Machine screws through thin metal plates
I have been wondering about something ever since I got my electric drill and started drilling all over the place, thus far mostly in wood and masonry.
If I want to fasten together two thin metal plates (2mm thick galvanized steel), I need to drill a hole through both and then fit a machine screw or bolt through them.
My questions are:
1. What should the size of the hole be in relation to the screw size? I always drill into wood with a bit that is 0.5mm or so smaller than the screw I want to put in so that the screw can grip tightly. How does it work for metal?
2. Do the holes need thread for the screw to work properly or is a plain hole good enough?
I could experiment but I don't like to waste material or drill bits - I need a theoretical push in the right direction. And I don't have the bits that make thread. Is it perhaps worth investing in a set?