Cold forming tap having internal finish cutting edge and method of producing the same

A cold forming tap for forming an internal thread on an inner surface of a prepared hole previously formed in a workpiece, by plastically deforming the inner surface of the prepared hole, comprising:

• a cutting edge for finishing a minor diameter of said internal thread to be formed by said cold forming tap, and an external thread portion in which an external thread is formed to extend in a helical direction of said cold forming tap and which has, in a cross section thereof perpendicular to an axial direction of said cold forming tap, a polygonal shape defined by outwardly arching sides, said external thread portion including radially outwardly protruding portions and relief portions which are adjacent to and have a diameter smaller than said protruding portions;
• wherein said protruding portions and said relief portions are alternately located in said helical direction, such that said protruding portions are spaced apart from each other at predetermined angular intervals in a circumferential direction of said cold forming tap, and such that said relief portions are spaced apart from each other at predetermined angular intervals in said circumferential direction, said protruding portions being formed into the inner surface of the prepared hole of the workpiece, for plastically deforming the inner surface whereby said internal thread is formed on the inner surface;
• wherein at least one of said protruding portions within each lead of said external thread has a root diameter larger than that of the other of said protruding portions, said at least one of said protruding portions being removed down to a root of said external thread as to provide a tangent surface which is tangent to said root;
• and
• wherein said cutting edge is defined by a flute which is formed in said tangent surface and which extends in said axial direction of said cold forming tap, such that said flute is located on a forward side, as viewed in a rotating direction of said cold forming tap, of a minimum diameter position lying on said tangent surface so that the said cutting edge is located in said maximum diameter position.

A cold forming tap for forming an internal thread on an inner surface of a prepared hole previously formed in a workpiece, by plastically deforming the inner surface of the prepared hole, comprising:

• a cutting edge for finishing a minor diameter of said internal thread to be formed by said cold forming tap, and an external thread portion in which an external thread is formed to extend in a helical direction of said cold forming tap, said external thread portion including radially outwardly protruding portions and relief portions which are adjacent to and have a diameter smaller than said protruding portions;
• wherein said protruding portions and said relief portions are alternately located in said helical direction, such that said protruding portions are spaced apart from each other at predetermined angular intervals in a circumferential direction of said cold forming tap, and such that said relief portions are spaced apart from each other at predetermined angular intervals in said circumferential direction, said protruding portions being forced into the inner surface of the prepared hole of the workpiece, for plastically deforming the inner surface whereby said internal thread is formed on the inner surface;
• wherein at least one of said protruding portions within each lead of said external thread has a root diameter larger than that of the other of said protruding portions, said at least one of said protruding portions being removed down to a root of said external thread so as to provide a tangent surface which is tangent to said root, and said cutting edge being defined by a flute which is formed in said tangent surface and which extends in said axial direction of said cold forming tap;
• wherein said external thread portion includes a tapered leading portion and a full-form thread portion which is adjacent to one of opposite ends of said tapered leading portion that is nearer to a proximal end of said cold forming tap, said tapered leading portion having an outside diameter which decreases in a direction toward a free end of said cold forming tap, while said full-form thread portion having an outside diameter which is constant in said axial direction of said cold forming tap;
• and
• wherein at least said protruding portions and a portion of said cutting edge which are located in said tapered leading portion are constituted by a cemented carbide which is bonded on a circumferential portion of a body of said cold forming tap which is made of an alloy steel.