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This is my process for gluing on the back, I can't reheat that as the rubber would probably insulate it and if I did apply enough heat it might snap. But I have thought about coming up with a new way. I've been doing this for 25 years though so it's a hard habit to break.

I'm kind of a hhg rookie, but I have been leaving braces clamped for about an hour (work in 2 hours) and I left my 12 st bridge clamped overnight and strung it up in the morning and it was fine.

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Pat

I only have 164g strength that StewMac sells at the moment. I'm assuming that's fine for everything else since its the only strength that StewMac sells.

Also how much does the open time reduce using the 315g stuff? I've gotten a bit better/quicker with it after gluing up several tops/backs by using a heat lamp and pre-heated parts but the x-brace still rattles my nerves even though I know I can wet, re-heat the braces and increase clamping pressure if need be. I don't think I'd be too comfortable re-heating a bridge and applying the same process on a finished guitar.

315g has crazy fast gel time. I've just been using some for regluing bridges with a vacuum clamp. It's been more of an experiment—successful, but unnecessary. The 162g has held great on the guitars I've built.

For 315g I microwave the bridge for 8-10 seconds to preheat, then apply glue and have it vacuum clamped within 20 seconds of starting to apply glue. Squirt, brush, flip it onto the guitar (aligning it with StewMac Acoustic Bridge Locating Pins) and press the vacuum frame into place.

http://www.stewmac.com/Luthier_Tools/Types_of_Tools/Clamps/Acoustic_Bridge_Locating_Pins_Package_of_6.html?utm_source=google&utm_medium=shopping&utm_campaign=2018-03-gp&gclid=EAIaIQobChMIoKr6kPaA2gIVh2x-Ch1q8AbDEAQYBCABEgKasvD_BwE

It's under vacuum for 10-15 minutes, then remove vacuum clamp to cleanup glue squeeze out. Then back under vacuum for 45 minutes to an hour. Remove the clamp and let it sit overnight into he next day. A minimum 18 hours.
The two so far this week look good, but I'll be going back to 162 or 192g.

This is just one way of doing it—there are many other successful approaches.

The principal advantage to 315 that we see is reduced time to loading (a few hours), but the gel time can be an issue without some additional heat. If the bridge is gently heated to 160-170 deg F or so, it will act as a heat reservoir to remelt the glue on the bridge patch nd slow down the gel process. We apply two coats to the bare, cool bridge patch, brush glue on the bridge, and set in place. By the time the clamps go on (15 seconds or so), all of the glue on the patch is liquid, as it will be for 5-6 minutes as the bridge cools.

A Salton plate warmer - found on Ebay for between $10 (used) and $80 (as new) does the job to heat parts for assembly. The one here in the shop is about 20" long by 10" (H-125 Fiesta model), and will handle bracing, bridges, fretboards, etc.

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

Woodie - are you using 315g HHG for work other than gluing the bridge? Or only for that one operation?

I've been looking at getting a vacuum clamp system but more for holding guitars while sanding etc. Pricey but they look handy and I already have a long expensive wish list as it is. That's an interesting setup though and it seems pretty simple and straight forward. How do you like it vs. clamps/cauls etc and what other clamping jobs do you use it for?

Bridges only - 192g works well elsewhere.

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

Joey,

I'm just getting started with vacuum clamping, but so far I'm very impressed. Very quick and easy - shorter clamp time, easy to work around the bridge to deal with squeeze out. I was fortunate to pick both of these pieces up used, but you could reasonably make you're own vacuum clamp if the new LMII is not an option.

DIY Bridge Vaccuum Clamp
http://acousticguitarconstructionforum.com/viewtopic.php?f=7&t=817

Almost any old diaphragm airbrush pump can pull enough vacuum for this work. You just flip the connection so your hose goes to the inlet, and your exhaust/muffler on the outlet. Keep in mind its nice to have a well regarded brand as you'll be running it full throttle for an hour or more.

For hot hide gluing a bridge it makes the work very, very easy. Hesh and Dave have another great method with the more traditional caul and sound hole c-clamp that they setup before gluing. Both directions work great.

I too plan to setup a vacuum fixture for holding the body when scraping binding. I'll likely machine something down at the shop, and will try to post about the process when I get to it.

Happy to answer any questions, or help advise on a setup if you decide to go that route.

There seems to be some debate over the clamping Force of a vacuum clamp on the bridge being enough. Something like, if you do the math on recommended clamping Force the vacuum clamp Falls Way short. But a lot of people seem to be using it with success, so there you have it

That's why I've been a little hesitant to use it. On a sale string anyways...

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Pat

My vacuum gauge reads 25 in of Hg when clamping, which is approximately 12.25 PSI*. On a 6" x 1" pyramid bridge thats 73.5 lbs. I guess it could be more as the rubber membrane bridges around the perimeter of the bridge, resulting in slightly more sq. in. of vacuum pressure.

Empirically this works, but I don't have hard data to back it up. I will say a really good fit to the radius of the top is key for any bridge gluing venture. Careful scraping to make it fit perfectly—We are not deforming any wood to fit the curve.

Do you have PSI specs you've read for adequate hide glue clamping? PSI for bridge clamping force?

Thanks Daniel, I'll keep that in mind. I've been eyeing the Festool and the LMI versions. I've only ever put hands on the Festool at Woodcraft and it's really nice. Pricey- but really nice. Checking out other brands they all seem pretty pricey. A diaphram pump is an interesting suggestion. There a bunch of them on Ebay but I have no idea what I'm looking at- I'll have to read up unless you can give me some basic specs to look for. Thanks

The assumption we use is that a good vacuum system can provide about 12.5 pounds (force) per square inch of differential pressure (psid - the difference between sea level pressure and local atmospheric pressure) at sea level, with a reduction as quality of the pump decreases or elevation of the user increases. Atmospheric pressure drops by about 1/2 for every 18,000 feet of elevation change, but I don't know of any builders closing boxes on top of Mt. Saint Elias! The more likely case is that builders see moderate decreases in achievable psid as their height above sea level comes into play. In terms of percent of that 12.5 psi sea level clamping pressure that will be seen as elevation increases:

1000' (vic Waddams Grove, IL, 1033'); 99.8%; 14.7 psi, or about 12.5 psi max clamping pressure
2000' (vic Frostburg, MD, 2070'); 93.0%; 13.7 psi, or about 11.6 psi max clamping pressure
3000' (vic Davis, WV, 3100'); 89.6%, so 13.2 psi, or about 11.2 psi max clamping pressure
4000' (vic Kanorado, KS, 3907'); 86.4%, so 12.7 psi, or about 10.8 psi max clamping pressure
5000' (vic Fort Davis, TX, 4900'); 83.2%, so 12.2 psi, or about 10.4 psi max clamping pressure
6000' (vic Stanley, ID, 6253'); 80.1%, so 11.8 psi, or about 10.0 psi max clamping pressure

So total vacuum clamping pressure on a typical 7.5 square inch belly bridge patch will vary between 87 pounds and 75 pounds for shops between sea level and 6000' elevation. If the joint is perfect, very little clamping pressure is needed, provided the glue line is thinned before the glue gels. If the joint is less than perfect (e.g., most repairs), clamping pressure needs to be increased - for average joints, 150 psi or more (and very effective cauls) might be needed to close and thin a Titebond glue line.

Pressure applied by the usual clamps seen for bridge work is much higher, with medium sized cam clamps providing about 300 pounds of consistent clamping pressure per clamp. For our glue-ups using two Ibex and three cam clamps, total available clamping pressure is over 1800 pounds, or about 240 psi over an average belly bridge. By comparison, the 100 or so 24" x 3/16" go-bars used in closing a box provide about 700-900 pounds of clamping force, depending on the deflection of the go-bar and the bar's composition.

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

Brad the reason I don/t use fish glue for my blocks , is the issue of humidity, I/ve run my own tests with humidity on samples of wood glued with fish. They did come apart with excessive humidity , I prefer to use the security of regular titebond which also is more forgiving of mistakes , and better at gap filling , I also use reg titebond for jointing tops and backs particularly for backs and tops of my ukes. Experiment and see what works best for you
!!!!

I have recently had an issue using hhg to join the fb to the neck. Carefully scraped and warmed both pieces, applied the glue, clamped quickly with many c clamps and cauls. I let it set over 48 hrs in the clamps. When I turned the truss rod the top 3" popped loose. I heated and removed the fb. I then deepened the truss rod channel 1/8" and glued a wood strip over the tr with tight bond. Same process to re-glue the fb. After several days I tried tightening the tr to flatten a slight hump and again I had the fb release, although in a very small area near the nut. I was very careful with glue/water mixture and temperature. A bit frustrated right now. The fb has remained secure everywhere else. I'm hesitant to try heating the small gap area for fear of breaking the bond near it. The gap is about a sheet of paper thick, 1/2" deep, and 1 1/4" long. I'm thinking of trying to push some hhg into the gap. Any advice or critique appreciated.

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Brad Hall
Mystic Dawg Guitars

How many C clamps are you using? I can't imagine being able to tighten a bunch of C clamps in time before the glue gells.

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Pat

I don't have any specific numbers, but 12 lb PSI just doesn't seem like very much on such a critical joint. I imagine a perfectly fitted joint is a must.

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Pat

Pat, I agree, when I do the math 12.5 PSI really low. BUT...
Rubbed HHG joints hold as well as any clamped joints I've glued. We have strong evidence of vacuum frames working for brace gluing on tops, and they are working with the same PSI/In of Hg.

In order of priority for a HHG joint I would suggested: (1) clamping before gel, (2) quality of fit (clean wood, total surface contact between pieces) and lastly (3) total clamping pressure. I have no hard numbers to back this up, but I would guess more HHG joints fail from a cold or poorly fit joint rather than low clamping pressure.

I would love to find some data about how much clamping pressure is needed, minimal pressure for full strength joint, for both HHG and Titebond.

But this one goes to 11...

Just an add about vacuum clamping- there's a LOT of pressure at 25" hg, so much that I can dent a bridge with the air fitting on my jig.

Also, its not just about pressure, its about vacuum. The air is dry, so th we hhg dries really fast in a vacuum. That's saying a lot as hhg dries fast already.

Vacuum clamping for braces is used at Taylor guitar and has been for many years. This is empirical evidence that shows it provides sufficient clamping force.

Hide glue works great for rub joints if the pieces match perfectly. Put a 75 pound weight on top of a bridge and tell me if you think it's pressing down enough to hold the parts together. Of course, it is !!!

In short, with either Titebond or Hide glue, vacuum clamping provides sufficient clamping force. There is absolutely no reason to doubt that statement. However, do make sure the fit is good - and this really goes for anything you do. Poorly mated parts are bad for tone.

It definitely has enough force to glue parts together. Although, the stresses that braces see is nowhere near those that a bridge sees. Is Taylor using vacuum clamping on its bridges?

My thinking has been like this: it's common for Bridges to pop off of guitars. When it happens within the first few months you can just blame it on a very bad joint. But when it happens 20 years down the road, barring some force other than string tension, the joint seems gradually compromised. So, without any long-term track record it's difficult to have much confidence in vacuum for that application.

Does anyone know when vacuum clamping Bridges became somewhat common? Are there many 20-plus year examples out there? I'm only speaking in regards to Steel strings here.

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Pat

Please don't take my comments concerning achievable clamping pressure with vacuum as a condemnation of the approach. I think we agree that a well-made joint needs little pressure once the glue line is thinned (as is done with a rubbed joint or during joint closure with a clamping system).

That said, better understanding of just how much or how little pressure will be applied by a vacuum system would seem to be desirable from the viewpoint of how best to leverage strengths (i.e., reduced drying time; very uniform glue line pressure) and offset limitations (i.e., low glue line clamping pressure re: other methods). Particularly for repairs, where bridge patches are seldom flat and prior repairs can make for some interesting challenges, knowledge of those limitations may help inform the repair person's approach to the task....as it does for us.

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

We should start a whole other thread on bridge failures... Lots of different ideas, I'm sure.

25" Hg is 12.3 psi...25" Hg sounds higher, but the applied pressure is the same, which is what does the work of clamping. Darn physics...no consideration of our feelings at all.

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

Oddly enough, the variable for me to switch to HHG is the vacuum clamp. I switched to vacuum first, then I researched HHG, and couldn't justify NOT using HHG, especially with vacuum clamping. Again, the premise for me was not the clamp, but the vacuum. Come to find out, HHG dries by evaporation, which is right in the wheelhouse of what a vacuum does exactly - extract moisture.

So I figured use a glue that is effective with a rubbed joint, then add a vacuum to the process -
it sucks the glue between the joint (for good coverage)
sucks the excess out (to clean up quickly)
provides pressure all around (so the bridge doesn't move)
sucks out moisture - quicker dry time

And yes, I have a jig for bracing, tops and backs - can anybody say "no go bar"?

Admission of guilt - I just wanted to build something else, and the "science project" aspect of it intrigued me, especially the venturi part. Not sure I'd be as game if I had an actual vacuum pump system. I think its cool to build tools for building, ie: cam clamps (currently redoing mine) - gonna glue those up with HHG!