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While not on a guitar, I've used epoxy to glue on two bandura bridges exactly because of the gap filling abilities.

Until a few years ago, I wasn't using a spherical dome to dome my tops, I was using something which I think was more like a solera. The perimiter of the tops was at the same "elevation" and I had carved some sort of dome into my workboard with planes and scrapers. (the reason I did that was because I didn't know any better).

Anyhow, one of the big side effects (and reasons I now use a spherical dome) is that fitting the bridge is impossible!!! Check out my avatar, the bridge on a bandura is roughly 14" long. You think getting a little guitar bridge to match is tough???

After spending a week trying to fit the bridge to the top with only moderate success, I decided that what I'd do is use epoxy to hold 'er down rather than HHG specifically because of its gap filling abilities. My thought was that a tight fitting bridge glued with HHG was probably best, but a poorly fitted bridge glued with epoxy was probably better than a poorly fitted bridge glued with HHG. That bandura sounded great and I've gotten many complements on it from pros.

Luckily, since switching to spherical domes, I no longer have the problem of poorly fitting bridges. I had Tracy make my a 25' domed workboard and my bridges now fit perfectly.

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http://banduramaker.blogspot.com

All the information, experience, and observations I've gathered so far suggest that creeping bridges are rare, and probably always the result of hot car syndrome (some other factors, such as a way-too-thick glue line when using PVA, or using glue that's well past its shelf-life, could also come into play in some cases). Personally, I've never seen it on any of the guitars I've built, owned, played, repaired, etc, the vast majority of which had their bridges glued on with PVA. So, yes, it's been shown that PVA CAN creep, but if, in the real world, that is a rare occurrence, and only happens when the glue is misused or the joint abused, I wouldn't make a decision on whether to use PVA on that basis.

As for epoxy, I will do more research on the likelihood of creep being a problem before making a decision about using it for bridges.

I don't feel the slightest bit compelled to try to build a guitar to be hot-car-proof. Anyone who's going to spend the $$ on one of my guitars knows you can't subject them to extreme heat or dryness (or I make sure they know it!).

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Thanks for sharing the details of your technique, Kent. Sounds good. I will try that.

Thanks, also, for the compliment on my bridge. Very much appreciated.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Thanks, Pat. I agree that bridge, bridge plate, and bracing design, and top thickness, too, all play a role, and I think you have some interesting ideas here.

I've put a lot of thought into the design of all of these components on my guitars, with structural integrity and longevity very much in mind (as well as acoustic response, of course). It would be a lengthy discussion for me to get into every aspect, but there are quite a few things about my designs that are different from what is commonly seen, and for reasons other than aesthetics or just being different. I don't think I've got it all figured out or claim superiority over my peers, but I do think there are real advantages to many of the things I'm doing, and I do believe I've stacked the deck in favor of my bridges staying on.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Thanks, Bob.

I'm not sure I'm clear on what you mean by "wetting" here. I confess I haven't had the time to go and read all the stuff that you and others have linked to in this thread - maybe it's explained there.

Or maybe I do understand and I'm just over-complicating it in my head at the moment.



This is at the crux of the method I'm proposing. Gluing both surfaces to a plastic shim (custom molded!) is exactly how I'm seeing it.

As I understand it, then, preparing a surface boils down to two things*: activating it by having it be freshly sanded (or scraped/planed), and making sure it's clean. My approach to the "clean" part, lately, has been vacuuming it off, then wiping it with an alcohol-dampened cloth. The alcohol wipe is an alternative to a tack cloth; it's not intended to remove oils from oily woods (like some people try to do with acetone), but mostly just to remove fine dust that the vacuum doesn't pick up, along with possibly other contaminants like oils from my fingers. It evaporates in seconds and doesn't leave any of its own residue (as far as I know), like a tack cloth might. Maybe that's not really an adequate method, though... ?

(*Aside from fitting the joint.)

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Thanks for your input, David. This is a valuable list of reasons why bridges come off or need to be removed - a great reminder for all of us of what not to do. I would comment, though, that the concluding statement doesn't seem to follow, to my mind. It seems to me that bridges come off, as you state, for one reason or another, i.e. there is always a reason. I'm not convinced that when everything is done right, bridges still come off (given that the guitar is not abused).

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

I've been using Smith's "All Wood Epoxy" to glue fretboards and the fretboard extension tenon on my bolt on necks. I mix it in medicine cups and the excess dries as hard as a rock. I doubt it would have a detrimental effect on tone. The obvious thing seems to be doing some trial bridge glue-ups on off cuts from your tops or or some yard sale instruments and seeing how easy it is to deal with the squeeze out, get them off with heat, how much heat it takes, what kind of mess it leaves behind, and report back.
Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Hey Todd, Filippo clarified my points well.
By ,"done right" I meant the things you can control.
After your instrument is out there, your bridge may need to be re-glued or replaced even if you put it on in a way that it will stay put without ,"abuse".
It may not be your- fault- as the builder but ,trust me, the repair person will most likely be muttering things while he works (hopefully only to himself) that will not be promotional material for your instruments.
I am sure a well executed Epoxy joint can hold on under normal use, maybe even better than hhg, but if it is abused and needs to be fixed I think you want something more than a legally defensible position. there are brands that are well known for their impractical reparability,(Ovation,a.k.a crap,etc.) I don't think you want to add your name to that list.
This is completely aside from any ethical arguments.

All the stuff you just asked sort of comes full circle, so let's see if I can make a combo answer that'll really help out!

Wetting is the ability of the adhesive to actually chemically spread on the surface. It's the amount of real contact the adhesive is making. The common diagnostic test for high surface energy (which equals high wetting) is to spray water on a surface and see if it beads up or spreads out. If it spreads out, that means it has an easier time sticking to the surface than to itself (which is because the energy on the surface overpowers the hydrogen bonds) and that's called good 'wetting' of the surface. If the water beads up, then it sticks to itself easier and that indicates low surface energy. As you can imagine, the situation in which the adhesive wants to self-spread on the surface is directly correlated with how strong the chemical bond will be between them.

Contamination reduces surface energy because the contaminants are sticking to the surface by way of those 'open bonds' that are present in a high energy surface. In other words, the contamination is taking up valuable bonding space for the adhesive! Similarly, dust on the surface can both block the adhesive from contacting the surface as well as eating up the reactivity of the adhesive in that area, and leaving it less able to attach itself to the surface. Plasma etching, which directly improves surface energy by essentially throwing extra electrons at a surface, automatically cleans the surface as a side effect.

So a surface with high energy must be a clean one, and high surface energy encourages wetting. Wetting indicates a strong chemical bond, and a strong chemical bond equals a strong adhesive bond.

So far as cleaning goes, actone tends to work best just because more stuff is soluble in it. Alcohols and water don't really dissolve oils, while acetone does. The best composites guy I know uses it as a matter of course on everything that it doesn't dissolve. Solvents don't help on semi-porous (absorbent) substrates, though, so in those cases it's best to just machine a fresh surface and do a dry cleaning.

Wood is such a surface, so I'd recommend against solvent use on it. I've found nothing that works nearly as well as high pressure air for cleaning wood. If you hold the nozzle close so there's lots of pressure on it, it's the best thing there is for removing dry contamination.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks, everybody, for the continued input on this thread. This is exactly what I'd hoped for - lots of good information, people challenging my thinking, etc.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Thanks for sharing your thoughts, Filippo. A lot of very good points and good questions raised, for sure.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Thanks for sharing your experience with the Smith's epoxy, Terence. Also, your suggestion for testing is spot-on.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Excellent points, once again.

I think a lot about making guitars that will hold up well over time. As just one example, I laminate my bridge plates (using epoxy, btw, but I don't glue them to the top with epoxy) in three layers with alternating grain direction, partly so they won't split, and partly so that they add more stiffness in the long-grain direction of the top, to make a more torque-resistant substrate for the bridge to sit on. I also use unslotted pins (and coat the bridge plate with CA, too) to prevent string ball damage.

I also think a lot about making my guitars easy to work on. For example, my bolt-on, butt-jointed necks with floating fretboard extensions make neck resets a quick and easy job. (Along with that, the body is engineered to strongly resist distortion [neck block/upper bout collapse, etc], and the heel is reinforced also, so it doesn't flex significantly.)

...The point being that I take the issues you raise seriously and very much appreciate your thoughts.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Crystal clear and extremely helpful, indeed, Bob. Thanks a ton!

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Bob Garrish wrote:
"The short of it is that there's no such thing as a cleanly sheared surface. At the micro scale, abrasive removal is the same process as planing, but with sharper cutters and less compression of adjacent material. "

The microphotographs I've seen of stock prepared with a sharp hand plane showed clean, sheared surfaces, for the most part. Obviously there will always be some loose ends; stuff that was not supported well enough to cut cleanly, but that's not common. Power planers do tend to leave a burnished surface unless the cutters are really sharp, and they don't stay that sharp for long. Scraping leaves a more burnished surface; with the cell structures bent over and compressed. Sanding with grits coarser than about 150 left a plowed field, covered with rocks. When you get to 220 the sanding scratches get to be about the same size as the features of the wood structure, so if the paper is sharp it's essentially the same as a good plane cut once it's been cleaned up. Naturally, I probably can't scare thos pics up: they may have been in 'Fine Woodworking' many moons ago, or even possibly in the Catgut 'Journal'. All of this becomes moot if the cutter is not sharp. You may be working from a different data set, of course.

Of course... & the 'ultimate' example of a cleanly sheared surface would be sections prepared for (electron or light) microscopy.
Those microphotographs in the 'wood books' look pretty clean, and they are sheared surfaces by either glass or steel 'blades'.

I think we're on the same page, Al, we were just talking at different scales. In mechanical material removal everything ends up either shearing or compressing. Sharper cutters cause material to shear easier, and smaller cutters cut down on compression by shrinking the amount of force experienced locally by the material being cut. Wood is soft and shears nicely, but on other materials (many metals) it can make a big difference. In the end on wood it is semantics past the realization that an equivalent surface can be made by either cutting with big knives or little ones.

The idea of abrasives as cutters is actually quite new to me (last December). There's a group here at the university doing grinding research for Pratt & Whitney and their setup is a wheel (I think Invar) with a single grain from a grinding wheel mounted on it. They need to measure the results with some crazy laser measuring gear, but they model the cutting as normal chip formation (the Merchant model) even at that scale. They definitely got to see my 'extra-surprised' face when I first found out about that one!

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Todd wrote:
"I don't feel the slightest bit compelled to try to build a guitar to be hot-car-proof. Anyone who's going to spend the $$ on one of my guitars knows you can't subject them to extreme heat or dryness (or I make sure they know it!)."

Sometimes they don't quite 'get it' though. I had a person trying out one of my classicals once on a few day's loan; a nice box in BRW/Euro spruce. He decided he didn't want it, brought it back on his lunch break, and left the case on the bench. The case was hot to the touch: clearly it had been left in a car seat in an unshaded parking area all morning on a bright, warm Septmber day. Fortunately, there was no damage to the guitar (surprise!), but the strings were dead.

Ultimately, my plea is for 'standard practice', unless the new way is clearly better. I simply don't believe that a well fitted bridge properly glued with HHG or AR will be any more likely to come off than one put on with epoxy over the finish, and maybe less. Meanwhile, you've added a complication that the repair person won't be expecting when (not if, IMO) it does come up.

Yup; abrasives cut. What else would they do?

Heh, I suppose, eh?

I guess I thought they'd be modeled differently, with some form of friction/shear equation or somesuch, but surprise is my favourite emotion!

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I can't even begin to read all these posts. I do not recommend gluing on bridges with epoxy or Ca. I may not be up on the development of epoxys,but back in the 50's I glued on a few classical guitar bridges. They flew off after a year or so. I think the epoxy kept on hardening until it got brittle enough to let go,then,WHAM. One hit me in the back as I was sitting with my back to the guitar when it let go.

I think modern epoxies are better, but you do raise a good point: there's a lot of variation within the family.

WEST system has been mentioned, and I've been meaning to point out that it is specifically designed to penentrate deeply into the wood before curing. It's original use was to bond fiberglass to wooden boat hulls, and the intent of deep penetration was probably to decrease the sudden change in hardness from the 'glass layer to the wood, which would help keep the 'glass from peeling off. I'm not sure I'd want that on a top/bridge joint.

Some epoxies harden much harder and more brittle than others. Some don't set up at room temperature at all. Some contain wax or other ingredients that are there for specific reasons, but may not be what you want. I've had some epoxies that won't harden properly when exposed to air.

In general, the longer the cure time of room-temperature epoxies the harder, more durable, more water resistant, and less irritating/allergenic they are. All epoxies have a limited shelf life, and may not cure reliably once they've passed it. All of them are also temperature sensitive: a brief time in a hot store room could eat up all of the shelf life of a new batch.

Whatever brand you use, KEEP THE PART OF THE MIX YOU DIDN'T USE. If it doesn't harden properly, you've got a problem. Uncured epoxy can be cleaned off with alcohol or acetone, in general.

I hate to drag this out of the coffin but.....

Back to the old, tired argument about sanded vs planed gluing surfaces, I had a thought.

A sanded surface essentially has much more surface area than a planed one. All those grooves, however small, really add up. So even if glue forms a chemical bond, not a mechanical one, wouldn't that bond be improved if there is more surface area to bond to?

Just a thought. Really, can't we all just get along

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I will not, I will not!!!

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Kent-
I think that microphotographs show that with a sanded surface the 'pores' in the wood are blocked more, more torn fibers lying on the surface, etc.
Otherwise, I'd agree with your point- a smoothly/cleanly 'serrated' surface would have more surface area.
Cheers
John

It's pretty much as John says. If the surface is clean (ie: the abrasive junk taken off) then a surface of equal surface energy but more area is more wettable and achieves a better bond...sometimes.

After reading up on this one more, I'm now completely convinced that a clean and finely abraded surface is superior to a sheared surface (but always with a caveat!): small-scale roughness increases wettability and makes better bonds...and large scale roughness makes worse bonds! This stuff is crazy interesting, though I'm sure the actual field work is about as exciting as watching paint (glue) dry.

The 'why' requires a warning:

<Nerd>

It has to do with the ratio between the part of the adhesive in contact with the air and the part in contact with the substrate, and it makes absolutely no sense unless you think of the adhesive (or any liquid) as having different properties at the micro and macro scales. It's like the capillary effect in both basic principal and execution: when the ratio between the surface area of a tube over the volume of the tube gets small enough, the surface tension of the liquid can pull it 'up hill' even though it makes no sense when considering its macroscopic properties. Wetting, surface tension, and surface energy are all very closely related.

</Nerd>

And...for the truly, truly nerdy:
This has some good technical info on wetting; it's from a course on metals but the mechanisms and formulae are the same across the board: http://www.ami.ac.uk/courses/topics/0122_mos/index.html

You could actually use the information on there to calculate the exact right grit to use if you knew the target thickness of your glue line, though I'm much too burned out right now to go through the motions (from helping build a race car all week...booyah!)...where's the dead tired emoticon?

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services