PURPOSE: To shorten a span of workhours as well as to solve a problem on hard labor due to manual operation by installing a cover on-off device for an abrasive tank.

CONSTITUTION: When a polishing machine is suspended of its operation, an abrasive tank 7 is stopped at a specified position by two positioning sensors 10, 10a and by means of electrical control. Simultaneously with this suspension, an arm lever 4a installed in a cover on-off device 4 of an automatic device A is extended, reaching to a cover 8 of the abrasive tank 7, and the arm lever 4a pulls this cover 8 toward the automatic device A, thus the cover 9 is opened. Simultaneously with this cover 8 opened, an upper conveyor 3 is traveled up to a specified position in the polishing machine as being turned round. A workpiece and an abradant are delivered out of an upper work storage tank 1, an abradant storage tank 2 onto this rotating conveyor 3, and they are carried into the abrasive tank 7, and when they are reached to the specified quantity, rotation of the conveyor 3 comes to a stop, thus it is restored to the original position.