J. Osmer

Ultra-Precision Diamond Cutting of Steel Molds

Excessive chemical tool wear occurs when steel alloys are machined with monocrystalline diamond tools. The basic idea of the presented research work is to avoid chemical reactions between the carbon of the diamond tool and the iron of the workpiece by establishing a chemical bond between the iron and another chemical element. Therefore a thermo-chemical process for modifying the chemical composition of the subsurface zone was developed. As a result the diamond tool wear was reduced by more than three orders of magnitude which by no other method has been achieved so far. The surface roughness obtained in diamond cutting of carbon steel was better than 10 nm Ra.

Tool path generation for ultra-precision machining of free-form surfaces

The generation of tool paths for ultra-precision machining is still a limiting factor in the manufacturing of parts with complex optical surfaces. In conventional machining as well as in complex five axes machining the application of CAD- and CAM-software for the generation of tool paths is state of the art. But these software solutions are not able to generate tool paths according to the high requirements of ultra-precision machining. This paper describes possible ways to generate tool paths for ultra-precision machining when the optical surface can be analytically described or when the surface data is derived from optical design software. Ultra-precision milling experiments with different tool paths have been carried out and the quality of the machined geometry has been evaluated concerning the achievable form accuracy.

Diamond Cutting of FeN-Layers on Steel Substrates for Optical Mould Making

The mass production of glass or plastic components by replication techniques, like hot pressing or injection moulding, requires inserts made of temperature resistant and hard materials. Generating an optical surface finish in these materials is time consuming and difficult. By using thermo-chemically treated steels as mould materials diamond cutting processes generating high form accuracies and low surface roughness can be applied without significant tool wear.

Diamond milling of nitrided steels for optical mold making

So far ultraprecision diamond machining of steel molds for optical applications is not possible because of excessive wear of the applied diamond tools. This article discusses a thermochemical approach for steel machining with single crystal diamond tools as well as the process of material modification. Ultraprecision raster milling experiments for the nitrided carbon steel C45 (AISI 1045) and the tempering steel 42CrMo4 (AISI 4140) are presented. In both materials a surface roughness of Ra<10nm could be achieved. The observed flank wear of the cutting tools was VB<2μm for all experiments after cutting work pieces with a diameter of 30mm.

Precision Mould Making – From Macro to Micro

Nowadays several qualified technologies have been established for the manufacturing of precision moulds. The fields of application can mainly be divided into moulds for non-optical and optical components. For optical moulding inserts the development goes from basic rotational symmetric geometries to complex surfaces like steep aspheres and freeforms which can additionally be overlaid with microstructures. The moulded components require a figure accuracy in the (sub-) micrometer and surface roughness in the nanometer range while moulds for replication also need advanced materials with high surface integrity. Here, diamond machining processes, e.g. diamond turning and milling as well as precision grinding and polishing are necessary for the manufacturing of precision moulding inserts from various materials. Depending on the material and application of the applied part to be replicated different replication techniques are used like injection moulding of plastics, hot embossing and precision moulding of optical glasses. For non-optical applications the current technical progress is driven by miniaturized products which are typically produced in mass production by replication techniques like hot embossing or metal forming. Each of these processes requires specific properties of the mould. Therefore, the surface topography and tribological conditions are of particular importance.

Ultra Precision Machining of Non-Ferrous Metals and Nitrocarburized Tool Steel

This paper presents results for the machining of materials typically applied in ultra precision machining in comparison to a nitrocarburized tool steel. Analyzing and evaluating the machining results regarding surface integrity lead to recommendations for the ultra precision machining of this new mold material. The influence of feed, depth of cut and cutting speed on surface quality, resulting cutting forces and tool wear have been investigated. The results show that the decisive factor for the ultra precision machining of nitrocarburized tool steel are the significantly higher cutting forces. In some cases the high cutting forces lead to vibrations during the turning process deteriorating the surface integrity. Therefore, tool nose radius and depth of cut have to be reduced to minimize the cutting forces and avoid the vibrations.

Ultraprecision machining of nitrocarburized steels

Ultra precision machining processes generate surfaces in optical quality and with sub-micron form accuracies. These specifications can be realised by applying single crystal diamond tools with nanometric edge sharpness. Typical workpiece materials are non-ferrous metals which can be machined without significant tool wear. But for optical mould making these materials have disadvantages regarding tool life in injection moulding of plastics. Alternatively diamond cutting of thermo-chemically treated steel is a new way to machine hardened steel moulds. This paper presents results from the machining of two thermo-chemically treated steel alloys. Analyzing and evaluating the machining results regarding surface integrity will lead to recommendations for the ultra precision machining of nitrocarburized steels. The influence of the thermo-chemically generated compound layer composition on surface quality and tool wear has been investigated. Therefore, diamond turning experiments have been carried out on a five axes ultra precision lathe in different depth beneath the surface. Here, both steels can be machined in optical surface quality with a surface roughness Sa < 10 nm, but the achievable surface quality strongly depends on the depth beneath the surface in which the machining takes place. The results show that with increasing depth beneath the surface the roughness values increase as well. Therefore, diamond machining at the edge between compound layer and diffusion layer has to be avoided to gain the best possible surface quality.

Diamond Machining of Nitrocarburized Steel Molds for the Mass Production of Optical Components

Optical components, e.g. lenses for cameras in mobile phones or illumination optics, are produced in high volume by repliation techniques in plastics or glass. Thus, the necessary sequence of process steps comprises all steps from optic design to mold making and the final replication process. Every single step in this process chain is accompanied by high resolution measurement techniques and quality management. An important factor in this process chain is the optical mold making by diamond cutting processes with single crystal diamond cutting tools. The drawback of diamond cutting processes so far was that steels which have very good material properties for replication (temperature resistance and hardness) cannot be machined because a catastrophic diamond tool wear occurs. If these materials are applied for optical mold making a complex process combination of grinding and polishing operations is inevitable. The substitution of this traditional process combination by diamond cutting processes can reduce costs and enhances the possibilities of optical mold making. This paper describes a possible solution for diamond machining of steel molding inserts for the replication of optical components by plastic injection molding and hot pressing of glass. Through a thermo-chemical surface treatment the chemical reactivity of steel and single crystal diamond cutting tool can be minimized. Beside the results of the diamond machining processes the occurring diamond tool wear is discussed. From these results a new process chain for the manufacturing of optical components is deduced. This novel process chain consists of nitrocarburizing the steel molding inserts and a subsequent diamond machining operation by turning or milling. Through the nitrocarburizing process the diamond tool wear can be reduced by three orders of magnitude and the machined molding inserts show an optical surface roughness Sa 10nm. These inserts can directly be applied for the replication of plastics or the pressing of glass which is shown by first application examples.

Chip Formation in Ultra-Precision Machining of Nitrocarburized Steels

For the replication of optical glass or plastic components moulding inserts with surface roughness in the nanometre range and form accuracy in the micron or sub-micron range are needed. Despite these requirements the applied moulding insert material has to suit further needs like high temperature stability and resistivity against abrasive and chemical wear. To satisfy the specific requirements of replication processes steel alloys can be heat treated in a way to meet these demands. Unfortunately, these steel alloys cannot be machined with single crystal diamond tools because catastrophic diamond tool wear occurs. In recent years good progress in the field of ultra precision machining of steel has been made by nitrocarburizing the steel alloy. This leads to a sub-surface compound layer which is diamond machinable with surface roughness Sa < 10 nm and reduced diamond tool wear. But the ultra precision machining of these nitrocarburized steels introduces new challenges caused by the high hardness of the compound layer. Typical values are about 1200HV0.025. Therefore, this paper presents results from ultra precision machining processes focusing on the material behaviour during the cutting process. Influences of depth of cut and material composition on the surface generation can be found by evaluating chip formation and the resulting chips. Furthermore, the sub-surface of ultra precision machined steels is characterized by metallographic analysis to evaluate the influence of the nitrocarburizing process on ultra precision machining. In conclusion this paper presents the results for a deeper understanding of the material removal mechanisms in ultra precision machining of nitrocarburized steels.