Formed rotary cutting tool having varying diameter and constant clearance angle, and method of forming groove by using the same

A formed rotary cutting tool comprising;

• a generally cylindrical body, portion having;
• (a) a plurality of flutes which are in an outer surface of said body portion to extend generally in an axial direction of said body portion and which are located around an axis of said body portion so as to be angularly spaced apart from each other, and (b) a plurality of cutting edges each of which is constituted by one of widthwise opposite edges of a corresponding one of said flutes, said cutting edges cooperating with each other to have a diameter which changes in said axial direction, so that said body portion has a desired configuration, said tool being rotated about said axis to form a groove whose configuration corresponds to said desired configuration, in a workpiece,
• wherein said diameter of said cutting edges alternately increases and decreases in said axial direction, so that said body portion has at least one diametrically large part and at least one diametrically small part which are alternately arranged in said axial direction,
• and wherein each of said cutting edges is defined by an intersection of a rake face and a flank face which has a predetermined clearance angle with respect to a line that is tangent, at a margin of said each cutting edge, to a circle in a transverse cross sectional plane perpendicular to said axis, said circle having its center at said axis and a diameter equal to said diameter of said cutting edges, said clearance angle being constant in said axial direction, irrespective of a change of said diameter of said cutting edges.