Selecting rational geometric parameters of a disk mill in machining high-temperature nickel powder alloys

Abstract

812 In milling parts made from high-temperature nickel powder alloys, the tool often fails, which may lead to loss of the parts. In finishing such parts, the working life of the mill must ensure complete machining of a single part without tool replacement. In machining such parts, the disk mill often undergoes not only abrasive and adhesive wear but also crumbling of the hard-alloy plates. Accordingly, it is important to select geometric parameters of the disk mill that reduce the wear, increase the resistance to crumbling, and hence ensure a mill working life sufficient to machine at least one part. It is found that, in machining high-temperature nickel powder alloys, contact of the blank with the cutting part of the disk mill is accompanied by jumps in the cutting force that are 6–8 times its steady value (Fig. 1). These jumps may be attributed to the strong vibration in the initial milling period. The cutting force gradually declines in the course of milling; running-in of the mill proceeds; and the optimal geometric parameters of the teeth for the given conditions are formed. When the mill is running in, chipping and blunting of the cutting part occur and determine its subsequent wear. In the milling of high-temperature nickel powder alloys, the running-in period is very important. If less wear appears in that period, it will develop more slowly thereafter (in steady conditions). In machining high-temperature nickel powder alloys, the mill profile in Fig. 2 performs well. This profile, with facets f 1 at the front surface and f 2 at the rear surface, allows the mill to resist increased cutting force and thereby practically eliminates any negative consequences of the initial wear. In the running-in period, a mill of this profile is generally characterized by little blunting, and subsequent wear is of strip type, without crumbling of the blade (Fig. 3). In machining EP741NP alloy, the tooth wear of the disk mill is mainly observed at the rear surface, in the form of a strip of constant width (Fig. 4). We may assume that the highest temperature develops at these points. The wear criterion adopted is h r = 0.5–0.55 mm. Further mill operation results in accelerated abrasive wear and often in crumbling of the blade. The dependence of the force P z on the front angle γ is shown in Fig. 5a for the machining of EP741NP alloy by a mill with plates of the experimental VRK15 hard alloy (developed by the All-Russian ScientificSelecting Rational Geometric Parameters of a Disk Mill in Machining High-Temperature Nickel Powder Alloys