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I purchased a cheap project box made of ABS plastic (so, relatively soft material). The box has pilot holes (with counterbore) on one half and matching smooth bosses/protrusion for driving the screws on the other, but the box came without any assembly screws.

After measuring the hole diameter, which was 2mm, I went ahead and ordered a bunch of M2 screws, and I'm pretty sure they'll do the job, but then I started thinking about and now I'm wondering if I actually ordered the best size for the job.

Digging around tells me that M2 specifies a screw with 2mm Major Diameter, i.e. 2mm is the diameter across the outer thread. In a perfect platonic world in which all dimensions are perfectly accurate, this screw would just graze the smooth sides of the shaft when inserted, wouldn't it?

With dimension tolerances, the screws I ordered will probably bite in sufficiently to do the job. But what size screws should I have ordered for the "best" results? should I choose the screw so the root diameter match the hole diameter, to get the thread to embed as much as possible into the sides? Should I aim for the thread to sink in about half way into the sides, so that the plastic can deform into the recesses of the thread? How much does it matter? are these considerations different for different kinds of materials? How?

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2 Answers 2

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Screw manufacturers have this information

https://www.stanleyengineeredfastening.com/-/media/web/sef/resources/docs/other/threaded_fasteners_for_plastics.ashx

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  • $\begingroup$ Thanks. They suggest to aim for a thread engagement of 75-85% as a starting point, and they give a handy table by size as well. However, I'm a little skeptical because on p11. they suggest choosing a hole size to maximize clamp load. But Wurth (YT video fG-gp0WCZWI) recommends keeping clamp down as far as possible to reduce stresses. $\endgroup$ Jul 14, 2023 at 18:35
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Thanks to @TigerGuy for reference.

Vendor #1 - Stanley

https://www.stanleyengineeredfastening.com/-/media/web/sef/resources/docs/other/threaded_fasteners_for_plastics.ashx suggests 75-85% thread engagement as starting point. This means that the root diameter matters also needs to be taken into account.

The document contains a handy table on p13 "Hole Sizes Per Percentage of Thread Engagement". A few examples (80% TE):

Major D/Hole D
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2.5mm/1.8mm
3.0mm/2.34mm
3.5mm/2.7mm

Vendor #2 - Celo

Similar guidance from another vendor, this time sorted by material. Roughly, they recommend a hole diameter of 0.75d for softer plastics (like plain ABS) and 0.83d for harder plastics (like PC).

https://www.celofasteners.com/en/content/189-boss-dimensions-for-celospark

Vendor #3 - Plastics/Chemical company

See https://techcenter.lanxess.com/scp/americas/en/docguard/Joining_Guide.pdf?docId=77016

It suggests that several manufacturers recommend a hole size of 3.0mm for a major diameter screw of 3.5mm. which generally agrees with the previous doc.

So for screws around this size, a rule of thumb would be Screw Major diameter = Hole diameter + 0.5mm.

Damn, I should have ordered M2.5.

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  • $\begingroup$ Should you not have ordered self-tapping with tapered tips for plastic? $\endgroup$
    – Transistor
    Jul 14, 2023 at 19:48
  • $\begingroup$ By tapered I mean "pointy" rather than parallel threaded to the end which would be difficult to start. I haven't heard of "type B" before. $\endgroup$
    – Transistor
    Jul 14, 2023 at 21:46
  • $\begingroup$ Ah, I've seen multiple technical pages discussing self-tapping screws for soft plastics which explicitly state pointy tips are not appropriate if a pilot hole exists. Also several product catalogs for this specific purpose show products with a distinctly non-pointy tip. $\endgroup$ Jul 14, 2023 at 21:57
  • $\begingroup$ "Type B tapping screws have spaced threads and a blunt point with incomplete entering threads. They are used in thin metal, nonferrous castings, resinous plywood, plastics, and various composite boards." and "Type B points provide for more full threads to be engaged in the material being fastened." $\endgroup$ Jul 14, 2023 at 21:59

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