Sunday, April 17, 2011

Venco Reassembled

Yesterday I got the Venco totally reassembled and tried to pug some stoneware clay through it. The first thing I encountered was that my little taping of the screens was way too aggressive and I had to remove them and take all the tape off and then just retape the top 2.5 inches. What was happening is that clay was totally filling the slot which allows a vacuum to reach the clay. It works much better now.

But, and I accept suggestions here, it will not pull a vacuum. The vacuum pump, which is one of the old Clisbys, is working fine. If I put my finger over the hole it goes straight to 28+ inches of Hg. But with the NEW gasket in place, and just about every old and new gasket I could find or make from the stuff I have around, except the old one which I stupidly threw away, it will not go above 5 inches Hg.

Since the middle gaskets were of electrical tape, and since clay got real dry around the gaskets if the mill was not used for some time, I made new ones out of Buna-N rubber, the same rubber that separates the top and bottom pieces on my Shimpo stainless steel mill. I also made the vacuum chamber gasket out of this Buna-N rubber but it really leaks. Badly.

I have a really nice lap wheel and so took one of the metallic diamond pads and attached it to a flat piece of metal, and sanded the top of the vacuum chamber until it was flat, which took some time. Did not help a lot.

The top piece of plexiglas is the piece that came with the pugmill and it is not totally flat and many years old -- close to 25 probably. I tried some other pieces of thin plexi and they did not work either.

So. What to do? The first thing is that I will block off the vacuum chamber leading into the pugmill and make sure it's the gasket at the top of the chamber that is the problem. I think it is but it could be the Buna-N rubber which runs down the seam of the top and bottom pieces of the chamber, or the Buna-N pieces that protect the plates where the screens are extracted. I don't think it's this because the Shimpo works fine with the Buna-N.

I do not tend to have great mechanical ability. A problem like this one stumps me for some time. The mill has lots of clay in it so it's not open at the end, as a Venco can tend to be. Interesting problem.

Suggestions appreciated.


John Bauman said...

Any chance the leak is either in the hose or the connection between the plexi and the gauge? I can't imagine why your gasket wouldn't do the job. Is the relief valve open at the end of the hose?

John Tilton said...

Hey John,

No, if I plug up the hole in the vacuum chamber with my thumb, it goes straight to 28" Hg. I'm pretty sure it's the gasket between the top cover and the vacuum chamber.


John Bauman said...

Have you tried to seal it with silicone -- tub and tile caulk? It would be a mess, but it would clean up if it didn't work. (though I can't imagine caulk working any better than the gasket you already made for the thing.

John Tilton said...

Hi John,

I emailed Venco and they said that you have to use closed cell foam rubber and I bought two gaskets from them, which I hope were shipped today. I'll be able to make some porcelain in the meantime.

Looking for closed cell foam rubber was a Pandora's box of an enormous selection of different materials. I decided to play it safe and get it from them as I am sure it's the best kind.


John Bauman said...

Others have recommended a closed cell for me as well.

Ben Owen Pottery Blog said...

Hey John,

I came across your question on the pugmill and the leak. I have three 4" vencos and know many of the issues with leaks. Step 1... Make sure you wet down the gasket under the clear cover and clean the aluminum casing under with a sponge and that may solve it. Another common problem is the seal of the two halves of the barrels after cleaning and reassembling. I put two layers of electrical tape on each side of the barrel, 2x4 places = 8 pieces of tape. If the tape is not thick enough to make a good gasket, the vacum will not work properly. A third place that the pugmill can have a leak is the covers over the screens. Wet the foam gaskets prior to reinstalling after the screens are in place. If you are having any amount of vacum to work. Turn on the pugmill and let it build up and turn off with the vacum still on the clay. Listen quickly for any leaks at that point.
Hope this will help. We went to Florida in March for a family vacation and had planned to visit you but ran out of time. Maybe next time I will make it.
Ben Owen III

Ben Owen Pottery Blog said...


Also, Use duct tape to seal the top 2" of the screens and that should help with the clay coming up into the chamber. Make sure not to feed the pugmill too fast with clay and that will help also.

Ben Owen III

John Tilton said...

Hi Ben,

Thank you for this. I also have one of those little Shimpo pugmills for porcelain and the gaskets on that are Buna-N rubber. So I decided to try that because when the Venco sat around for months, the clay became dry at the tape gaskets, whereas, with the Buna-N of the Shimpo, it did not. So it has Buna-N gaskets instead of tape.

The other thing about the Shimpo is that it uses a very soft rubber between the Plexiglas top and the top of the vacuum chamber. It's very soft and it's glued to the Plexiglas top piece. I bought a piece of closed cell polyethelyne from MSC and it might work -- it would have to be glued to the top -- but it's not as soft as Shimpo's, which I have not been able to find out what it is from them. So I don't want to glue it and find out that it's wrong.

The problem was between the chamber and the plate covers. I have that fixed, at least I think. But there is still leakage at the top gasket.

I bought some new gaskets from Venco, but they also seem stiff and not able to get a perfect vacuum. I get about 25" Hg.

The polyethelene sheet also seems stiff, though the vacuum will just suck the gasket in, so it's not that stiff. But it's stiffer than the one on the Shimpo.

If Anne helps me and we pug for several minutes, I can get acceptable clay with the Venco gaskets, but it's not great when I'm alone.

Also made a top plate from 3/4 inch clear Plexiglas and it's nice to be able to see into the chamber again. It's very stiff and straight.

The top of the screens are taped down about 2.5 inches.

Thank you for the help here. It will be perfect before it's over.


Jered said...

This may be for the next time.
Use a colored soap to spray lightly around all over the outside of the chamber while the pump is running and the seals are as good as you can make them. Clean off the soap from the outside and take a look inside. It seems the hardest part for me is finding the leak. I have used this method for pressures (gas lines, etc) as well as vacuums.

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Greg McRitchie said...

I had a Venco and re-did the seals in the manner you described. A little vaseline on the vacuum plate helped to seal it. If you really want to find all the leaks, use reverse pressure and soapy water.