blade positioned at inner perimeter of the active filler zone such stapling can eventually lead to plugging or otherwise interrupting the uniform flow of fibrous material into the active refining area accordingly a taper is formed at the end of blade 46 a and also as defining margins of notches 49 of blades 46 b and 46 c since after a bar set is cut in half each tapered notch margin becomes a blade inner end as is apparent in fig8 and 10 once the bar set cluster envelope 42 is defined and the desired blade and spacer widths have been selected a precise number and length of blades and spacers are stacked to form a parallelogram of particular width length and bar set angle theta as in fig8 the dimensions of the bar set are such that the bar set may be cut in half along line c c to form two identical bar clusters in the specific example of fig8 blades 46 and spacers 48 are assembled alternately within the parallelogram blades with tapered ends 50 are put into position outside the barset cutting line c c blades intersecting the cutting line are notched with the notch 49 occurring where the cutting line passes this is shown in fig8 where the cutting line passes notches in blades 46 b and 46.