Off Axis Grinding Machine
The precision and accuracy requirements for the off-axis aspheric generator are based on the need that the surface is ready for interferometric testing as early in the subsequent polishing process as possible. Using HeNe laser sources with a wavelength of 0.6328 micrometre, ground surfaces with four to eight wavelengths (2.5 to 5 micrometre) of surface error can be processed to converge rapidly to the required figure while the surface microroughness targets are being achieved.
This surface accuracy requirement includes the local undulations (or cusps) caused by the discrete passes of the grinding wheel (which is effectively spherical), together with the effect of grinding wheel wear. Target accuracy therefore for the machine motions in the full workzone of 2.5m x 2.5m x 0.61m was set at 2.5 micrometres as measured at a typical workpiece surface.
It can be shown from a simple error budget that it is unrealistic to expect the above target for workzone accuracy to be achieved using a conventional machine tool design approach in which geometric accuracy is determined by the integrity of the structure and guideways. Two approaches to enhanced accuracy were therefore considered.
- Error compensation by means of periodic characterisation of the accuracy performance.
- Use of an independent metrology frame concept.
The former approach relies on a predictable stability of the accuracy performance between each characterisation/calibration operation but, in view of the many uncertainties, the latter approach is selected.