Post 4 More on the vacuum pan
Now that the vacuum pan machining in the lathe is finished the pan is removed. The next step is to drill an internal (L) shaped hole in the pan that enters the side of the pan and exits into the pan. (1/8" or less in diameter hole) see diagram.
l--l inside pan l l--l l l ------ll-------l l ll l l=== ll--------------/---l---- l---------------\-------- Valve fitted to side of pan ^ l to hold vacuuum -----------------\---l \ \-- 1/8" hole in pan to pull vac.
A small valve such as a good gas valve is fitted to the entrance hole on the side of the pan. A valve that had 1/8" pipe thread was fitted to the pan by using a drill and pipe tap. I had problems with some valves as they would leak causing loss of some vacuum. To fix the problem I disassembled the valve,laped the parts and reassembled it with laboratory vacuum jar grease. A short length of small diameter hose is attached to the valve which is used to suck the vacuum.
Last step in making the pan is to lap the seal rim. Lay a piece of plate glass on some carpet. With some fine grit grind the pan rim against the plate glass. Check the rim at many points around it for flatness. Get the rim flat to .0001" or less. Use the spherometer that was made to do this. This is the only critical part in making the pan and like mirror making it is ground in. Also important is to check this at about the temperature which you will be grinding and polishing. I painted the inside of my pan with black lacquer. This helps when checking for pits as the plate is polished out.
I will talk a little about the glass in my next post. Bob Pfaff