Machine screw into aluminium – what size hole?

Technical experts! If I’m mounting something in aluminium, presumably I can size the hole in the aluminium so the machine screw bites into it and avoid having to put a nut on the back? If that’s ok, what size should the hole be vs the machine screw? I think it’ll be an M3 screw, as that’s what we’re using elsewhere (where it’s going into threaded inserts), into 3mm aluminium plate.

Machine screw won’t self tap into the aluminium. Well, it might, but not easily.

You can tap it if you go buy an m3 tap. Needs a 2.5mm tapping drill.

Or use a self tapping screw.

Pilot hole wants to be the metric thread diameter less the pitch of the thread – so M3 is 0.5mm pitch = pilot hole of 2.5mm dia, then run a tap through it to form the thread.

How thick is the ally? General guideline in steel is that the thread depth wants to match bolt dia (so an M10 nut is 10mm thick, an M3 nut is 3mm thick) for standardised strength. Ally is softer so obviously wants to be deeper, but its also subject to how much load you’re actually planning to put on it.

As per Jon – M3 thread into 3mm thick alu sheet is a no go based on my experience – it wont take much load or vibration to pull it through. You need a nut on the back, or if you can’t get in, a nutsert/rivnut would be ideal.

As per Snotrag above….depends on the application as well – in this instance as the 3mm sheet/plate isn’t very thick. Is the component regularly demounted/removed? Is the mounted component heavy? Subject to loadings/vibration?

Rivnuts would be a better solution if you can afford the protrusion of them.

……or just put a nut on the back

The general rule of thumb for female threads is 2xD (where D is the diameter of the screw/bolt) in this instance an M3 into 3mm plate is 1xD, you could tap it and it would hold, but I wouldn’t consider it reliable, if it’s going to see any loading or movement.

My typical answers would be consider using a nut (and washers), consider using a rivnut, consider thickening the plate? Do any of those suit the application?

Can you rivnut it?

<p style=”text-align: left;”>M3 into 3mm is doable, but the issues are M3 doesn’t leave much tolerance for error, and most common grades of 3mm sheet/plate are fairly soft so will strip easily.</p>
If you wanted to try tapping it, although 2.5mm is the correct size tapping hole, a cheap 2.5mm drill drilled freehand is very likely to produce an oversize hole. I’d suggest trying a 2.3 and possibly even a 2.2mm drill before tapping.

Depending on access/aesthetics, rivnuts or chimney nuts are likely to be far more durable.

Thanks, that’s super useful! Nut and bolt it is then. It’s for the mains input on an amp so it’s essential that it stays in place for its lifespan!

In that application I’d use a locking nut, nylok or Aerotight, depending on how hot it will get.

just a suggestion,  you could have a local sheet metal company  press in a clinch stud , have a look at Pem fasteners they have a huge range of sheet metal fasteners