a threshing unit is arranged in a concentrically spaced relation within a perforated rotatable grain separating cylinder . an elongated axially fluted impeller located below the threshing unit and within the separating cylinder accelerates the speed of travel of the threshed material to substantially the peripheral speed of the separating cylinder to reduce grain cracking and fracturing . the grain is centrifugally separated through the separating cylinder for removal by a clean grain auger located therebelow and the chaff is discharged by air flowing axially between the threshing unit and the separating cylinder . tailings from the threshing unit are moved rearwardly along the inner peripheral surface of the separating cylinder for pickup and return to the threshing unit . a pair of oppositely arranged axially extended nozzle units direct concentrated blasts of air inwardly of the separating cylinder to lift the chaff and grain from the inner peripheral surface thereof whereby to continuously clean the cylinder perforations for clean grain travel therethrough .