O. Riemer

Polishing of Structured Molds

Abstract High precision molds for the replication of structured optical elements like Fresnel lenses or prism arrays are generated by diamond machining or precision grinding. In some cases surface quality of the replicated components is not sufficient to meet the increasing demands concerning surface roughness and form accuracy for optical applications. Subsequent polishing of the structures may therefore be necessary. Within this work structured molds were finished by a newly developed abrasive polishing process, by laser polishing, and by abrasive flow machining. This paper focuses on the material removal mechanisms and achievable surface quality in abrasive polishing. Surface quality is compared to that achieved by laser polishing and abrasive flow machining.

Measurement of optical surfaces generated by diamond turning

Abstract For the manufacture of ultraprecision parts an entirely controlled machining technology is required as well as the capability of a complete evaluation regarding microgeometry and integrity of the generated surfaces. Optical and stylus profilometry are the commonly used technologies for determining surface roughness. Furthermore, atomic force microscopy AFM is frequently applied for 3D characterization of the microtopography. In this paper the application of atomic force microscopy in the field of diamond turning is presented by means of measuring and manufacturing examples. Typical applications are the manufacture of metal mirrors from aluminum, copper and electroless nickel substrates by diamond turning and fly-cutting operations using monocrystalline diamond tools. The atomic force microscope is used for the evaluation of the microtopography of these mirrors, which is decisive for their optical properties and quality. Additionally, the atomic force microscope can be used for measurements of microstructured components and the diamond tools itself. In this case, AFM is not only used for roughness measurement, but also for inspection and verification of the actual, respectively, the produced geometry of the components and the tools. The examples given in this paper illustrate the great potentials of atomic force microscopy in the field of ultraprecision machining.

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.

Material removal mechanisms in abrasive vibration polishing of complex molds

Optical and medical industries are demanding a large variety of optical elements exhibiting complex geometries and multitude opto-functional areas in the range of a few millimeters [1]. Therefore, mold inserts made of steel or carbides must be finished by polishing for the replication of glass and plastic lenses [2]. For polishing theses complex components in the shape of localized cavities or grooves the application of rotating polishing pads is very limited. Established polishing processes are not applicable, so state of the art is a time consuming and therefore expensive polishing procedures by hand. An automated process with conventional polishing machines is impossible because of the complex mold insert geometry. The authors will present the development of a new abrasive polishing process for finishing these complex mold geometries to optical quality. The necessary relative velocity in the contact area between polishing pad and workpiece surface is exclusively realized by vibration motions which is an advantage over vibration assisted rotating polishing processes. The absence of rotation of the pad opens up the possibility to machine new types of surface geometries. The specific influence factors of vibration polishing were analyzed and will be presented. The determination of material removal behavior and polishing effect on planar steel samples has shown that the conventional abrasive polishing hypothesis of Preston is applicable to the novel vibration polishing process. No overlaid chemical material removal appears.

Fabrication of Diamond Micro Tool Array (DMTA) for Ultra-Precision Machining of Brittle Materials

A novel conditioning technique to precisely and effectively condition the nickel electroplated mono-layer coarse-grained diamond grinding wheel of 91m grain size was developed to fabricate a Diamond Micro Tool Array (DMTA) in ductile machining of brittle materials. During the fabricating process, a copper bonded diamond grinding wheels (91m grain size) dressed by ELID (electrolytic in-process dressing) was applied as a conditioner, a force transducer was used to monitor the conditioning force, and a coaxial optical distance measurement system was used to insitu monitor the modified wheel surface status. The experimental result indicates that the newly developed conditioning technique is applicable and feasible to generate required wheel topography of less than 2μm run-out error and grain geometries. The taper cutting test on BK7 proves the fabricated DMTA is capable of realizing ductile machining of brittle materials.

Tools and Tool Setting for Improved Surface Finish in Diamond Turning

Abstract In ultraprecision machining operations monocrystalline diamond tools are used with nose radii typically in the range of 0.5–10 mm. These tools allow the use of the entire cutting edge along the nose arc and a simplified procedure for tool adjustment. A disadvantage of this kind of tools is that, as a result of the kinematical conditions, the tool nose radius deteriorates the surface finish of the workpiece by generating feed marks. Also, radius tools are not very effective in smoothening vibrations which usally dominate surface roughness. In order to improve the surface finish, prismatic diamond tools with a straight minor cutting edge have been used in ultraprecision turning of aluminum plane mirrors. By setting the angle between the minor cutting edge and the workpiece surface to values in the order of 0.02°, an opitimal smoothening effect was achieved. Due to the plastic deformation of the workpiece by the minor cutting edge, voids and scratches from material inclusions are leveled flat.

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.

Herstellung von mikrostrukturierten Spritzgussformen für ein Operationsleuchten-System

Kurzfassung Für die Fertigung komplexer optischer Oberflächen und Mikrostrukturen sind Diamantbearbeitungsverfahren wie das Drehen und Fräsen mit monokristallinen Diamantwerkzeugen fest etabliert. Für spezielle Anwendungen sind diese Fertigungsverfahren allerdings aufgrund ihrer geometrischen und kinematischen Limitierungen nicht immer ohne weiteres einsetzbar [1]. Der folgende Artikel fokussiert auf die Weiterentwicklung eines Konturbohr- und eines Profil-Umfangsfräsverfahrens zur Herstellung von Spritzgussformen für einen strukturierten Ring-Reflektor eines großen neuartigen offenen Operationsleuchten-Systems. Das Ziel des neuen Beleuchtungssystems ist es, eine Ausleuchtung bei hoher Homogenität im Fokusbereich ohne thermische Beeinflussungen und eine hohe Anzahl an Fokuspunkten in der Tiefe zu erreichen. Das System besteht aus einer Gasentladungslampe in Kombination mit einem offenen multi-konvexen Ringreflektor.