Machining a square section O ring groove

you should be able to cut this on your lathe using a "trepanning" tool - you can grind your own

If that nasty old rotary table confuses you, I'll take care of it for you :-).

Trepan. You need a trepanning tool, which is much like a cutoff tool except that it accounts for the curve of the cut it makes. Then you get to find out how rigid your lathe is.

If you aren't comfortable cutting straight in with a square tool you could grind a triangular bit like a threading tool but about 20 to 30 degrees between sides and cut in a little at a time with it. Angle the tool so only the tip cuts, run in with the carriage until it starts to complain, back off a little and cut in or out almost to the other wall. Once you have roughed out most of the groove you can use a square-ended bit narrower than the slot so only one side cuts at a time.

The pointed tool I use is rounded on the end and nearly flat on top so it cuts in both directions. The angle is ~30 degrees. I use it to rough out belt grooves on mandrel-mounted pulleys, which won't stand much cutting force.

jsw

You'd best use HSS and low cutting speed, and the lathe is definitely better than the rotary table and mill. The O-ring has to seal against the bottom face, so the finish of that part of the cut is critical. Sidewalls don't need to be exactly at 90 degrees, usually they're low-tolerance.

Is the O ring groove on the inside diameter of the big tube? If so I have done it the wrong way in the past. I think to cut a gland, you are supposed to have a boring bare bit ground to the correct width, and just make one cut.