convex and concave surfaces the convex surface progressively protrudes from the edge of the top or bottom surface toward one tip the concave surface progressively recedes from the edge of the top or bottom surface toward the other tip as shown in fig6 a the tip portions viewed from above show envelopes profiled by the points corresponding to the line 21 of the polyhedral member the envelopes form unique loci of the members of this invention when the members are rotated about the shaft 6 or axis b the hatched cross section of the polyhedral member 1 of fig5 b shows a cross section cut along the line indicated by the arrows shown in fig9 as shown in fig7 and 8 the opening 3 is closed as follows 1 as the polyhedral members 1 rotate in a direction indicated by the arrows the opening 3 becomes gradually smaller 2 soon the leading ends of the convex surfaces 21 fig6 b of the tips 2 34 collect at the center a so that the opening disappears fig9 3 and at this time recesses 22 fig5 b and 9 surrounded by the slopes 15 16 of the convex surfaces 13 appear around the center a on each of the upper and lower surfaces of the members as shown in fig1 and 14 when the members continue rotating the recesses 22 disappear and the upper and lower surfaces of all polyhedral members meet at points 21 39 fig6 b this continuous closing process containing a phase wherein the recesses 22 appear and a phase wherein the upper and lower surfaces 17 replace the recesses is a feature of this invention it forcedly induces the coating material to move to the top and bottom parts of the.