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.
Step 2: Reverse the piston disk
Notice that the piston disk is actually a shallow cup shape, with a lip around it. That lip flares out and seals against the cylinder on the downstroke.
(1) Unscrew the nut on the end.
(2) Pinch the rubber and metal disks together, and pull them off the end of the shaft.
(3) Turn them around.
(4) Put them back on the shaft.
(5) Replace the nut and tighten it up, hand tight.
Now you need to get that stuff back in the cylinder, and you don't want to scratch the piston disk on the sharp inside edge of the cylinder, so:
(6) Pinch the rubber disk from the sides, into an oval shape,
(7) Angle the shaft and the disk about 30 degrees relative to the open end of the cylinder, and insert one end of the oval, and
(8) Gently work the rest of the disk into the cylinder, pushing it inward anyplace it seems to catch on the edge, and straighten the shaft.
Now you can wipe the nasty grease off your fingers.
(9) Push the handle in some, straight, and
(10) Screw the cylinder top back on.
You're half done.
Step 3: Remove the hose fitting & hose from the pump
Step 4: Remove the hose from the hose fitting
But first you need to get the hose off, to get at the fitting to modify it.
There is a piece of thin metal crimped around the end of the hose, to clamp it onto the fitting. You need to get that off, so you can remove the hose. Cut the thin metal with tin snips, nippers, a rotary tool, or whatever you have available that can cut thin metal. (You might even be able to tear it apart with a small screwdriver and needle-nosed pliers, with some effort. Pry and bend and tear it apart bit by bit.)
Once you've got the crimp off, work the hose off the fitting. You'll find a "hose barb" there---a hollow cylindrical bit that sticks up into the hose, with a flared part to help it grip the hose from the inside.
Step 5: Remove the check valve parts from the hose connection fitting
(You probably can see the ball, just barely, when you tip the hose barb down... it rolls almost to the end of the hose barb, and stops short when it hits a couple of extra bits of metal, which partly block the end of the hole.)
To get the ball out, you need to make the hole as big as the ball all the way to the end of the barb.
The straightforward way to do that is just to drill it with a 1/8" drill bit; just drill down into the hole to take away any extra bits of metal that retain the ball, and you can roll the ball right out.
For my Slime 2060-A, it was easier than that. I noticed that there were a couple of slivers of metal on opposite sides of the end of the hose barb, which had just been bent inward to make the retaining parts. All I had to do was bend them outward again.
I did that by inserting a screw into the end of the barb, and turning it with a screwdriver while holding the fitting with a wrench. The threads of the screw gripped the two bent-in pieces of metal and pulled them outward enough to make room for the ball to come out. (For other pumps of a generally similar old-school design, you may have to drill the hose barb to make the hole big enough to release the ball.)
Once you have the ball out, check to see if you have a hole clear through the fitting. There may also be a piece of rubber in there that needs to be removed. In my case, I tapped the fitting on the table a few times, and a little piece of rubber just fell out.
If you have a different brand of pump, you may need to drill yours to tear up the rubber, and blow the bits out. Drilling straight through fitting is not difficult, and may be a good idea anyway, if you want to go to the trouble. (See the next step.) Whatever kind of check valve is in there, drilling right through it is bound to help remove it.
Step 6: (Optional) Drill the hose fitting out a little
This doesn't make much difference most of the time, because the resistance there is usually small relative to how hard you have to pump anyway, for any substantial vacuum level. It does make rapidly pumping larger volumes of air at lower vacuum levels slightly easier. (Like near the beginning of evacuating a vacuum bag or the vacuum tank for my vacuum former.)
At any rate, I just drilled through the existing hole with a 1/8" drill bit. As usual with drilling metal, you should to do that in little spurts, at fairly low RPM's, and frequently pull the drill out to get shavings out of the way and let the bit cool. This took about a minute of drilling in several goes spread out over a few minutes, drilling from either end while holding the fitting firmly with a wrench. (I used titanium-coated but very cheap drill bit from Harbor Freight.) If you drill too fast, or for too long, you'll just heat things up and dull the bit faster.
Once you've drilled it, blow the metal shavings out. You don't want them getting in the pump.
Step 7: Screw the fitting back on the pump
Step 8: Attach the hose and the new check valve
If you're using 1/4" braided PVC like me, the hose will probably fit well enough to make a seal, but not tightly, and it's a good idea to put a hose clamp around it. That will ensure that it doesn't lose its seal, and that it doesn't slip off. (Under vacuum, it probably won't lose its seal because vacuum will suck it inward onto the barb; if it slips off, though, you're hosed.)
Now attach the new check valve to the barb:
(1) Make sure the arrow on the check valve is pointing TOWARDS the pump, so that it will allow air flow INTO the pump, but not back out through the intake.
(2) Insert the appropriate end of the barb into the litttle piece of hose. It should be snug; work it in most of the way.
Once that's done, you can attach the long remainder of the hose to the other barb, in the same way; the other end is what you attach to whatever you're sucking from.
Now you have a vacuum pump. Try it out.