Knowledge of the tool wear state in machining has become an important issue in research and industrial application. Current systems use the spindle power or cutting force as measured variable and refer it to a taught set point. However, this method lacks the ability to adapt to new work piece geometries. A new approach focusses on the tool instead of the work piece, and uses a sensory tool holder with integrated strain gauges. This tool holder provides polar figures whose shapes relate to the engagement conditions and whose area is a function of the tool load. As the tool load increases with tool wear, the area of the polar figures provides information about the tool wear status, and with knowledge about the engagement conditions, the model can be calibrated.