Feedback needed for Vacuum Fixture Idea

[bmcnabb]

Hello all,

I am currently working on a custom vacuum fixture for a new and potentially large customer. I was planning on using a modified plenum and would like to have my vacuum inlet on the side of my 2" thick Delrin I will be using as my base. My concern is that the vacuum will not fill the channel up completely if I do this, and that it will be very strong suction near the vacuum hose and weak suction further away. Can anyone with more knowledge on this tell me if my design will work, or should I go in from the bottom dead center of my channel? I've attatched a PDF to show my concept.14442

[stump]

It should work just fine in either location. Vacuum works by lowering the air pressure in the chamber. The pressure will lower equally in the chamber no matter where you draw it from. Leakage will result in higher air pressure in the entire chamber. Any differences should not be noticeable IMHO unless you have sealing problems.

[bmcnabb]

Thanks Miles. Not very familiar with vacuum so it helps to have someone varify this for me.

[knight_toolworks]

the only problem I see is there is not much surface getting vacuumed. is the middle of the part cut away? if not then I would open it up and just have spacers to keep the material from sagging. You want as much vacuum area as possible.

[dana_swift]

Brandon- the trick on a dedicated vacuum fixture is to use gasketing.

Look into All Star adheasives:
http://allstaradhesives.com/

They give quit a bit of info on their web site. But my experience has been really good using vacuum with their products for dedicated vacuum fixtures.

In simple english, create "evacuation" channels that act like drainage ditches to move residual air out from under your part. These flow into your main vacuum plenum like you have drawn in your pdf. The evacuation channels only need to be 0.020 deep or so, just enough to get air molecules into. Deeper is needed if you plan to cut through and have significant air flow.

One other secret to using vacuum: use an accumulator. When you open your valve to initially pull down your part there needs to be a space for the air to go. Also use large piping so the air at low pressures has very little friction. 2" PVC seems to be a good size for me.

My main accumulator is a discarded 100 pound propane bottle. I suck that down to 20" Hg or better. Then I open the valve to the fixture. "Thump" everything makes up tight. If your gasket is good you should be able to maintain a very high vacuum. Your vacuum level will stabilize at the point where your leakage equals the CFM for your pump/blower at that pressure.

With most of my fixtures before cutting the system will pull down to 24" Hg - then the limit switch turns off the pump to stop wasting electricity. After a while it bleeds back down to 18" Hg and the pump kicks on again. Cycling based on how much leakage there is.

If you cut through your part, or work with porous material the leakage will change (if you cut through it can get dramatic.) You will learn from experience how much vacuum is required to hold your parts still on your fixture while you cut them. When I make the final cut through on my parts the vacuum quickly drops to about 10" Hg. At that level with my particular parts everything is still good. You will learn some other value. At 10" Hg I am counting on the CFM capability of the pump to hold the vacuum with holes in my part. The 2" PVC vacuum lines really pay off there.

You may discover you need more pump, or less. In my case if I need less, the pump just shuts off automatically and cycles every minute or two. Once the sustained pressure drops below 24" the pump runs continuously at its rated CFM for that pressure.

FYI, my main vacuum pump is a 5hp Beckker vane pump. That can provide 100CFM open, and high vacuum when the system is solid. Its the best of both the high CFM and high vacuum worlds, but is not cheap. I started out using inexpensive air conditioning vacuum pumps which worked fine but didnt last as long as I want, they also put out a potentially explosive air/oil mixture which has to be vented outside. The Beckker pump I use does not produce any oil mist.

Once you get accustomed to vacuum its hard to go back to mechanical hold down. My vacuum fixtures bolt to my main spoilboard making them easily changable, so I can use standard mechanical hold-down for things that require that, and vacuum for specific dedicated parts.

Key factors: gasketing, accumulator, large pipes, big pump.

Hope that helps-

D

[dana_swift]

One other thought, I have posted elsewhere on the forum. I have found that industrial 3/4" garden hoses will not collapse under almost perfect vacuum pressures. They do get a little smaller! But the opening is good.

If you need flexible hoses, industrial "red" 3/4" garden hoses were the cheapest thing I could find that works. And they work great. I am still on my original set after about 4 years now.

I use several in parallel to make a large cross section if that is needed for flow, and just one by itself if little flow is expected.

Thought I would add that comment-

D

[knight_toolworks]

Also use the thinnest gasket material you can to keep the part from jiggling. 1/16" or 1/32" is best if you can do it (material is smooth)