. The 3400 Harmon-i-Cut allows the capability to rough and finish with one tool and is stocked with FC-18 coating.4-Flute - Variable Helix - 3400 Harmon-i-Cut HP End Mills, FC18, RH Spiral, Crnr Radius, Stndrd, 3/4 It features a variable helix, variable index, and variable rake which maximizes tool life and removal rates by minimizing the effects of harmonics.