120b on the other hand is provided with crests 121 of an inverted v shaped cross section and troughs 122 of a v shaped cross section arranged in a plurality of lines in an axial direction c c so that the crests 121 and troughs 122 are alternately arranged in a circumferential direction d d of the roll 120b the upper and lower rolls 120a 120b are engaged with each other as they rotate clockwise and counterclockwise respectively specifically to say a line of pyramidal cones 123 of the upper roll 120a arranged in the axial direction a a thereof are received by the corresponding trough 122 of the lower roll 120b as the line of pyramidal cones 123 have rotated down to a bottom position as viewed in fig9 whereupon the crest 121 of the lower roll 120b is received between two lines of pyramidal cones which are adjacent to each other in the circumferential direction b b fig1 11 and 12 illustrate various manners in which the upper and lower rolls 120a 120b are engaged with each other in the case illustrated by fig1 each pyramidal cone 123 of the upper roll 120a has substantially the same cross section as those of the crest 111 and the trough 112 of the lower roll 120b so that a gap formed between the upper and lower rolls 120a 120b is substantially uniform in the case illustrated.