Comment by Animats
2 days ago
For small screws, in the millimeter range, the jump between metric sizes is too big. So, in addition to M1, M2, M3, M4, M5, etc. standard metric screws include M1.4, M1.6. M1.8, M2.5, and M3.5 (rare) to fill in the gaps.
Screw sizes and drill sizes should have been sized by a ratio, like resistor values. But that would have been a pain for manual machining.
Yep, as it is some sizes are easier to work with.
Domestic drill sets don't seem to be designed for tapping holes but if you stick to M3, M6 and M10 the tapping sizes do correspond with the 2.5, 5 and 8.5mm drills[1].
I guess if it was based on a ratio system you would need special tapping drills for all of them.
e.g. M4 needs a special 3.3mm tapping drill already.
1. According to my trusty Zeus tables.