In article ,
Doug White wrote:

Joseph Gwinn wrote in
:

In article ,
Doug White wrote:

Joseph Gwinn wrote in
:

In article ,
Doug White wrote:

I need to take the slop out of a couple Delrin devices that are
subjest to vibration. They are a bit loose in the hole they fits
into. A nylon set screw threaded into a hole tapped into the
Delrin would do the trick, but it will probably work loose pretty
quickly.

It it was metal, with a metal screw, I'd just put some Locktite on
it & be done. I've seen spectacular failures with regular
Locktite getting near some plastics. They do make a special
version for plastic parts, but McMaster wants almost $20 for a
bottle that will spoil long before I use more than 10 drops.

My current thinking is to stack a pair of nylon setscrews to lock
against each other, but adjusting them will be tricky unless I
bore all the way though and can adjust the locking screw from the
far side. I can try to thread the holes _just_ deep enough to
hold the screws at the right height, but if I overshoot, I'm stuck
with Plan A.

Any ideas or suggestions?

Drill the hole a bit undersize, and run the screw in without
cutting threads, using the screw to roll its own threads.

Delrin one of the few plastics for which press fits work, according
to the datasheet. The datasheet (or app notes) tells how
aggressive one should be.

I couldn't tap the Delrin with a nylon screw, but I could find an
undersized steel screw & make a tight tap. Then I could use the
nylon set screws. It might take a little experimenting, but I think
that would work OK.

It also occurs to me that a trilobular (thread-rolling) screw might
work well in delrin. After rolling, the plastic will shrink back a
bit, capturing the rounded-triangle shaped screw enough that it won't
easily work loose.

My instinct is that threadlocking compounds like locktite will fail in
delrin, which is very slippery.

My original concern was that it would attacke the plastics, either the
Delrin or the nylon screw. I've seen very spectacular failures when
liquid Locktite was used with polycarboante (Lexan).

I'm going to try the thread forming tap approach. Just got a couple taps
off eBay.

If you don't already have it, Du Pont publishes some very good
application notes on how best to use delrin.

Joe Gwinn