In this paper, a new desktop NC machine tool, which has abilities of compliant motion and stick-slip motion, is first presented for finishing small metallic molds with curved surface. The NC machine tool consists of three single-axis robots with a high position resolution of 1μm. A thin wood stick tool is attached to the tip of the z-axis. The tool tip has a small ball-end shape. The control system is composed of a force feedback loop, position feedback loop and position feedforward loop. The force feedback loop controls the polishing force consisting of tool contact force and kinetic friction forces. The position feedback loop controls the position in pick feed direction, e.g., z-direction. The position feedforward loop leads the tool tip along a desired trajectory called cutter location data (CL data). The CL data are generated from the main-processor of a CAM system. The proposed NC machine tool has realized a compliant motion required for the surface following control along a spiral path. In order to improve the finishing performance, a small stick-slip motion control strategy is further added to the control system. The small stick-slip motion is orthogonally generated to the direction of the tool moving direction. Generally, the stick-slip motion is an undesirable phenomenon and should be eliminated in precision machineries. However, the proposed NC machine tool employs a small stick-slip motion to improve the finishing quality. The effectiveness of the NC machine tool was examined through an actual finishing test of a LED lens mold with a diameter of 4 mm. It was observed that the undesirable cusps can be removed uniformly. And, it was confirmed from the results that the proposed finishing strategy by using the stick-slip motion control is effective to achieve a higher quality surface like a mirror finishing.
Desktop Cartesian-Type Robot with Abilities of Compliant Motion and Stick-Slip Motion
