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.

Monday, April 4, 2011

Cleaned Out Venco Pugmill

I've not cleaned out my Venco Pugmill for several years and it just seemed like the time to do it because I was not getting a good vacuum and there looked to be lots of corrosion. I had decided to install Buna N rubber gaskets all the way around to try to get the vacuum tighter, and to try to have less air seeping through the joints. When I don't use the pugmill for several months there is a noticeable dryness at the horizontal seam that runs the length of the pugmill. Venco recommends electrical tape for the joints, but over time, air gets through and it causes problems with stuff spalling off the sides into the clay, and the hardness I mentioned.

One test that I used was to take a pug and twist it, and in so doing there were air bubbles and delamination. So I decided to tape the screen aggressively so that the total amount of volume that needed to be deaired was less than before, and was nearer the vacuum chamber. As luck would have it, Rick McKinney of MKM Tools stopped by and he noticed that the screens were polished where the most clay went through and suggested that I tape with this in mind and so I did. Not sure how this will work, and for sure the mill will be slower, but I hope it helps. The mill is too fast anyway -- hard to keep up with for one person.