Around the mid-1990s, something fundamental changed in the nature of machining. Machine tools — specifically, CNC machining centers — became widely capable of milling at sufficiently high spindle speeds and metal removal rates that the dynamics of the machining system started to matter. Process characteristics related to vibration and natural frequency became potentially significant for determining the performance of the cut. Machining fundamentals have never quite caught up. Machining fundamentals as they are typically taught still tend to assume a milling pass as it might be performed on a manual mill, in which the tool can be modeled as a rigid cylinder and the part can be modeled as a rigid prism. The reality: In CNC machining, if the milling pass is programmed to run at what might be considered a reasonable production rate in aluminum or steel today, then the tool is deflecting and oscillating, the workpiece likely is as well, and these motions have to be appreciated as a potentially meaningful factor for the performance of the cut.