Pilot hole size for threading an ASME B16.8.4 type B or AB thread forming screws into plastic

[debun]

Does anyone know of tried trued and tested pilot hole sizes for using ASME type B or AB thread forming screws in plastic? I was thinking simply 0.8*the major dia would suffice but I have changed that theory for the reasons below if you care to read on.
I see design guides for all the proprietary plastic screws i.e. plastite, remform, hi lo etc. etc. and there is a huge variation within even just one size of screw e.g. 0.02” pliot size variation between a #6 plastite 48-2 and 45 screw and that’s from the same vendor. Even if you use the same screw e.g. a #6 remform has ~0.02” pilot size hole variation based upon the material you are threading into. Pilot hole size seems to change based on the modulus of the plastic and the thread depth. What do you do if you are using a regular ASME thread forming screw? I may not be able to source another screw that has a good design guide and I’m limited on development time for testing.

 

[MadMango]

The variations you are seeing for pilot holes comes from the amount of thread engagement you desire, 100%-40%.

I've never seen any standard guide, due to the issues you stated above, base materials, varying thicknesses, etc. When I am in a time crunch and can't source Plastite screws, I always take the Maximum Minor diameter and add 20%. So say a #6 Type AB screw has a Max Minor diameter of .104", +20% (.021"), yields a pilot hole of .125". This works pretty well for me into basic ABS, Nylon (non-glass filed), and Delrin sheets. When screwing into the material off our 3D printer, I usually add 24% to prevent cracking bosses.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[btrueblood]

What Mad said, plus, when time is tight, you usually are constrained to one or two standard-size drills to make the pilot holes. Sometimes I just order some of both size drills and make a few test articles, looking for cracking when tightening the screws and testing (however crudely) for pull-out strength.

 

[debun]

I’m completely new to the world of plastics and I’ve been trying to find the performance differences between AB/B and specialty plastic screws. I’m told that anything other than a specialty plastic screw will strip out in polpopylene. Any truth to this? How much can I gain by increasing screw length? I know in metals the return diminishes rapidly after 4-6 threads.

 

[Screwman1]

I would never consider using an AB/B type screw in polypro. You will have terrible strip-out problems because there is too little differential between the drive torque and the strip torque. I would go with a Hi-Lo or a Delta PT in that material. Plastite is better for higher moodulus materials.
I always want to do testing on any new application in plastic because there is so much variability between different resins. Try to keep draft angle to a minimum.
You need to test enough parts to find the mean peak drive toque and the mean failure torque and then set your seating torque at half the difference between those two numbers. Watch out for process capability when you add in your seating torque tolerance.

 

[mcgyvr]

I second the hi-lo/delta pt recommendation.

Here is a very "generic" recommendation based on the materials..I've saw this floating around somewhere and put it in my excel memory bank.
Flexural Modulus..
200,000 psi or less = thread forming
200,000 psi to 400,000 psi = tri-lobe or hi-lo
400,000 psi to 1,000,000 psi = thread-cutting
1,000,000 psi+ = type t thread cutting.

and some good info here in the PDF file "threaded fasteners for plastics" on that page.

http://www.infastech.com/Products/B...ead-forming-rolling-tapping-screws/Delta-PT-R

 

[Screwman1]

Hey Mac; you must have attended one of my training sessions! I recognize those numbers! Those go back a long time.

 

[debun]

Yeah Dupont has a guide on choosing the right screw Link where they mention flexural moduls but I also found another reference on the internet that said to use the pitch diameter of the screw and the pilot hole size. That seems to make the most sense to me.

 

[Screwman1]

You have to factor the flex modulus in too. Otherwise you'll end up using the same % engagement and fastener design for polypro as 33% GF nylon. The fastener and hole size vary with the material.