Ralf Gläbe

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.

Advances in Precision Machining of Steel

Abstract Precision machining of steel with geometrically defined diamond cutting tools is handicapped by excessive tool wear. We have revisited solutions proposed in the literature and investigated some new approaches. Care has been taken to identify the mechanisms responsible for chemical and abrasive wear and to quantify tool wear in well defined cutting experiments.

Deposition of alumina coatings on monocrystalline diamonds by sol–gel techniques

Abstract Machining of steel or iron-based alloys with diamond tools leads to rapid tool failure — probably due to chemical wear. The use of monocrystalline diamond tools has, up to now, been obligatory for precision machining. Coating the diamonds with a thin but hard and chemically inert alumina film may overcome the problem. Alumina coatings were deposited by sol–gel techniques. It was shown that a very thin TiN intermediate layer, deposited by reactive sputtering, results in a good adhesion of the alumina coatings to the monocrystalline diamonds. The microstructure of the coatings was characterized by field-emission scanning electron microscopy (FE-SEM) and by transmission electron microscopy (TEM). The deposited coatings showed a nanocrystalline, dense microstructure. The hardness of the coatings was investigated by ultramicrohardness measurements (UMH).

Holographic projection based on diamond-turned diffractive optical elements

We introduce an approach to generate holographic data for diffractive optical elements fabricated by means of a diamond-turning process. The aim is to project a predefined intensity distribution in the far-field domain of the corresponding diffractive surface. The method takes into consideration typical constraints that result from the fabrication process, such as the spiral path of the turning tool and the fact that only the phase distribution of the incident light can be manipulated.

Diamond machining of micro-optical components and structures

Diamond machining originates from the 1950s to 1970s in the USA. This technology was originally designed for machining of metal optics at macroscopic dimensions with so far unreached tolerances. During the following decades the machine tools, the monocrystalline diamond cutting tools, the workpiece materials and the machining processes advanced to even higher precision and flexibility. For this reason also the fabrication of small functional components like micro optics at a large spectrum of geometries became technologically and economically feasible. Today, several kinds of fast tool machining and multi axis machining operations can be applied for diamond machining of micro optical components as well as diffractive optical elements. These parts can either be machined directly as single or individual component or as mold insert for mass production by plastic replication. Examples are multi lens arrays, micro mirror arrays and fiber coupling lenses. This paper will give an overview about the potentials and limits of the current diamond machining technology with respect to micro optical components.

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.

Design of diamond-turned holograms incorporating properties of the fabrication process

Recently, the fabrication of computer-generated holograms by diamond face turning with a nanometer-stroke fast tool servo (nFTS) has been demonstrated. Existing methods for the design of diamond-turned holograms account for their spiral-shaped surface topology and the fact that only the phase of a wave field can be modulated. Here we present an algorithm enabling the additional consideration of two important fabrication-related properties: the shape of the diamond tool used and the limited control frequency of the nFTS. Our method is based on the generalized projections method and enables the design of holograms for the reconstruction of arbitrary intensity distributions in the far field. Experimental results are presented, demonstrating the advantages of the method.

Material aspects for the diamond machining of submicron optical structures for UV-application

The paper discusses material aspects governing the diamond machining process for submicron structures which serve as diffractive optical elements (DOE). Therefore, first the cutting process for fabrication of DOEs is introduced. The microstructured surface is generated by diamond turning using a nano Fast Tool Servo (nFTS), which enables a variation of the depth of cut up to 350 nm at a maximum frequency of 10 kHz. Such DOEs are typically used with a wavelength of 632 nm. A new application for UV-radiation with a wavelength below 350 nm requires alternative materials with sufficient reflectivity. Cutting experiments and UV-reflectivity tests were performed in order to identify suitable materials. High machining accuracy and UV-reflectivity were achieved with ultrafine grained aluminium as workpiece material.

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.

Merging Technologies for Optics

The market for optical and optoelectronical components is a rapidly growing global market. The development of new manufacturing technologies for the fabrication of optics enables the fabrication of optical components with nanometer roughness and submicron form accuracy for mass markets like digital cameras, video projectors and automotive applications, even for low cost applications with short product life cycles. Therefore, the process chain for fabricating optical high quality mass products has to be deterministic flexible as well as in order to suppress cost intensive and time consuming iterations within the manufacturing chain. This paper introduces several approaches to take up this challenge.