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Anyone know the stats? Wondering if it has the right amount of pressure for TB...

Robbie explains it here.

https://youtu.be/9FTG5p2dsBA

M

1 bar or 14.73 lbs/sq.inch for a perfect vacuum.

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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.

Oh wow! So about 1/10 of the psi required for titebond...

I figured that's what was coming next...

Vacuum clamping, 15psi or close to it is your max. at sea level.
13psi is prob more typical. Bridge clamps or C clamps typical psi
is in the range of 200+ psi. Is 13psi enough for bridge clamping? I really don't know.
The only time I've had a bridge peel up at the back was when I used the vac. clamp. Once with
hide glue and once with Lepages. Can't say without a doubt that vacuum clamping was the culprit.
I went back to using the old Fox bridge clamps with narry a peel up.
Wondering... who here using vacuum clamping for bridges highly recommends it?
Btw. the hide glue in question was 192gm to be fair.
While 13psi is no doubt fine for veneering and such, I like a bit more oomph on my bridge clamps.
Sorry, I didn't really answer your question, just my experience.

The 228 lbsin the video is for the area of the fixture, which is irrelevant. The actual pressure on the bridge would be 14.73 lbs x about 6 for a classical bridge or 88lbs.

I've used vacuum for about the last 20 guitars, with HHG. The bridges are well-fitted to the dome and slightly concave along the length. Nary a problem.

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formerly known around here as burbank
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http://www.patfosterguitars.com

Pat, wondering.. classical or steel string? and gm strength of your hide glue?

I've used TB and an LMii vacuum clamp for the last 9yrs with only one failure (50+ instruments) which was due to contamination of the guitar top during buffing.

Last weekend I got to visit a guitar I built around 8 years ago, and it's doing just great! The guitar is gigged weekly, and spends a lot of time in the studio as well. The finish is absolutely trashed (EM6000) and will be coming in to have its neck refinished as soon as he can spare it. The thing sounds great!

A good fit it essential for sure! The vacuum clamp will not pull down a poorly fitting part. The literature on the TB site says use 'Enough clamping pressure to bring joints tightly together (generally, 100-150 psi for softwoods'. If it's a tight fit already do you still need 150 psi?

M

I have glued Classical bridges with HHG and a rubbed joint (no clamps) and they held fine. Some videos show traditional methods using rope and wood cauls after bracing the soundboard internally. I have been thinking about using a Home Depot ratchet strap to do something similar. I have also been thinking about gluing the bridge to finish as some factories do and wondering what the consequences would be.

If you put a caul spanning the surface of the membrane of the vacuum clamp and between it and the bridge could you transfer more pressure to the bridge?

One of the differences about vacuum clamping is that the pressure is evenly distributed over the item being clamped.
You should also make sure to use a breather fabric between the the vacuum membrane and piece to be clamped. The breather should extend over to the inlet. This insures that the vacuum membrane doesn't seal itself off somewhere thus reducing the the pressure.

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Jim Watts
http://jameswattsguitars.com

That's a great idea! Shoulda thought of it...

I‘be only glued one with vacuum, but it’s holding up well after three years. Tightbond original. Well-fitted joint.

Sent from my iPhone using Tapatalk

If there's much of a a gap (more than the top can flex) between the caul and the top, chances are high you'll break your top depending on the amount of overhang.

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Jim Watts
http://jameswattsguitars.com

That's what I was thinking WRT to using a caul to expand the clamp area. No free lunch here either.

Ken, I do the same on steel strings and classicals, with 192 g HHG.

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formerly known around here as burbank
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http://www.patfosterguitars.com

Testing required for sure

I got this from a Titebond data sheet, or one of their techs, can't remember which...The 150 psi is based on making irregular surfaces conform, where the idea of irregular was stuff like the ripples from machine planers - typical industry stuff. So 150 PSI was enough to make the high spots yield and bring both surfaces into 100% contact. If you have more conforming surfaces, you need a lot less pressure to bring them together, but you need enough pressure to make the glue squeeze out and thin out the glue line. The same source said something like is was impossible to starve a Titebond joint provided it had initially been properly wetted and not re-opened. So the trick to using Titebond (and some other water based glues) is to ensure both surfaces are wetted with glue, the surfaces conform well and sufficient pressure is applied quickly so that the glue line can be thinned out before water is absorbed into the wood and the glue is too viscous to flow. If you can do that with a vacuum clamp, that's fine. I have vacuum capability but still use screw/cam clamps on nearly everything, including bridges, because I can better thin the glue line that way and a thin glue line is a strong one.

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

http://www.goreguitars.com.au

Just throwing this out

I recently made a wall cabinet with rounded staved doors and veneered them. I chose a hand powered vacuum pump and it was a great success. If you have half a mind (which I am sure you do have) look at this slide and the next half dozen or so - there is text below

https://www.flickr.com/photos/ruby1638/47721099272/in/album-72157706524329895/

Here is a guitar tutorial on the company's website

https://www.roarockit.com/woodworking-tutorials-categories

On my second try I got the pump to hold pressure for the 45 minutes that they recommended

Ed

Here's a vid more about the venturi press itself, but zip forward to about 2 minutes in and you'll see the cheap ass clamping jig and the vacuum press in action, similar to Robbie's vid linked above.

My opinion, vacuum pressing and HHG go hand in hand. I've used it with TB and it works well. I've switched to HHG, and use 192 for the braces on the top and back (in another jig of course), and 315 for the bridge.

Besides the vacuum action with the glues (yes, even TB, and epoxy), what I like about the vacuum press is it provides complete and even clamping pressure as Jim mentioned above.

And its so fast. . .