Surface Micro-Groove Topography Generation Method and Anti-Friction Performance for High Speed Ball-End Milling

The residual topography with certain characteristics can be generated on high speed ball-end milled surfaces due to the structure and moving way of the ball-end milling cutter. In this paper, the formation mechanism of micro-groove features on the high speed ball-end milled surface was explored through the combination of high speed ball-end milling tests and MATLAB simulation method on the one hand. The selected material was the commonly used cold working die steel Cr12MoV with hardness about 61 HRC after quenching and tempering treatment. The milling tests were carried out on a five-axis high speed machining center DMU 60P duoBlock. On the other hand, the relationship between the bearing capacity of ball-end milled surfaces with micro-grooves features and sliding friction speed and radial depth of cut, together with the friction reduction mechanism, was analyzed based on the hydrodynamic lubrication theory. During the analysis process, the Fluent simulation method and the ring-on-block sliding friction tests (Tester type: MRH-3) under lubrication condition were performed. Results showed that the surface topography of the high speed ball-end milled surface was closely related to feed per tooth and radial depth of cut. With the increase of radial depth of cut, the surface topography with micro-groove features can be generated for given feed per tooth condition during the high speed ball-end milling process. The simulated surface topography was consistent with the actual machined surface topography. Furthermore, the bearing capacity of micro-groove was gradually strengthened with the increase of sliding friction speed benefiting from the increased fluid flow rate. When the sliding friction speed varied from 30 m/s to 45 m/s, the dynamic pressure of the oil film increased about 54.87%. However, the bearing capacity was firstly improved and then attenuated with the increase of radial depth of cut. This can be attributed to the combined action of the wedging effect and the reverse flow phenomenon occurred during the sliding friction process. For surfaces ball-end milled with a small radial depth of cut, the vortex area was relatively small due to the small inertia resistance of lubricating oil. This meant that the wedging effect played a more important role in leading to the significant bearing capacity. Nevertheless, the wedging effect for surfaces high speed ball-end milled with a large radial depth of cut was seriously weakened because of the improved inertial resistance of lubricating oil and the strengthened reverse flow phenomenon. In this paper, the surfaces high speed ball-end milled with radial depth of cut 0.3 mm showed the best anti-friction performance from lubricating oil bearing capacity point of view. The findings of this paper can not only guide the selection of cutting parameters of high speed ball-end milled surfaces with anti-friction performance requirements, but also contribute to the realization of high-performance manufacturing of large and complex dies and molds.