Many air compressors make good vacuum pumps if you can find the air intake, enclose it, and attach an appropriate hose or fitting.
In this instructable, I'll show how to convert a 12-volt "tire inflator"-type air compressor into a vacuum pump. This makes a vacuum pump suitable for vacuum bagging laminates and composites (like fiberglass), or for evacuating a tank for a small vacuum former.
The vacuum created is several times stronger than any vacuum cleaner can produce, and most of the way to a perfect vacuum. (About 25 "inches of mercury" out of a possible 29.9, or 12.3 pounds per square inch---or 1768 pounds per square foot.)
It is strong enough to achieve professional-quality results for many processes that require vacuum.
I got my little air compressor for $2 at a Goodwill Blue Hanger store (a.k.a. "Goodwill Outlet Store"). New, it would cost about $20. Converting it to a vacuum pump required a few dollars worth of parts & glue.
In addition to the pump, I used:
a few feet of 1/4" inside diameter braided PVC tubing
a nylon fitting with a hose barb for 1/4" I.D. tubing, and
some J.B. Weld steel-filled epoxy
All of these things are available at home improvement stores.
Since this is a 12-volt device that draws almost 4 amps, it requires a fairly hefty (DC) power supply. I run it off my 6-amp car battery charger. (Or sometimes off of a 12-volt deep cycle, trolling motor-type battery, for vacuum forming in locations where A.C. power isn't available.)
Thanks to Doug Walsh and his book "Do It Yourself Vacuum Forming for the Hobbyist" for the basic idea.
I've done very similar conversions of "nebulizer" air compressors (for medical equipment) from thrift stores. They're quieter, but don't pull as hard a vacuum. (About 17 inches of mercury or 8 pounds per square inch.) That's still several times harder than a vacuum cleaner can suck, and good for vacuum-bagging things like RC model airplane wings, but only a little more than half the ideal vacuum.) The upside is that they're quieter and run cooler, and will likely last longer.
NOTES(added in light of comments below):
If you use a really, really cheap tire inflator, such as the $10 "mini air compressor" from Harbor Freight, don't expect too much. Really dirt cheap inflators may only run for a few minutes before overheating. (Better inflators can run for up to an hour. ) Err on the side of not running your pump for too long at a stretch. If you don't know if it's rated for more than 15 minutes, only run it for 5 or 10 minutes at a time, giving it 5 minutes to cool down before restarting it. Ideally, you'd like a pump with a heavy finned aluminum cylinder, a cooling fan, and a powerful motor, rated for continuous long runs. (Really ideally, you'll get it for $2 at the Blue Hanger.) Failing that, be gentle with your cheap little pump.
Some tips on keeping the workload within your pump's limitations:
For vacuum bagging: (1) don't expect to use this pump for things like full scale airplanes, or to cope with substantial leaks, (2) use a modest-sized vacuum reservoir so that you don't need to run the pump all the time, or for a long time just to build up vacuum in the reservoir. Either use a vacuum switch to top off the vacuum automatically now and then, leaving the pump off most of the time, or do it by hand. If the pump is running most of the time, something is wrong.
For vacuum forming: (1) don't expect to empty a 30-gallon water heater tank with this thing. (I use a 7-gallon $20 Wal-Mart air carry tank for my 12 x 18 inch vacuum formers.) (2) Use a two-stage plumbing system to reduce the load on the vacuum pump and make your small tank go much further. (Like this one, using a vacuum cleaner to suck most of the air out, and an evacuated tank to pull the plastic down hard: http://www.tk560.com/phpBB2/viewtopic.php?t=424 .) (3) Don't run the pump until it tops out at 25 inches of mercury or so unless you're forming thick plastic around tight details. 20 inches is plenty for most vacuum forming purposes, and the last few inches take longer, and wear out your pump that much faster.
Step 1: Open the case
(The way some cases are put together, you may have to remove rubber feet that are glued over the recesses where the case screws are. Both of my nebulizer pumps were put together that way.)
Inside you'll find an assembly with a little motor, a couple of gears, and a little piston pump. In this picture, the motor is near the center, and the pump cylinder is on the left, with the compressed air hose coming out near the top.
Step 2: Find the air intake
Some pumps have a hose barb or other fitting connecting to a muffler. If yours does, that's great---you can just use the hose barb that's already there.
This pump just has four little holes in the top of the cylinder. (Actually, there's a little filter under the holes, but we can ignore that.)
Unfortunately, the holes are down in a funny-shaped recess, so we can't just glue a hose barb directly over them. We also want to make sure that the glue doesn't slop into the holes, so we need a couple more steps to make things fit.
Step 3: Find/make an appropriate hose fitting
To fit the hose, I wanted a hose barb for 1/4" I.D. tubing. There are many fittings (also in the plumbing department) with a hose barb on one end.
I chose a nylon double-ended hose barb (or "butt splice," for connecting two hoses), and cut off the extra barb with a razor knife. That left me with one barb with a flanged base suitable for gluing down, for about a dollar and a minute's work.
Because of the funny shape of the recess on top of the pump cylinder, I also shaved down two opposite sides of the flange, so that it would fit down in the recess. That took another minute.
Step 4: Prepare the intake for the vacuum fitting
First I prepared the surface, swabbing it with alcohol on cotton swabs to remove any dirt and especially oils.
As you can see in the picture, the J.B. Weld was a bit runny at first, and almost flowed over a couple of the holes. Oops. I should have waited until it set up somewhat, to a more putty-like consistency. I pushed it back from the holes with a cotton swab a couple of times. Once it was thick enough to stay where I put it, I shaped it into a circular wall that the hose barb flange could sit on.
Meantime, I mixed up a little more J.B. Weld, so that it would thicken a little, too, in preparation for gluing the hose barb on.
This was the time-consuming part---waiting about hour or so for J.B. Weld to thicken. I had other things to do, though, so I only spent about 20 minutes actually working on this project. If you're in a big hurry, you could probably use faster-setting epoxy, but I like J.B. Weld because it's fairly good at conducting heat. (I didn't want to insulate the top of the cylinder too much. That probably doesn't matter, because a pump used for vacuum doesn't heat up as much as one used for compression---highly compressed air gets very hot---so you might try 30-minute epoxy instead.)
Step 5: Attach the vacuum hose fitting
Step 6: Attach vacuum hose
You don't generally need a hose clamp to hold it tightly to the barb, if your hose is fairly rubbery. (That's one reason I chose the flexible PVC.) Vacuum will tend to suck the hose inward onto the hose barb, making a seal, rather than stretching it outward and making a leak.
Once the hose is in place, you need to figure out how to route it out of the case.
For this pump, there isn't much room to route the hose through the case without kinking the hose or stressing the hose barb, so I chose to just run the vacuum line straight out the top.
Step 7: Make a hole in the case for the vacuum hose
This was easy, since the hole I wanted was at the seam between the halves of the case. I just used nippers to make roughly semicircular holes at each of the mating edges, so that they'd make a roughly round hole when put together.
Step 8: Put the case back together
Step 9: Cut off tire inflator end of compressed air hose (for now), stow hose
(If I ever want to use the pump for inflating tires, I can splice it back together, using the same kind of double-ended barb shown before.)
Then I coiled up the hose and stowed it in the hose-and-cord storage compartment.