What are metric drill bit diameters on sale in the UK based on?
In a typical pack I might see drill bits with the following diameters.
2.0 2.5 3.0 3.2 3.5 4.0 4.2 4.5 4.8 5.0 5.2 5.5 6.0 etcI thought those odd sizes were due to some equivalence with inch measurements. But I don't think that is the case.
Nor do they seem linked to the area of the hole cut by the drill bit.
So what are they based on?
I would assume they're based on how many bits they think they can sell you, plus what they have ready supplies of. The sizes seem to match values here quite well:
NT
That's hardly a typical pack. Most just have the 0.5mm increments.
They're not even the tapping sizes for common thread values. Where have you seen such a pack on sale? That might give a clue as to its purpose.
4.2 is for M5 and 3.2 is for 1/8 pop rivets.
It can be difficult to get 4.2 alone so I bought a pack of 10 from SF (lots of 5mm on bikes).
i have a large tray of drill bits i bought in germany from a hobby fair a few years ago, about 300 bits in total, about 10 of each size for the small uns, 5 of the ones over 5mm, and they all go up in 0.1mm steps,
i only bought them 'cos they were a good price, i dont do anything that requires 0.1mm presision with a drill bit, if i need a larger hole than the bit in the chuck, i wiggle it about till i get the requires hole size :)
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