The problems arising in sifting and classification, as well as a pertinent portion of the state of the art, are described in EP 0 328 067. This published patent protects rollers whose sheathing surface is provided with tapering projections separated by tapering indentations. Neighboring rollers are arranged such that the outer ends of the projections are always opposite each other and are separated by an axially parallel gap; this gap defines the thickness of the chips to be sifted.
The prior art machine is described in U.S. Pat. 2,566,267. Here the annular grooves with a flush base as well as the crosspieces with a flush sheathing surface are always oblique to the roller axis.