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.