Abstract: A system and method for monitoring tool wear in CNC machining operations by monitoring spindle power and extracting instantaneous cutting geometry. The method is based on a physics-based two parameter process model and measuring at two different cutting conditions. The process model parameters are measured with easily accessible spindle power. In contrast to spindle power alone which is influenced by many factors, most especially by variable cutting conditions and by the tool condition, the process model parameters are independent of the geometrically variable cutting conditions and provide a simple and direct measure of tool wear. The two process model parameters change differently depending on the mechanism of the tool wear, specifically flank wear versus cutting edge degradation. This provides a diagnostic for tool wear.