On Thursday, January 30, 2020 at 12:16:47 PM UTC-5, Klay Anderson wrote:
So--I've a tap and die set for plastic pipe (think sprinkler system). Any ideas for the best lube for threading same? Dry alone snaps the pipe. I think petroleum products will attack the plastic. Silicone?

Three things:

a) warm the pipe. Even a few degrees will make a large difference. Ideally, you will want the pipe to be just below the plastic stage, but unless you have a "bending box" don't try this at home. About 100F is close enough for your purposes. Anything below about 60F, it will be very brittle.

b) If you run lubricant over the pipe while cutting - such as with a Ridge Cutter - there will be no issues.

c) And if you never (repeat, NEVER) will need to use solvent fittings on that pipe, good old Vaseline will do the trick nicely.

Then, there is the obvious: Cut slowly. Cutting too fast overheats the pipe, and if you overshoot into the plastic stage, all you will do is make lots of scrap. Use only (repeat, ONLY) Schedule 80 or Schedule 120 pipe. Schedule 40 pipe is too thin to take a safe thread and also withstand any level of operating pressure. Did I mention: Use ONLY SCHEDULE 80 pipe, or better.

Peter Wieck
Melrose Park, PA