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Minimum HG for using a vacuum clamp?

I have a very nice vacuum jig that Don Williams made me. I mean it's NICE! Aluminum frame, brass fittings.
I wanted to be able to mount it under my work bench on drawer slides and pull it out when I need it.

The problem I have is HG. With my tiny little pump I can only pull 10 hg, not nearly enough. SO I have a larger pump now
and have spent some time double checking all the fittings, re (Teflon) taping some and sealing up any corners where a leak may be. With the new tweaks and larger pump I can pull 17 hg.
What is the minimum or recommend hg needed to use this jig?

What do you guys that use vac pumps, particularly for back and top bracing pull?

Thanks so much!
Lance

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Lance@LuthiersForum.com

Lance...I think you are going to have issues with the hide glue cooling down too much before you can get all braces located and the vacuum applied...unless you're going to work in a room that is 140 degrees! Or you only glue a brace or two at a time, which is going to take a couple days to complete a top.
I think hide glue is best used in a go-bar situation and/or clamps for holding pressure due to the time/cooling issues. Also warming the braces will extend the time for the glue set up, but you knew this already. However, I don't think they would stay warm long enough to do an entire top and them get the vacuum cover in place without some "cold joints" forming.


OOPS! I mistook HG for hide glue. You mean vacuum force!

NEVER MIND......

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Hank Mauel

17 will do...but where are you measuring it? Odds are you're measuring it at the pump, as most are, and in that case the actual pressure at the part is likely significantly less. You can get away with using a lot less vacuum if you use a less flexible membrane (one that can't pull itself down to level around the braces), but you're also at higher risk of the membrane pulling things over.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks Bob,
The membrane is from LMI and its quite heavy.

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Lance@LuthiersForum.com

I built the EVS system as on Joe the woodworkers site: http://www.joewoodworker.com/veneering/EVS/parts.htm

It pulls 21 inches of mecury very quickly and since it's regulated does not run continously.

I bought the pump used on ebay.
Here is one with an $80 buy it now: http://cgi.ebay.com/3-5cfm-THOMAS-2619- ... 7C294%3A50

Hope this helps.

Yea 17-22 works. I am selling the one got from LMI. It will pull that much. I bought one from Joe and it is one size bigger. Was gong to run two jigs (for back and top) but not now. One thing make sure base is sealed (which I know Don would do) or can lose suction/vacuum. I like he LMI tan rubber more than the black. Thicker. If ever have to replace it, get one extra wide on it. It can be a pia to change if not. Joe sells good closed cell foam if have to replace that ever (which you will) Get 50' roll cheap. I have done 3 rigs with one roll and still have enough for few more. Lance another idea got from Bourgeois is getting a radius glue dish. I have them as they did by cutting round shape more rectangle shape and then can make hold downs for the X braces. Also cauls for bridge plate and UTB. I will send some pic's for you on some other things for placing braces and holding them so don't slide around. Once go vacuum, won't go back. I do have a go bar setup should pump go down or somthing. Joes bigger pump at $139 good deal, more than enough pull.

Thanks guys, after some more sealing, I was able to get 18.5 hg.
Here are a few shots of what it looks like with the vac on.

Sorry to sound so dull on this subject, its just that I have never used one and want to
be sure what I am seeing is going to work.

Thanks again!
Reading of the gauge at the pump


Here are a few shots of a back in the jig under 18.5 hg. Using the LMI thick rubber. Its the older stuff that Stan
talks about.

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Lance@LuthiersForum.com

bump

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Lance@LuthiersForum.com

I just use vacuum for glueing the bridge although I have thought about doing what you are. My vacuum is a venturi type from joe woodwooker and I pull about 21 Hg sometimes more and I'l not sure what that is dependent on.

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Tickle your guitar daily, and it'll tickle you back.

Lance, based upon the 18 1/2" HG, the clamping force you are applying to your bracing is ~9.09 PSI. The math is pretty straighforward: 29.92" HG = 1 ATM, 14.7 PSI = 1 ATM (at sea level..)

Setting up a simple proportion:

29.92" HG / 14.7 PSI = 18.5" HG / X PSI

Solving for X yields 9.089 PSI.

This pressure would now have to be multiplied by the total effective area of the bracing you are clamping to give you the total clamping force. I would think that if you have good fitting joints with minimal gaps you are in good shape.

My go-bars give me about 14 lbs. of force each and I spread them out across each of the braces. I obviously can't place them edge to edge to allow the PSI figure to be anything close to that. However I do place them on top of a sheet of 1/8" plexiglass between the go-bar tip and the bracing to help spread the clamping load. My guess is that your vacuum system with the 18.5" HG is yielding better clamping forces than my current go-bar system. ..... YMMV....

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Peter

mine usually pulled around 21 or so and I never had any issues. If you're getting 18.5 at the part itself you've got nothing to worry about.

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John Mayes
http://www.mayesluthier.com

Thanks again guys!

im getting 18.5 to 19 hg at the pump. I will be testing it out this week on the guitar my daughter is building

I will report back...

PS< what do you recommend for a sealant strip? Im using black weather stripping now, but somehow I think there might be something better.

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Lance@LuthiersForum.com

Filippo, I went at it like a mad man with two tubes of cyclone caulking!
Every corner, pulled every screw and PUMPED In caulking, and put the screws back in. Made sure the rubber sheet was glued down to the frame. And I spread caulking over every joint. I think my only possible leak now is the foam seal.

I hear ya tho on the driving your self crazy!

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Lance@LuthiersForum.com

Hey Lance, I believe the max hg you will get is also dependant on your altitude.

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Tickle your guitar daily, and it'll tickle you back.

Thanks John, here is what I got from our township site.
920-925 above sea level

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Lance@LuthiersForum.com

Vacuum gasket is all neoprene foam, but the weather stripping stuff might be lower quality. You get a better seal if you're using gasket with its 'skin' against the two sealing surfaces (extruding the stuff makes a skin on it, but cutting it exposes the less sealy core). I've thought about using 10 durometer rubber to increase durability, but the setup cost is high and I'm still on my first roll of good foam gasket.

Adding a resevoir is only going to hurt your chances of getting a high vacuum unless your frame seals perfectly before vacuum is applied. The resevoirs are only there to get a quicker evacuation, but if there's not a perfect seal then it's just making your pump work harder and longer to get the pressure down. Generally, the frames seal better when under negative pressure and so a tank might even prevent a small pump from 'locking in' the frame enough to get a better seal at all.

You can hear leaks if they're significant, but it's harder to hear them the lower the negative pressure is (less pressure differential = less airflow = less noise). If it's going to leak there are only a couple options so eliminate them one by one. Either the frame is leaky (completely avoidable by using a one-piece of welded up frame), the membrane isn't sealed well (use more screws, more glue, or more whatever...it never hurts to overdo it here), or the gasket material is improperly applied (the butt joints need to be TIGHT).

The maximum pressure differential does go down as altitude increases (less air pressure), but it's not large enough to affect the vacuum pressures we use in clamping (it's mainly in whether you can get over 28-29 mmHg)

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I saw LMI vacuum sheeting mentioned and for what it's worth, I expiramented a little with a frame press. Not wanting to to spend a lot of money on something I wasn't sure that I really wanted to do, I bought a clear vinyl shower curtain at the dollar store. For the press floor, I lined it with vinyl window screen to help keep it from self sealing. It all seemed to work fine .

Rene

Maybe if you got a gluing setup like Taylor's where they have a template and you snap each braces to the template, using a heated glue roller to allow you to apply glue very quickly, then you could use hide glue with vacuum clamping since you could have all braces ready in less than 30 seconds then just close the lid. I don't think too much clamping pressure is needed however for hide glue, since the glue itself will pull the joint in as it dries and I think go bar system should be sufficient.

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

Are you saying that if you plug your air inlet you get 10 mmHg? With the inlet closed (finger over the tube you'd run to a frame) you should be getting at least 20-27 depending on the kind of pump you have.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Filippo, you have a leak somewhere. Make sure all of your filtings are tight and have some type of sealant on them, teflon tapes can work ok. If I had to guess based on the picture, I'd look where the fitting go into the PVC caps. You can start by checking just the pump before any of the accumulation tanks and plumbing.

Good luck.

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Jim Watts
http://jameswattsguitars.com

Well I was an organic chemist in my former life and used high vacuum pumps all the time. I have not used pumps for clamping, but I find it curious that the gauges you are using are measuring backwards. With vacuum, you want a lower number on the gauge. For example, in my lab I could get less than 0.1 mm Hg on my manifold, which is very high vacuum (760 mm Hg = 1 atm). It seems these gauges measure the apparent pressure being applied by the atmosphere from outside the vacuum system. Very strange to me.

I also hate to respectfully disagree with Bob, but it doesn't matter where you measure it, the pressure is the same throughout the system, unless you were measuring it at the pump without the bag hooked up. Then you would not be measuring the entire system with all its inherent leaks.

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Randy Muth
RS Muth Guitars Website
RS Muth Guitars Blog
Facebook Fan Page

Have you tried sealing/caulking it while it is under vacuum? This can help pull the sealant into the spots where it is leaking. I had a similar problem sealed it up and now I can pull 27hg or so before it kicks off.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

You're right it's measuring the pressure applied to the outside of the system. This is pretty standard for course vacuums. For someone who works with fine vacuum this would be confusing no doubt.

NB: Fitting and connections are evil in vacuum systems.

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Jim Watts
http://jameswattsguitars.com

I was able to pin down a couple of leaks in my deck by sprinkling baby powder around the frame while it was sucked down. Clean everything really well first, then carefully open the press & see where the powder has accumulated.

I'm a big fan of extreme overuse of teflon tape. I tape up my fittings until they look like I dipped them in frosting, and then screw them in. I get squeezeout on my teflon tape. It's not elegant, but I always get great seals. Using tapered pipe thread fittings and taps (NPT) is important as well, as well as having enough wall thickness to ensure a good seal. Using a vacuum tank in membrane or bag clamping situations is disadvantageous, IMO.



In the lab we tend to be more concerned with stuff (ie: how many particles per CM^3) where in industry they're more concerned with suck. Reading backwards from atmospheric measures suck pretty well, and measuring upward from vacuum measures stuff pretty well.


Disclaimer: This is for Randy, and it's going to get REALLY geeky, so be warned!
<GEEK>
There is definitely a pressure gradient in the system, and I've even done empirical testing to determine which factors effect it. I've been a bit too stingy to go out and affix a bunch of vacuum gauges at various places on a setup to determine the exact amount of difference, but I'll get around to it sometime. I had the same reaction the first time I was told about the pressure drop in long air hoses. Most of what we learn about pressure in science degrees is only applicable to non-flowing systems (which generally works, for lab and HVAC setups). As soon as there's flow, everything changes and there's a pressure gradient (relatively speaking, vacuum clamping systems are like sieves compared to lab and HVAC setups). It was actually John Watkins who turned me on to the 'what' in this one (that adding a bit of airspace under parts makes the vacuum work better) but it took quite awhile to figure out the 'why' given how that conflicted with my academic knowledge (the flow is the 'why')!
</GEEK>

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services