Optical Materials: Double-Sided Lapping and Polishing | Optics | Photonics Handbook | Photonics Marketplace

The slurry performs the stock removal as the abrasive grains roll and slide between the two plates and the workpiece. For polishing, the abrasive type is changed, the particle size is reduced, and polishing pads are applied to the lap plates. This combination results in slower stock removal rates and provides a much improved surface finish. Lapping and polishing slurries can be recirculated depending on cost and process performance. Sequential removalEach step removes the subsurface damage incurred during the previous process and prepares the parts for parallelism, surface roughness and thickness for the next process. By the completion of the process, the workpiece becomes a mirror image of the lapping plates. Ideally, the plates should match in that one is slightly concave and the other equally convex.

The most elementary variables, aside from optical material, involve part geometry and size. Parts can be square, rectangular, trapezoidal, round, elliptical, or polygonal. Round parts yield better results because they can rotate around their own axis. Also, minimum thickness limitations are lower with round parts. Parts can be processed using DSLP from 0.025 mm (0.001 in.) in thickness up to the limitations of the specific machine, with a general consensus of about 50 mm (~2 in.). With thinner parts, the limiting factor is the aspect ratio of the part’s largest dimension (diameter or diagonal) to the thickness. The higher the aspect ratio, the higher the probability that the glass will distort or become damaged during processing, and the more difficult it is to obtain uniform surface conditions on both surfaces. Pricing increases for parts with higher aspect ratios. Table 2 demonstrates the minimum thicknesses generally possible on DSLP machines, machine specific, of course.