Method of working fresnel step

Abstract

In connection with a Fresnel lens in which concentric pitch base lines are drawn on a curved lens member at certain pitches and a number of radial partitioning lines are drawn so as to pass through the center of the concentric pitch base lines and to cross these lines so that a surface of the lens member is partitioned into a large number of arcuate Fresnel step design sections, and an angled Fresnel step is formed in each of the Fresnel step design sections so that light transmitted from a focal point of the lens at its rear side forms a substantially parallel flux of light through a refractive or reflective prism, the invention relates to the cutting of Fresnel steps in a mold element for forming the lens by causing a tool attached to a multiple axis milling cutter and having an acute point to perform reciprocating or circumferential cutting for every width of each of the inclined surfaces of said Fresnel steps while causing the tool to move radially to perform scan-cutting of the inclined surfaces. The cutting inclination angle at the point of the tool attached on said multiple spindle milling cutter may be made equal to the angle of the Fresnel step at its steep surface side. Alternatively, the tool axis may be varied during cutting to allow the tool angle to correspond to the given angle or angles of the inclined surfaces.