Effects of tool deflection in the high-speed milling of inclined surfaces

The present paper looks at the dimensional errors resulting from tool deflection in the high-speed milling of hardened steel surfaces. These errors are measured as the difference between the theoretical surface and the high-speed milling machined using ball-end mills. The effect of various factors on this dimensional error is investigated. First, account was taken of the workpiece material and the slope of surfaces; the values chosen were those normally used in injection mould manufacturing. The workpiece materials were of 30 and 50 HRC hardness, with slopes of 15degrees, 30degrees, and 45degrees. In this manner, results may thus be of utility to the mould and die industry. The selected tools were solid ball end mills of sintered tungsten carbide, coated with TiAIN. These were of various diameters and lengths, and accordingly exhibited various degrees of slenderness. A great value for this latter parameter is a restraint on the potential application of the high-speed milling technique. This is the main reason for this work. Tests were carried out using three machining strategies, namely, upward, downward, and z-level (horizontal), as well as with two cutting types, downmilling (also called climb milling) and upmilling (or conventional milling). In all cases the resulting roughness was also measured. Dimensional errors in several flat slope planes were measured, comparing with those obtained by simulation. The results of these tests have been applied to the prediction of error in the high-speed milling of two industrial parts. Knowledge of error magnitude may be useful when NC programs are prepared for the machining of mould complex surfaces, since it may then be attempted to enhance accuracy. Reference is made to various practical problems that were necessary to resolve in order to achieve measurement errors less than 20 mum in a process as complex as that of high-speed milling in three axes machining centres.