Geometric adaptive straightness control in the CNC end milling machine

Abstract

Geometric Adaptive Control is carried out with a CNC end milling machine under the two different feeding conditions: well and poor controlled feed drive mechanisms. The off-line control scheme models the relationship between tool deflection and the feedrate by a modified Taylor``s tool-life equation and compensates for machining straightness error in the finished surface due to tool deflection and guideway error generated by the peripheral end milling process. The time-varying parameters of the tool deflection-feedrate model are estimated, without a priori knowledge of the cutting process, by an exponentially windowed recursive least squares method using only post-process measurements of the straightness error by a gap sensor. The location error is compensated by moving the milling bed according to a numerical control command before mitting. The waviness error is regulated by the manipulation of optimal feedrate obtained from the proposed GAC method during machining. With the poor controlled feed drive mechanism, a straightedge is used to identify the guideway error and the tracking control method is employed to compensate for the guideway error of the end milling machine, in which the workpiece is traveling on the guideway and the tool and spindle system is stationary. To reduce the drift of cutting parameters a single parameter adaption method is introduced. To make the system insensitive to the machining error of high frequency, a low pass filter based on FFT, harmonic filter, is applied to the feedrate control inputs. Developing a simplified variance perturbation method for the well controlled feed drive mechanism and a GAC for the poor controlled feed drive mechanism case, the GAC can be applicable to a batch type of production, where the characteristics of the machining system will vary noticeably within a production lot. Experimental results show that the location error is controlled within the range of fixing error of the milling bed on the guideway, an...