Cutting force and dimensional surface error generation in peripheral milling with variable pitch helical end mills

Mechanics of cutting force and dimensional surface error generation are presented for variable pitch helical end mills. It has been known that the variable pitch cutters reduce the chatter vibrations by disturbing the regeneration mechanism in dynamic milling. In this paper, it is also demonstrated that the variable pitch cutters reduce the dimensional surface errors caused by the static deflection of slender end mills. Since the pitch angles between the adjacent flutes are different, each flute experiences different chip loads, hence leading to the varying surface errors imprinted at each tooth period. The excess surface errors left by the flutes, which have larger chip loads, are removed by the following flutes with smaller effective chip loads due to variable pitch. Unlike in the case of uniform pitch cutters, the regeneration of chip thickness mechanism is always present in milling with a statically flexible, variable pitch end mills. A mathematical model of force and surface prediction, which considers the chip regeneration mechanism under static end mill deflections, is presented with experimental verifications. Up to 20% reduction in surface errors are achieved by using variable pitch over uniform pitch cutters.