Official Luthiers Forum!

Schrader valves are designed to resist pressure, are they not? If there is a vacuum at their back end, won't that be the same as pressure (atmospheric, that is) on the inlet? So when unhooked, the valve will try to bleed atmosphere toward the vacuum.

I use ordinary quick-connect fittings throughout my vacuum system, and I'm going to say that I see only the slightest drop in efficiency in use, an inch or less. Plus I have the option to string all my air and vac lines together to pump up that soft tire out front.

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Dan
http://www.acme-archtops.com

Nice work Filippo!

Hi Fillippo,

I like that rig! I think you bought the last of the vac bases, I got the 2nd to last. I have trouble (even with 27" HG) keeping a strong enough grip. I was able to pop a maple guitar off the vac jig a little too easy. I would have no problem for glueing bridges or braces I guess. I just am very uncomfortable using it to hold a guitar for scraping etc.

I am thinking that the round vac head should be quite a bit larger diameter, for one thing. I was strictly using a dehumidifier compressor and no tanks. Just wondering what your experience is with the jig. PM me if you wish.

Terry,

I have the one you have as well as the LMI hold down clamp. Like you said, the clamping area could be larger on the black one thus providing more clamping pressure. I also like the way the LMI jig folds down to a full 90 degrees. The other one won't do that. I run them both on a venturi valve and get about 21hg here in Colorado.

Hi Robbie,
Thanks. I think I'm going to make one like LMI's outa hard maple. And after seeing Fillippo's nifty rig, I'm looking forward to finishing mine up. Thanks Fillippo! I've enjoyed watching your new shop take form.

I just have barbs on my vacuum fixtures and slide the vinyl hose onto whichever one I'm using. Unless it stays on a barb for a couple weeks (ie: when I'm doing production runs of necks for my bigger clients) the hose just pulls right off. If it gets stuck, I cut the last 1/2" off with a razor or heat it up and yank it off. No leaks this way, and I might go through one short piece of vinyl hose a year. It also makes it easy to run T fittings etc. if I need to split my vac up (like on multiple-part fixures).

With regards to pulling a guitar off of 27"hg: your gauge isn't telling you the whole truth. If you were to put gauges all over the underside of your work, you'd notice a pressure gradient possibly including some relatively high pressure (read: low vacuum) zones. I'm going to make a testing rig to figure these differences out one of these days, but for right now I've just been designing my vacuum fixtures to alleviate the problem (though I can't quantify it outside of 'it holds better now!' yet). It's very easy to have 25" at the gauge and maybe 5" of actual holding pressure with certain fixture designs, which makes it way too easy to peel up an edge and break the seal.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Filippo, I think the stem of a schrader valve is the valve body, so you couldn't (guessing, here) reverse the core to reverse the action, but that's all right, because you'd have to enclose your shop assistant in the vacuum bag to press on the stem to allow the vacuum "out". In any case, I don't think those things necessarily depend on air flow to open them - I think the clip-on female fitting presses the core stem open, so that when it's disconnected, the valve stem can close.

And as for the air line connectors I use, sorry, no url because no big online deal. In the past I've purchased my connectors from HD, Lowe's, Sears, auto supply shops, and just about anyone who carries air tools, hoses, compressors, and so on. They DO come in several different shapes, but if your salesperson understands that you want the commonest, the standard, the economy version, you'll find that the parts from any manufacturer will interchange with any other. Sorry I can't come up with the exact trade designation, but maybe someone out there knows what it is.

Oh, have a look in MSC, Grainger, and McMaster-Carr.

Don't buy cheap (often blue or yellow) plastic hose, pay more for the limpest, most flexible (probably black) rubber hose you can find, I stick with 1/4" hose and it passes all the compressed air and vacuum I need. If you're going to also run high-capacity air tools, then see what the manuf. has to say, but 1/4" works for everything normal, including lacquer spraying.

Don't worry about flexible hose collapsing under vacuum - hasn't happened to me.

Terry, no surprise, your guitar popping off the vac clamp. I can actually hear air flowing through the wood, and I'm talking across five layers of veneer and four cured glue lines, so there's no telling what the pressure has dropped to inside the clamp head. Try a larger diameter clamp. I swing my vise so the bulk of the instrument is over the carpeted bench top.

By the way, beware using thick weatherstripping to seal the edge of your clamp head - I've cracked backs when the unsupported plate is pulled into the clamp, and that might have been only 1/8" or so.

I just read Bob's post, and he's right on, although I do have guages all over, and while I can see a drop just before the clamp head, I generally get good enough holding power that the part won't pop off UNLESS it starts to peel - just break the seal at one small spot on the perimeter, and "break" will become just one of the new words the shop cat will learn.

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Dan
http://www.acme-archtops.com