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The thread on removing finish for gluing on bridges got me thinking about using epoxy for bridge gluing. The method I'm envisioning is simple:

1. Mask the bridge area to within 1/16" or so of the actual bridge footprint.

2. Send the guitar to Joe White for a top-notch polyester finish; when he's done, he removes the mask, and the finish on the top is VERY thin (I've measured it, but I forget exactly what it's been - maybe .003" or so, iirc)

3. Sand bottom of bridge to fit curvature of top, using 80 grit

3. Position bridge and trace it's outline with alcohol-soluble marker; apply either masking tape dams or pins to reposition bridge for gluing

4. Remove bridge and scuff-sand the lip of finish that extends within the outline (I believe I could do that easily with a small shop-made sanding stick, much faster than I could score and remove the finish by any method); also scuff-sand the spruce

5. Vacuum and then wipe gluing surfaces down with alcohol to remove dust, other contaminants, and the marker

6. Mix up some slow-setting epoxy and brush it on both the top and the bridge

7. Thicken the remaining epoxy with silica powder to somewhere between mayo and peanut butter and brush it on the bridge

8. Reposition bridge (using either the masking tape dams or pins applied in step 3); use hand pressure to press it on well enough to get most of the squeeze-out out; wipe away the squeeze-out

9. Put my vacuum clamp on there and apply vacuum

10. After maybe half a minute (just giving it time to squeeze out any more glue that's going to squeeze out), remove clamp and clean up squeeze-out, using alcohol on a swab to get it really clean

11. Reapply clamp

12. Check back in a half hour or so; if there's any more squeeze-out, clean it up with alcohol

13. Leave it clamped for a few hours, then let it sit for a day before doing anything else to it

The reasoning behind this is that the thickened epoxy will fill the thin gap between the bridge and the top that exists because the lip of finish is left there and the bottom edge of the bridge is not relieved. This, in effect, creates a PERFECTLY fitted joint, in that the thickened, hardened epoxy leaves no voids or areas filled by a glue that is ineffective at gap-filling.

In addition, as added insurance, the epoxy will also bond to the lip of finish, which itself has very good adhesion to the wood, so the bridge will be bonded to the top all the way out to its edge (an advantage, in my mind, over the rout-a-little-rabbet-around-the-edge method).

I believe the bond would be more than strong enough and would only fail if the guitar were subjected to very high temperatures (which puts the whole guitar at risk, anyway, so that's just something an owner of a fine handmade guitar should know to NEVER do).

The advantages of being much faster than removing the finish out to the edge of the bridge footprint, while still having the bridge glued down all the way out to its edge, and having a totally clean and sharp looking joint around the edges of the bridge, all make this idea sound very appealing to me.

Disadvantages...

1. Harder to remove bridge - I see that as a good thing, as I don't want my bridges coming off, ever.

2. Harder to remove old glue and re-glue bridge if it does lift - well, I think it's extremely unlikely that it would ever lift, so that doesn't really worry me. Even if this ever did happen, it might be a pain, but not an unsolvable problem.

3. Epoxy is nasty stuff - I use it anyway, and I take care to protect myself.

4. Possibly detrimental to sound - surely some people will be convinced that a few thousandths of epoxy between the bridge and the top will spell death to good tone, but, as I see it, that is a matter of conjecture and personal belief, and I don't believe it would make any difference. Silica-thickened epoxy cures hard, and I'm not convinced that a few thousandths of that material sandwiched in there would produce any significant damping (I'm not a believer in the "hide glue is better for tone" idea in general).

Can any of you think of any pitfalls or disadvantages I'm overlooking? Any other thoughts on the idea?

_________________
Todd Rose
Ithaca, NY

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

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

PLEASE don't do this.

Epoxy adheres like any other glue: it's primarily a chemical bond, not a physical one. The fact that epoxy does not shrink as it sets up, and will thus fill gaps more effectively then water based glues doesn't make a poor epoxied joint stronger then a properly done joint with HHG, or even Titebond. I'd bet it's weaker.

Bridges do pull up, in spite of our best efforts to keep them from doing so. Ever see an Ovation with the bridge pulled off? Those are epoxied on over the epoxy finish of the top. They are not a pretty sight: usually there is a fair amount of collateral damage to the surrounding area of the top, and you can't touch it up short of refinishing the top.

As any repair guy will tell you, the best way to deal with a lifting bridge is to get it _before_ it flies off and kills the cat. If you see it starting to lift, you can usually get in and peel it gently up with whatever means is best to get the particular type of glue loose. For HHG that's often just prying it off dry, and Titebond and it's ilk will release with heat. So will most cold-cure epoxies, but I think they take a higher temperature, and you're still left with the problem of getting all the stuff off the wood so you can do a proper job of gluing the bridge to the top. Not fun: there's no solvent that will get the stuff loose for one thing, the way you can soften Titebond with acetic acid, and HHG with water.

Epoxy has it's uses, in places where other glues won't work (such as gluing to end grain) AND where the joint is really going to be permanent. We'd all like to think of the bridge joint as being permanent, but we have to acknowledge that it seldom is. Let's not make the inevitable repairs any more difficult than they need to be.

Please?

That would be a mechanical bond, I think- and probably not a very good one at that. I doubt that they rough-sanded the epoxy top finish before bonding the bridge to it. Epoxy will not bond to another epoxy chemically once the initial cure is complete. Partly-cured epoxy will chemically link to an additional application of epoxy. (I wasn't aware that epoxy bonded chemically at all to wood , but I'll take Alan's word on it!)

And, of course those non-pin Ovation bridges are much more likely to pull away than a pin-bridge (or a classical bridge with the same gluing area), no?
Cheers
John

Any good glue joint is a chemical bond, not a mechanical one. The mechanical joint meme is why a lot of people fail with epoxy, and adhesives in general. Looking up 'surface energy' and 'wetting' on Google will end land you at some good stuff about adhesives, but here are a couple links I found that explain it pretty well:

http://www.aitechnology.com/faq/adhesiv ... principle/
http://forum.woodenboat.com/archive/ind ... 60952.html

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks for your reply, Al. I greatly respect your knowledge and opinions, so I hope you won't take my questioning or my being not-yet-convinced as disrespectful or dismissive at all - or defensive, either, for that matter. I'm just responding with my thoughts and continuing to explore this idea. I very much appreciate your input.



My understanding is that what makes epoxy different, in terms of bonding strongly where gap-filling is involved, is that it bonds very strongly to itself as well as to other materials, whereas other glues, like HHG and PVA glues, do not. In other words, it becomes a solid, strong material that is strongly bonded to both the bridge and the top - not unlike if one were to glue a few-thousandths thick shim of hardwood under the bridge (using HHG or whatever) to fill that gap. I'm not suggesting that it would be stronger than a well-made wood-to-wood joint glued with HHG or PVA, but I'll need more convincing that it would be significantly weaker.



It seems to me that the vast majority of bridges that pull up are most likely the result of faulty design, faulty methodology, faulty materials, faulty technique, or abuse to the instrument. I may be convinced otherwise, but at the moment it seems to me that the method I've described is sound and would be extremely unlikely to result in a failed joint unless the instrument is subjected to abuse. I suspect that those failed Ovations are the result of a combination of many or all of the above factors (faulty design, faulty methodology, faulty materials, faulty technique, and/or abuse to the instrument). There are many things I'm not going into depth on here that all contribute to a bridge staying on - bridge design; bracing and bridge plate design; proper selection, drying, and handling of woods; etc - and, given how meticulous I am in all these areas, I have a hard time imagining my bridges meeting the fate of an Ovation bridge, in spite of the common factor of using epoxy as the bridge glue. In my experience, Ovations are crap in almost all respects (and are also likely to be subjected to unintentional abuse by the uninformed people who buy them).



I've been doing repairs for a while myself, and I understand this. If a bridge did lift, it would be a pain. But not the end of the world. The bottom of the bridge could be cleaned up with sandpaper on a convex radiused sanding board, and the epoxy on the top could be mostly routed off. Repair people are great at problem solving, and I don't see this as being a huge problem, really. In any case, it'll take more to convince me that this would be at all likely to occur in the first place.



I hear what you're saying, but I don't see bridge joints as seldom permanent or failed bridge joints as inevitable. My own observations suggest that lots and lots of bridge glue joints never fail - and I'll bet, that, by and large, they're the ones that don't suffer from faulty design, faulty methodology, faulty materials, faulty technique, or abuse to the instrument. I haven't made enough guitars yet for this to mean much, but none of mine have failed yet. Moreover, I know many expert guitar makers (including some in our midst here on the OLF) who have never had a bridge joint fail.

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Todd Rose
Ithaca, NY

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

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

I am not sure about using epoxy for gluing bridges (I use hide glue), but I have experience in composite aircraft construction so I can comment on your proposed methods.

If you are filling with cab-o-sil or glass bead fillers, there is no need to wet the surface before buttering up with the thickened epoxy. The filler is a colliodal mix and the epoxy will wet the surface if enough clamp pressure is applied.

I would not remove clamps and remove squeeze-out until the epoxy is mostly cured. in West Systems 105/205 I would leave clamped from 40 min after mixing at least 12 hours before removing clamps for any reason. If you remove the clamps and something moves (and it will) after the epoxy has gelled but before the epoxy has significantly cross-linked the shear strength of the bond will be significantly compromised. The load on a bridge is in shear.

If you are vacuum clamping, you can use peel-ply and an absorber (paper towel) to wick the squeeze-out away. with west systems, you can also warm with a heat gun and scrape away squeeze-out relatively easily after 12 hours. if you wait longer (24 hours) epoxy will be much stiffer and squeeze-out more diffiicult to remove

With vacuum clamping end epoxy, the most difficult problem is controlling where the excess epoxy will go. in addition to the squeeze out of the filler epoxy, you will likely get some low viscosity unfilled squeeze out that will want to wick out and wet everything. use of a breather (peel ply) and absorber (paper towels) can help control this by giving the excess glue a place to go, but the best control is to apply the correct volume of adhesive. Otherwise it is very likely the adhesive will be wicking op the sides and onto the top of the bridge.

Alcohol is a lousy solvent for cleaning up epoxy.

using a thin bond line of high-quality epoxy, the sound will likely not be compromised.

If you are looking at epoxy as a permanent glue, you might also think about using a urea formaldehybe glue suce as DAP weldwood plastic resin glue. For wood, it has most of the same advantages as epoxy but is easier to work with and is safer. Like hide glue (and unlike epoxy) a wide variety of glues will adhear to cured urea formaldehybe glue, so it will be repairable (if you can get it apart)

-jd

Thanks for your input, Todd.

I'm not having trouble keeping bridges on, so that issue doesn't have anything to do with my reasons for considering the epoxy method I've described.

You raise a good point about certain cases where one might want to remove a bridge to repair cracks on the top. That scenario is well worth considering.

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Todd Rose
Ithaca, NY

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

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

Thanks, Bob. I'll check out those links.

I understand that with many glues, such as HHG and PVAs, the best joint is made with smooth surfaces, rather than "toothed" ones, due to the chemical nature of the bond. My understanding is that epoxy is different in that respect - that it will adhere better to many materials if they are roughed up, and that, even with wood, it will adhere better to a fairly coarsely sanded surface than to a planed or scraped surface. If that is mistaken, I certainly appreciate being enlightened.

Also, as I said above, my understanding is that epoxy differs from HHG, PVA, and some other glues in it's ability to bond strongly to itself in the way it behaves as a gap-filler, especially when thickened. Again, if that's mistaken, I appreciate the eductation on that.

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Todd Rose
Ithaca, NY

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

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

Why not use CA instead of epoxy?

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

jd, thanks a lot for your input.



Okay, so I'd be wise to leave it clamped for 12 hours rather than just a few hours. Still, I should be able to unclamp and remove squeeze-out within the first half hour to 40 minutes. That concurs with my observation that the slow setting epoxy I've been using (System 3) hasn't really begun to stiffen up until more than 1/2 hour from when I mix it up. Maybe vacuum clamping would not be the best option with this method, but I think I might try a test run with scrap to see if I have any of the problems you describe with epoxy wicking up and making a mess.



Can you elaborate on that? I've used alcohol to clean up epoxy with good results.



Thanks for that suggestion. I'll investigate.

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Todd Rose
Ithaca, NY

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

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

That may be an idea worthy of more consideration as well, Steve. I have been thinking of trying the method of gluing the bridge on with CA on top of the finish, but have shied away from that. A method much like I've described here (in which the bridge area has been masked for finishing) using thick CA rather than epoxy might be a viable option, though. I've read that CA can form strong bonds where gap filling is needed, but I'd have to look into that more. Also, I find epoxy clean up with alcohol to be a lot easier than CA clean up, although CA can be cleaned up with CA solvent even after it fully cures.

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Todd Rose
Ithaca, NY

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

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

System 3 is a bit more viscious that the West Systems, so wicking may not be as bad. (System 3 also gives me hives unless I wear a resperator and barrier cream, so I won't use it.)

Alchohols can be used as a thinner for epoxy but have a couple of problems. First the are hygroscopic (absorb water vapor from the atmosphere) and the water is not compatible with the epoxy -either causing hazing and at higher concentrations inhibiting a full cure. Second the alchohol will thin the epoxy, but not chemically attack it, so you end up smearing around a thin film. You may find that cleaning up with xylenes, toluene or mild acids such as vinegar works better (test for compatability of your finish and use proper safety measures before using solvents that dissolve partially cured epoxy). Also note that cleaning up with alchohol can help transport uncured epoxy thru your skin and exacerbate any health issues.

Definitely do some test pieces first. I've built airplanes, surfboards and sailboards using epoxy, but other than installing truss rods, I do not use it on guitars.

-jd

Bob-
Thanks for those links- lots of interesting information there.....

My impression is that glues which will bond well to smooth surfaces (highly planed wood, glass) are bonding chemically; those that require penetration into the substrates are depending on mechanical keying and bonding.

From the WEST epoxy website:
http://www.westsystem.com/ss/surface-preparation/
(Underlining and bold face are mine...)

"Surface Preparation

Whether you are bonding, fairing or applying fabrics, the success of the application depends not only on the strength of the epoxy, but also on how well the epoxy adheres to the surface to which it is being applied. Unless you are bonding to partially cured epoxy, the strength of the bond relies on the epoxy's ability to mechanically "key" into the surface.
That is why the following three steps of surface preparation are a critical part of any secondary bonding operation.

For good adhesion, bonding surfaces should be:

1. Clean


2. Dry

3. Sanded -Sand smooth non-porous surfaces—thoroughly abrade the surface. 80-grit aluminum oxide paper will provide a good texture for the epoxy to "key" into. Be sure the surface to be bonded is solid.
Hardwoods-Sand with 80-grit paper.
Teak/oily woods-Wipe with acetone 15 minutes before coating. Solvent removes the oil at the surface and allows epoxy to penetrate. Be sure solvent has evaporated before coating.
Porous woods-No special preparation needed. If surface is burnished, possibly by dull planer or saw blades, sand with 80-grit paper to open pores. "

etc....

This seems quite different to the procedures for using adhesives like hide glue or CA, which I assume bond chemically.
A quick test with a couple of pieces of smooth-planed hard maple and epoxy, vs rough-sanded samples, will tell you a lot when you destroy the samples with a hammer.

Cheers
John

Adventures in aviation dept.:

http://www.westsystem.com/ss/bonding-gluing-clamping/

"Single-step bonding

Single-step bonding is applying the thickened epoxy directly to both bonding surfaces without first wetting out the surfaces with neat resin/hardener. We recommend that you thicken the epoxy no more than is necessary to bridge gaps in the joint (the thinner the mixture, the more it can penetrate the surface) and that you do not use this method for highly-loaded joints or for bonding end grain or other porous surfaces."



EDIT- Windsurfer- I was typing as your reply was posted. I think we basically agree on the alcohol 'thing'...EDIT

I agree in general with your comments about vacuum-bagging with epoxy, and also about clamp time. Alcohol will do the job for cleaning up epoxy- probably better than most alternatives. (Vinegar is often recommended but I've tried it and it is terrible for cleaning around glue joints- it's good for removing accidental epoxy on your skin). If you are not careful, you can get epoxy/alcohol spread all over the place, or, worse, get alcohol penetrating into the epoxy in the joint.
Removing excess epoxy mechanically (sharp stick, etc) is the first step, before getting out the (roll of) paper towels and the alcohol. Avoid cleaning your skin with alcohol, or other solvents, as they aid penetration of chemicals into your skin and can promote sensitization.

Cheers
John

That's not much advantage compared to the disadvantages. It really doesn't take much time to remove the finish. Working out the bugs of a new system would take me more time than removing the finish on several years worth of guitars.

_________________
http://www.chassonguitars.com

I agree-
Is it really so important to get the 'factory look' around the bridge? I'd rather have the bridge all glued to the guitar top than to start messing with rabbeting the edge of the bridge, gluing to finish, etc. . Aren't those things just shortcuts used in industrial settings?
With the method of leaving a ring of finish under the bridge edges, it seems a 'short distance' between a too-small rabbet making for a thick glue line, and a too-deep rabbet filled with glue.

Whether epoxy is a good glue for bridges is a separate issue, though.

Cheers
John

[quote="JohnAbercrombie"Adventures in aviation dept.:

http://www.westsystem.com/ss/bonding-gluing-clamping/

Single-step bonding

Single-step bonding is applying the thickened epoxy directly to both bonding surfaces without first wetting out the surfaces with neat resin/hardener. We recommend that you thicken the epoxy no more than is necessary to bridge gaps in the joint (the thinner the mixture, the more it can penetrate the surface) and that you do not use this method for highly-loaded joints or for bonding end grain or other porous surfaces."

[...]

Cheers
John[/quote]

I forgot how much good stuff West Systems has on their web site.

The reason Single-step bonding does not work well with porous surfaces because the thin epoxy tends to wick out of the joint and into the pores, leaving the remaining filler with insufficient epoxy to achieve a good joint.

Single-step also doesn't work well with low clamping forces or overfilled epoxy because the surfaces don't get wetted

With correctly mixed epoxy and vacuum clamping, you should be fine without having to butter the surfaces with unfilled epoxy.

But this does bring up a good point; adding filler does not increase the strength of the matrix, it only improves the ability to fill gaps. a well fitted joint with a thin bond line will still be stronger than a thinker bond line with lots of filler.

-jd

I appreciate the continued input from everyone here. At this point, I'm seeing two main issues to consider in this:

1. Do we WANT the bridge glue joint to be easily reversible? Todd Stock raised the point that there are cases where a crack repair dictates removal of a bridge, even when the bridge joint itself is still in perfect condition. Refinishing the top would be another case where you'd want to remove a bridge. There are probably other times that I'm not thinking of off the top of my head. My own repair experience is not extensive enough for me to say how frequently these situations arise, where you'd want to remove a bridge even when the bridge joint itself is still in perfect condition (hasn't come up for me yet), so I have to ask around to get a better sense of that. If it's a rare situation, I'd have to weigh that against the advantages of a method of gluing on the bridge where one result of the method is that it makes a permanent, not-easily-reversible joint (this would apply to using CA as well, whether glued on top of the finish or not), bearing in mind that any bridge and glue can be removed if it were really necessary, using a router or whatever method necessary. If it's a fairly common situation, that would dictate to stick with HHG or PVA without further question.

If any others who do lots of repair would like to comment on this issue, letting us know how often you find yourself removing bridges for reasons other than the bridge joint itself failing, that would be very helpful.

2. Would a joint such as I described, if done properly, be plenty strong and reliably permanent, except in cases of abuse (namely, subjecting the guitar to extreme heat)? My understanding of how epoxy, specifically thickened epoxy, works has been such that I have been going on the belief that it would. I've been looking at the joint I described as really being two joints and three materials: The bridge wood (material #1); the joint between the bridge wood and the layer of cured, thickened epoxy (joint #1); the few-thousandths-thick layer of cured, thickened epoxy material itself (material #2); the joint between the epoxy layer and the top wood (joint #2); and the top wood (material #3).

Some of you have called my understandings and thinking on this into question, so I have more studying to do. Any further input or insights into this from any of you would be helpful.

_________________
Todd Rose
Ithaca, NY

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

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

Todd,

no doubt a well-executed epoxy joint for the bridge will be strong enough, stiff enough and essentailly permanent. Same can be said for urea formaldehyde glues. Advantage for the Urea glue is that it is more repairable if you ever decide to remove it for other reasons (epoxy joint must be repaired using epoxy)

As you know, the real question is not whether it can be glued with epoxy, but whether is should be.

-jd

long and involved thread about wood glues. Not about guitars, but contains some interesting information and is worth a read:

http://www.sawmillcreek.org/showthread. ... rmaldehyde

-jd

Thanks for your input, Kent. I appreciate your perspective.

I came up with this idea as I've been brainstorming possible alternative ways to address the issues people, including myself, have with removing finish and achieving a result that looks flawless around the edges of the bridge. In my case, I'm a perfectionist and my own toughest critic, and I'd like mine to look better, even though they already look very good and very few people would ever notice the things I can see and am not entirely happy with. The main thing there is that, even cutting carefully with a fresh Xacto blade, I find that the polyester finish near the edge kind of pulls up in little ripples as the knife is drawn across (which is then exacerbated when I lift the finish off with a chisel if I haven't cut quite deeply enough in a spot here and there), and also that there is a very slight bit of white line or ragged edge that can be seen after the bridge is glued on if one looks extremely closely. Another thing for me is that I haven't yet become confident that I'm always cutting to exactly the right depth, and I worry that I may have cut into the spruce fibers a tiny bit (though I haven't had any bridge joint failures). Obsessing about minutiae? I suppose - I can't seem to help that - but I'm not alone, as evidenced by the people who hide these kinds of things by leaving a lip of finish under the bridge and routing a tiny rabbet around the underside edge of the bridge to sit on top of the finish.

In these photos, you can't see the rippling, but in the angled shot of the ziricote bridge, you can see a bit of a white line from the cut edge of the finish.

Finally, though I would never cut corners to save time if it sacrificed quality in any way, any sound method that saves a significant amount of time is a good thing in my book. I find doing a clean job of removing the finish to be significantly time consuming, though I am getting faster at it with each guitar.

_________________
Todd Rose
Ithaca, NY

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

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

Lots of stuff in this thread, some of which I just skimmed, as I need to get back into the shop! I'll reply with a few general points, rather than anything specific.

Epoxy is, as I understand it (I haven't done the experiments myself, nor seen the papers) a reasonably strong material in itself, and possibly stronger than other dried glue film materials. That's one reason you can get away with using it to fill gaps: you don't sacrifice much strength to the material. The other reason is that it doesn't shrink, so it won't create gaps. But remember, a good glue line is stronger than the stuff it's glued to: the glue fills in the air space in the wood, and that strengthens that part of the wood.

Sanding an epoxy surface when bonding something new to it is not done to 'key' it for the mechanical strength: it's done to raise the surface energy by breaking chemical bonds. The test is to lightly spray the surface with water, which is a polar molecule. If the water beads up it's more attracted to itself than the surface, and that tells you that the surface enrgy is low. If it spreads out into a film, the water is more attracted to the surface; the the energy is high.

The shearing stress in the glue line between the bridge and the top is highest at the leading and trailing edges of the bridge. The wider the bridge is, the farther apart those edges are, the lower the stress at the edges. There is also tension at the back edge, and compression at the front edge, due to the torque. It's the combination of upward tension on the back edge of the bridge and the high shear stress that causes the joint to fail. No glue is very good at resisting peeling, and once the joint starts to fail the distance from the front edge to the back is decreased, so the shear stress rises, and the lifting force increases due to lower leverage. You could say that any bridge that lifts does so because of poor design and/or abuse, but IMO it would be difficult to design a bridge that would work well acoustically and be absolute proof against peeling up. These things are always compromises, and we accept a certain percentage of failures in return for decent sound.

All the Ovation bridges I've seen fail have done so with nearly 100% wood shear. The thick epoxy finish (around .04") causes the wood of the top to break out in a much larger area than the bridge footprint.

As to cosmetics: we certainly should try to get the best appearance possible consistent with good acoustic and engineering practice. IMO cosmetics drive far too much of what we do. I have always been leary of removing _any_ wood from the top where the bridge goes (such as routing the top for the bridge), and, for the reasons given above, have also resisted leaving any sort of finish 'lip' under the bridge: that's just a stress riser IMO. Some finishes show less of a line than others, some can be touched up later to look better, but, ultimately, if it's a choice between a 'perfect' look that tends to come apart, and something a little less perfect that will hold for the longer term, I'll go with the latter. If the customers object, I think that's something we need to educate about. We, of course, can always strive to be better than we are, but we do need to be realistic, IMO.

Todd, one thing I do to prevent seeing the white edge of finish is to score with the blade at a bit of an angle toward the bridge. By the time you go through two layers of tape, the outline is the exact size of the bridge or a bit smaller. If the blade is perpendicular, the score will be wider than the bridge by the thickness of the blade and you may see a white edge.

I then put a very small bevel on the edge of the bridge to make sure the bridge drops in flush. The bevel is only about the thickness of the finish. It also makes it a lot easier to get a knife under the bridge without damaging the finish should you need to remove it.

Nice bridge by the way. I can see how it would be a challenge to get a good score around the curves on the top edge.

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http://www.chassonguitars.com

This is a bit off from the original topic of this thread.

Alan brings up a good point about design and peeling bridges. I've thought about this for a long time especially when repairing a lifting bridge. Seems to me that conventional bridge/top/bridge plates are prone to failure because of their inherent design. We could consider the bridge plate and the top as one component for the sake of this argument. With the stiff bridge pulling up on the less-stiff top/bridge plate, we have a joint just waiting to peel apart, since the top/bridge plate unit is more apt to distort than the bridge.

A more robust system would have the stiffer component on the underside of the top, something with the stiffness of our current conventional bridges, with the less stiff component on top, which would have only enough stiffness to hold the saddle and bridge pins. But I don't know how it would look or sound, or if anyone would buy a guitar built that way.

I think the Yairi bridge design addresses some of these issues, though in a different manner, by using a string anchor that installs from the inside, and is separate from the saddle portion of the bridge. With his system, the peeling force we commonly deal with is instead pulling up on the inside of the top. Looks like it might be pretty heavy though.

Food for thought. Or not.

Pat

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