In this paper, various models of the milling process in frequent use are reviewed. They are classified in order of increasing sophistication and accuracy as (1) the Average Rigid Force Static Deflection Model, (2) the Instantaneous Rigid Force Model, (3) the Instantaneous Rigid Force, Static Deflection Model, (4) the Instantaneous Force With Static Deflection Feedback Model, and (5) the Regenerative Force, Dynamic Deflection Model. In each case, the validity, possible applications, and limitations of the model are discussed. In the final section, several applications of the Regenerative Force, Dynamic Deflection Model are used to illustrate the wide range of applicability of this model.
This paper compares various sensors and shows that a microphone is an excellent sensor to be used for chatter detection and control. Comparisons are made between the microphone and some other common sensors (dynamometers, displacement probes, and accelerometers) regarding sensing of unstable milling. It is shown that the signal from the microphone provides a competitive, and in many instances a superior, signal tht can be utilized to identify chatter. Using time domain milling simulations of low-radial-immersion, low-feed, finishing operations it is shown that for these cuts (especially at relatively high speeds) chatter is not adequately reflected in the force signal because of the short contact time, but that it is clearly seen in the displacement signal. Using the dynamics of existing production milling machines it is shown how the microphone is more suitable to chatter detection than other remotely placed displacement sensors, especially in cases that involve flexible tooling and workpieces. Aspects important for practical implementation of a microphone in an industrial setting are discussed. Limitations of the microphone are addressed, such as directional considerations, frequency response, and environmental sensitivity (i.e., workspace enclosure, room size, etc). To compensate for expected unwanted noises, commonly known directionalization techniques such as isolation, collection, and intensity methods are suggested to improve the ability of the microphone to identify chatter by reducing or eliminating background and extraneous noises. Using frequency domain processing and the deterministic frequency domain chatter theory, a microphone is shown to provide a proper and consistent signal for reliable chatter detection and control. Cutting test records for an operating, chatter recognition and control system, using a microphone, are presented; and numerous examples of chatter control are listed which include full and partial immersion, face-and end-milling cuts.
This paper is based on previous work of the author and his associates which was published in a series of papers, mainly on those given here as references [2–6], dealing with time domain simulation of chatter in milling, with cutting process damping and with stability lobes. These matters are reevaluated here from the particular point of view of high-speed milling. First, the derivation of limit of stability of chatter in the frequency domain is recapitulated, and lobes of stability explained. These lobes should lead to substantial increases of stability at high speeds of milling. Further, corrections to the results of the simple theory using time domain are presented as they are obtained by time domain simulation which takes into account, in a very realistic way, all the main aspects of milling. It is shown that still, in many instances, high gains of stability are achievable by determining and using a particular spindle speed such that the cutter tooth frequency approaches the frequency of the decisive mode of vibrations as measured on the cutter. The usual modes of vibration of a spindle with a long end mill are discussed, and it is shown how a long end mill stabilizes cutting at medium speeds but becomes a flexible element strongly involved in chatter at higher speeds. In the following section, cutting process damping is discussed which has a very strong stabilizing effect at low speeds but is also partly effective at speeds presently in use. This damping is lost in high-speed milling. Typical cases of high-speed face milling of steel and long end milling of aluminum are discussed and a need of about seven times more stiffness for spindle modes and 14 times more stiffness for the end mill mode derived. The former should be achieved by spindles with larger diameter roller bearings while simultaneously the technology for the design of these spindles running at high speeds must be developed. Present research work shows good promise for this development. For the latter, methods of maximum use of the lobing effect should be developed as well as methods of increasing the damping of the end mill mode.
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