The sharp semi-circular edge of a wiper die determined by the intersection of the radius face with the cavity. Because this intersection occurs at the line of tangency (see the entry under aero-cut for an alternative design), theoretically the thickness of the feathered edge should be zero, but this is not practicable for either the manufacture of the wiper or its end use. In actuality, the origin point of the radius face is offset slightly from the centerline of the tube cavity to give the feathered edge a slight thickness. This is known as simple-sweep geometry. The formula for determining the optimum thickness is F = t x .004 where “F” is thickness of the feathered edge and “t” is the tube diameter.