Tool offset optimisation for the machining of free-form optics with a non-zero gradient at the centre

Abstract

Free form surfaces are now commonly used components in optics applications and can be widely found in fields such as ophthalmics, car illumination and head-up display systems and laser optics. The machining of free form optics on a 3-axis diamond turning machines is made possible with the use of tool servo machining which synchronises either or both the axial and radial motions of the tool and surface positions (X & Z axes) to the angular position of the spindle (C axis). However, the machining of surfaces with non-zero gradient at the surface centre is particularly troublesome because the tool is still subject to a relatively large amplitude motion when reaching the central area of the surface. As a result, a small tool offset in either X (radial) or Y (height) creates a particular central signature that can be readily identified, measured and subsequently corrected by the machine operator. In this paper, we report on a method to optimise the tool offset (X axis) in the particular case of non-zero central gradient and illustrate our discussion with simulation and measurement results.

Diamond machining of a single shot ellipsoidal focusing plasma mirror

“…This introduces a number of positional or dimensional errors such as the tool radius uncertainty, tool waviness, tool offset and tool height. 4 • Orthogonality or more generally the residual misalignment of each axis with respect to the other.…”

“…The ellipsoid has been designed to have a zero gradient at the centre to avoid possible cutting artefacts induced by residual tool offset and tool height as discussed in. 4…”