Ductile mode single point diamond turning (SPDT) of binderless tungsten carbide molds

Ductile mode grinding is a finishing process usually being applied to generate molds in brittle materials (e.g. tungsten carbide) to be used for precision glass molding (PGM). To that aim, ultra-precision machineries (UPM) are applied controlling depth of cut not to exceed a critical value, hcu, crit (e.g. 160 nm for tungsten carbide). Recent process analyses of the ductile mode grinding process of brittle materials have demonstrated that the critical indentation depth hcu, crit, that determines the transition from brittle mode to ductile mode removal, can significantly be shifted to higher values by adjusting process parameters such as the type of coolant and its pH value: e.g. for tungsten carbide up to 1600 nm and for BK7 glass up to 350 nm depth. This paper reports on a feasibility study to extend the process window of ductile mode material removal. Applying optimized ductile process parameter sets, enabling values of the critical depth of cut larger than 1 micron, single point diamond turning (SPDT) of binderless tungsten carbide molds has been successfully tested applying UPM machineries. Experimental data will be presented demonstrating that by controlling and adjusting ductile process parameters only, it is possible to extend its process window into regimes that are today not yet machinable: binderless tungsten carbide molds for precision glass molding have been processed in a ductile removal mode by SPDT generating surface roughness levels of less than 2 nm rms. An analysis of the adjustment of the critical process parameters will be presented together with a detailed description of the First Light experiments towards SPDT of binderless tungsten carbide molds.