Investigating diamond turning process to achieve optical flatness in an aluminium alloy

Abstract

Optically flat components are very versatile and used in many applications

such as: inspection of gauge block accuracy and its wear, testing several modules

containing lenses, mirrors, prisms, filters etc. They also meet recruits for high

demanding interferometry where exceptionally flat optical windows are needed.

Such optically flat components require a high degree of surface preparation;

characteristics such as surface finish and flatness often have to be within fractions of

light wavelengths. Such demanding surfaces can be fabricated by machining

processes such as the Single Point Diamond Turning (SPTD) process.

The aim of this project is to improve flatness in ultra-precision machining

(diamond turning) of an Aluminium alloy optical component. Experiments were

carried on an ultra-precision lathe, with single crystal ultra-sharp diamond tools.

Surface characterization is carried using interferometry to identify the degree of

flatness. Process parameters, fixturing techniques and material effects (where

possible) are varied to understand their effects on the surface generated. This is

expected to be of interest to companies such as Qioptiq (Singapore).

Form accuracies of less than or equal to 0.15Jlrn (6") and finish tolerance

reaches less than or equal to 5nm Ra are shown using proposed improvements. It is

shown that by providing a very strong, dynamically stiff fixturing using Light

Activated Adhesive Grippers (LAAG) technology, a relatively new work holding

technology, the workpiece can be gripped with minimum pre-loaded distortion, with

maximum approachability and dynamic rigidity during the machining process.

Additionally, such joints can be cured or destroyed using light on demand within

seconds.