Method for manufacturing mold for retroreflective element and method for manufacturing retroreflective element

A method for manufacturing a mold for a retroreflective element having plural polygonal faces, the mold having plural polygonal faces having a common vertex, the method including the steps of:

• roughing of a polygonal face in which cutting is carried out such that a predetermined cutting amount in a finishing process is left with respect to a shape to be obtained, the polygonal face corresponding to one of the plural polygonal faces of the retroreflective element;
• and
• finishing of the polygonal face in which a blade portion having two cutting edges, an angle formed by the two cutting edges being substantially identical with an interior angle of the polygonal face at the vertex and a length of each of the two cutting edges being equal to or greater than a length of each of the edges of the polygonal face meeting at the vertex, is made to move relatively towards the vertex while an angle between the polygonal face and a surface of the blade portion edged by the two cutting edges is kept within 1 degree and the blade portion is kept oscillating such that cutting of the predetermined cutting amount of the polygonal face is carried out using the two cutting edges and flatness of the polygonal face is kept tolerable,
• wherein a depth of cut for each one-time cutting operation is 2 micrometers or smaller, and a movement of the blade portion is a combination of a linear motion towards the vertex and an oscillation, and
• wherein a distance travelled by the blade portion in a direction of the linear motion during a period of the oscillation ranges from 0.01 micrometers to 20 micrometers.