The net atmospheric force on whatever you evacuate will be over 11 pounds per square inch (or over 1500 pounds per square foot), which is enough for many tasks requiring vacuum.
(For those of you who are not metrically impaired like most of us in the U.S., that's over 7700 kilograms per square meter.)
(1) Vacuum pumps made this way are in use around the world, in non-industrialized areas, for vacuum packing food. (Especially to save seeds for the next year's planting, by killing bugs and preventing germination. See http://www.plenty.org/soy/vacuumpump.html )
(2) A manual vacuum pump can used to make a "vacuum press" to clamp laminates together with a ton or more of force while the glue sets. (A smaller manual pump, designed for evacuating wine bottles, is often used for laminating skateboard decks, using a commercial vacuum bagging kit from Roarockit called the "Thin Air Press". Our bigger pump pulls about as hard, but requires considerably less pumping.) It can also be used for vacuum bagging composites like fiberglass, to squeeze out excess resin and make stronger, lighter parts.
(3) You can use it with a vacuum former, to form thicker plastic sheets than you could with just a vacuum cleaner. (Doug Walsh shows how to apply this to vacuum forming in his book "Do It Yourself Vacuum Forming for the Hobbyist," available from http://www.build-stuff.com )
(4) You can implode various things with it, or attach it to a vacuum chamber and expand or explode things in the chamber; that's often fun and/or instructive. (Try Peeps.)
The vacuum generated is not strong enough for some tasks, like evacuating refrigerant systems or degassing difficult-to-degas liquids. For those tasks you need a pump with more leverage, that takes almost all of the air out.
A plain bike pump is just a piston pump rather like a syringe, with a rubbery disk sliding in a cylinder. Drawing back the disk sucks air into the cylinder, and pushing it in pushes air out.
To make this syringe-like thing work as a pump, sucking from one place and blowing to another, two "check valves" (one-way valves) are used. One lets it suck in from an air intake, but not blow out the intake. The other lets it blow out the exhaust, but not suck in from the exhaust.
Unlike most piston pumps, a bike pump uses the rubbery disk both as a piston and as one of the check valves. When you pull back on the pump, the disk flexes inward and allows air around it, into the cylinder ahead of the piston. When you push inward, the disk is stopped from flexing the other way by a metal disk (like a big washer); its edge seals against the inside of the cylinder, so that when you push it down, it compresses the air in the cylinder and forces it out the exhaust.
(There's a normal check valve at the exhaust, to keep it from sucking the air right back in on the upstroke, and more air flows in behind the piston through a hole in the cylinder top.)
To convert the pump, we'll need to do two things:
(1) Reverse the piston disk and the metal disk that backs it up, so that it seals on the upstroke (to create vacuum) and flexes on the downstroke (to let air around it and out of the cylinder).
(2) "Reverse" the exhaust check valve, so that it lets air in but not back out, and we can use the old exhaust connection as the new air intake. Actually, it's usually easier just to remove the check valve, and replace it with one that does the right thing, so that's what we'll do.
To make it easy to do both of these things, we'll want a simple, cheap, old-fashioned bike pump with no frills (like a pressure gauge) to complicate things.
(If you want a small electric vacuum pump, have a look at my other instructable on converting a 12V "tire inflator" air compressor: http://www.instructables.com/id/E791HNXF23Z39P6/ )
Step 1: Get the right pump, and a few other cheap things
You want a simple cheap pump with:
(1) a shaft that's a metal rod about a 1/4" or so in diameter, not a 1/2" plastic tube
(2) a cylinder top that unscrews, or can be released by unscrewing a few small screws
(3) a rubber disk and metal plate that are held onto to the end of the rod (opposite the handle) with a nut, so that you can just take them off and reverse them
(4) a metal fitting where the hose attaches, with a six-sided base, which unscrews like a nut. This is both a hose barb for attaching the hose, and a check valve we'll need to gut
(5) reasonably long throw and reasonably large diameter (for a bike pump); a skinny pump will be slower, and
(6) no pressure gauge. Pumps with pressure gauges often use a different kind of check valve, and have air space around where the gauge attaches that may affect how much vacuum you can pull once the conversion is done. (You can fix those things, and I have, but it's easier just to get a no-frills pump.)
If you have an Auto Zone nearby, go look for a Slime 2060-A "Floor Pump"; it's cheap and you'll be all set. Otherwise, look at the pictures in this instructable, and try to find a very similar pump.
(I have no connection to Auto Zone or Slime, and no reason to think that other brands aren't just as good, but if you use exactly the same pump, you should have zero difficulty following the directions.)
Looking for a "floor pump" at an auto parts store is a better bet than looking for a "bike pump" at a bike shop.
You'll also need:
(1) Three or four feet of 1/4" I.D. braided PVC hose. (Lowe's sells it by the foot in the plumbing department, for about 30 cents a foot, so you'll want a dollar or so's worth.) That's a kind clear flexible tubing with braided reinforcement; small diameters stand up very well to vacuum, unlike some other small hoses.
(2) A check valve with hose barb ends to fit 1/4" I.D. tubing. McMaster-Carr sells these online. Mine cost about $4.00. (Part number 6079T53 from http://www.mcmaster.com )
(Some people use an aquarium check valve, which costs about $2 at a pet shop. I tried that and mine leaked, as well as restricting flow a little more than I like, but some people are happy with them. If you go that route, you'll need to use smaller-diameter hose to connect it; let me know how it works out for you.)
(3) A small hose clamp. I got mine for 25 cents at a local tool place, but you'll likely have to buy a 2- or 4-pack at most hardware or auto parts stores, and pay a dollar or two.
You'll also need few tools:
(1) An adjustable wrench, or a non-adjustable one that fits the hose fitting on your pump.
(2) Something that can cut thin metal, such as tin snips, a nipper, a hacksaw, or a rotary tool. (You might be able to get by with a file and a pair of pliers, or even just some needle-nosed pliers.)
(3) (maybe) a power drill with a 1/8" drill bit suited for drilling a little bit of metal, or for the 2060-A, a screw and a screwdriver.
Rags or paper towels are good, because you'll be dealing with greasy things.