A sensor-less tool-workpiece contact detection method for an ultra-precision machine tool based on Kalman Filter (KF) is proposed in this study. The servo thrust signal of feed axis is processed using KF, and its Kalman Residual (KR) is calculated in real time. Parameters of KF are analyzed to achieve the optimized contact detection performance. A cumulative Kalman Residual (CKR) is constructed, and its threshold is experimentally determined, to detect the external disturbance of feed axis under a constant feed rate. Experimental studies were conducted in which the workpiece moved towards the tool initially and retracted in the opposite direction as soon as the tool and workpiece got in contact. The experiments were implemented with three different feed rates. The experimental results validate that the proposed contact detection method can achieve the micro-level tool-workpiece detection.