How to build your own vacuum pump for under $50

555 wrote this terrific article:

There comes a time in the life of every dedicated and thrifty windsurfer where a board gets pranged badly, and the thought crosses their mind - "I wonder if I could fix this myself?"

I am one of those windsurfers, and one of my favorite tools for the job is my vacuum pump.

Windsurf boards are awkward shaped things, and clamping a piece of stiff divinycell over a curved core while the epoxy sets is a very tricky business if you want to be sure that it's pressed down evenly so that there are no nasty voids between the two layers. G-Clamps just don't do it!

A vacuum pump and bag allows you to exert relatively massive force evenly over the surface of whatever you put in the bag. My pump will pull 26 inches of mercury, which means nothing to most people. BUT if you work it out, that's equivalent to 890 grams of weight on every square cm, or over half a tonne stacked on an area the size of an A4 sheet of paper. The best part about vacuum, is that it doesn't just push 'down' it pushes 'in' all over the surface of whatever is in the bag.

All that for under $50? Sound a bit much like I'm going to throw in a set of Ginsu 2000 steak knives that can cut steel pipe?

Okay, here's the list of major stuff you'll need to acquire:

1 Fridge compressor (free - find some old fridge that's been chucked out, pinch off the lines so the freon doesn't escape, and liberate the pump with a hacksaw, keep the power cord attached, you'll need that!)
1 Vacuum advance unit of an old carburettor($5 at a car wrecker)
1 reasonably grunty tension spring (maybe off a car seat at the same wrecker?)
1 Vacuum guage (Supercheap auto have them if you get stuck)
1 mains voltage microswitch ($4 at Dick Smith or Jaycar or wherever)
2 'T' junction air fittings (these are the most expensive part of the whole thing!)
Some plastic tubing that doesn't squash under vacuum (I found some in my collection of "stuff that might be useful one day")
A sheet of plastic, or a bag, and some tape. Caulking compound is handy too, but we'll come to that later.

Fridge compressors are abundant, often found growing on the side of the road, or in clusters at your local landfill. I got my first one through a friend who's grandmother was getting a new fridge because the door seals had gone on her old one. I recommend not stealing the one from the kitchen, or your grandmother, otherwise fixing your board might be the least of your worries.

Apart from being environmentally conscious and crimping the refrigerant lines to keep the gas in before you cut them, the only other thing to watch out for when liberating a pump is that you keep it up the right way in the car on the way home - they're full of lubricating oil, and that's not good on the carpet! Once you've got your pump home, plug it in and turn it on - be aware that a small amount of oil and gas may come out the exhaust, so be ready to catch that, and don't check for it by peering down the line while you turn the pump on. If it doesn't go, you might need to find the regulator, and bypass it. If the reg was still okay, it'll just think that the fridge is too hot, and will let the pump run indefinitely.

Hopefully, if all is well, you should find that one of the copper lines is blowing air out, and the other is sucking it in(often the bigger of the two lines).. That's the one we're interested in.

That was the easy part. Now we need to make up the vacuum switch so that the pump doesn't crush whatever is in the bag completely. (how many A4 sheets to cover a windsurf board?)

This is where that vacuum advance unit comes into play. If you apply vacuum to the line, the diaphragm inside will be sucked in, and the coupling on the bottom will move. The idea is to hook it all up so that this movement will trigger the micro switch and turn the pump off when the desired vacuum has been reached. Of course, the vac-advance unit was only designed to see around 8 inches of mercury in it's last life as part of a car, so it's well bottomed out by the time your pump has got to 26". The answer is to use that spring you found to pre-tension the diaphragm. I've hooked one end of my spring up to a piece of threaded rod (a bolt is just as good) and then passed that through a piece of steel so that I can tighten or loosen the spring to adjust the vacuum cut off point.

Now cut the power cord in the appropriate spot, and run one of the power wires through the microswitch so that it's 'on' when the switch is pushed down, and turns 'off' when the switch comes up. Remember.. this is mains voltage you're playing with, so be sure to unplug it from the wall before cutting, and make sure it's well insulated once you're finished. Mount the switch so that some part of the tensioned coupling is pushing on it, and will stop pushing on it when the diaphragm is sucked in.

It may also be a good idea to think about 'what if' at this stage.. if something goes wrong in the middle of the night while you're sleeping, and the switch fails to trigger into the 'off' position.. suffice it to say that your board might lose a bit of volume. A second switch that will act as a failsafe if the diaphragm is pulled too far in, or that triggers a siren or something isn't a bad idea!

Plumb in the vac-gauge, and adjust the spring tension so that it's shutting the pump off at no more than 15". In my experience 15 is the max you'd ever want to pull on styrofoam. 5-8" is ample for most jobs, and 26" will make your board very, very flat. Don't take my word for it though, experiment on some scrap foam.. I take no responsibility for any squashed boards!

You'll have to use a bit of imagination to hook this all up, but the ideas and principles are there.

It is also possible to use a 'bleed valve' to control the vacuum, but this means that your pump will be running all the time. They do get hot, and also use a bit of power. When I get a good seal on a job, my pump will cycle once every 8 minutes, for about 0.5 of a second. A nice to have addition to the setup is an old LPG cylinder which you can plumb in to the system - this acts as a vacuum reservoir, and will mean that your pump will cycle less often, but for longer. I haven't found it necessary.

Alternatively, if that's all a bit hard, feel free to blow $600 on a 'proper' pump and fittings.

Also have a look at these:
www.dream-models.com/eco/vacuumpump.html (Thanks to Decrepit for the link)
www.berkut13.com/sucker.htm (This one uses a bleed valve though)

This is my pump - covered in epoxy dust from the last repair I did. I left it mounted on the bracket it was on in the fridge, and just bolted everything else to that. It's not especially pretty, but it works well.

It didn't start out with a gold compressor, but I had to deal to some surface rust, and gold was the first colour out of my paint box...	Around 8 inches of Mercury - I used this to bag the bottom skin onto my AHD board. All wiring junctions are tucked inside the grey box, or the black box on the side of the motor itself. Except for the switch, which is well wrapped in insulation tape, and tucked under the guage.
Detail of vacuum regulation - the rightmost nut sets the pre-tension, and the vacuum switchpoint. The hose coming out to the right connects to the vac-bag.
	The switching tolerance can be adjusted by changing which part of the lever arm is contacted by the bolt. If I shift it so that the bolt rests against the top part of the spring arm, it will cycle in 1/4 inch of mercury. But that is too frequent. 3 inches is good enough.