Three wagons method applied: optimization of level of surface roughness within ductile mode grinding of tungsten carbide molds

Precision glass molding, PGM, enables an optics fabrication with a high level of stability and large batch sizes while keeping cost at low levels. This technology is relying on the generation of highly accurate mold surfaces made of brittle materials, e.g. tungsten carbide or silicon carbide. These mold materials are often finished by ductile mode material removal applying ultra-precision machineries (UPM) eventually generating low levels of surface roughness of a few nanometers root mean squared. While the process of ductile mode grinding is usually controlled by keeping tool indentation depths below a so called critical depth of cut hcu cnt, there is no method for predicting or optimizing the level of surface roughness being generated. In this paper, we report on a process analysis of ductile mode grinding analyzing the influences of critical process parameters on the level of surface roughness being generated for tungsten carbide and BK7 glass. To that aim, the "three wagons method", a recently developed method for process optimization in optics fabrication was applied. That way, critical process parameters were identified determining the eventual level of surface roughness within the ductile process window of UPM machining. Experiments have been carried out proving that the level of surface roughness generated strongly depends a.o. on the type of coolant used, the pH value of the coolant and the cutting depth h (with 0 < h < hcucrit). Based on the experimental data collected, a formula was developed enabling the prediction of the level of surface roughness eventually being generated by ductile grinding applying UPM machineries. Applying this formula, an optimized set of critical process parameter values has been determined predicting a minimum level of surface roughness on tungsten carbide (CTNO1L) by ductile mode material removal of < 1 nm rms; subsequently, this parameter set has been applied experimentally generating Ra = 0.83 nm, a value usually obtained by fresh feed polishing. The developed formula enables a better predictability of level of surface roughness within the process window of ductile mode grinding and is currently being extended to other materials.