This paper presents a new methodology for the calibration of an untended machining system. The methodology requires the calibration conducted under both static and dynamic loading conditions. The transformation matrix method was used to establish the mapping function between the performance measure of interest (input) and the control signal (output). A prototype system, building on an CNC machining center equipped with a magnetic spindle, was developed to demonstrate a strategy for monitoring the tool wear progress through the cutting force prediction. The effect of phase distortion due to the presence of time lag on the output prediction was studied. A performance index to characterize the phase distortion error was suggested for further improvement of untended machining.