Ultraprecise micromachining of retroreflective structures

Abstract

To meet stringent automotive safety requirements, car taillights typically incorporate retroreflective elements. In addition to their retroreflective role, these structures are also used for lighting/aesthetic/styling purposes. The most common type of automotive retroreflector (RR) – also known as reflex reflector – is characterized by a corner-cube (CC) geometry that has been fabricated for more than 60 years through a conventional pin-bundling technology. While accurate, this manufacturing approach remains time-consuming, expensive and over-constraining in terms of the RR design. To address this, alternate RR fabrication pathways have been developed and this study outlines the capabilities of a novel approach including milestones, setbacks, advantages and disadvantages. Corner-cube geometry includes three mutually orthogonal facets that meet at a common vertex/apex. This configuration precludes the use of most material removal techniques involving rotational tools. To address this, an alternate RR shape called right triangular prism (RTP) was proposed. This geometry is amenable to diamond-based single point cutting approaches, but its optical performance proved to not be identical with that conventional CC RR. The successful fabrication of RTP RRs was demonstrated in acrylic and quality/functionality of the prototype were assessed through both metrological and optical means. Surface quality Ra of less than 20 nm was achieved through an adequate combination of multi-axis machine tool kinematics and ultraprecise single point tool geometry. This cutting technique worked well on non-ferrous, but not on ferrous materials. Nevertheless, an alternative strategy involving micromilling has been developed for cutting RTPs in ferrous substrates. The successful fabrication of tooling inserts has been completed such that injection molded replicas of RTP RRs will be produced in the future. It is expected that the development of cutting-based RR fabrication strategies along with the associate knowledge on the underlying cutting mechanics will enable a broader diversity of RR designs in the future.