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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?

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  • Get a copy of Zeuss tables.
    – Solar Mike
    Oct 11 '20 at 20:30
  • Thanks for the suggestion. I ordered two copies.
    – Zhro
    Oct 12 '20 at 0:46
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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.

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  • LOL, if you just want the tap and screw to just pass through... Oct 12 '20 at 2:46
  • 7/64" seems an unlikely size drill for a metric screw thread.
    – brhans
    Oct 12 '20 at 3:15
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    I don't make this stuff up: www-eng.lbl.gov/~shuman/NEXT/MATERIALS&COMPONENTS/… But forming taps are not the "usual run of the mill," either. Note the metric drills over on the "inch" side of the chart if that makes you happier.
    – Ecnerwal
    Oct 12 '20 at 3:43
  • Metric screw tapping drill sizes are simple (for normal cut threads): the nominal diameter minus the pitch. This means that of the small sizes we often encounter only M3 and M6 are likely to have tapping drills in a typical set of drills increasing in 0.5mm steps.
    – Chris H
    Oct 12 '20 at 13:53
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Get yourself a tap & drill chart. I think every tap manufacturer gives them out.

https://www.aimsindustrial.com.au/sutton-tapping-drill-wall-chart-free

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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.

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For an M3 tap, you want a 2.5mm drill.

metric tap and drill table

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