Dragging the Boat Anchor

Well the Boat Anchor is at it again. Simple little thing, pick out some
screws to use to fasten a flat top on a polypropylene tank that is about
3/4" thick. He picked out 1/4-20 Washer head type F thread cutting screws.
These are the ones with the little gashes down the point that are supposed
to cut the threads. I tired to explain to him that in a material like
polypropylene the little gashes will just plug up and the screw will
basically just mush it's way in. It'll hold for one use but if you take the
top off a couple of times the threads are just going to tear out.

He reads the McMaster Carr catalog where it says the type F are for metals
and plastics, completely ignoring the next line that say that Type 25 and
Type 27 are much better for soft plastics. They both have a large cut out
that basically forms a 1 flute gun tap with lots of clearance. I'd probably
choose a thread forming screw though in Polypropylene.

I bet him lunch on this. So plastics guys, do I get to pick an expensive
restaurant or eat crow?

Gary H. Lucas