I just did a job using a new 2-flute solid carbide slot drill in 6061 aluminium on my CNC mill.
A quick check with a caliper said about 3.85 mm, so I "assumed" it would cut 4 mm.
Not so, it actually cut like a 3.85mm - duh !

So, measured carefully the flutes, also 3.85 mm.
5/32" is 3.97 mm, 9/64" is 3.57 mm, so it does not appear to be imperial.

Is it normal for cutters like this, probably purchased cheap somewhere, to be somewhat smaller than the expectation ?

I will start checking actual cutter dimensions for future jobs.

Keep well,
John


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