Converting an Old Compressor to a Vacuum Pump

Advice on cobbling together a vacuum bag veneering press out of available equipment and materials. January 8, 2010

Hi there. I'm trying to set up a vacuum bag system cheaply and have an old electric air compressor with a 1/2 hp motor fly-wheeled to a single piston. Would such a pump have anywhere near the power to create 1 atm. of suction assuming I rig it up properly with bag/fittings/seals?

Forum Responses
(WOODnetWORK Forum)
From contributor K:
The answer to your first question is yes you can use an old pump. My first vacuum pump was just that. It is probably capable of pumping more air then most vacuum pumps. One thing that can make a vacuum draw faster is to use a larger hose. Since vacuum at its best is only about -15 psi, it is only going to flow into the end of the hose at that rate. Doubling the diameter increases area four times. So a pump with a 1/2" hose will be four times as fast as the same pump with 1/4" hose to the bag. If you press the bag down expelling air before you seal the bag, the speed is normally not the issue as much as the open time of the glue you choose, and its open time.

From the original questioner:
I may well try 1/2 inch tube instead of 1/4 given the bag size required to handle my arch radius. Out of curiosity, did you install a vacuum switch on your first setup? I'm in Guatemala, and such units are spendy, thus for testing purposes I'm planning on a vacuum gauge in-line with a one-way valve (if find-able) and triggering the pump manually to maintain pressure after the initial draw-down or just letting it run. Any thoughts?

From contributor R:
If you use that compressor and intend to use the intake side for the suction, don't completely block off the intake port with the suction pipe, tee it in with a restrictor plate. Or build an adjustable spring loaded relief valve so as not to load the compressor down too much. I had an old refer pump hooked up that worked but it was slow. I don't need a vacuum system often so I just use my truck. I hook a hose from the brake booster vacuum line to an extra brake vacuum check valve connected to my bag.

From contributor K:
You can just let it run. For a pump that is accustom to pumping up to 130 psi, it won't even know itís doing any work at -15 psi. You can feel of both the motor and the pump ever so often for heat to alert you to any problems. I have only tried one oil-less pump before, and it didn't work. If you are using an old compressor, you may want to shut it down and check the oil ever so often to make sure the vacuum isn't pulling the oil around the rings the first time, or you can just disconnect the line out of the pump, and set a cup under it to see if anything comes out. Every so often one of the local clubs will ask me to do a weekend workshop. As part of the demo, I usually show how to use a compressor with the tank open. A hose barb > hose > clear plastic drum-liner, for a startup demo. I can buy a box of 80 clear plastic drum-liners at the Lowes or HD for under $15. They are about 3' x 5', and are pretty tough, and nice for small parts like cabinet doors.

From the original questioner:
Good call on the barrel-liners. I'll see what can be found down here in the next days and will report back on how the test goes.

From the original questioner:
Just to let you know the practice run went fairly well. However, I need better vinyl, more diligence sanding down every frame corner, and a new motor for the creaky compressor. Incidentally, I went with 1/2 " tubing as I discovered some 1/2 " plastic tap (as in bathroom) screw/washers in the plumbing department. They came as a pair and made an excellent valve when sandwich-screwed together from either side of the bag, and then just a 1/2 brass fitting / hose barb and likewise on the pump end.

Related query: as my form radius is 18" (except the tips), would I be better off using clamps for initial bend and leaving the form outside the vac.bag? I ask because the pre-vac gap between the laminates and form were severe enough that I had to basically sit on them to avoid the bag pinching between lams and form while vacuum built. (I did see an excellent photo of such a hybrid set-up in the WOODWEB archive, but it seemed better suited for a constant curve rather than varied radius arch).

From contributor K:
I am not sure I followed all of what you are up against, but I will say that I like to use the form outside the bag whenever I can, because the form can be much easier to build. It only has to hold up to the force of bending the stack of thin plys before they are stuck together. However, if it is in the bag, it needs to be able to withstand a ton per sq ft.

Most of the things that I do, I try to make the plies thin enough that I can press them down to the form by hand before the air is drawn from the bag. Once the vacuum is drawn, it gets rigid enough that it doesn't need the form, although I usually do leave it on, with a few clamps, just in case I lose the vacuum without knowing. The more you do it, the more tricks you will learn along the way.

I always try to make my parts oversize by a few inches. When I cut the parts out, I always break the off-fall into pieces, and inspect them to see how the breaks occur, just as a form of QC. If you have bad glue lines as you bend and twist, they will turn loose first.