High Precision and High Efficiency Machining of the Scroll Compressor Pieces (Improvement of Machining Accuracy in Constant Cutting Point Control)

Abstract

Scroll compressor components have spiral grooves known as scroll wraps. Since their shape is determined by an involute curve and their accuracy dominates the compressor performance, they are produced by precise contour milling using end mills. The progress in NC device for milling involute shape has enabled a feed speed control at a cutting point and a position control, keeping a cutting point constant relatively to the end mill center. Instead of the conventional method reported formerly which controls the feed speed at the center of the end mill, more precise milling process has been required, which is based on a new control. While milling the inside face of the curve, even if the feed speed is controlled so as to keep the projection cutting area constant, the cutting force increases in connection with the tool contact length. This increase in cutting force deteriorates the profile accuracy. Though controlling the radial depth of cut is one solution, the minimum depth of cut near the involute center is so small as the surface roughness, and the accuracy is not stable. In the present paper, the relationship among cutting area, tool contact length and cutting force is analyzed by the multiple linear regression analysis, and the cutting force was simulated. The result of the cutting force simulation showed a good coincidence with the experimental result in tendency. Using the result of calculation to control the feed speed at the cutting point so that the cutting force can be kept constant, it was possible to improve the profile accuracy from 8μm to 3μm.