Friday, June 6, 2014

A big jar of nothing.

 There it is with the view ports welded in. I went for one to view filament and on to view object being coated. (I didn't do the welding, I thought, as it was critical I leave it to the expert!)

 There was some distortion when the welding happened so I had to sand the flanges flat again. I made that little sanding thing to help. It took a while but I got then flat enough for the o-rings to take up any of the remaining irregularity.

 The strangest thing was the bottom of the jar was no longer flat after the welding process. It shrank upwards towards the welded pipe. I was very surprised. It was across a section of at least 50mm. I spotted it when I did a test pump down and the bottom gasket was buzzing as air rushed in through the gap! Back to the counter top with sand paper as described before.

 Above is a view of the o-ring seal on view port. The o-ring are viton(€16 each woops). 6mm cross section. The glass is a square in 12mm, normal type. I just got a glazier to cut it for me.
So the tube sits proud of the flange by about 3mm. This keeps the o-ring in place. I cut the clamping rings from 3mm aluminium. Drilled the top one tapped the underside one. It is split as you can see from the side view.
Oh and I pumped it down today, its tight! Yipee!

Tuesday, April 8, 2014

Cheapo (metal) Bell Jar the beginnings...

After a long stretch of solid work I got a chance to get back to vacuum for a while.
I was getting a bit nervous of my glass belljar imploding so I decided to go for a metal one instead.
After seeing a guy on youtube who built a mildsteel box to coat telescope mirrors in I decided to give the gas cylinder a second try.
I bought a used gas cylinder 380mm diameter just right for my base plate at 400mm.
I let the remaining gas out and unscrewed the valve. Don't forget to fill it with water to displace the remaining gas before you go a cuttin'

I marked the line by wrapping a yoga at around the cylinder to get a parallel line. That wasnt a good idea... you need to use some thing not so stretchy, that defeats the purpose! I marked away and used the bottom end with the complete dome.
I put the cylinder on it side and cut it with the water still inside. It was pouring out all over the place. I trimmed much of the foot ring off too. I wanted to get rid of excess weight. It is heavy.
Once it was cut I dried it straight away. We don't want any rust forming, very bad for outgassing.
Then on to leveling. Even though I paid great attention to the cutting I still spent 3 hours leveling the cut to get it nice ond flat. For this process I spray glued strips of al'oxide paper to a scrap of countertop. Its nice and flat and very ridged. I did a couple of hours of 80grit and an hour with 120...
Very tiring on the arms! During the sanding I regularly ran a black marker along the edge so I could see my progress clearly. I would watch the colour sand off, and the black patches get smaller and smaller. Eventually I got down to a consistant edge. I went then, by hand with a finer sand paper to get rid of the 120 scratches. I rubbed along the edge insted of across it. If there are scratches left they are better not to be radial across the edge as it is more likely to leak in that case.

The whole interior had a dark crust on it. I don't know what it is, possibly passivated with phosphate? I diligentlly sanded this away. There was still a bad smell coming from the tank. This got less and less as the crustyness came off.
I cut a new flat rubber gasket and greased up and sure enough it pumped down to 40micron the limit of my pump....