Search

Strong thermal–mechanical coupling and rapid tool failure easily occur in the milling process of hardened steel with solid end mills. This work investigates the effectiveness of minimum quantity lubrication (MQL) during high-speed milling of hardened mold steel. Three kinds of spraying modes were designed in the experiment. They are namely the spraying ways to the flank face (MQL-F), to the rake face (MQL-R) and to the rake and flank faces (MQL-FR). The cutting forces, tool wear, and chips when using the MQL modes and compressed air cooling mode (CAIR-F) were compared, and the wear mechanism and lubrication mechanism were further analyzed. The results showed that the tool wear values and milling forces of several MQL modes were significantly lower than those with CAIR-F. The milling force and tool wear values were successively reduced using MQL-R, MQL-F, and MQL-FR, with better lubrication of the rake face resulting in a smaller curvature radius for the chip. Abrasion wear, adhesion/attrition wear, and flaking/chipping were the main wear mechanisms. The MQL-FR method was the optimal method of the MQL modes. It could effectively reduce the cutting contact stress and unit cutting energy, and obviously reduced wear phenomena such as adhesion and chipping.

The machining efficiency of hardened steel molds can be significantly improved through high-speed deep milling. However, under these circumstances, the temperature and milling force increase greatly, and the end mills are subjected to rapid wear. A feasibility study of the oil-on-water (OoW) method in the high-speed end milling of P20 hardened steel was conducted. The experimental results show that the tool life of OoW is significantly longer compared with that of dry cutting and compressed air cooling. Moreover, the OoW method promotes tool life under low- and high-speed milling conditions. Thermal wear phenomena (e.g., adhesion) are evidently suppressed, and the development of the milling force and tool wear is considerably slowed down in OoW processing. The main reason for the excellent cooling and lubrication performance of the OoW method is that a two-layer film (oil film + water film) is formed when the OoW droplets collide with the cutting interface. The evaporation of the water film removes much heat and prevents the high-temperature failure of the oil film. Moreover, the single use of oil mist and water mist leads only to a single-layer film, resulting in an evidently shorter tool life than that of the OoW method. In the OoW cutting process, the chipping belt of the flank face is discontinuous, and the wear of the adhesion/attrition is less severe compared with those of the other processing modes. In addition, the tool wear and chipping belt occur with a certain amount of material peeling, and the thermal fatigue and mechanical behavior of the tool substrate in a certain temperature range are the main reasons for the discontinuity of the chipping belt in the OoW process.

Hollow spherical corundum granules were introduced into a vitrified bond diamond grinding wheel as pore former. The influences of content and particle size of the hollow spherical corundum granules on porosity, flexural strength, and hardness of the vitrified bond diamond wheel samples were systematically investigated. The grinding performance of the designed vitrified bond diamond cup wheel on sapphire and 6H-silicon carbide (SiC) substrates was examined, and morphological properties of the ground surface were evaluated. Results showed that higher content and smaller size of hollow spherical corundum granules could lead to the increase of the porosity and the decrease of flexural strength and hardness. Sapphire and 6H-SiC substrates could be effectively ground by the vitrified bond diamond cup wheel containing 8% hollow spherical corundum granules. During the grinding, although a brittle mode dominated the material removal of two substrates, more ductile removals and lower surface roughness could be identified from 6H-SiC substrate than from sapphire.

The progressive cutting based on auxiliary paths is an effective machining method for the material accumulating region inside the mould pocket. But the method is commonly based on the radial depth of cut as the control parameter, further more there is no more appropriate adjustment and control approach. The end-users often fail to set the parameter correctly, which leads to excessive tool load in the process of actual machining. In order to make more reasonable control of the machining load and tool-path, an engagement angle modeling method for multiple-circle continuous machining is presented. The distribution mode of multiple circles, dynamic changing process of engagement angle, extreme and average value of engagement angle are carefully considered. Based on the engagement angle model, numerous application techniques for mould pocket machining are presented, involving the calculation of the milling force in multiple-circle continuous machining, and rough and finish machining path planning and load control for the material accumulating region inside the pocket, and other aspects. Simulation and actual machining experiments show that the engagement angle modeling method for multiple-circle continuous machining is correct and reliable, and the related numerous application techniques for pocket machining are feasible and effective. The proposed research contributes to the analysis and control tool load effectively and tool-path planning reasonably for the material accumulating region inside the mould pocket.

The toolpath for a sharp corner is important in pocket machining during high-speed machining with numerical control. When milling a sharp corner, a longer tool engagement length generates larger engagement angle and milling force, which reduces the machining precision. To reduce the milling force at a sharp corner and clear up uncut materials, the toolpath at corners must be optimized. In this paper, a new method of generating the toolpath with circular cycle transition at a sharp corner is put forward to provide a new numerically controlled toolpath pattern for pocket milling. On the basis of a common contour-parallel toolpath, minimum radius arcs and circular cycles are added in sequence at a sharp corner. When the cutter comes to the sharp corner, the corner material is removed step by step with a sequence of circular cycles. The validity of the proposed method is confirmed through simulation and tests. The proposed toolpath method is realized by employing the Institute of Manufacturing Technology (IMT) computer-aided manufacturing (CAM) system. The toolpaths at sharp corners generated through commercial software and IMT-CAM software are tested by comparing the corresponding milling forces. It is illustrated in the results that the proposed method reduces the milling force effectively and is of great significance when it is applied in high-speed machining.