PURPOSE: To check the polishing condition ocularly and enhance the working effectiveness by making the two ends of a barrel from acrylic resin plate furnished on the inside with a roughened surface.

CONSTITUTION: A hexagonal barrel 2 made of acrylic resin is equipped with flanges 1, l', and to one of them 1 an acrylic resin plate having the same outside dia. as the flanges is fastened using six butterfly nuts 4. The inside of this acrylic resin plate 3 assumes a roughened surface 5. The acrylic resin plate 6 having the flange 1 has also an inside which assumes a roughened surface. Polishing is made using a neutral polishing/lustering liquid as a polishing medium or as compound. The polishing condition can be checked during the work without stopping the machine or opening the lid of the barrel, to provide an enhanced working effectiveness. It is not restricted to that only the two ends of the barrel are made of clear acrylic resin, but the whole barrel may be made of clear acrylic resin.