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Bridge pulls off!
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Author:  crich [ Mon Oct 22, 2007 1:38 pm ]
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Ok, this was the second time I had to re-glue my bridge with HHG and after 5 days I decided to tune up to pitch when the back side of the bridge rolled off the soundboard with a snap, again. This happened on my last one, I was using a steel deep jaw clamp and figured that maybe the weight of the clamp was distorting the top so I attached two screws to my caul that ran up the outside string holes and used wing nuts. I'm using granular HHG and mix it up fresh and haven't had any trouble with plates,braces,stacked heal or scarfed headstock. But this is probably alot more pressure. The HHG granulars are almost two years old. Could this be the problem? Are my tops too thin and when strings are brought up to pitch the top deflects and pop the glue joint? Man this is de moralizing....clinton
Author:  KenH [ Mon Oct 22, 2007 3:12 pm ]
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Clinton,


I have to ask a question, because I did it also and got the same result. DId you make sure that you were gluing to bare wood and no finish was under the bridge?  The glue wont stick to lacquer.... been there, done that.
Author:  JJ Donohue [ Mon Oct 22, 2007 3:16 pm ]
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Clinton:

Adhesive failures are always difficult to pinpoint in a forum situation. A few things I'd look for in trying to identify the cause:

1) Contaminated surfaces...I always scrape the surfaces prior to applying glue. It removes any wood oxidation and activates the surface. BTW...What wood species is your bridge?

2) Glue gelling before surfaces are joined...I heat up the bridge prior to applying glue and get them clamped up within 30 to 40 seconds.

3) Clamping force...It actually could be too tight (starved joint) or too loose. What kind of squeezeout are you getting? 2 wing nuts might not be applying uniform clamping force. I use 3 Klemmsia clamps and a curved caul on the BP...I believe that might produce more force than you are using.

3) Bad glue...Are you using 192 gram strength HHG? Are you storing your glue granules in a cool dry place in a sealed container? How old is your mixed glue solution? What viscosity...too thick or too thin?

I have not experienced a failed bridge using HHG on 8 guitars so I can't really be sure what is causing your failure. I use fresh glue on clean and warm surfaces and clamp as soon as possible while making sure there is a small amount of uniform squeezeout.

One thing to try is to glue up some samples and make sure that the wood tears when trying to destrpy the joint.

Good luck and hang in there...HHG is definitely the way to go so don't lose faith.

Author:  David Collins [ Tue Oct 23, 2007 12:29 am ]
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The strange thing here is that with hide glue you should be able to force
a non-radiused bridge on to a contaminated, poorly prepared surface,
and it should still hold well for at least a few years. It sounds to me like
the glue is gelling before you're getting it clamped. make sure surfaces
are warmed, and try to have everything prepared so that you can be
fully[/] clamped within 30 seconds.

JJ mentioned gram strength - a low gram strength should certainly hold
for a while if glued properly, but a higher gel strength could make the gel
speed too fast. Then there's the mix, temperature and viscosity. Too cold
or too thick and it will gel to fast, too hot it will denature the collagens
and weaken the bonding (though would still hold better than yours had).
Too thin and there won't be enough glue in the water. What's the viscosity
when it's up to 140-145 F? Molasses, Honey, maple Syrup, motor oil,
milk? And what glue is it? The more details on the materials and process,
the better.
Author:  crich [ Tue Oct 23, 2007 5:49 am ]
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I got the HHG from someone here on the forum and I remember they said it was 192 g. I'm scraping off French Polish and using an ebony bridge. I timed myself and had the two wing nuts tightnen in 20 seconds then I tighten the two set screws on the wings which takes another 10 seconds. I heated the bridge, a little bit, on a hot plate. As far as viscosity my wife says it'more like maple syrup. BTW, that's a pretty cool anology. I do store the granulars in a sealed bag sealed in another zip lock bag in a dry cupboard. Maybe the purchase just wasn't quite right? Because I did notice there was no wood fiber pulled. Clinton
Author:  grumpy [ Tue Oct 23, 2007 6:50 am ]
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Maple syrup is too runny. try for hot glue that runs about as thick as oom temperature Titebond does. Really...

And NO hot plate. It's far too easy for there to be contaminants on it. Use a hair dryer on bot the top and bridge, and scrape the surfaces with a fresh razor blade just prior to warming and gluing.
Author:  JJ Donohue [ Tue Oct 23, 2007 7:04 am ]
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Clinton...I'd still recommend making some trials and be CERTAIN that you have good hide glue in the first place. As a matter of fact, since you are unsure about the origin of your current stuff, you might just want to order a fresh supply. I order mine from LMI.

You're learning...just like all of us did...and still are.
Author:  crich [ Tue Oct 23, 2007 9:58 am ]
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I will try it on a scrap piece. What worries me is that I have glued up two guitars,ie: plates,braces,heals and the only failure ,SO FAR, is the bridge. Hopefully that will be the extent of problems. Clinton
Author:  Mattia Valente [ Tue Oct 23, 2007 11:54 am ]
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Silly question perhaps, but: you are using a matching caul inside the body, right?
Author:  McCollum [ Tue Oct 23, 2007 12:40 pm ]
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How smooth is the bottom of the bridge?
You should cross hatch it!
Lance
Author:  crich [ Tue Oct 23, 2007 1:44 pm ]
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I never heard of anyone cross hatching a bridge? I could try that on a piece of scrap. I'm using a caul that I radiused in my dish. I didn't use a clamp but two bolts coming up from the first and sixth string holes then a fitted caul on top of the bridge and wing nuts. I then fitted screws in inserts over the wings of the bridge to hold them down. I really was impressed with myself...? Clinton
Author:  Steve Walden [ Tue Oct 23, 2007 1:50 pm ]
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Hesh said: "I switched the large clamp that I use in the sound hole for another one. "


Is that the one you just sold me?....


Consider that the cross-hatching of the bottom of the bridge increases the surface area to be glued.  The HHG would have more grip on it then?  I am no expert (former drip under pressure) but my "engineering judgement" has come into play.
Author:  McCollum [ Tue Oct 23, 2007 2:41 pm ]
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Well, I cross hatch all of my bridges, all of my friends do, and the only bridges I've ever seen come loose in my years of repair are those where the bottoms were smooth. The cross hatching doesn't need to be deep, just scored, for the glue to get a grip on.

Lance

www.mccollumguitars.com
Author:  grumpy [ Tue Oct 23, 2007 2:51 pm ]
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It's been shown many times that PVA and hide glues have better bonds on freshly planed/scraped surfaces. No need for any 'grip'.
Author:  David Collins [ Tue Oct 23, 2007 3:00 pm ]
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Cross hatching is what used to be done long ago at factories like Martin,
and was really based on a misunderstanding of how adhesives like hide
glue really work. Glues like hide glue or pva work primarily by specific
adhesion, and not mechanical adhesion. Cross hatching is designed to
benefit glues with good mechanical adhesion and less specific adhesion.
It is a good thing to do with adhesives like epoxy, but they will definitely
weaken a hide glue joint.

Fifty years ago this was common practice, and it is a tradition that has
been passed on through generations. Knowing more about the chemistry
of adhesives today it is much more clear that hide glue does not benefit
from a scored surface to "get a grip" in to. To the contrary, however small
those scores are they still amount to a series of miniscule gaps, where
hide glue actually has very little strength. Direct surface to surface
contact provides the best surface for the hydrogen bonds that the amino
acids create between tightly matched surfaces. Freshly scraped surfaces
are the best surfaces for most wood glues, and certainly for hide glue.
Author:  McCollum [ Tue Oct 23, 2007 3:38 pm ]
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I'm not going to argue that point, but I've built over 300 hundred guitars and been reparing for a long time and I know what works for me!
Lance
Author:  Blain [ Tue Oct 23, 2007 3:51 pm ]
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You might check to make sure you're heating the glue hot enough.

After my first attempt at gluing a bridge using HHG I noticed that it was starting to pull up so I immediately took the strings off and removed the bridge. I looked Hesh's tutorial over and over and finally realized that I wasn't heating the glue hot enough. My thermometer was in the water and not in the glue so my glue temp was actually a lot cooler than what I thought. I then stuck a thermometer in the glue bottle and one in the water and then properly heated the glue to 140ish and reapplied the bridge. It's been holding strong since.

If you made the same mistake I did, I could see the HHG working on other things that don't have as much force pulling on them. But for the bridge, it must be accurate.

Just my thoughts...
Author:  Mattia Valente [ Tue Oct 23, 2007 6:37 pm ]
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What David and Grumpy said.
Author:  JJ Donohue [ Wed Oct 24, 2007 12:55 am ]
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Good info, Todd. Your "CV" as well as your woodworking skills speak volumes and I always appreciate your backup when you make your well-conceived points.

BTW...I'm curious...what's your rationale for using 315g HHG on bridges...and how are you getting 3-4 minute open times? Also, what's the viscosity like for this stuff?

TIA
Author:  crich [ Wed Oct 24, 2007 1:31 am ]
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You guys made this a great thread!
Todd, I also would like to know your viscosity and your thinking on 315g HHG. I purchased mine from someone here on the OLF so I'm sure it's the "norm" for guitar making. I use 220 grit sandpaper on ebony. Actually I get it close then I lay sandpaper on my soundboard and without distorting or pushing down on the soundboard I sand the bridge to fit. I mix my HHG by wieght and it's usually 50%/50%. However it seems a bit thin according to from what I'm reading here. Clinton
Author:  Pwoolson [ Wed Oct 24, 2007 4:59 am ]
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If I had to guess, I'd say you're doing it right with maybe the exception of not getting all of the FP off of the top. When the bridge pulled, did it leave your HHG on the surface of the bridge or on the top of the guitar. If on the bridge, your top wasn't prepared well.
Author:  Alan Carruth [ Wed Oct 24, 2007 6:18 am ]
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Todd:
Many thanks. I had read about surface energy in an Experimental Aircraft Association magazine a few years ago, and had, perhaps, been discounting the importance of 'keying' a bit too much. I generally use a freshly scraped bridge surface, and have had only one bridge come up; it was definitely a cold joint, though.

To explore another aspect: somebody mentioned wood type. I know some people feel that some 'oily' woods need a solvent wipe to get proper glue bond strength. I don't wipe down the glue surface, as I've heard conflicting reports. Some people say it just brings oils to the surface, and spreads them around, so that you end up with more problems. Any comments anybody?
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