Automatic dwell control in computer numerical control plunge grinding

Abstract

Accuracy in precision grinding is strongly affected by variations in the normal grinding force. The force gives rise to deflections of the machine, grinding wheel and workpiece. To allow relaxation of the deflection a dwell period is included at the end of the grinding cycle. Estimation of the time constant during grinding allows the automatic selection of the correct dwell time for the individual workpiece and current force level. A new strategy has been developed for the estimation of the time constant based on power measurement during dwell. The strategy employs the weighted least mean squares technique together with weightings based on classification of the power level into bands. The power bands are designed so as to employ lower weightings to be applied to power samples in the regions of the power curve most prone to causing an inaccurate estimate of the time constant. The complete system was implemented on an adaptive system comprising a PC and an Allen Bradley 8200 CNC (computer numerical controller). The time constant identification and dwell control algorithms were executed within the PC and synchronized to the grinding cycle executed under the control of the CNC. The system was successfully tested under laboratory and industrial conditions. It was shown to produce a reliable and accurate estimate of the time constant with workpieces exhibiting time constants in a range from 0.7 to 55 s. The system was shown to cope with this wide range of time constants without user intervention and to be tolerant of the high signal noise levels typically encountered in an industrial environment.