Abstract: An improved method of producing a rotary dispersion member comprises providing a frusto-conical from (23) with a cylindrical base (25), operating a cutting wheel (27) to cut a groove (28) in the from (23) by moving the cutter along a line (29) parallel to the side of the frusto-conical form until the straight part of the groove is cut to full depth and then moving the cutter away from the form at 90.degree. to its central axis (34), the end of the groove thereby being formed as arcuate, grinding away the periphery of the base (25) until the outer generated curve (32) is at the required angle to a surface normal to said axis (34), mounting the worked from on a plate (36) to produce one part of a mould tool, fitting the one part in another mould tool to define a cavity therebetween, and injecting material into the cavity. The resultant dispersion member can be formed with known teeth by spark eroding said another mould tool.