A sieve guide assembly and a method of fractionation using a sieve guide assembly comprising a circular sieve screen (190) and a sieve guide 310 mountable in a fractionating device for fractionating granules for solid dosage forms, such as tablets, capsules or sachets, wherein the fractionating device comprises a drive adapted for: (i) in combination with a sieve screen without a sieve guide, inducing a lateral flow of granules defining lateral streamlines 31a.1. 31b.1, 31c.1, 31d.1 and an orbital flow defining orbital streamlines 31a.2. 31b.2, 31c.2, 31d.2, 31d.3 on the sieve screen (190), and (ii) in combination with the sieve guide assembly, inducing a guided lateral flow of granules defining guided lateral streamlines (331.3) and a central guided orbital flow defining central orbital streamlines (331.2) on the sieve screen (190), whereby the sieve guide assembly is adapted to provide a uniform, controlled and effective exposure of the granules to the sieve screen.