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Thread: Best thread inserts for delrin?
03-15-2010, 03:48 PM #1
Best thread inserts for delrin?
I am making a few parts out of delrin and need a few blind tapped holes. I was thinking of using helicoils but was wondering if anyone has any experience with the press in style inserts. McMaster-Carr I am mainly concerned about the tear out strength of the insert. Any advice would be appreciated.
03-15-2010, 04:26 PM #2
not answering your question directly, but there are other inserts besides Helicoils. Some are solid whereas you know Helicoils are wound wire. Space to install and the size of the part may be limitations.
03-15-2010, 04:39 PM #3
Well non-scientifically speaking, if it takes you one hammer hit to get that insert in, its going to take that much energy (or probably less) to get it forced out.
Keenserts are a type you may want to look at, steel tube like bodies with both an internal and external thread that get threaded into the part.
03-15-2010, 04:45 PM #4
Are these inserts for repetitive use? (running screws in and out often) or just for an assembly?
I've used the self threading inserts in acrylic, polycarbonate, and PTFE (Teflon). They are the style with McMaster P/N: 99362A700
The example shown are 1/4-20 but they have many more to choose from.
These were for repetitive use with thumbscrews. Otherwise, my experience with Delrin is that it is strong enough to thread as though it were aluminum.
The style I've shown are not quick to install however, so you may be looking for a press fit style like you said.
03-15-2010, 04:46 PM #5
I wouldn't want to use a press-in/crimp-in type insert in Delrin. There's too strong a chance that it would just pull out. Helicoils or clones thereof should work very well. We've even used them in Oak for retaining handrails with Stainless machine screws.
03-15-2010, 04:53 PM #6
+1 for key locking insert. I've used it a lot in aluminum and UHMW with good success.
03-15-2010, 04:59 PM #7
I used to make parts for NAVY SUBs and one item was Delrin Bearings for the snorkel, The NAVY specified Helicoils in the surface that had fitting stock on it... so that's what I'd use...
If your application is using the bolt holes and changing the bolt often, I'd look at the type that have the larger thread on the OD and your screw size ID...that type should give you better life of the tapped hole.
03-15-2010, 05:31 PM #8
Helicoil trouble in delrin
Am I the only one with experience of helicoils being single-use parts?
Half the time we need to remove the screws from a helicoil in plastic, the coil comes out by a turn or two and the assembly has to be torn down and sent back to the shop. For my money, use either a self-tapping solid insert (not a press-in), or a keensert. Either way, you save the special tap and frequent replacement.
03-16-2010, 08:40 AM #9
For attachment purposes tapped holes work fine in delrin. Use very sharp taps and a coarse thread. Go at least 2D deep in good thread and counter bore top to help ensure that Ham Handed Harry doesn't cross thread things. I've gone down to M2 with every success but its real easy to break at tap. Big problem is the ease of cross threading on insertion and the destructive results. Even at M6 'ole Harry won't notice cross threading.
Proper inserts for plastics, especially delrin and the other slippery ones, should have anti-rotation and anti-pull out features. In smaller sizes these are usually engaged by expanding the bottom part by driving the screw right home, the insert being made with side slits in the bottom third or so to permit expansion. Allegedly the threads on a proper brass insert properly fitted will strip before the insert pulls out. I've seen the heads of brass M4 and M5 cheese head screws sheared off without evident damage to the insert or its retention.
Mr Mighty Big Manufacturer seems to use ultrasonic and similar methods to fit inserts with exterior patterns to resist rotation and pull out. Many and various patterns out there, each better than the others according to the respective makers.
Helicoils and delrin don't sound a good match. Timeserts or similar should do better. Preferably glued as well as expanded to help retention.
03-16-2010, 08:59 AM #10
With Delrin you want a shouldered insert so that when you tighten the bolt it pulls up against the insert NOT the plastic. This will keep you from pulling out. I use the heated inserts that you melt into a hole.
03-16-2010, 09:13 AM #11
03-16-2010, 11:05 AM #12
I prefer these for Delrin; Mc Master Carr 90248A024. They are thick wall, and have an adhesive seal. No special tools are required to install them, a screwdriver works just fine. They are also available in stainless, and extra thick wall if you have room.
03-16-2010, 04:28 PM #13
UPDATE- In typical engineering fashion there has been a change in material. Now the parts are to be made of UHMWPE so that it is static dissipative. Now I am leaning more towards Plastite screws as I have never had much success with inserts in PE. Thanks for all of your input.
03-16-2010, 05:01 PM #14
These are made of brass and I THINK are inserted hot to seal in the plastic:
MSC Item Detail
There are 8 pages of inserts in this section.....What size thread are you looking for?....
I know where there 1-2 thousand brass, 3/8-16 inserts available for about $.20 each.....