Composite screw tap

The invention discloses a composite screw tap which comprises first tap teeth and second tap teeth. A sleeving hole coaxial with the first conical teeth is formed in the first conical teeth, and firstpin holes penetrating through the first conical teeth are formed in the radial direction of the sleeving hole. A second screw tap neck smaller than the second taper teeth in small diameter is arranged at the upper end of the second taper teeth, and second pin holes penetrating through the second screw tap neck are formed in the second screw tap neck in the radial direction. The second screw tap neck and the first pin holes can be sleeved in a sliding and rotating manner in the axial direction. Pin columns are arranged in the first pin holes and the second pin holes. The breaking strength of the pin columns is smaller than that of the second screw tap neck. According to the composite screw tap, internal threads of two specifications can be machined under the condition that the screw tap isnot replaced; the breakin