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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.
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 the “three wagons method”, 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 binderless nanocrystalline tungsten carbide molds. 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.
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 < hcu,crit).
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 (CTN01L) 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.
In this paper, we publish first results on ductile machining of tungsten carbide dies on a standard CNC grinding machine.
This approach enables the pre-machining and finishing of tungsten carbide dies in one machine, in one clamping with the same tool.
In 2017, the so called “grinding process validation approach” (gPVA) was introduced to determine suitable parameter windows. The method allows the definition of parameter windows for grinding tools and materials. Parameter adjustments for optimum results are possible due to the experimentally determined dependence on specific chip volume and tool pressure.
The system was originally developed to describe brittle grinding processes on standard grinding machines. Tests on an ultra-precision lathe provide process parameter data on ductile mode machining of tungsten carbide using UPM machines. This paper reports on gPVA being applied to transfer the ductile machining process from UPM machineries to a standard CNC grinding machine.
There are a several approaches and methods to determine SSDs known in literature. However, many of them inevitably lead to the destruction of the workpiece. Although others are non-destructive, but very complex in design and/or associated with large investments. Likewise, only a few are suitable for determining SSDs on ground rough surfaces.
Filled-Up Miicroscopy (FUM) is an alternative approach to approximating the depth of SSDs, even on rough surfaces without destroying them. At a first glance at the method, the procedure is described in detail and all necessary steps of preparing the samples are shown. A first comparison with the known Ball Dimpling Method confirms the functionality of the concept.
Based on experimental investigations a formula for the critical depth of cut, relating the material specific properties Young’s-Modulus E, material hardness H and fracture toughness KC was developed by Bifano et. all [1]. Even when the influence of cutting conditions, like tool or process characteristics, are neglected the formula is widely used for setting up UPM machines ever since. However, previous investigations have shown that hcu,crit strongly depends on coolant fluid characteristic as well as on the compressive stress applied into the cutting zone by the use of tools with e.g. negative rank angles [2].
In this paper, we report on a ductile process analysis applying a recently developed method for process optimization in optics fabrication [3]. Following that trail, critical process parameters have been identified and their influences on the critical depth of cut hcu,crit have been tested experimentally in fundamental ruling tests.
Among others, following parameters were identified and tested: (a) characteristics of the coolant used, (b) the pH value of the coolant, (c) the tool specifications 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 dcmax = 1600 nm.
That way, a new formula was developed, which allows the prediction of the critical depth of cut depending on critical process parameters while machining binderless nanocrystalline tungsten carbide. The formula was set up based on experimental results and is one step towards extending Bifanos formula taking the influences of critical process parameters into account.
Bifano et all. demonstrated the possibility to apply ductile cutting mechanism while machining hart and brittle materials by the use of ultra precision machines which enable highest precision in feed control and indentation depths. Based on the investigations a formula relating the material specific properties Youngs-Modulus E, material hardness H and fracture toughness KC was developed [1]. For this, the influence of the cutting conditions and the process environment were not taken into account. But previous investigations have shown a high influence of the cutting fluid while machining binderless nanocrystalline tungsten carbide through ultraprecision diamond turning [2].
Concluding, calculating the transitions point by using the formula shows insufficiently results and cannot be used for setting up a stable and efficient process. To adjust the formula and to allow a better prediction of the cutting process, further investigations are necessary.
This paper focuses in particular on the influence of water with different pH-values (acidic, neutral and alkaline) on the transition point for some brittle materials, binderless tungsten carbide and two different kinds of glass. Basic ruling experiments with a steadily increasing depth of cut have been performed. Measurements by white light interferometry have been done to analyze the influence on the cutting performance of mono crystal diamonds. As a result, a deeper understanding of the chemical influence on the mechanical process of diamond machining is created.
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