Positioning structure between parts of different materials and assembly structure of motor

A positioning structure between parts of different materials that positions a first part formed of a first material to a second part formed of a second material, which is different from said first material in degree of modification due to environmental variation, by contacting surfaces of said first and second parts, comprising:

• at least three solid projections that are disposed on the contact surface of said first part at nearly equal angular intervals along a circle whose center is coincident with a reference point, which reference point is defined as a point where the relative position between said first and second parts does not change even if environment varies, said projections being projected in a direction substantially perpendicular to a plane including said circle;
• and
• grooves that are disposed on the contact surface of said second part so that the respective projections are fitted therein and that have wall surfaces to which said projections can contact at the inner and outer sides of said projections when viewed from said reference point,
• wherein the width of said grooves in a radial direction of said circle is determined so that said projections contact the inner or outer wall surface of said grooves even when said first and second parts expand or contract in size in different degrees due to environmental variation,
• wherein said first and second parts are positioned by contacting the surfaces thereof while fitting said projections into said grooves,
• wherein each of said projections has a predetermined width in the radial direction so that the projections contact the inner wall surface and the outer wall surface of said grooves when assembling at a normal temperature,
• wherein the environmental variation includes temperature variation or humidity variation.