Ductile mode grinding is usually applied for finishing of e.g. tungsten carbide molds used for precision glass molding (PGM) by controlling depth of cut on feed controlled machines. Bifano et all. demonstrated the possibility to apply this mechanism while machining hard and brittle materials by the use of ultra-precision machines (UPM). Based on experimental investigations a formula for the transition from brittle to ductile cutting mechanism, also known as the critical depth of cut hcu,crit, relating the material specific properties Young’s-Modulus E, material hardness H and fracture toughness KC was developed  and is widely used for setting up UPM machines ever since. However, the influence of cutting conditions, like tool or process characteristics, are neglected leading to discrepancies of the value of hcu,crit between the prediction and the actual machining results of up to 200%. Furthermore, previous investigations have shown that hcu,crit strongly depends on coolant fluid characteristics as well as on the compressive stress applied into the cutting zone by the use of tools with e.g. negative rank angles . In this paper, we report on a ductile grinding process analysis applying a recently developed method for process optimization in optics fabrication . Following that trail, critical process parameters have been identified determining the process window of feed controlled ductile grinding applied on State-of-the-Art UPM machineries. The influences of the critical process parameters on the critical depth of cut hcu,crit have been tested experimentally using an ultra-precise SPDT machine. Among others, four critical process parameters could be identified determining the transition between brittle and ductile mode grinding: the critical depth of cut depends substantially on (a) the type of coolant used, (b) the pH value of the coolant, (c) the tool tip radius of the applied diamond and (d) whether ultrasonic assistance (US) is being switched on or off. Depending on the applied set of process parameters and for the experimental data collected, maximum ductile mode material removal rates could be achieved with hcu,crit, max = 1600 nm. That way, a new formula was developed, which allows the prediction of the critical depth of cut depending on critical process parameters, a.o. tool parameters and cutting fluid characteristics, while machining e.g. binderless nanocrystalline tungsten carbide or BK7 glass. This formula was set up based on fundamental ruling test results and is one step towards extending Bifanos formula taking the influences of critical process parameters into account.