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Vacuum clamping-good or not?
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Author:  Steve Saville [ Wed Jan 31, 2007 10:02 am ]
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[QUOTE=Howard Klepper] If clamping force is critical, then I would think that vacuum is not the way to go. With a good pump, you are only going to get about 11 psi. You can get several multiples of that with mechanical clamping. By contrast, Franklin recommends in the range of 150 psi for Titebond, iirc.[/QUOTE]
(I thought I would bring this subject up with a recent quote from Howard Klepper.)

Vacuum clamping systems are getting a lot of attention.
With the best vacuum system, you'll get close to 14.7 psi. That is less than 1/10 the recommended clamping force for many adhesives.
Based on that, vacuum clamping systems seem like a bad idea for building guitars. I know that many of you use these systems and are obviously happy with them.
How do you rationalize using very low clamping forces for critical joints?
Author:  Don Williams [ Wed Jan 31, 2007 10:26 am ]
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Dude, take a 14 pound weight and set it on your thumb and see if you think that's enough to clamp a brace to a top or back...now imagine that same force on every square inch of the brace....literally.

You get the idea. It's plenty.....and it works.

Don Williams39113.7706018519
Author:  Shane Neifer [ Wed Jan 31, 2007 10:30 am ]
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I guess another test would be to try and pull the brace away from the top while the vacuum is on....it sticks pretty good! Might be taht 14 psi distributed over the ENTIRE piece makes up for the point loads of clamps. Just a thought, I have nothing to back this up!

Shane
Author:  SimonF [ Wed Jan 31, 2007 10:42 am ]
Post subject: 
Hi Steve,
I wondered the same thing myself. I think the truth is that in lutherie you
make parts that should have perfect joints. In this situation, all you really
need is just something to hold the joints together and apply enough
pressure to squeeze out the unnecessary glue.

I can get up to 22 psi with LMII's Venturi vacuum pump with my
soundboard clamping fixture. And even more than that on my bridge
clamping jig. Also what Shane and Don said is true, you get this pressure
applied every square inch and not just in a localized area.

Correction: As Steve points out below, you can't have more than around
14.7 psi or 1 ATM - but you can have more than 14.7 mmHG - maybe
I should look at my gauges a bit more carefully.

And a lot of the brilliant and pioneering luthiers like Fox, Ryan, Olson,
and Bob Taylor use vacuum clamping extensively. To me that is more
than enought reason to have peace of mind in vacuum clamping's
effectiveness. It also makes bracing the top and back a breeze.

Peace Out,
Simon SimonF39114.0040856482
Author:  Don Williams [ Wed Jan 31, 2007 10:44 am ]
Post subject: 
Shane that's part of it. It's not the downward pressure that makes vacuum clamping so good. Literally, the forces act in all directions on the brace. That means that even the sides of the braces have atmospheric pressure on them. It's on everything in there...it's not the rubber membrane that is clamping it is literally the atmosphere. It's a hard concept to grasp, but it works.

Don Williams39113.7813888889
Author:  Steve Saville [ Wed Jan 31, 2007 11:20 am ]
Post subject: 
[QUOTE=SimonF] In this situation, all you really
need is just something to hold the joints together and apply enough pressure to squeeze out the unnecessary glue. [/QUOTE]
That's my impression too. I suspect that the higher clamping force asked for by glue manufacturers is more for imperfections in the joint than anything else. I seems to me that when the adhesive dries, it shrinks and tends to pull parts together.

[QUOTE] I can get up to 22 psi with LMII's Venturi vacuum pump with my soundboard clamping fixture. And even more than that on my bridge clamping jig.[/QUOTE]

The most you can get is 14.7 psi, or one atmosphere. If you are getting more, something is wrong with your gage.

Another good thing about vacuum clamping is that it will dry the adhesive much faster because any water in the adhesive with turn to gas very fast at this low pressure.
Author:  Bill Greene [ Wed Jan 31, 2007 11:25 am ]
Post subject: 
[QUOTE=SteveS] [QUOTE=Howard Klepper] If clamping force is critical, then I would think that vacuum is not the way to go. With a good pump, you are only going to get about 11 psi. You can get several multiples of that with mechanical clamping. By contrast, Franklin recommends in the range of 150 psi for Titebond, iirc.[/QUOTE]
(I thought I would bring this subject up with a recent quote from Howard Klepper.)

Vacuum clamping systems are getting a lot of attention.
With the best vacuum system, you'll get close to 14.7 psi. That is less than 1/10 the recommended clamping force for many adhesives.
Based on that, vacuum clamping systems seem like a bad idea for building guitars. I know that many of you use these systems and are obviously happy with them.
How do you rationalize using very low clamping forces for critical joints?[/QUOTE]

OK, I'm a dope (which I freely admit) but exactly how would one go about getting 150 lbs of clamping force on a 1/2" x 12" brace, of spruce, without crushing, cracking or denting it beyond repair? I'm not being dense, or difficult, but how would you get that kind of pressure on a fingerbrace, or rimset?
Author:  JohnAbercrombie [ Wed Jan 31, 2007 12:12 pm ]
Post subject: 
[QUOTE=Bill Greene]

OK, I'm a dope (which I freely admit) but exactly how would one go about getting 150 lbs of clamping force on a 1/2" x 12" brace, of spruce, without crushing, cracking or denting it beyond repair? I'm not being dense, or difficult, but how would you get that kind of pressure on a fingerbrace, or rimset?
[/QUOTE]
Not a dope at all, Bill!

Most people don't have much idea how much force a single screw clamp can exert- 100 lbs is not unusual at all.

Even a single go-bar stick (not fiberglass) can exert about 8-10 lbs of force, unless you make them too long.

6 square inches of any wood (except balsa, perhaps) can certainly support more than 150 lbs without crushing.

Also, a vacuum system can produce at most 90 lbs of (downward) force on your hypothetical 6 sq in brace.

Admittedly, no joint in a guitar should need this sort of force if it's fitted properly.

It's not clear to me how you would use vacuum to safely glue on a back or soundboard to rims- surely you don't want to apply that 15psi across the entire top/back?

The main drawback (for me) of vacuum clamping systems for gluing is the difficulty in cleaning up squeezed out adhesive. I don't like to remove clamps when a joint is partly cured (pulling the vacuum to remove squeezeout) and then re-clamping. Vacuum systems are ideal for laying up fiberglass boat hulls in molds, applying veneers with epoxy, etc but I don't see the point in a 'hand-building' instrument shop situation (as opposed to a factory).

My vac pump doesn't spend much time away from its home under the bench.

John

Author:  Steve Saville [ Wed Jan 31, 2007 12:13 pm ]
Post subject: 
[QUOTE=Bill Greene]OK, I'm a dope (which I freely admit) but exactly how would one go about getting 150 lbs of clamping force on a 1/2" x 12" brace, of spruce, without crushing, cracking or denting it beyond repair? I'm not being dense, or difficult, but how would you get that kind of pressure on a finger brace, or rimset?
[/QUOTE] I don't think you are a dope....

That 1/2" X 12" inch brace is 6 in?. All you'd need to do is evenly apply a 900 lb force evenly over the brace, or about 75 lb every inch.

That might be a little too much force. Is that makes the point you are trying to make?   

Does anyone know what force your average go bars exert? I'm pretty sure you'll not get that 900 lbs for you 12" brace.
Author:  JohnAbercrombie [ Wed Jan 31, 2007 12:20 pm ]
Post subject: 
[QUOTE=SteveS]
Does anyone know what force your average go bars exert? I'm pretty sure you'll not get that 900 lbs for you 12" brace.
[/QUOTE]
Steve-
Check out the rodbuckle.xls spreadsheet at
http://www.ukuleles.com/Technology/sprdshts.html

I decided to make by go-bar deck 'less tall' after about 2 min playing with the spreadsheet and seeing the effect of bar length on pressure.

Cheers

John
Author:  Frank Ford [ Wed Jan 31, 2007 1:03 pm ]
Post subject: 
Sure enuf, the technical guys will say that 100 lb./sq.in. is appropriate for
hide glue. And, they claim up to 10,000 lb./sq.in. of tensile strength.

Wouldn't you think that the most stressed guitar joint probably doesn't
approach that kind of load?

What is the most stressed joint? Bridge? Brace? Neck?

I'd guess that it is the center joint - top or back. Mighty few square
inches there, and a LOT of stress when the humidity drops, you betcha.
But, what's the tensile strength of wood across the grain? I dunno - bet
it's no ten grand, though.

Far as I've seen, abject glue failure is due more to high heat exposure, or
some serious contamination of the joint, rather than too high or low
clamping pressure. Lots and lots of instruments are being built with
vacuum clamped braces and bridges, and they seem to be doing quite
well. Personally, I'd leave it at vacuum for braces and use my good old
heavy mechanical clamps on bridges.

Author:  burbank [ Wed Jan 31, 2007 1:32 pm ]
Post subject: 
[QUOTE=JohnAbercrombie] [QUOTE=SteveS]
Does anyone know what force your average go bars exert? I'm pretty sure
you'll not get that 900 lbs for you 12" brace.
[/QUOTE]
Steve-
Check out the rodbuckle.xls spreadsheet at
http://www.ukuleles.com/Technology/sprdshts.html

I decided to make by go-bar deck 'less tall' after about 2 min playing with
the spreadsheet and seeing the effect of bar length on pressure.

Cheers

John[/QUOTE]

John,

Did you also shorten the bars?
Author:  James Orr [ Wed Jan 31, 2007 1:50 pm ]
Post subject: 
I wouldn't glue a top or back on with a vacuum. When I was veneering
drum shells with a vac and bag, I had to make molds out of construction
foam (foam without air or something like that) to fill the inner diameter of
the shells so that they wouldn't shatter and implode from the pressure.

The shells were 5/16" maple plywood.
Author:  Bob Garrish [ Wed Jan 31, 2007 2:02 pm ]
Post subject: 
We'd better not decide vacuum clamping is no good after I spent all day yesterday setting up to make clamping fixtures for the upcoming store...

The technical reasons vacuum clamping works well are a combination of uniform clamping pressure and that the vacuum very much discourages any excess glue in the joint (which would weaken it). In the case of bracing, depending on the thickness of your membrane, you're also getting more than strictly atmospheric pressure because the membrane is often incapable of pulling down into the corners beside the brace and so that extra area is also pulling down on the brace top.

I've heard some interesting tales of people who got the 'clever' idea to vacuum clamp on a top or back, but that's definitely -not- an application of vacuum clamping unless you're willing to get very clever.
Author:  JohnAbercrombie [ Wed Jan 31, 2007 2:30 pm ]
Post subject: 
[QUOTE=burbank]


John,

Did you also shorten the bars?[/QUOTE]

I was thinking about using the underside of a high shelf for the top of a go-bar deck, and changed to a more typical design. Wood bars were cut to length later.
John
Author:  tippie53 [ Wed Jan 31, 2007 4:06 pm ]
Post subject: 
     Most gobars are about 8-10 lbs per. I think 150 psi is rediculous. A glue starved joint will fail . I have been using a vacuum clamp for the last year. My joints are cleaner , more secure and the work is much neater . I highly recommend vacuum clamping.
      If you think about that 150 psi , you should be applying about 1/2 a ton of force to a bridge to glue?
   john hall
Author:  Mike Mahar [ Thu Feb 01, 2007 1:31 am ]
Post subject: 
Violin makers glue the center seam of their tops with hide glue. They spread the glue on both surfaces, rub the the boards together until the glue is spread evenly, and set the top aside. They don't clamp it at all. Some of those joints have withstood the pressure of violin strings for a few hundred years without fail.

I know of at least a few guitar builders who just do a rub joint on the center seam of their backs and sides with great success.
Author:  crazymanmichael [ Thu Feb 01, 2007 2:03 am ]
Post subject: 
the reason a rubbed joint is effective with hide glue is that it is almost self clamping, it draws the parts together. not recomended with ar though. if the sceptics don't want to believe it just try to separate one.

actually saw a guitar on a site a few weeks ago that had centre seams on the sides, and dovetailed ones at that!   as was just about every joint on the guitar. rather busy looking, but an extreme example of precise workmanship.
Author:  Dave Rickard [ Thu Feb 01, 2007 4:31 am ]
Post subject: 
Sounds like Howard
Author:  Don Williams [ Thu Feb 01, 2007 4:50 am ]
Post subject: 
it was...
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