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I need to affix a plywood frame (depth ~1.25 cm or so) to an office entrance door. The door is metallic, but not a solid metal block: it's hollow. The metal face of the door is about 2mm. This is the kind of door I'm talking about (not this particular make or model, but that's the basic idea.

Which kind of screw should I use? And should I use metal ring(s) somewhere?

  • how thick is the metal where you intend using screws? consider adding reinforcing plates... – Solar Mike Feb 21 at 7:25
  • @SolarMike: See edit. – einpoklum Feb 21 at 11:01
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I ended up using 4mm-width, 30mm length wood screws, i.e. screws with a conical head and a cross shaped recess (for Philips/Posidriv screwdrivers). So, something like the second from the right in this image: enter image description here

I used a 3.3mm metal drilling bit - narrower than the screw. Since the metal is thin, and likely not very hard, I was expecting to be able to widen the bore using the screw itself, in a way that would help it fasten itself to the bore. I first drove my drill bit through the plywood frame; when I got to the door, I didn't try to push forward wil a lot of strength immediately, but rather leaned into the drill with mild extra force, and let it take a while to do its work. It took a good few seconds for the bit to go throw the outer metal layer.

I then drove the screw in with a powered screwdriver some of the way, then fastened manually. The fastening required a bit of strength; it compressed the plywood, so that more than the code now appeared sunken; and did seem to "catch" onto the metal - the most important point of the whole exercise.

I did not use any rings.

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  • It would be usual to use a self-tapping screw, if you'll be doing more of this in the future. – Andrew Morton Feb 21 at 11:57
  • @AndrewMorton: Can you link to an explanation of what that is? I'm not in an English-speaking country, so I might know it under a different name. – einpoklum Feb 21 at 12:43
  • Here's Wikipedia's take on the subject: Self-tapping screw. Also, I think that what you refer to as "rings" are known as washers in English. – Andrew Morton Feb 21 at 13:49
  • @AndrewMorton: How do self-tapping fasteners differ from those featured in the photo? Perhaps I was using a self-tapping screw fastener already? – einpoklum Feb 21 at 13:53
  • It would have been easier to drive the screw in. As what you did worked, there is nothing to worry about now. – Andrew Morton Feb 21 at 13:56

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