Threadlocking in Plastic?

How about using the same thing Mauser did on rifles? Add a small lock screw that has to be removed or turned into position befure the main screw will turn.

like this:

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.

Joe Gwinn

"Steve W." wrote in news:ka36k5$ndf$ snipped-for-privacy@dont-email.me:

I don't see how that works well for fine adjustment. I want to tweak it so I have a slip fit with as little play as possible.

Doug White

Joseph Gwinn wrote in news: snipped-for-privacy@news.giganews.com:

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.

Thanks!

Doug White

Doug White wrote in news:XnsA124D2B687CC7gwhitealummitedu@69.16.186.7:

It turns out Permatex has a plastic threadlocking compound for ~ $6 on Amazon, so I'll probably get some of that & possible combine it with the home made tight tap idea.

Doug White

Years ago they used CA glue (super glue) on screws in plastic on R/C racing cars. When I've done it, I assembled without the glue and then applied the glue to the bolt head and car chassis after assembly. This holds well but breaks away cleanly with a little screwdriver force.

RogerN

You can also use thread locking helicoil inserts.

I have a 3 year old bottle of Gorilla Glue that gets used occasionally. It spends all of it's time in a freezer between the use times. Store opened epoxy's and glues in a freezer. Take the out just before you want to use them.

Threading in Delrin usually produces undersized holes, so they grip pretty well to start with. See what happens there before going into extra hold-downs.

Enjoy, DoN.

But why NOT use Loctite?

Delrin is about as inert as plastics come.

Easy enough to try it ans see.

Can you slot or slit the part and use a grub screw to expand it to the size you need?

Greetings Doug, I machine lots of Delrin and Acetron. What Joseph says about rolling threads is correct. When I'm making parts for a customer that wants a tight thread I just use a form tap, AKA roll tap, to tap the hole. Often even a cutting tap will leave threads undersize enough but a form tap always makes for tight threads. Eric

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.

Joe Gwinn

Joseph Gwinn wrote in news: snipped-for-privacy@news.giganews.com:

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.

Doug White

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

Joe Gwinn