Ultra-fine grinding as a prerequisite for producing polishable free-form optics

. In many conventional optical manufacturing processes, lapping is still a standard method to provide polishable surfaces. With the increasing use of CNC technology, efforts are being made to substitute the lapping process in the process chain. The paper presents studies that compare the lapping and fine grinding processes and provide an assessment of the subsequent polishing process. By using fine grinding with resin bond tools, polishing times can be significantly reduced and subsurface damage structures minimized. Ultra-fine grinding is also an important shaping process for the production of complex surface geometries, such as free-form optics.


Motivation and research approach
In order to obtain polishable surfaces, certain requirements must be met with regard to the maximum shape deviation and the micro-roughness of the components.This objective is particularly important for an efficient manufacturing process of optical components in order to be able to perform the final polishing step with high efficiency.In classical process chains, pre-and fine grinding are used for shaping and often a lapping step is inserted before the polishing process starts.An important objective of the lapping process is the geometric shaping of the workpiece with a surface structure that is adapted to the subsequent polishing process.For this purpose, lapping compounds with an average grain size between 23 and 3 µm are used.The processing objectives are thus to refine the surface structure, to approximate the final shape as far as possible, and to achieve a surface suitable for polishing.[1] Current requirements of the optical industry are the production of smaller quantities and the machining of more complex optical surfaces, especially aspheres and free-form surfaces.For this purpose, CNC machines with several simultaneously operating machine axes are used.Lapping operations are not or only partially suitable for these machining requirements.Thus, an important research task is to answer, can polishable surfaces be completely provided by grinding operations ?To this end, two different approaches have been taken in recent years.On the one hand, ductile grinding has been developed by BIFANO, and on the other hand, tool manufacturers, such as Effgen, have provided polymer bonds that enable ultra-fine grinding of optical surfaces.[2] With both methods, surface qualities can be achieved which exhibit a lower roughness after ultra-fine grinding compared to the lapping process.While the right lens of Figure 1 has a typical matte surface after lapping with F800, the left lens geometry already has a partially transparent surface that comes close to a pre-polished state.If such surfaces can be processed with full-surface tools, for example spheres and prisms, it can initially be assumed that the mid spatial frequencies behave similarly.However, it is interesting to ask to what extent subsurface damages can be minimized by the ultra-fine grinding process in order to be able to carry out the downstream polishing process with limited depth.

Comparison of the methods
Lapping is a machining process with loose grain distributed in a paste or liquid, which is guided on the lapping tool in paths that are as non-directional as possible.This gives the glass surface a surface typical of the process, with a stochastic distribution and a relatively high proportion of material carried by the surface.The relative movement causes the workpiece to move under pressure over the lapping tool.In the process, the lapping grains start to roll and cause localized and time-limited contact with the workpiece.The lapping pressure becomes so great in geometrically small zones that it exceeds the compressive strength of the glass, resulting in a large number of microcracks.As a result, the cracks converge, lapping small pieces of glass and finally blasting them out of the workpiece surface with the cooperation of the water.The summation of the individual micro-abrasion craters results in shape removal, whereby the shape to be created is transferred to the glass component by the lapping tool.In ultra-fine grinding, on the other hand, the cutting grains are directly integrated into the bond.By using resin bonded tools (see figure 2), the grinding tool obtains a much higher ductile behavior in contrast to a metallic or ceramic bond Fig. 2. Plane grinding tool for machining with bonded grain, with a resin bond, Effgen company.
In their interaction with the glass surface, the grains have a much smoother effect on the glass surface than in the case of lapping.Thus, it can be assumed in principle that the microcracks have a lower penetration depth and a lower depth of subsurface damages is achieved.

Investigation of lapping and ultrafine grinding on plane workpieces
The same average particle sizes were used for the investigations.The respective process parameters, in particular rotational speed and feed rates, naturally differ for both processes.Figure 3 shows a simple comparison of the micro-roughnesses of both processes based on a tactile measurement.If ultra-fine grinding is additionally used in the process chain instead of the lapping process, the subsequent polishing time can be significantly reduced.The total processing time can be reduced by about 40%.(figure 4) Fig. 4. comparison of conventional polishing time between samples which were ultrafine-grinded using resin bond tools and samples being grinded with metal bond D35 tool (material fused silica)

Conclusion
Ultra-fine grinding with resin-bonded tools offers a very good prerequisite for producing polishable free-form geometries (see figure 5).Due to the kinematic principle of action, lapping processes are often not suitable for free-form machining.

Fig. 5. Ultra-fine ground free-form geometry
High-quality surfaces can be produced with ultra-fine grinding and subsurface damages can be minimized.In combination with a 5-axis grinding machine, the grinding tools investigated offer very good conditions for polishing complex surface geometries.