Abstract: A rotary cutting tool such as a helical end mill, having pockets arranged to overcome harmonic vibrations. The tool has at least two and in one embodiment all three of three spacing irregularities. In the first irregularity, columns of pockets, typically associated with flutes, are staggered circumferentially out of even spacing. In a preferred embodiment having three flutes, the flutes are mostly not centered on one hundred twenty degree intervals. In the second irregularity, at least some pockets are arranged at different radial rake angles. In the third irregularity, at least some pockets are arranged at different axial rake angles.