In the actual chip load calculation, bit diameter doesn't come into play. e.g. Whether you've got a 1/2" tool or a 1/16" tool (let's say two fluted), if you move .001" in one tool revolution, you will cut off .001" of material with a chip load of .0005" irrespective of the diameter of the tool. This is not to say that tool diameter doesn't matter, it just doesn't enter into the calculation of chip load.
Bit diameter is a factor in the calculation of "surface feet / min" or SFM. This is the speed at which the edge of the tool travels with respect to the material you're cutting. Different materials and bit coatings can dictate different optimal SFM.
-Andy B.
http://www.birkonium.com CNC for Luthiers and Industry http://banduramaker.blogspot.com