Official Luthiers Forum!

I would also be interested in how peak stress across the nylon string guitar bridge patch (whether tensile, compressive, axial, shear, etc.) can exceed that seen with steel strings, given bridge patch area and saddle height are similar, and medium gauge steel string tension is about twice that of hard tension nylon strings.

_________________
A constellation only takes shape when one maps the whole.
- Beth Brower

I have had some experience with flat sawn ebony, garboon and macassar alike, and moisture. I have to say the wood reacts more violently than any other wood I have worked with. Of course that is an major factor when using it, especially flat or riff sawn.

That's not to say some have not been able to overcome it. I know Rick Turner uses CA on his bridges, not sure why but it would help with the moisture issue.

If I really want an ebony looking bridge I use African blackwood. Being a rosewood, it has much better expansion/contraction numbers than ebony.

With bridges, and any wood or glue, you still need to be sure your fit and clamping practices are good.

_________________
Joe Beaver
Maker of Sawdust

Hey Joe, one of my favorite songs.

No one is disputing that flat sawn ebony may curl. What I've been disputing is that no one knowledgable and intentionally would use it for bridges. Everyone can have a bad day and looks like Ed may have had one. By the way I am calm.

Bridges do curl and for a plethora of reasons. Some of these are loose braces, top distortion, worn pin holes, creased bridge plates and RH swings can cause or contribute to some of the above too.

Bridges also curl in the removal process from the differential expansion of heat being applied to only one side and quickly too. This is with even well quartered lumber.

It never was being disputed by me that bridges curl or flat sawn ebony might curl with excess moisture. What is actually being disputed and Gore understands this regardless of his attempts at misrepresentation.... is my contention that who would intentionally use flat sawn ebony for a bridge? Gore's going down the path of how much clamping is required to clamp a curled bridge is just one more deflection from his incorrect contention that a common cause of lifting bridges is flat sawn ebony and excess moisture in HHG. That's complete BS. Bridges are of course fitted, or at least that's what I've always advocated and written even here, hence the toot on fitting bridges that's been here for nearly a decade now.......

I'm also disputing the false notion that HHG is somehow problematic for bridges and that Titebond and Titebond like glues are superior. Like Woodie we too see plenty of examples of plowed finish from Titebond or Titebond like glued bridges. On balance do you want your bridge subject to plowing finish and lifting.... and how difficult that can be to repair under warranty...... or would it not simply be a better practice to use quartered lumber and HHG. Of course observing open times, best practices for clamping, etc.

There also is the notion of not observing any specific mix ratio for HHG and the excess moisture that may be present because of this. As David F. rightly said knowledgable luthiers thin or thicken HHG at will for different applications. Woodie's crew use higher gram strength for bridges and that's something that I'm interested in now too.

Along the way the idea of a rub joint with HHG is brought up but no one has answered my question regarding naming any notable producer of STEEL string instruments, that after all is what Ed's guitar in this thread is.... that uses a rub joint for bridge installation.

Rub joints with HHG for classicals has been done for well over 100 years but it's not done if the bridge has lifted and has to be reglued or at least we wouldn't. In the repair world our surfaces that we encounter are not always perfect as they are for builders. Clamps are prudent for any reglued bridge in my view AND especially any steel string bridge sands the CA glued examples on polyester finishes that can take it.

And lastly, for now... Woodie and I are also questioning the contention by Gore that a classical guitar with a rubbed joint bridge has more tension (or stress) than a steel string. That's utter bullcrap and disappointing as well coming from someone positioning themselves as an authority.

There apparently is an ivory tower, academic view of Lutherie being promoted here that does not square with what Luthiers see in the trenches every day. It's likely that ole conflict between those with only building experience and limited repair experience.

Going back to bridges curling, sure they do as mentioned. Luthiers use differential expansion intentionally exploiting this phenomena for repair work fairly frequently as well as intentionally messing with HHG viscosity. The well known Martin B string crack that Luthiers across the planet may encounter can be repaired with differential expansion of even a well quartered top by applying water to the top side and controlled heat under the warped area. FYI these cracks are also caused by the differential expansion/shrinkage of the dissimilar materials of pick guards and guitar tops.

So to be clear since Gore seems to like to attempt to manipulate a losing argument by intentionally attempting to misrepresent the written words of others..... a bridge lifting because of slab swan ebony and too much moisture from HHG is an absurd notion. Sure it can happen but who uses slab sawn ebony intentionally??? HHG's moisture content is not problematic to those who adhere to mix ratios and bridges can be clamped down well regardless of what we saw here, everyone has a bad day.

Interesting thread. Having seen older posts describing how moisture from glues can result in deformation, I've taken to testing for that myself. So far I've not seen any movement but that doesn't mean it won't ever happen and I like the added confidence I get from checking.

My process of mating the surfaces is similar to what bobgramann outlines above. I've only done a couple dozen bridges, not the hundreds or thousands that some here have completed, but the only one to come loose on me so far was my first-ever use of HHG. I attribute that to simple operator error. After refining my processes everything has gone smoothly.

Personally, regardless of how we got here, I think we're all incredibly fortunate to be part of the lutherie community and to be creating these wonderful instruments from such beautiful materials. Let's hug it out and try to keep things positive.

_________________
George :-)

Not intending to take sides, but whilst string tension is obviously higher on a steel string guitar than a classical, the tensile stress on the glue joint is less on the typical PINNED steel string bridge because of the anchoring of the strings on the bridge plate, and those who do a pinless bridge tend to use a larger footprint.

Jeff, I'm not a engineer but what you say seems intuitive to me. I'm in.

Hesh, I appreciate your imput. As always I count on it being based on your broad experience and knowledge. But again speaking from my intuitive side, I suspect the following.

You and others are right about there being many causes of bridge lift. Most of what you are saying seems to apply to guitars that have been exposed to some form of poor treatment and happens after they have been around awhile.

Trevor and others are right about the likely causes of bridge failure in newer instruments. In those cases it most likely reflects on material used (including the bridge material & glue), bridge footprint, glue surface preperation, clamping force and technique.

For me the casual builder it is a Win Win !!! Thank you to Ed for posting the thread and for all the sincere posts!

_________________
Joe Beaver
Maker of Sawdust

George I'm all for civility and just when I thought that I was out..... Gore dragged me back in...

I'm happy to be civil unless someone misrepresents my writings and does so intentionally. When that happens folks should expect the possibility of a response from anyone.

Hey Jeff thanks for that and I agree with you however steel string string tension being higher, substantially higher in some examples contributes to other issues that can be structural and contribute to lifting bridges. A perfect storm of sorts contributed to by high string tension. Classical guitars don't suffer from enlarged pin holes from string balls migrating through the plate, top and eventually bridge as steel string guitars do.

It's often the case that even inferior finish clearing, not observing best practices with gluing and clamping still won't be the root reason why a steel string bridge fails. Instead the nature of how a steel string is built and has to withstand the higher string tension when the structure fails can create top distortion to the point where a bridge simply can't conform and lifts. Or, in other words it's often the case in the repair world that bridges lift for a combination of reasons including how they are cared for.

It's true that stress on the glue joint of a classical is higher and that's well known but it's also true that higher string tension on steel strings contributes to bridge failures for other reasons. This is something that classicals don't suffer from to the same degree or for the same reasons as steel strings.

So with this said who here is willing to risk their STEEL string builder reputation on agreeing to only use a rub joint for bridge installation going forward? We even now agree that the stress on the joint is less, any takers?

No problem Joe but we reglue bridges on new instruments at times too that lost their bridge in the store....... We also are known for doing the ghost final assembly for some Luthiers at times. Dave built the first 100 prototypes for a major US guitar manufacturer and over 100 additional production guitars. Between the two of us we've produced several hundred instruments.

Not arguing with you my friend but it's not that simple that one side knows new stuff and the other side doesn't.

Point being we work with new instruments frequently and do not agree with Gore that the major cause of lifting bridges has anything to do with slab sawn ebony being used or the moisture content of HHG. That's absurd. Builders do not commonly use slab sawn ebony and everyone we know tends to be religious... about HHG prep and the parameters, strict parameters for successful usage.

Hesh,
Read Trevor's orignal post again. You'll see he says there are many causes for the a bridge lifting, the less obvious are 1) wood curling from moisture, worse when it is flat sawn, 2) smaller footprint of some bridges, and 3) glue choice. Once again, these are the less obvious causes, per Trevor, not the major causes.
Once again I say you are both right, no need to challengle the others position.

_________________
Joe Beaver
Maker of Sawdust

Rank amateur instrument maker here, but long-time woodworker. Why not favor the back (trailing) edge of the bridge when gluing it? It seems like the glue at the front (soundhole) edge of the bridge is not doing much. Favoring the back edge slightly would ensure that if any of these things were to happen it would be the front edge that suffered the adhesion problem.

Ed

I'll repeat what Gore wrote "If asked to choose the most likely failure mode, I would be guessing something along these lines."

I'm challenging Gore's assertion because he's wrong. People don't use, intentionally... or if experienced slab sawn ebony and HHG should not be getting a bad rap for moisture content here, that's misleading too.

Millions of ebony bridges have been glued on with HHG and remain on to this very day. This is a misleading statement and a very obscure causation for a lifting bridge.

We challenge others as I am often challenged because we are both passionate about what we do and we may despise snake oil and the tendency of some to further same on Internet forums. Sorry if it offends you.

Good point! And at the very least be sure as shootin that the back edge is down and stays down. This is why I examine my back edges (on bridges...) with a flash light right after the deboogerization of the bridge reglue and before hanging the thing until the next day. That back edge as you are correct Ed does the heavy lifting against bridge rotation.

I'll add that with dovetail fitting having a decent fit that may require a light tonk to set it is important for the entire dovetail. But just like that back edge of a bridge the lower part of the heel of a dovetail is going to have more pull on it from string tension and the neck wanting to fold into the body than the part of the dove tail closest to the fret board. As such at the very least be sure that the lower part of the dovetail is well engaged for the same reason as the bridge trailing edges need to be down.

Of course with a bridge we want the entire thing down but I would suggest to others as Ed points out that the back edge is the most important part to be sure it's down. Consider inspecting same after every bridge glue up or reglue, we do.

I surrender

_________________
Joe Beaver
Maker of Sawdust

I wasn't going to get back into this thread due to the increasingly acerbic tone, but...

As for slab sawn Ebony, I encourage anyone to go the the LMII site and see what they have to say about the grain orientation of the Ebony that they sell. This bridge almost certainly came from them. It's nigh on impossible to tell what the grain is doing sometimes in Ebony just by looking.

Not necessary or desired. People have every right, all of us including you to respond to what others share with us. That's what I'm doing and will continue to do. Not attempting to offend anyone but I am also not going to be misrepresented by anyone, ever.

My replies in this thread will stop when I am no longer addressed.

Hey, kind of reminds me of that guy with the tweets....

Forgive me if I am excessively obtuse this morning, but I do not believe anyone has presented an explanation as to why bridge patch glue line stress would necessarily be higher in a classical bridge. Stating that the pins in a pinned bridge transfer load seems likely, but then we also know that a properly fitted unslotted pin may be removed at full string tension and the instrument played without harm. Perhaps a little more in the way of explanation?

_________________
A constellation only takes shape when one maps the whole.
- Beth Brower

I will jump into this inferno by saying that there could be a whole bunch of reasons for failure. I/ve had hhg failure on a thinner bridge, using a 192 g strength glue. I/ve also repaired quite a few bridges as well. I would say it all depends on the glue , clamp time, atmospheric temp/humidity, open time , types of clamps used. cauls ?? etc . I reglued a classical oriental gtr that had some mystery glue on it.It took me 3 tries with HHG before I got it right, One problem I overlooked was a super clean top before regluing, I thought I had cleaned it all up only to discover vy thin shreds of old glue embedded in the spruce that I had missed. So back to the deglue goo which has saved my butt more than once. Oh yes good lighting, aimed at the top so one can see any anomalies like dips bumps dirt , uneven curve from front to back and side to side .A perfect fit is superior to one that has been force fit to align with the tops convex surface. Not a fan of the rabbeted ledge either, as I have seen too many older bridges that have unsightly gaps particularly in the back end of name brand bridges

Nothing to do with the pin. It's the fact that the ball end is anchored on the "other side" of the bridge glue joint, taking some of the stress off the joint.

wow. I would think the high point would be in the middle there. oops. you probably have no idea what I'm talking about. I just looked at a picture of a bridge sanded, Probably a few pages away now. Senior moment?

_________________
"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

"It's the fact that the ball end is anchored on the "other side" of the bridge glue joint, taking some of the stress off the joint."

I agree that the strings being secured under the soundboard take - some - of the stress off the joint. But there are forces trying to rotate the bridge and lift the back of it , and with twice the tension those forces are greater with steel strings.

Thanks for that concise, insightful, well reasoned and eruditely argued post. It speak volumes about you.


Thanks, Joe, for taking the trouble to try to explain that to someone who clearly doesn't want to listen. If Ed had thought that was a "common or garden" bridge failure, I doubt he would have posted. After all, he's not short of experience. He was asking if the glue line indicated anything in particular. Assuming he knew the standard stuff...

...I suggested some of the less obvious, like this one...

And of those less obvious possibilities, this seems the most likely...

You've got to try pretty hard to interpret those words any other way. It won't stop someone intent upon misrepresenting you though.
It's a shame you got bullied into that.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Clay, the Rotational forces (torque)on the bridge/ soundboard as a unit are obviously greater but when you look at the resultant shear (sliding) and tensile stresses on the glue line for a pinned bridge , the shear stress is much greater for a steel string (unless the footprint is increased, which is often the case). Because the string ball ends anchor on the bridgeplate and not on the bridge the DIRECT tensile stress on the glue line is very low.
How often do you see a steelstring guitar with virtually no intact glue at the back edge still maintaining string tension
When this happens on a classical the bridge blows off quite dramatically.
I am not denying any secondary effects from distortion etc which cause peeling at the glue joint.

Tension, stress, rotate and flat sawn. Titebond it, I say! And that thing better stay down!

Just for the record Trevor, I think Hesh owns the “pull your head out/arrogant SOB comment, not Woody.

Keep posting man, I for one value your comments.

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

Once again, I am left considering a statement rather than reviewing an explanation, so please bear with me. Note that while I work in a shop with at least one engineer present - and often too many - my academic accomplishments lie in other fields, and I am bootstrapping my way through this discussion with borrowed textbooks and a flurry of text messages and sketches exchanged with MMS.

My understanding (to be corrected if I have erred):

There are two externally applied loads on the bridge/saddle system, assuming we collect the separate, parallel forces of the strings and string end balls. The rest of the forces acting on the system are in response to these external forces.

For medium gauge strings, we have a 186 lb tensile force virtually parallel to the plane of the bridge/top joint and applied 1/2" above that joint, as well as another 186 lb tensile force acting at right angles to the bridge/top joint and at about the midpoint of the bridge. I've included the pins in the bridge/saddle system, with the expectation that play an insignificant roll unless the joint fails or creeps.

If I understand the method for collecting forces and moments, we can collect all of the forces and any moments about a common point, such as the front-face-of-bridge-pin/bridge patch intersection. For the string tensile load, we can replace the 186 lb force tensile load applied above and parallel to the bridge-top joint with a 186 lb tensile force applied along the bridge plate joint and a moments of 93 inch-pounds in the anti-clockwise sense (viewing the system in profile with peg head to left and tail block to right). For the 186 lb tensile string ball force, which already acts through the common point.

For the classical bridge, the forces applied are just the string tensile load of 90 lbs acting (once again) parallel to the bridge/top joint and 1/2" above it. Resolving the applied forces to a common point of application (the lateral mid-line of the bridge patch), the resulting system is composed of a tensile force of 90 lbs applied parallel to the bridge-top joint and a moment of 45 in-lbs anti-clockwise. For the purpose of the analysis, a typical Martin-style belly bridge has a net gluing area of 7.6 square inches, while a classical bridge is more typically about 8.8 square inches.

The figure is the sketch of the two systems as I understand them, showing both the applied forces and the force and moment system about the common point. If one of more of you talented gentlemen and ladies lurking would be so kind as to provide an explanation as to how the forces and moments depicted translate into tensile, compressive or shearing stresses in the respective glue joints, as well as the magnitude and direction of those stresses (to include peak stresses), I would be deeply in your debt.

_________________
A constellation only takes shape when one maps the whole.
- Beth Brower