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vacuum pressure needed for braces
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=25299		 Page 1 of 1
Author:  sprouseod [ Mon Jan 04, 2010 11:34 am ]
Post subject:  vacuum pressure needed for braces
My current vac press that I have built pulls 28"hg at the front end of the pump. with the press I am down to 15 or 16"hg. Is this adequate pressure for gluing the braces? Why the significant drop? IS there just some leakage or is this normal?

thanks
Richard
Author:  Michael Dale Payne [ Mon Jan 04, 2010 11:42 am ]
Post subject:  Re: vacuum pressure needed for braces
if you have a gauge measured 28" at the pump port but are only holding a gausge measured 16" at the fixture and you are not reducing port size between the fixture and pump then it sounds to me like you may have a small leak. now if you are reducing your tubing size then there will be a reduction but 12" of reduction seems to me as a larger loss than I would expect. I personally would line to see at least 20" at the fixture but 16" is probabbly good enough as long as it is constant.

Also gauges can be out of calibration so you may want to swap gages to see if you get the same result.
Author:  stan thomison [ Mon Jan 04, 2010 11:46 am ]
Post subject:  Re: vacuum pressure needed for braces
Richard Not knowing the material made of or how made. Mine is alum and I lay it on a sealed on an old wood frame had from previous unit. I would say vacuum leak. If laying it on porous slab or frame is wood that can be the problem. I just had to reseal my frame with new foam I get from Joe the Woodworker guy. I would just go through and try figure out where leak may be. Most of the time I can hear it. Check foam and make sure it is connected all around and no splits or tears.

Your pressure now may work, but I look for over 20 and if get 22 or so happy with that. I had LMI compressor, but went to bigger unit. I do not have it on a system like he shows to build. Basically plug it in, let it suck out air and run 15=20 minutes and then remove clean and good to go.
Author:  Bob Garrish [ Mon Jan 04, 2010 6:10 pm ]
Post subject:  Re: vacuum pressure needed for braces
If you have a pump that 'locks' when it's off, it's a lot easier to tell how much leakage you have. My Venturi releases instantly when I turn it off, but my 'real' vac pump keeps a seal when it's turned off so I can watch and see how long it takes for the air to leak in. That gives me an idea of how leaky a particular fixture is.

That sort of pressure drop, barring broken gauges, is surely a result of a leak though (sucking air through a porous surface counts as a leak!)
Author:  sprouseod [ Mon Jan 04, 2010 7:21 pm ]
Post subject:  Re: vacuum pressure needed for braces
Michael Dale Payne wrote:
if you have a gauge measured 28" at the pump port but are only holding a gausge measured 16" at the fixture and you are not reducing port size between the fixture and pump then it sounds to me like you may have a small leak..


I never thought about reduction in the port size. It looks like the dia of the hose is really small compared to the fitting, and everything was stepped down from 1/4" to 1/8". I had a buddy help me set it up and I didn't pay attention to the fittings he used. I will try that first.

Thanks
Richard
Author:  dunwell [ Tue Jan 05, 2010 3:47 pm ]
Post subject:  Re: vacuum pressure needed for braces
I disagree about the port/hose size. As far as I can see that will effect the rate at which you can pull it down, the flow rate, but not the final pressure. Given enough time to accommodate the flow it should pull down to what ever the pump will pull or whatever you have the cut off set to.

I'm suspecting leaks. If the leaks are big enough then yes, the flow rate and hose size can come into play, but you really need to fix the leaks first. Are you using a vacuum frame or vacuum bagging? I'd pull it down to what ever it will go to and then turn the shut-off ball valve and see if it starts to loose. With my vacuum bagging set up with a new bag and fresh seals it won't kick in the pump (~3psi drop) for about 10 minutes.

And I envy you folks down at sea level getting 28 inHg, up here in Colorado I'm real pleased to pull 21. Just less air stacked up at 8200 ft :o

Alan D.
Author:  Bob Garrish [ Tue Jan 05, 2010 10:04 pm ]
Post subject:  Re: vacuum pressure needed for braces
dunwell wrote:
And I envy you folks down at sea level getting 28 inHg, up here in Colorado I'm real pleased to pull 21. Just less air stacked up at 8200 ft :o

Alan D.


>29"...I can see the sea from my shop, and it's only a couple hundred feet down :D
Author:  Ken McKay [ Wed Jan 06, 2010 1:59 am ]
Post subject:  Re: vacuum pressure needed for braces
Yes, you have a big leak.

As a couple have advised, draw the vac, clamp the tube, turn off the pump and watch the pressure drop. It should be very slow. Listen for leaks, a stethescope works but I suspect a big seal leak.
Author:  sprouseod [ Wed Jan 06, 2010 8:57 am ]
Post subject:  Re: vacuum pressure needed for braces
I am going to try and figure out the leak today. It sound like it is in the fitting at the press or in the foam stripping. The unit still pulls between 25 and 28 inHg at the end of the hose.

have a few of question:
1) Will quick connect fittings work or should I use something more permanent?

2)What foam stripping do you suggest?. I would rather pick something up around town Homedepot/autosupply vs. order form veneersupplies but if that is the problem I can definitely order it.

3) I built the vacuum press from the instructions from the link below It recommended butting the foam stripping together at the ends then gluing with thin CA. Even though I used a small amount the joint seems to be quite rigid where I think a possible leak may be. How should I connect the stripping together?

Here is a link to the site http://www.kennethmichaelguitars.com/vacuumclamp.html

Thanks for all your help.

R
Author:  Bob Garrish [ Wed Jan 06, 2010 9:25 am ]
Post subject:  Re: vacuum pressure needed for braces
I don't trust most quick connects not to leak a little under vac, at least the ones meant for air. A lot of them use the air pressure to force the seals shut. I use brass barbs and just pull the hose off of them (you only need it over one or two ridges of the barb to get a perfect seal)

You want closed cell neoprene foam gasket. Going to a gasket or rubber place rather than a HD or such is a good idea. You want stuff with the 'skin' on the gluing and contact surfaces if possible. Lately I'm of the opinion that round cross-section works best. I glue all the ends together with CA and it works fine, though I recommend medium rather than thin for this application. There's definitely some technique involved in making a good joint on neoprene cord, and it's important to use absolutely minimal glue.
Author:  jayluthier [ Sat Jan 16, 2010 2:25 am ]
Post subject:  Re: vacuum pressure needed for braces
You can also use surgical tubing laid in a shallow channel on one side of the press. Against a smooth complementary surface...it will suck (grip) like nobody's business!IE very good seal. use thin wall 1/2" (you can find it online from fishing supply places (used for spearguns), or medical supply places)
Author:  sprouseod [ Sun Jan 17, 2010 5:33 pm ]
Post subject:  Re: vacuum pressure needed for braces
I just gave up [headinwall] and bought a 2'x2' vacuum bag from veneer supplies. Now it pulls 28inHg in the bag. :D

Richard
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