Official Luthiers Forum!

Personally a fan of 5 of these. http://www.stewmac.com/Luthier_Tools/Ty ... lamps.html

If you apply HHG to the underside of a bridge it will move, more so in ebony than rosewood but both will move. That's why I glue size with thin HHG, allow it to dry and then check the underside of the bridge. With a little straight edge I virtually always find that the bridge needs to be scraped again and resized. I do it until the bridge stops moving, which is two or three times. Wetting the outer side of the bridge certainly helps a little but may not entirely eliminate it. Try the straight edge test on your next bridge, across it's width.

Actually, with the diminishing quality of available Ebony, there is a very real chance of slab cut Ebony and I have in fact encountered that problem on more than a few occasions IRL. HHG, with its excess water (in comparison to Titebond), can indeed immediately begin to warp slab Ebony, and it can be very difficult to tell orientation by looking at it. I also make my HHG a fairly thin blend as it seems to hold heat longer, someven more water.

I use HHG over Titebond simply for future serviceability. Well, and I like that HHG is a fixed target overnight when measuring the monopoles, whereas Titebond is a moving target for several days.

And Hesh, speaking of smoking the drapes...calm down, man:)

^^^^All my money goes there Ed. ^^^^^

It's not staying hot longer, it's gelling at a lower temperature.

There could be other things contributing but thinner hide glue is trouble.

My experience has been the opposite, the thicker the glue the faster the gel time. But I really don't know much more about it than what one reads on the web...

I'm putting my money on uneven clamping pressure, judging by the glue pattern on the top and the bridge.

My experience has been that it is easy to starve the joint with thin HHG

I'm with Trevor in the bridge curling: I've seen it. Fingerboards too. I don't scrape the bottom of the bridge flat, but hollow it just a little (a little!) from front to back for this reason.

The stress at the back edge of the bridge depends a lot on the length of the glue line along the line of pull; in other words, the width of the bridge. Increasing that length a little cuts the max stress at the edge a lot, and that's what the 'belly' bridge does. Once the bridge starts to peel the stress at the edge becomes even higher, and the failure moves faster, so the trick is to not let it get started. That's why it's so important that the edges be down tight, and why a little bit of swelling caused by water uptake can be such a problem.

Mario Proulx pointed out once that Martin used to 'tooth' the bottom of their bridges. This was supposed to strengthen the joint, but actually makes it weaker, so it contributed the problems they had with bridges lifting when they went to steel strings. The narrow bridges work fine if the surfaces are smooth, fresh, and fit properly.

Which brings up the subject of 'surface energy'. When you work a surface and remove material, you're actually breaking chemical bonds. Those open bonds create high 'surface energy'. It takes a while for these to find something to link onto, and if you hit them with glue they'll link to that. The Forest Product Labs found back in WW II that if you glue within 15 minutes of working the surface the bond is stronger.

The test for surface energy is to spritz the surface with a mist of water, which has polar molecules. If the water beads up the surface energy is low. If you use a microscope you can actually measure the angle between the drop and the surface at the edge and put a number on it. If the water spreads out in a film, the surface energy is high.

I once made a guitar with a fossil mammoth ivory bridge. The literature that came with the blank said that the stuff was hard to glue. It occurred to me that with material that old most of the chemistry had already happened, and the surface energy might be low, even on a pretty fresh surface. I tried the spritz and the water beaded right up. Scraping the surface got it to spread out, but not for long; if I dried it and came back later for a re-run it would bead up. So when it came time to glue the bridge down I had the clamps all set, got everything warm, gave the bridge bottom one last light scrape with a very sharp scraper, slapped on the hide glue, and got it down in a hurry. No problems.

Heating with a hot air gun helps by drying the area to counteract swelling from glue application, as well as giving longer time before gel

I've always been very wary of of heating the parts, fearing local changes in RH as well as slight shrinkage, but maybe I'm overthing it. But it only takes ~10-20 seconds to get the clamps on anyway so I haven't worried bout it.

Given that we haven't had a chorus of unanimous answers from the pic, it'll have to fall in the 'could've been' category. And again the smooth edge at the back makes me think it just wasn't clamped right.

It's back in place making noise again, and if it lifts again I'll know about it as the owner is fairly local.

Thanks

I think your misunderstanding what I'm trying to say.

Watery glue has a longer open time not because it is staying hot but because it gels at a much lower temperature. If your adding enough water to give a noticeably longer open time than the full strength mix, it'll be weaker.

On violin family instruments, hide is thinned to deliberately make it weaker for some joints. It's very effective.

What's your glue an water ratio?

I've never measured. I just add water til it behaves like I want it to...

When I see a hide glue joint failure like that my first thought goes to technique - was it FULLY clamped before it cooled off to the gel state? That's a really big deal, and in my experience, the biggest problem in using the stuff. . .

_________________
Cheers,

Frank Ford

FRETS.COM
HomeShopTech
FRETS.NET

Great thread! I usually warm my bridge on a heating blanket at around 150 degrees via surface thermometer and warm the top with a hair dryer. Get it clamped up fast. Glue around 145 and about like honey.

The cupping with moisture is something I was not aware of although I've used BZ bridges almost exclusively for a long time and they are usually pretty well quartered.

It was interesting in the Goodall video posted recently to hear him extoll the greatness of Tightbond for bridges echoed by Trevor in this thread. Hummm-

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

Well, I'd like to think so, but...

Ed--

I had some bridges lift after using hide glue, and in order to keep it from happening again, I took a hard look at all aspects of the process and increased my awareness of everything I could, did everything a bit better, and I don't have problems any more. I guess my point is that you can drive yourself nuts trying to pin down the one thing that allowed this to happen, and you will probably never know for sure. But you can do everything in the future with an eye toward preventing this problem, and probably solve it.

For instance, you should fit the bridge and the footprint really well. You should use a thicker mix of glue. You should heat the parts. You should work fast. You should watch out for cupping of the bridge (this is a materials selection issue for new guitars). You should use multiple clamps. You should figure out a way for the weight of the clamps to not distort the clamping forces. If you watch out for all those things, I bet you won't see this problem any more. And yet you will remain in the dark about which specific thing (or combination of things) caused those failures in the past.

I'll add one more precaution, Don. Make dead sure that all traces of finish have been removed from the footprint. This can be more difficult than it sounds. An extremely thin trace of lacquer, for example, can be hard to see or feel. In the few instances where I've had lifting using Ebony and HHG, the problem was traces of finish.

_________________
It will probably be alright.

. I think you've had enough already.


Still in denial! Plenty of respected people have seen this. If the bridge curls and you don't spot it, no amount of the type of clamping likely to be pre-set for a HH bridge glue-up is going to give you conformity. If you do spot it, all you can do is abort and do something similar to what Michael does.

Ed's done way too many bridge glues, with too much success, to be caught out by the basic stuff. Sure, we can all improve our processes, but this failure has all the trademarks of a joint not properly closed, most likely because it was un-closeable because the bridge curled.

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

http://www.goreguitars.com.au

Probably better to understand the difference between tension and stress before you make a comment like that.

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

http://www.goreguitars.com.au

Ed's done way too many bridge glues, with too much success, to be caught out by the basic stuff. Sure, we can all improve our processes, but this failure has all the trademarks of a joint not properly closed, most likely because it was un-closeable because the bridge curled.

I'd like to believe it wasn't me, but I've worked with myself way too long to believe that.

I did reuse the old bridge, so if the problem is indeed slab Ebony, I'll find that out again one day I'm sure. Fortunately the guitar in question belongs to a friend so no big deal if it does.

I am inclined to think that I clamped it wrong, and that the glue may have been too thin, and that gluing a flat bottomed bridge to a domed top isn't the best idea even though it usually works fine and that maybe I'd be better off with the light horseshoe style SM clamps rather than the big F clamp I use now.

I'm putting it into the 'any number of things' pile and moving on.

Thanks everyone, peace out...

In the book (Build, p. 20-8, penultimate paragraph), I talk about a jig I use for shaping the bottom of a bridge. I laminated a bunch of thin ply into a building dish, using a lot of go-bars, so I had a convex surface when flipped. Stick sand paper onto that and you can rapidly get a spherically curved bridge base to match your top. Different radii/shapes to match the appropriate top curves, of course. The lump of wood beneath is so I can hold it in a vise. Only takes a couple of minutes of sanding, so not much impact on your production. Be sure to clean off the sanding dust.

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

http://www.goreguitars.com.au

I used to do something very similar to what Trevor describes above to fit the bridge to the top. But, even though my shop humidity is closely controlled and I glue the braces and bridgeplate in a dish, I found some variation in the top shape when it came time to glue the bridge. I really wanted the surfaces to meet perfectly, so I have been sanding the bridge bottom to the shape of the top on the top. I use a fresh sheet of 120 sandpaper held in the bridge location, mark the bottom of the bridge with crosshatched pencil lines, and sand the bridge against the sandpaper on the top until the pencil lines disappear. I sand in very small circles (a few millimeters in diameter) so that the bridge bottom picks up the local topography of the top at the bridge foot print. I test the result by pulling thin strips of paper between the bridge and the top with the bridge lightly held on position. If the paper doesn’t drag, the bridge doesn’t fit. I thought everyone did it this way. It does take me some time, but I get the fit that I want.

We use a 28' radius dish for the top (with 28' radius braces on all but the 60' upper transverse brace), so making 28' radius convex sanding form is easy - just sand a 10" x 8" piece of 18mm MDF in the dish and mounting PSA 80 grit. The mating surface of the bridge plate is also milled to 28' radius, so top, bridge plate, and bridge all conform to the design top radius.

Despite what might be a lower moisture content for Titebond where a lower solids mix of hot hide is used, we will not usually glue or reglue bridges with thermoplastic glues. The primary issue with Titebond for constantly stressed joints is that even with thin glue lines, these glues are prone to both cold and hot creep. We see enough cold creep on recently made instruments that separation of the lacquer along the front edge of the bridge due to shear forces occurs. Both the SCGC 000 and Martin 000-28EC - owned by conscientious musicians that do not subject their instruments to adverse storage or use conditions - came in for repair required extensive touch-up around the edge of the bridge patch to restore the finish.

_________________
If someone is aggressively selling a technical product whose merits are dependent on complex experimental data, he is likely lying.

- Burt Rutan

Trevor probably better to pull your head out..... and not be the arrogant SOB that you clearly are before you make a comment like that..........

You need to spend some time in the trenches doing copious amounts of repair work and stop breathing your own air. As Rick Turner used to say on this forum you don't know **** until you learn to do some repair work.....

Bob you mean like this: http://www.luthiersforum.com/forum/viewtopic.php?f=10117&t=25513&p=344237&hilit=fitting+a+bridge#p344237

UPDATE: I no longer fit bridges this way because it takes too long and after a while you get pretty fast at fitting them simply using a single edged razor blade and careful observation. Fitting bridges is something that we do nearly every day making for lots of practice.

The other change is the bridge rabbiting that we do with most finishes that have any thickness to them. I may nix the rabbiting on FP shellac or other very thin film finishes where there is a very minimal finish ledge.