[CNCWNTTOBE]

Hi,

Can anyone tell me where I can get recommended pilot hole diameters for M3 and M4 screws that will self thread into an injection molded nylon part? Is there a general rule for hole size when threading into plastic - nylon in specific? Thanks for your help.

[sigma relief]

Your pilot hole dia depends a lot on the type of screw and the specifics of the Nylon. "Recommended" is also a vague term as it depends what you are optimizing for, pull out strength, stress cracking of parts, or repeatability of install.


Are the screws standard machine thread, thread cutting/thread rolling, or sheet metal style. In general, I would go at least 75% thread depth for machine screws and as close to the shaft diameter as you feel comfortable for coarse thread sheet metal screws.

The other side of the equation is hoop stress in the area around the hole. If the hole is in a large solid area, this is moot. If the hole is in a smaller area or has a raised boss, this is a real problem. Nylon is a bit forgiving in this case, but the expansion from forcing a traditional or thread rolling screw in can crack the parts. If you are using a glass filled nylon, this is a much more severe problem, do the calculations on the hoop stress.