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I just discovered that in addition to standard #10-12 sheet metal screws, there are #10-16 and even #10-24 versions available. I'm planning to use them on 0.06" thin aluminum, 0.12" nylon and a sandwich of both. Will a higher TPI version be more resilient to thread stripping with excessive torque? In my cursory analysis, higher TPI, given constant thread depth, should provide higher thread surface contact with surrounding material, resulting in a stronger bond. Is that correct?

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Yes. Thin materials and stronger fastening are both reasons fine pitched fasteners are used. I don't know about sheet metal but in normal fasteners, #10-24 is the regular coarse pitch. #10-32 is the fine pitch. But you won't even get 2 threads in the aluminum and I think you want at least 3-5.

Also, which material is going to be tapped? You only have thread the hole in the material farthest from the head of the screw/bolt.

given constant thread depth

Threads depths are not constant. You can tell by looking at the recommended tap drill sizes for the same diameter screw/bolt with different thread pitches.

Personally, I would try and find an alternative approach to tapping the the aluminum. Like a thin nut, or a PEM nut, or something else depending on the situation. Can you do something like use a pin instead of a screw and and flare the pin with a punch?

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  • $\begingroup$ I'm looking for the simplest approach. There will be no tapping, just a pilot hole for a sheet metal screw. I just found and tested #10-16 on 1/32" aluminum sheet, and I couldn't strip the thread with a regular screwdriver. I consider that good enough. I ordered #10-24 SMS for comparison. $\endgroup$ Sep 12, 2022 at 4:11
  • $\begingroup$ @PaulJurczak Even then the equivalent is to drill a larger clearance hole in the material closest to the screw head so it does not get tapped by the screw. $\endgroup$
    – DKNguyen
    Sep 12, 2022 at 13:07
  • $\begingroup$ Yes, that's the plan. The mounting plate contacting the screw head has a large diameter hole, not engaging the thread. $\endgroup$ Sep 12, 2022 at 23:19
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You need at least 2.5 to 3 threads of engagement for the rated force. This only allows part of 1 turn.

The worm gear compression effect may warp the material. So a lower pitch or finer thread pitch will improve matters. The weakest link is likely to be the thread rather than the sheet.

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