Discovery of a secondary roughness phenomenon during an experiment confirming a new hypothesis concerning the “Ball nose milling with a tilt angle milling strategy”

Abstract

Background: During ball-nose milling using the presently popular tool-dragging strategy (by this is meant ball nose milling with a tilt angle milling strategy ‒ in the further text, the shorter designation "tool-dragging strategy" will be used), a considerable force component acts in the direction perpendicular to the normal plane of the surface of the machined part and, actually, also to the vector of feeding speed. They tend to cause tool vibration in the direction of their action. Aim: The aim was to confirm, by an experiment, the theory that during ball-nose milling using the tool-dragging strategy, its additional lateral tilt might improve the cutting conditions and contribute to the stabilisation of the cutting process. Thanks to that, it may be possible to achieve higher productiveness in machining using a slim tool. Methods: The main experiment was a full-factor experiment with repetition and randomization. It had three factors: The main factor was the angle of lateral tilt, which was in 19 levels, the other two were the tool RPMs and the feeding speed, both of them in three levels. The total number of tests was 342. As responses, cutting forces, acoustic emissions (in a limited extent), and surface roughness were measured. During the experiment, it was discovered that with positive lateral tilt angles of the tool, its mark was divided into two perceivably separated bands, one of which was of significantly higher surface quality than the mark tool with no lateral tilt.Results: For the cutting force response model, the experiment confirmed the hypothesis with a determination coefficient value of 93 %. For the models of the other responses, the factor was lower, still, they were in conformity with the hypothesis. Conclusion: The results of the research may confirm that, if using a slim ball nose tool for finishing operations is necessary, suitable lateral tilt could help achieve better cutting stability and surface quality or higher machining productiveness. As a side effect, the phenomenon of a secondary surface with startlingly better roughness value was newly observed when a lateral tilt was used. This is also described in the article.