Method of manufacturing an elbow, cutting tool, and elbow

A method of manufacturing an elbow including an inner side surface having an axial center bending inward at a predetermined curvature, the inner side surface having a circular shape in a cross section, wherein

• the inner side surface of the elbow is formed by providing a plurality of grooves side by side in a circumferential direction of the cross section of the elbow, the plurality of grooves being formed by moving, on an inner side surface of a roughly formed elbow, a cutting section of a rotary cutting tool along an axial center of the elbow to be finished,
• the rotary cutting tool includes the cutting section and a shaft section, and the cutting section rotates around a rotation shaft that is coaxial with the shaft section,
• the cutting section includes a plurality of cutting edges,
• the plurality of cutting edges are disposed such that respective outermost ends of the cutting edges are positioned on a surface of a sphere having a smaller diameter than an inner diameter of the elbow to be finished, or the cutting edges are positioned on a circle perpendicular to the rotation shaft and arranged along the rotation shaft, the circle having smaller diameter than the inner diameter of the elbow to be finished and
• the moving of the cutting section is performed by moving the cutting section along the axial center of the elbow to be finished relatively in a direction from one end to another end of the roughly formed elbow or from the another end to the one end of the roughly formed elbow while rotating the cutting section around the rotation shaft.

An elbow, comprising:

• an inner side surface having an axial center bending inward at a predetermined curvature, the inner side surface having a circular shape in cross section;
• and
• an outer side surface formed along the inner side surface, the outer side surface having a diameter larger than a diameter of the inner side surface,
• wherein the inner side surface has the same flatness in a surface on an inner side of the curvature, and in a surface on an outer side of the curvature.