The basic machine consists of a drum rotating within a drum. The rotating drum is fitted with flights arranged in a spiral on the periphery. These flights or cleats agitate the material and by virtue of their design, convey the material to the machine discharge. The outer shell or stationary drum has similar flights. The clearance between the fixed and rotating cleats establishes a threshing action as the material travels through the unit. The conveying action allows the machine to clear itself and hence makes the unit self-cleaning. The flights were manufactured with a hardened replaceable insert. This provides excellent wear resistance and simple maintenance. The only additional maintenance consists of periodic greasing of the pedestal bearings.