Re-print of what I sent to Gary last year:
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BradyVac “Ova Da Hump” Valve
A sucky short story about the marriage between a Fein & a Gast…
This is just a quick write up for you guys @ Gary’s camp. Sorry I can’t make it, but family duty calls…and work is really busy…
Here’s a little contraption that I came up with a while back to aid in holding down non-porous material while using a low CFM, high Hg” vacuum pump, like a small Gast or Welch ½ HP or even a venturi type system that works in conjunction with your air compressor. This setup is designed to be used in what I call a ‘closed system’, meaning that it is NOT intended to be used with a bleeder board or BradyVac v1. You can use a BV2 if you use non-porous materials. (For more info on BradyVac configs, visit my write up on the SB website and/or forum) Plastic jigs and setups work best.
In my experience, sometimes part hold-down is difficult if your material is not absolutely dead flat. For example, the stippled-back of fiberglass or carbon fiber woven layups prevent a high suction/low CFM vacuum from ever getting over the hump and sealed to the gasketing. This prevents your low CFM/High Hg” (LFHS – Low Flow High Suction) setup from being able to make a seal against the gasketing, and your high CFM, Low Hg” Fein or other vacuum source (HFLS) doesn’t generate enough negative pressure to hold the part down while machining. Many times, holding your hand on the part to get it to seal is a pain, and depending on material, it may not seal at all with this method.
Using a HFLS vacuum source, like a Fein Turbo III, the material pulls down and ‘gets over the hump’- but with low negative pressure, it is not enough to hold your small parts. The HFLS vacuum is EXCELLENT at sealing off the material to the jig & the LFHS (Gast) is great at keeping that part held down, provided that leakage is kept to a bare minimum.
Using the good ole principles of pressure differentials, I sought out a way to have the vacuum automatically switch from HFLS to LFHS. This would let me seal off my part to the jig and then build the necessary vacuum pressure to keep my finished parts held down against the forces of the cutter. I discovered that a regular old sewage check valve was just the ticket. It was cheap (I think around $7) and it allowed me to directly connect the Fein to it. (Even came with hose clamps!) A short length of 2” PVC pipe was drilled and tapped to accept a small quick release hose fitting. The open side of the PVC is connected to my vac jig and the LFHS vacuum is piped to the quick release nipple. The valve is situated so that the Fein (or other HFLS) opens the flapper valve when it is turned on.
In operation, the Fein pulls the flapper valve open and begins building vacuum. Once the Fein tops out (around 7” Hg), the small LFHS pump quickly builds vacuum until it tops out, providing that there are no leaks. As the LFHS pulls past 7” Hg, the pressure differential in the sewer valve causes the flapper to suck closed. This seals off the Fein from the rest of the system and at that point it can be turned off. The Fein can be left on as a sort of insurance policy, if your part is barely held down with 7” Hg. If the vacuum pressure drops to under 7” Hg, the valve is then sucked open by the Fein and pressure is then maintained up to 7” Hg, by the Fein unless leakage is really excessive.
I know that this isn’t the only use for this type of setup. I bet that it can be used in other ways in conjunction with anything that moves a lot of air and something else of higher suction. I have confirmed that it works really well with vacuum bagging, especially where open glue times are short, and even thermoforming where you need to generate vacuum quickly to keep production up.
Note that since I made this, I discovered that the setup really benefits from using larger tubing on the Gast inlet nipple. Go as large as you can since the pump can choke and lose a lot of it’s air flow (CFM) if the tubing is too small. It will generate vacuum just fine, but the larger tubing helps to get the most out of the pump.
Hope all of that makes sense…
-B
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