This design is an attachment for a set of calipers (dial, vernier, digital..) that allows for the measurement of 3-flute cutters. The formula for converting the caliper measurement into the diameter of the 3-flute cutter is:
Cutter Dia = Caliper Measurement * 1.5
Some of the design criteria may need to be modified to your own specifications as required for precision and caliper size. For instance, this model is designed for a caliper jaw width of .125" and can only measure diameters down to .100".
There is also a 5-flute design available on my profile.