its output shaft is extended downwardly and at the lower end of the shaft is fixed with a rotary polishing buffer 21 in this embodiment as the abrasive material of the rotary buffer a material tradename kenmaron produced by sankyo rikagaku co 3 1 3 chome marunouchi chiyoda ku tokyo a combination product of aluminum oxide powder glass wool and adhesive resin is used an adequate cooling fan not shown is included in the body 20 39 of the polishing apparatus 20 and on the lower side of the support plate 23 supporting the body 20 39 a slide plate for example nylon plate 24 is fixed and slidable on the upper slay 2 39 of the metal reed and four sets of rollers 25 in the drawing only one set is shown are disposed on both sides of the slide plate 24 in riding manner on the upper slay 2 39 in order to precisely hold rotary buffer in a predetermined relative position against the air guide recess surface 4 in contact mode reference numeral 26 denotes a handle and numeral 27 denotes a power source cord the description will be made in the following to explain how the guide path 4 of the blade 10 is to be abraded by the rotary buffer 21 to form the shape of the cross section having the inclinded surface 4 34 as mentioned above fig3 is an enlarged view of an essential portion for the description of the condition of the blade in the abrasive polishing process and a large number of blades 10 already assembled are arranged with their rectangular surfaces 4 39 now the particular mode of operation according to this invention will be started as the following.