I have a sherline lathe and want to build a venturi vacuum generator, to be used with an air compressor. This will be for vacuum bagging composites. Something like this:
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Can I just drill a hole in a cylinder lengthwise, and another one perpendicular to it (or at an angle), drill and tap for the 1/4 NPT connections and be done, or is there more to it? What kind of hole diameters work best? Should the holes be tapered?
IMHE, a venturi is the wrong way to make vacuum for vacuum bagging.
A venturi is most efficient when working with a large flow at higher pressures. It's not the right choice for pumping down to a vacuum (even when this is merely 1/2 an atmosphere) and when there's negligible flow. You also need to leave this pump running long term when the resin is curing, which is annoying with a venturi pump.
IMHO, find a different sort of pump. If the bag is reasonably sealed then a hand pump or a fridge compressor is perfectly adequate (don't leave the fridge compressor running permanently though).
"Andy Dingley" (clip) IMHO, find a different sort of pump. (clip) ^^^^^^^^^^^^^^^^ I agree with Andy. A great many woodturners use vacuum chucks, so the question of what is the best pump has been thoroughly discussed. These jet pumps require a large compressor. The compressor plus the pump is noisy and inefficient. Better to use a Gast vane pump or a compressor from an old refrigerator. It is also possible to adapt an automotive air conditioning compressor.
A pump that works really well for vacuum bagging, well my opinion anyways and what I use is a surge vacuum pump for milking cows, and you can leave it on for as many ours as you need, they work awesome. You can pretty much find them in like a milking supply company or a local farmer, might have just sittin around since nobody uses the surge vacuum pump for milking anymore they use a rotary pump nowadays, and every old farmer that milks has one laying around its a given! Good Luck
I did all my vacuum bagging of hull and rudders using a 2 stage Gast vacuum generator. I have no idea how to build one. I originally thought it was a cheap way to have a vacuum pump. By the time I assembled the whole contraption, "I could have had a V8".
Here's a pic:
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To minimize cycling of your compressor, you really need to incorporate a vacuum reservoir which means you need a tank, hi-flow check valve, gauge, vacuum switch, and solenoid valve. Basically the way it works is that you create vacuum in a tank and when you get to the desired vacuum, the solenoid valve shuts off the line to the air compressor. The check valve is so you don't loose the vacuum. As your vacuum bag inevitably leaks, the volume of the tank is enough to compensate and keep the vacuum. Eventually the vacuum drops below a specified level and then the vacuum air hose kicks in again. With the exception of when the air compressor runs, and when the vacuum generator runs, the system is completely silent. That makes it real easy to find leaks.
If I were to do it again ... I'd probably get a vane vacuum pump with good volume. Check out eBay.
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