What is high-speed machining?
High-speed machining (HSM) may be defined in various ways. First, with regard to attainable cutting speeds, it is suggested that operating at cutting speeds significantly higher than those typically utilized for a particular material may be termed HSM. Second, theoretical and experimental analyses have shown that increased local stability occurs when the tooth passing frequency of the cutter is equal to the natural frequency (or any of its fractional harmonics) of the most flexible system mode. Selection of the maximum available spindle speed that corresponds to one of these stable tooth passing frequencies is also referred to as HSM.
The latter definition of HSM relies on the concept of regeneration of waviness as a primary cause of instability (i.e., self-excited vibrations) in machining. This waviness regeneration occurs when a cutter tooth encounters an undulating surface left by the previous tooth. The prediction of system stability depends on the phase relationship between the displacement of the current cutter tooth and the waviness it encounters. For certain phase relationships, the succeeding tool vibrations diminish, while for others they increase until either failure or a system non-linearity (e.g., the tool leaves the cut) limits the motion.
A typical stability lobe diagram, which predicts system stability as a function of spindle speed and machining parameters. Both stable and unstable regions are seen depending on the selected spindle speed and chip width, or axial depth of cut in peripheral end milling. This diagram may be calculated using analytic or numerical time-domain techniques. In either instance, knowledge of the machine (and sometimes workpiece) dynamics is required. In many cases, the system dynamics are obtained using impact testing and modal analysis. The direct frequency response function is measured at the tool point and multiple modes fit to the results. Modal parameters (i.e., mass, m, stiffness, k, and damping ratio, x) for each of the selected modes are extracted and used as input to the stability lobe analysis. Another possibility is milling experiments for direct stable speed selection. Here, machining tests are completed to locate chatter frequencies and select stable spindle speeds. In all cases, however, the results are specific to the selected components (e.g., tool, holder, workpiece, spindle, and machine) and boundary conditions (e.g., holder force and drawbar force).