If I want to drill a hole for use with an M3 (or other metric size) do I need a special lookup table or do I simply use an M3 bit with an M3 tap?
If you intend to tap the hole, it needs to be "a special drill size" and you can look that up in any of a bajillion sources, many online.
Google says 2.5mm for M3 - I
haven't bothered to check for actual charts, but it will likely be correct. Yup. That's for the usual cutting tap. 7/64" for a forming tap.
If you just want the screw to pass through, M3 (i.e. 3mm) will do that.
Get yourself a tap & drill chart. I think every tap manufacturer gives them out.
No, at least for metric screws, the nominal size is the "major" diameter of the thread. The hole size for tapping is smaller.
Looking at the metric tap set I have here and doing some googling it seems the rule is.
- Take the major diameter
- Subtract the thread pitch.
- For M8, M9 and M12, round up to the next tenth of a millimeter (note: for M12 my kit rounds up, but the source I found on google rounded down)
M3 has a thread pitch of 0.5mm and M6 and M7 have a thread pitch of 1mm, so the math works out nicely for those sizes. For other sizes though it results in an oddball size.
If you are buying a tap set and don't already have a set of "engineering" drill bits in finely spaced sizes then I would suggest getting a set that comes with corresponding drill bits.