood with denatured alcohol just before the glue up. I also glued a piece of 1/4 by 1/2 spruce on the plate to simulate a brace (Titebond and DA wipe). After a couple of days I tried to break both joints - neither failed exactly at the glue line, there was some wood failure on both. However on the butt joint where the glue had squeezed out (I used waxed paper on both sides while I clamped it) I was able to chip some of it off the coco with a scraper - I'm guessing that there was still some of the oil on the plates.

I'm going to humbly suggest making a couple of dummy bridges and gluing them to some scrap spruce, then apply a shearing, twisting torque similar to what a bridge would experience. You could make some sort of fancy jig to simulated it, I would probably just clamp down the "top" and grab the "bridge" with vice grips and twist. If it failed easily at the glue interface I might consider that a bad choice and try something else.

The bridge to top interface is probably the most stressed part of any acoustic guitar and I see a lot of failures in my shop. I think it makes sense to do everything possible to make this joint sound (and I'm amazed how many manufacturers do such a poor job).

The problem with adhesives science as applied to wood bonding is that even the experts admit that their understanding of of the why's is poor, even where the experience-based knowledge of the how's is fairly well documented. After a half dozen guitars with coco back and sides, and another dozen or so using coco binding, fretboards, bridges,etc., as well as a number of repairs on guitars built with coco, our understanding of the how's that work is as follows:

- Solvent treatment after planing, scraping, or sanding does not appear to us to increase bond reliability in coco, as the resins and other extractives thought to reduce bond strength are mobile in both polar and non-polar solvents

- Clean, freshly prepped joints (minutes - not hours before closing the joint) which are very closely fitted give the best chance of success. Prepping the top is just as important as the bridge.

- In our experience, all common luthiery glues work (we've used AR/PVA, hide, PU, CA, and epoxies with coco), although we glue bridges with 315g hot hide glue for both the reduced time to full strength and what we believe is a stronger, more load-resistant joint over time

- Most joint failures we have seen are prep failures - poor surface prep, poor joint fitting, or poor glue-up technique - versus something related to the type of glue used.

As for the why's of successes and failures, those here with the broadest range of experiences suspect that the wide variety of outcomes with bonding cocobolo have more to do with variation in preparation and glue-up technique than choice of adhesives, which could be seen as explaining at least some of the issues reported that seem at odds with convention. In other words, success or failure is often determined by getting enough things right, rather than any one thing 'wrong' (e.g., adhesive choice) .

A test of wetability for waterbased adhesives (wetability is one good indicator of energy state and cleanliness of the surface) using distilled water droplets on a few coco scraps was very useful to my understanding of how choices in joint preparation might impact final joint strength. There is a very marked difference in how a drop of water behaves on an unprepared, a prepped (sanded and cleaned with air), or a solvent-washed after prepped surface. The faster a droplet loses cohesion and spreads across the surface, the better the wetability, usually due to a higher surface energy state and lack of extractives residue or other contamination on the surface. Chapter 9 of the Forest Service's Forest Products Laboratory Wood Handbook has a very useful discussion of wood adhesives.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.pdf

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For the times they are a changin'

- Bob Dylan

I stand by my words.
Elmers carpenter glue

Alan Carruth wrote:
"Mechanical bonding assumes that most of the bond comes from the actual strength of the glue material itself, filling in gaps, which needs some sort of 'tooth' to hold onto. This was the rational behind the use of a 'tooth plane' to scrub the surface of bridges, which is what Martin used to do. "

Are you sure this was their rational for toothing the bottoms? Toothing planes were often used for veneer work where high strength bonds weren't required. My thinking is that toothing was used to give a place for the excess glue to go to achieve a thin glue in the areas still in contact with the substrate.

I’ve only seen evidence to support chemical bonding for wood glues (hydrogen bonding/Van der Waals interactions in huge numbers). It’s well supported by the fact that bond strength increases as glueline thickness decreases.

Even with a super oily species such as cocobolo freshly machined surfaces are normally not a problem, but if you have more time than a factory spend a few extra minutes on surface prep. Hit it with acetone on a rag until it wipes off clear. Then glue it up, clamp it to all hell and leave it alone for a day or two.

Any plain PVA carpenters glue should work, and the same goes for hide glue. Steer clear of anything waterproof (type II/III wood glues, epoxy, and polyurethane). My personal favorite is titebond extend: longer assembly time, superior strength and thermal resistance because it contains wood flour as a filler.

huh...I've always viewed wood glues as a mechanical bond...you clamp the pieces together and the glue is forced into the grain structure...when dry it holds together because of the spider web like tendrils of glue interlocking within the grain...

never thought of it like a chemical bond as the stuff comes easily enough off my body with little effort, scrapes off the plastic screw on cap with a little effort with a fingernail, doesn't bond plastic together worth a flip, etc...

considering how poorly it holds a joint together that's not tight of course a thinner glue line is best!

Mike_P wrote:
"never thought of it like a chemical bond as the stuff comes easily enough off my body with little effort, scrapes off the plastic screw on cap with a little effort with a fingernail, doesn't bond plastic together worth a flip, etc..."

The test for high surface energy is to give the surface a spritz with a fine mist of water. If the surface energy is high, with lots of open bond sites looking for something to latch onto, the water will spread out into a film. If the surface energy is low the water beads up. In fact, the way they measure surface energy is to use a microscope to find the angle that the drop makes where it intersects the surface.

Most plastics have pretty low surface energy. Your skin has oils on it. Water based glues down usually stick to such things well.

Several years ago one of my customers sent me a fossil mammoth ivory bridge blank to use on his guitar. There was a flyer with it that talked about the problems of gluing the stuff, and giving some recommendations; iirc one of those was to use either polyurethane or epoxy; I don't remember which. It occurred to me that after a few thousand years in the ground all of the chemistry had probably already happened in this stuff, so the surface energy was likely to be intrinsically low. I spritzed it and, sure enough, the water beaded up like it does on wax. After I scraped it the water spread out well enough, but that didn't last long. I did a really careful job on the gluing surfaces, got everything nice and warm, gave things a light scrape, slapped on some new strong hot hide glue, and got it clamped down in record time. So far as I know there has not been any problem.