Hi everyone: I’m fighting with my vacuum bag and I suspect others are too. I’ve been bagging for about fifteen years now and(even though I’ve tried) have never been able to master it’s idiosynchrosies. Some times it cycles too often to maintain the vacuum, sometimes it settles down and turns on infrequently(every few minutes). I have, over the years, replaced the components under the hood, tried all the suggested method of finding bag leaks(none found), and wished for a timer that would turn the pump on at the interval that I choose to maintain a reasonable vacuum. I send my main suspicion to the pressure switch that turns the pump on and off. If only it allowed the vacuum to drop 5 or 6 points instead of two or three. Yesterday I was bagging a rather radical curve that probably put alot of streach on the bag itself. The pump wouldn’t stop cycling and I suspect it was the pressure the bag was putting on the system that was drawing the vacuum down so quickly. I would like anyone who has had problems of this nature to put in their two cents and maybe we can solve these problems once and for all.
Many thanks, Bill
Replies
Bill,
I use a vacuum pump from VacuClamp, http://www.vacupress.com. It is included in their hobby system. It looks like a large aquarium pump. It runs continuously, no tank and no cycling. It is not marketed as a professional pump, but I have used it quite a bit, frequently for 8 to 10 hours at a time, with no problems. It works fine on my 5' x 9' bag. I seem to recall it has a long life expectancy, in the thousands of hours. Mine pulls about 12" of mercury. Since it isn't a high volume pump, you have time to position the bag after you start it up. I have a couple of other larger pumps, but this one does the job for me.
A bigger problem for me is getting everything into the bag with out dislodging something. I have learned that if I don't use a water base adhesive, I can do one side at a time without warping the panel. I can then do the backside later for balance. Also, the veneer doesn't tend to roll up like it does with water base.
Bob
Bob,
What glue do you use that is not water based?
Thanks
I use 2 part epoxy from West Systems. I avoid the 5 minute stuff, I find the fast set regular to be about right. They have a neat metering system based on simple hand pumps (one squirt from each can, mix and use). I don't take the pumps out once I open a can, the shelf life is very good. You can get the amount you need, and easily mix up a little more if you run short. There is some bleed through on porous veneers, but a light sanding removes the excess, and it helps fill the pores. I don't notice any objectionable effects on the finish. I do put waxed paper or a piece of poly over the veneer to prevent bonding to the bag. When using epoxy in this situation, don't believe the set times on the container. When you spread it this thin, the set time will be longer due to less heat generation. I usually leave it in the bag with the pump running overnight. I took a panel out in about 2 hours once, and found the glue wasn't set.
I have a friend who swears by polyurethane glue for veneering, also not waterbased. Presumably the pressure from the bag prevents the foaming action, but I haven't tried it myself.
Either way, you need to experiment to get the right amount of glue.
Bill:
I am not sure I understand the problem, or question you're asking. Describe your rig--self contained unit, or venturi with air compressor, vinyl or poly bag?
I use a venturi with the air compressor, and I find that depending on setting with both the compressor and the venturi unit, I get different results. But it is fairly consistent and predictable. In theory, when the system reaches equilibrium, the pump should stop regardless of what you have in the bag.
Bill, The cycling of the pump is only related to leaks. The more you have and the larger they are, the more the pump will run.
When I seal the bag, and draw the vacuum, I feel of the wrinkles in the bag. If they are tight and hard, I know that I have a good seal. I then check the gauge to verify the hg.
If the wrinkles feel soft in one area, that means there is a leak near there. If I suspect there is a leak in a certain area, I hold my ear as close as I can to that area, and run my hands over the surface trying to hear if there is a change. Once I find a leak, I usually just put a piece of tape over it for the moment.
Other things to look out for are leaks in the system. You might want to just plug the end of the hose, and turn it on without the bag even being a part of the system. You may have leaks elsewhere. I have a manifold where I can hook about six hoses up to the same pump. I have found, after franticly searching the bag for leaks that one of them was just slightly open.
You may have a leaky check valve allowing air to flow back through the pump, or just leaky plumbing etc. that could be corrected. But if your pump does not cycle without the bag, you can assume that the leaks are all in the bag, or they way you seal it after loading it.
Bill,
Do you have a Manufactured bag and pump setup? Is the bag vinyl or poly? If the pump runs off and on without pulling enough vacuum, you may have a pump problem. Or if the pump runs continuous, without pulling enough vacuum, you have a hole somewhere.
Disconnect the hose, put your thumb over it and turn the pump on. If the pump operates properly and pulls enough vacuum, you have a bag problem.
Earl
If you feel or just want to check your bags. Seal the bags over a low pressure air source. Inflate the bags and lightly cover the whole bag with soapy water (outside surface). You will get bubbles blown from any leaks. Dont have to drench it, just make sure entire surface is wet, or at least area you thing might be leaking. This will find even the smallest leaks you can't hear.
I have not used vacuum for woodworking but used them fairly often for composite work on aircraft. When we suspect a bag to be bad, we inflate and check for leaks with leak detector (nothing but soapy water in a bottle). Also in aviation we have seal putty. Can use flat plastic, and seal down to a smooth flat surface. That way we can do repairs on aircraft, or on a smooth flat table for large items. Our pumps run constantly, no cycling.
On my rig, when the pump starts cycling too often, it generally means that the mouth of the bag isn't sealing well. The inner surface of the bag gets dust and glue particles and other schmutz on it, and that prevents the closure from working properly. I clean it out with one of those kitchen-sink scrubbies, and that usually does it.
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