A system for threshing and separating a harvested crop, the system including: a rotor; and at least one concave comprising: - a concave frame having two side walls and two end walls connected together to define a top opening, the two side walls comprising a top edge having a radius; - a grate connected to top edges of the side walls and end walls over the top opening of the frame, the grate having an inside radius conforming to the radius of the top edges of the side walls; - a functional surface on the interior surface of the grate for contacting a crop; and - helical cutouts in the grate defining a percent open area having a helical geometry; - helical mounting framework in the grate separating the cutouts, the helical geometry of the helical cutouts and of the helical mounting framework being generally in the same direction, and substantially parallel to a flow path of a spiraling crop flow; and characterised in that said concave further comprises: - rub bars mounted to a surface (38/40) of the grate and connected to the mounting framework such that a length of each of the rub bars is substantially perpendicular to the helical cutouts and the helical mounting framework, wherein the rub bars comprise segmented rub bars that comprise straight pieces having a length that does not span the width of the concave allowing the segmented straight rub bar to be mounted to a surface of the grate in a helical fashion without bending or twisting of the rub bar or grate. a rotor; and at least one concave comprising: - a concave frame having two side walls and two end walls connected together to define a top opening, the two side walls comprising a top edge having a radius; - a grate connected to top edges of the side walls and end walls over the top opening of the frame, the grate having an inside radius conforming to the radius of the top edges of the side walls; - a functional surface on the interior surface of the grate for contacting a crop; and - helical cutouts in the grate defining a percent open area having a helical geometry; - helical mounting framework in the grate separating the cutouts, the helical geometry of the helical cutouts and of the helical mounting framework being generally in the same direction, and substantially parallel to a flow path of a spiraling crop flow; and - a concave frame having two side walls and two end walls connected together to define a top opening, the two side walls comprising a top edge having a radius; - a grate connected to top edges of the side walls and end walls over the top opening of the frame, the grate having an inside radius conforming to the radius of the top edges of the side walls; - a functional surface on the interior surface of the grate for contacting a crop; and - helical cutouts in the grate defining a percent open area having a helical geometry; - helical mounting framework in the grate separating the cutouts, the helical geometry of the helical cutouts and of the helical mounting framework being generally in the same direction, and substantially parallel to a flow path of a spiraling crop flow; and characterised in that said concave further comprises: - rub bars mounted to a surface (38/40) of the grate and connected to the mounting framework such that a length of each of the rub bars is substantially perpendicular to the helical cutouts and the helical mounting framework, - rub bars mounted to a surface (38/40) of the grate and connected to the mounting framework such that a length of each of the rub bars is substantially perpendicular to the helical cutouts and the helical mounting framework, wherein the rub bars comprise segmented rub bars that comprise straight pieces having a length that does not span the width of the concave allowing the segmented straight rub bar to be mounted to a surface of the grate in a helical fashion without bending or twisting of the rub bar or grate.