Deviations Induced by Tool Sharpening in the Profile of Three Screw Pump Rotors

Three screw pumps represent an important family of positive displacement rotary pumps. They are quite easy to construct, even if the use of a shaped milling cutter for the rotor machining can cause some problems when the tool is not new. In fact the sharpening of the tool modifies the total geometry of the milling cutter by reducing its diameter. The shape of the single cutter does not change, but the cutter radial position is shifted towards the tool center. This causes deviations in the profile cut and the screws consequently need a long breaking-in. In this paper, the problem is analyzed from a quantitative point of view, using a method that allows us to determine the machining error as a function of the tool-geometry variation. Moreover, some alternative solutions to this problem are suggested.