OD: Refers to the actual Outside Diameter of the tube as measured with a caliper. CLR: stands for Center Line Radius, meaning the bend radius as measured from the apex of the bend to the middle of the tubing. Minimum Wall:This column represents the minimum recommended wall thicknesses for bending mild steel. For any other types of material, call our tech department at +1 (813) 986-9000. Maximum Wall: The maximum wall thickness that the die is rated for. Bending square tubing with a wall thickness of more than 0.083" requires special dies. Please call our tech department at +1 (813) 986-9000 to order.