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Is there a rule of thumb, or a table which helps one determine what is the maximum strength thread size to use for aluminium sheet?

I am interested in threading 3mm, and 2mm thick sheet aluminium. I am wondering what size of thread I should go for. E.g. M3 in 3mm sheet, or could I go M4, M6? With 2mm I would guess M3, but not M4?

I have not found an authoritive source, just answers ranging between equal to the thickness (M3 in 3mm), down to 50% of the thickness (M3 in 6mm).

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    $\begingroup$ Generally you want at least a thread diameter's worth of engagement in a nut/tapped surface. But it also depends on what clamping forces you need or loads you'll be subjected to. 2/3 diameter of thread engagement is a good absolute minimum, so M4 in 3 mm and M3 in 2 mm. $\endgroup$ – jko May 5 at 15:06
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    $\begingroup$ sheet metal screw? machine screw? $\endgroup$ – Pete W May 5 at 16:33
  • $\begingroup$ @PeteW, I thought M3 only referred to machine screws, but I am open to using sheet metal screws if they can be tightened more. $\endgroup$ – run_the_race May 5 at 16:57
  • $\begingroup$ I think so too, wasn't reading too carefully. Reason is you might get away with less with a sheet metal screw, like a single thread pitch. With machine screws, think in terms of number of threads. 6 is rule of thumb for getting full strength, 3-4 would be the bare minimum, but factor into this that the ends of the hole aren't perfect, so the actual thickness would be slightly more, which is in line what jko said. $\endgroup$ – Pete W May 5 at 17:58
  • $\begingroup$ Pem inserts are a good solution for industry. pemnet.com If I remember correctly stainless steel inserts work better with aluminum than steel inserts and can take higher loads and torque. $\endgroup$ – Jim Clark May 9 at 14:40

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