Official Luthiers Forum!

Got one back with a lifting bridge. It had come up to be just hanging by a thread at the leading edge. I'm sure RH may have started it, but the way it came almost all the way of was interesting. I managed to remove it cleanly, and I'm wondering if the glue pattern indicates anything in particular. To me, it looks like my clamping pressure may have been inconsistent. Might be time to step up to the aluminum Fox style clamps...

I've had a few (3) others as well so I'd like to get this licked...

I haven’t had any lift since I started using 315g hide glue instead of 192g for bridges. Also, to deal with the problem of uneven clamping pressure, I vacuum clamp the bridges. Every now and then, I would have a distortion problem with regular screw clamps. The vacuum clamp solved that.

Perhaps one day I can afford that. I'd love to have a vacuum table for bracing...

I did this same thing earlier this year. The clamping pressure must have been biased towards the front area of the bridge, and I did not get a good glue surface contact on the back edge. The bridge was sure easy to glue back on though. Easier than refinishing the top since when it released inside the case the bridge scratched up the top pretty good.

I think I need one of those vacuum clamp thingies too.

I bought the vacuum pump for $65 on ebay. The pump is a continuous duty diaphragm pump, so no reservoir was needed. LMI sells the clamp for $130 but you can make one for next to nothing if you can get a sheet of rubber for the membrane. It’s worth the trouble and expense.

Here is a question for the repair experts:

Let's work from the assumption that having perfectly mating surfaces is the goal; that's a given, and not my question. But, let's also assume that perfectly mating surfaces sometimes are not quite there.

My question is: Isn't it better, all things being equal, to have three clamps instead of just one clamp? If there is decent clamping pressure in the middle, and differently adjusted clamping pressure on each wing, that feels to me like it can do a better job of dealing with any imperfections in the mating of the surfaces than just one clamp can.

I ask because I see a lot of gluing setups that rely on one clamp in the middle and a caul spreading that clamping pressure out to the wings. I feel like that puts a lot of trust in the quality of the caul, the work that was put into making the surfaces mate, the resistance of the bridge to warping once glue is applied, etc. I think I put more trust in the clamps than I do in all of these other things.

Also, I have used the "one clamp, but the caul has threaded plungers in the wings" type setup, and I'm not crazy about it. My concern with this is that the threaded plungers only push the bridge wings down; nothing out at the wings is mashing the top up to meet the bridge. Well, that might not do the trick. I like having an actual clamp out there.

I look forward to reading the thoughts of those who do a lot of bridge repair work.

What’s the finish and is there a rabbet on the ledge? Asking for Hesh.

Serious question though.


Sent from my iPhone using Tapatalk

_________________
Insta - https://www.instagram.com/cbcguitars/
Facebook - https://www.facebook.com/cbcguitars

On bridge repairs I use at least 3 clamps but would use 4 if I had another one and could get it in there.

_________________
Steve Smith
"Music is what feelings sound like"

I like the team Woodie method of putting cam clamps on the wing screw Downs.

_________________
Pat

I size both the soundboard and the bridge with thin HHG first. Bridge usually has to be scraped back because HHG can distort the bridge, then sized again - until it stops moving. I use one clamp and a caul that clamps the wings.
Check the fit of the bridge surface and the dilution of your glue. Make sure you are getting it all done before the glue gels.

Looks like the top gave out as much as the glue did, but it does look like uneven stickage. What gauge strings was it strung with? One reason Martin went with the belly bridge design was to give a little more real estate for the bridge to grab onto.
I have some pretty Engelmann tops that don't like steel strings and narrower bridges. They are soft and lightweight and probably better for nylon strung instruments. The dense stripey tops seem to work fine for steel.

Finish is polyurethane, no rabbet as it's only ~.001 thick, strung with 12's...

Looks to me as if there was not enough glue, the glue residue looks pretty minimal and I see the trailing edge has less residue and that's where the heavy lifting is done. The fibers pulled up are minimal too.

In Michigan November is a bridge lifting, top cracking, sharp fret end making month big time. Our furnaces are now on, humidifiers either not yet deployed or not present. Often bridge lift for a combination of reasons or that perfect storm of conditions.

No vacuum clamping here but like McDonalds perhaps over 500 bridges served and the only ones that ever came back were a batch of fish glue glued bridges over ten years ago now.

We use the rabbit, clear finish to .010" of the perimeter or so and we use the Stew-Mac single clamp caul with an Ibex modified ball end matching the caul with wing screw downs. As Don said this clamping arrangement does have the possibility of uneven pressure if you are not aware of the flex and predisposition to do so.

If you are aware of it and here is my big contribution to the thread.... fit your bridge so that you have full wood -to-wood contact everywhere with only slight finger tip pressure and be mindful of the HHG time constraints we've never seen the need to go greater gram strength or more clamps. None of that hurts however and is likely more helpful if the fit is not perfect.

We also scrape our bridge patch and bridge bottom within 15 minutes of gluing, preheat the bridge, and use a methodology where our clamp is set in about 10 seconds or less. Lastly we also preposition our clamp in advance so that it's slap the bridge down in a well of tape and twist the clamp, next.

I did have one of my Heshtone bridges come up but it was left in a trunk of a car in Nashville for a week in the summer with mediums on it in 100F heat. Forensics also showed that my installation had a lack of glue on the trailing edge and a bridge shape that make it easy to not have the back edge clamped well. Live and learn....

Ed you also have a great, small shape there not unlike many of the vintage guitars. Because of the small foot print like Mario P's bridges he used to say that all things must be perfect, glue technique, fit, clamping.

If the top was sealed with shellac or some other sealer was used under the poly is it possible you thought you were down to wood with your prep but weren’t?

I’ve been fooled as the color of the sealer can look a lot like spruce. I go by the color plus the feel under a sanding block to be sure I am down to bare wood everywhere.

Also it does not take much of a finish edge to prevent full contact. That has happened to me where even with a rabbit I didn’t square off the corners of the wings properly on a bridge like yours and it lifted early.

Lastly I have gone back to two clamps in the center of the bridge and one on each wing after a short affair with the StewMac screw down wing design. I know it works for some.

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

I can't see how there would be not enough glue. I run a blurt on the top and a blurt on the bridge, a quick finger smooge to cover all bare wood, then clampy clamp quick like.

It's possible that the fit wasn't great, for a long time I was gluing flat bridges on domed tops and clamping hard (which actually can work) but since a report or two of lifts I've gone back to radiusing the bottoms. But that doesn't explain the lack of contact on the back edge. Perhaps the general fit is not as good as I think...

Lots of possibilities for things to go wrong when gluing a bridge. I'm sure you're familiar with most of those possibilities.

Some of the less obvious:

1) Bridge curling across its short dimension when hide glue is applied. Ebony (which it what it looks like you have) is particularly bad for this, worse if not quartered, which can be hard to see in ebony. That curl precipitates a bunch of knock-on problems: virtually impossible to flatten under clamp pressure to get good overall conformity; if you clean up wet, greater possibility of washing the glue from the joint. If asked to choose the most likely failure mode, I would be guessing something along these lines.

2) Minimal bridge footprint. The glue area behind the bridge pins, where most of the stress is, is really small, and then gone at the holes. It is easy for cracks in a glue line to propagate in a brittle glue like HHG with just a little flexing on that back edge. How much of a stress concentration exists there is largely dependent on your bridge plate design. Having a straight back edge to the bridge doesn't help dissipate the stress. The (Martin) belly bridge works better at this for a couple of reasons: more glue area behind the pins where you really want it; a curved back edge. For gluing SS bridges, I use a "strong back" sort of clamp design, similar to the Stewmac Fox thing, with a fully fitted caul above the bridge bolted through the bridge pin holes. There's also a caul inside which holds the bolts up. I can be certain I've got pressure on the back edge, where it really counts. The wings can be done with either clamps or screw downs like the Stewmac thing.

3) Glue choice. Hide glue is doesn't allow for much, if any, process variation. The mix, the fit, the temperature, the timing all have to be right with little latitude for variation. You're achieving this 99% of the time. To improve on that, consider a different glue. I use Titebond. I've had one failure ever, which was on first string up of my second guitar, which taught me about the importance of new surfaces on critical joints. I've never seen one of my bridges creep, which is the usual criticism of Titebond and that includes all the classical bridges which are arguably under even more stress (smaller footprint, straight back edge, usually no bridge plate, direct string pull) and I've never had a classical bridge fail. I recently refinished a classical and a steel string due to impact damage and decided to take the bridges off to do this. Even after routing the bridge down to less than 0.5mm left, the glue lines didn't give up easily under heat and moisture. The usual argument about being able to reactivate HHG doesn't count in this situation, IMHO, as cleaning old glue off a bridge/bridge footprint is trivial. Used well, Titebond introduces less moisture into the equation.

A few ideas to contemplate...

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

http://www.goreguitars.com.au

We see curled bridges often but it's a tell tale of top distortion that came first, not the bridge curling. We also only see it on instruments that have been alive for some years.

To be more specific bridge plate pin hole damage where because of slotted pins permitting string balls to begin to migrate upward through the pin holes bridge plates can split or crease. This creates an unlevel topside surface for the bridge and it starts to lift. When a back edge lifts and it's still under string tension the bridge can start to curl and they commonly do. The curl in my experience doesn't care if it's ebony or rosewood.

It's very common (and prudent) for us when regluing a bridge to have to address the root cause such as above, there are others too and then flatten the bridge bottom before carefully shaping it to the top. Older instruments have domes, dips, peeks, etc and we fit the bridges to all of the above so long as the underlying causes inside the box were addressed.

I'm commenting Trevor because I have never seen a new bridge curl on the short dimension on a newer instrument. This takes time, damage inside the box, etc. to happen in my experience. Or in other words I believe the curl to be a function of the lifting and top distortion not the cause of same. What came first the chicken or the egg FWIW.

It's possible to have fitted a curled bridge that was never flat to a domed top I suppose but the idea that HHG caused the bridge to curl fairly quickly is not something that I've ever seen or experienced. Instead the bridge being a major brace tends to take the shape of the top for better or worse provided that it's down everywhere.

One more comment.

When we clamp bridges through the sound hole and if the clamping contact area both inside the box and topside is minimal the clamp(s) because of gravity and no support over the sound hole tend to want to fall into the sound hole and NOT be level in respect to the bridge. This takes clamping action off the trailing edge of the bridge and can result in a poorly clamped trailing edge of a bridge.

And even one more thought. We've seen bridges that were not clamped properly on the trailing edge because the X-brace intersection was high enough inside the box that it tended to lean the clamps backwards since they could not clear the X-intersection. This one is why some clamps have adjustable bottom jaws to elevate this for the X-intersection. Not that they have this by design mind you for Lutherie and bridges but these clamps with the adjustable lower jaw height work better for generically gluing bridges with instruments with very high X-intersections.

Nonetheless if the clamp was unable to be level because it could not clear the X-intersection pressure on the back edge of the bridge is compromised. I've had to tape a block of wood to the lower jaws of some clamps at times for some imports with high X-intersections.

As a rule and practice I get out a flash light and inspect the entire bridge glue joint after deboogerizing it and before hanging it for tomorrow to be sure that it's down everywhere. Another reason why vacuum would not work for us since it would obscure the view in the short term.

You've not seen it, therefore it can't happen?

Well, it does. I've seen it. If you want a second opinion, check out Somogyi's Fig. 17.13 in his Responsive Guitar book. To quote part of the caption to save you looking it up ..."An example of a slab-cut ebony bridge that warped from taking on glue-moisture as it was being glued on..." (his emphasis, not mine). Further, he says "It left a noticeable gap in the glue joint that could not be closed with clamp pressure". It's pretty well known that if you increase the moisture content on just one side of a piece of wood it will curl, some wood more than others. Likewise, if you heat a bridge to get it off, the top side will dry which can result in a curl in the same direction as a wet base.

A good reason (if any more are needed) not to use ebony in bridges and one more reason to have CF in your bridges and bridge plates...

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

http://www.goreguitars.com.au

Please don't misrepresent what I wrote.....

You said that you consider this kind of a failure because of HHG application and the bridge to not be quartered and ebony "If asked to choose the most likely failure mode, I would be guessing something along these lines."

I disagree. Not only have I not seen it, and please stop with the BS reply about it not being possible because I didn't see it.... with hundreds and hundreds of repaired bridges I've never seen the real cause of a bridge lift being because it's ebony, not quartered, and HHG was used. Instead bridges lift for a plethora of other reasons, often RH related, internal structurally related but never have I seen a relatively new instrument lose a bridge over the conditions that you (or Ervin) suggest.

Sure it's possible but the primary or even likely cause, no way.

Trevor if you are going to be smoking the drapes please have the decency to share?

Regarding CF augmentation that's your opinion and what you like to do, produce unserviceable instruments?

Bottom line is we don't see slab cut bridges and for the life of me I'm sure Ed is not producing them either. So back to earth here and this thread what does this have to do with Ed's lifting bridge? Nothing!

Not to join the "Pick on Trevor Bandwagon" but:
"classical bridges which are arguably under even more stress"

I've used a simple rubbed joint with HHG and no clamps for classical bridges and not had them fail. I'm too chicken to try this for steel strings.

Completely agree. String tension is not difficult to calculate, classicals have much less string tension. Can anyone name a steel string producer who uses a rub joint for the bridges?

We've seen a few like this on small shop built instruments, with most arriving from the builder with the bridge separating, or in one case, a repair on a vintage Martin from a relatively new repair shop that did not have much experience working with hot hide (and should not have been accepting repair work on anything older than the principal). We also see Titebond joints like this, where the rear portion of the joint (in the tail block direction) shows either a glossy surface on both bridge and bridge patch or a 'laced' appearance.

The conjecture here was similar to Mr. Breakstone's suggestion of inadequate clamping pressure, which would likely result in a joint that was not closed (glue line thinned) quickly enough. Warming both bridge patch and the bridge would be useful, with the bridge providing enough thermal mass and heat storage to remelt the glue that is applied to the bridge patch just before the bridge is brushed with glue and the joint closed.

My own experience with the classic Stewart-MacDonald bridge clamp is that it may not apply adequate or uniform pressure to some bridges, and may need 2-3 supplemental clamps and another caul to achieve uniform glue line pressure on, for instance, the belly area of a belly bridge. Most guitar sound-holes seem to accommodate an extra clamp on the center line if the sequence of clamp placement is carefully considered.

On vacuum clamping, maximum clamping pressure for a 1 x 6 rectangular bridge is only going to be a maximum (at sea level) of 80 pounds, and usually less without a pump that can pull at least 27" Hg. Most vacuum pumps give differential pressure in inches mercury or pascals, so the maximum clamping pressure will be PSID/29.92" x 14.7 psi = clamping pressure (at sea level at 59 degrees F and 29.92" Hg standard day temperature and pressure). For a pump that will pull 26" Hg, that means a maximum of 12.8 psi, which is so low that the joint must be perfect, and perfectly closed if an uneven glue line thickness and a weaker joint is to be avoided.

For new construction, the bridge patch and bridge usually mates well and 12-13 PSI glue line pressure is enough. For repair work on instruments where the bridge patch is not flat, we do what we can to flatten things prior to bridge re-installation, and shoot for closer to 100-120 psi average glue line pressure, which is well in excess of what vacuum can achieve outside of placing the instrument in a pressure chamber such as a large autoclave.

Like Mr. Breakstone, we ensure a level clamp at glue-up that is not resting on or loading other internal structure, and we hang the guitar after bridge glue-up to avoid what can be an unbalanced load from the clamps on the bridge area due to the weight of the clamps. Aluminum clamps are fairly light, but the total clamp weight - almost all of it forward of the bridge pins - can load a lightly built instrument if left horizontal and clamps not shimmed to transfer the load generated by that unbalanced weight directly to the rim.

_________________
For the times they are a changin'

- Bob Dylan

I may not count given my low production quantity, but I've had good luck with rub joints. Even this steel string harp guitar with pointy shaped bridge has stayed stuck down just fine for 4 years now. And I don't do any humidity control other than what the air conditioner incidentally does when I have it on during the hottest part of summer, so they suffer the full swing from muggy spring rain with the windows open, to bone dry when the outdoor temperature drops below 0 in the winter.

It does take some fiddling with sandpaper and scraper to get the fit just right. I also scrape the underside slightly concave to compensate for the moisture expansion that Trevor mentioned.

IIRC, I seem to remember a post suggesting wetting the top of the bridge (or fingerboard) when gluing can help with potential cupping, and I've been using this tip.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.