Until now I have been using hot glue to connect tubes to the vacuum manifold. This works, but the joint is mechanically weak and will leak if the tube is even slightly bumped. When the bakeout oven is used, bumping the tube is inevitable, so the hot glue required constant reheating and re-sealing throughout the bakeout process. This was irritating, so I made a reusable adapter out of some parts from the hardware store. The parts and completed adapter can be seen above.
The construction of the adapter is very simple. I used my lathe to drill a 13/64 inch (slightly larger than 5mm) hole through a short 3/8 inch bolt, and another hole of the same diameter through a matching cap nut. I squared off the head of the bolt and chamfered the hole at the end of the bolt as well. I then drilled a 1/8 inch hole into the end of a 3/8 NPT plug, and joined the plug and the bolt with ordinary two-part epoxy. The hole in the plug is smaller than the hole in the bolt so that the tubing doesn't fall through the adapter and into the pump. To use the adapter, a small o-ring is coated in silicone grease and pushed onto the end of some 5mm glass tubing, then the tubing is put into the adapter and the o-ring is compressed against the tubing and the bolt by the cap nut. The adapter works well, and the seal is stable even when the tube being evacuated is mechanically disturbed.