A desktop NC machine tool with a position/force controller using a fine-velocity pulse converter

In this paper, a new desktop NC machine tool with the ability of compliance control is presented for finishing of metallic molds with small curved surface. The NC machine tool consists of a three-axis robot whose single one has a high position accuracy of 1 μm, which means that it can perform higher resolutions of position and force, compared to general industrial robots. A ball-end abrasive tool is attached to the tip of z-axis through a force sensor. In order to first confirm the application limit of an industrial robot to a finishing task, we evaluate the backlash that causes inaccuracy in position at the tip of the abrasive tool, by simply measuring the position and force. The basic resolution of the proposed NC machine tool is shown through the similar position and force measurement to one used in the conventional robot. Next, a CAD/CAM-based position/force control method with a fine-velocity pulse converter is proposed for a pulse-based servo controller that actuates the single-axis of the robot. We further introduce a systematic method for tuning a desired damping, which is coming from the critically damped condition used in a static force control system. The desired damping is the most important factor when an impedance model following force control is applied. Finally, to evaluate the characteristics of the NC machine tool, a profiling control experiment is conducted along a spiral path for a plastic lens mold with axis-symmetric concave areas. Consequently, it is confirmed that the proposed NC machine tool has a good performance, which can be applied to the finishing process of plastic lens molds.