How to Build a Small Vaccum Holding Table?

Hi, everyone. Love this Forum but haven't been on in so long.

I make a small item in my business (more on that later) that is made of plastic and about 1.5" in diameter. I am looking to possibly building a small vacuum table to see if that would me hold it in place for sanding and polishing.

I have a small compressor and have seen venturi attachments out there that say this would work, and a buddy of mine also mentioned a cheap Harbor Freight vacuum pump.

If anyone has a link here or elsewhere online for some simple instructions, they would be so great. I want to start out holding down just one piece as an experiment, then grow it for more units if this works.

Thank You,
EG

I saw this a while back on Make's site that relied on the porosity of MDF as the hold down: https://makezine.com/projects/make-4...d-down-system/ this works with an ordinary shop vac (or shop vac motor as in the build pictures. ) This clearly holds the part against some fairly significant forces from the CNC.

I don't know if this would scale down to the small size you need, though. Depends on the lateral forces that might end up being applied to the part as it's sanded and polished. 1.5" isn't a lot of area.

Quote:

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Originally Posted by bruce.desertrat

I saw this a while back on Make's site that relied on the porosity of MDF as the hold down: https://makezine.com/projects/make-4...d-down-system/ this works with an ordinary shop vac (or shop vac motor as in the build pictures. ) This clearly holds the part against some fairly significant forces from the CNC.

I don't know if this would scale down to the small size you need, though. Depends on the lateral forces that might end up being applied to the part as it's sanded and polished. 1.5" isn't a lot of area.

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Bruce, thanks so much for taking the time to post and respond, and even more for supplying a link. I'm the run right now but promise to check this out right away.

Here's the good news. We don't need a lot of suction. This is merely to hold a small part in place for sanding from above -- no kind of CNC or cutting would be involved. So this might very well merit a close look.

Thanks Again. Promise to post again after reviewing this.

Be Well,
EG

A google search of "ac compressor as vaccuum pump" turned up numerous YouTube videos.
Years ago I messed around with an old car ac pump as a vaccuum pump driven by an electric motor. Never did anything serious with it but it would build strong vaccuum.

I made a post on this subject a while back
https://www.homemadetools.net/forum/...589#post128299
There is a link to a video which also discusses possible vacuum sources including the venturi type that you mention.

For a small item I would not even entertain the idea of using the porosity of MDF. That works OK when you have a large area such as holding a large sheet on a router table.

I've played with this quite a bit on my CNC machine, needless to say it's been quite a learning process.
Here's the factors you're working with:
Imagine your object just laying on a table, it is fully surrounded by air. Assuming you're somewhere near sea level, all the air surrounding the object is at atmospheric pressure at 14.7 pounds per square inch. Unless the object is sealed to the table, atmospheric pressure is pushing equally all around the object.
With vacuum hold down, what you're trying to do is to remove as much air as possible that is underneath the object. If you can remove all the air that is between the object and your vacuum holding fixture, the air above the object will push the item down on the fixture at 14.7 pounds per square inch.
You can add all the "suction" you want, but once you are able to remove the air beneath the object, you will never hold it down more than 14.7 pounds per square inch. If I did the math right, your 1.5" dia. disc would have a max of 26# of atmospheric weight holding it down.
Conversely, if there is a lot of air leakage between the fixture and the disc, you'll never remove enough air between the disc and the table to have the full weight of atmospheric pressure bearing down on it. For small objects, a moderate size shop vac will supply more than sufficient air flow. Venturi units are better than vacuums for prolonged running (with a sufficient sized air compressor) but the trade off is the amount of airflow.
Either way you need a good gasket between the fixture and the object.
The other important thing is the amount friction between the fixture and the disc. The more contact area between the fixture and the disc and the "grabbier" the contact area is, the less chance of the disc sliding off the fixture.
With a little ingenuity and experimentation, you can build a simple vacuum hold-down fixture out of abs plastic. You should be able to purchase online an abs flange like you would use to hook your vacuum hose to a piece of machinery. Buy a scrap of abs sheet stock the same dimensions as the flange (preferably smooth both sides), drill a 3/8" diameter hole through the middle of it and glue it to the flat side of the vacuum hose flange. Make sure there aren't any gaps in the glue. If you're using a vacuum pump or a venturi unit, you can make a fixture out of 3 layers of abs glued together with the middle layer being used to create an internal passages from an air fitting on the edge of the glue-up to a center hole.
Source a small piece of 1/16" thick closed cell foam gasket material, preferably with adhesive on one side. http://allstaradhesives.com/
Here's where the experimentation starts. You need a slightly larger than 1-1/2" diameter ring of gasket material, maybe 1/4" wide, centered on the hole to seal around the perimeter of the disc. You might need more contact area with the gasket material to provide friction to keep your object from sliding. So you can fill in some of the space inside the 1-1/2" diameter ring with more pieces of gasket material, leaving little channels between the pieces to facilitate the removal of air between the disc and the fixture. Maybe you'll need to enlarge the 3/8" diameter hole.
If you are only polishing one side of your disc, sometimes it helps to leave the paper on the other side for more friction, sometimes it doesn't because the gasket material could possibly create a better seal with a smooth plastic surface. If you are polishing both sides, be aware the often the final polishing compound has silicone in it which can make your disc really slippery.

Some experimentation should bring you to a combination that works.
Good luck.
Al Waddell