Christian Wenzel

NURBS Based Ultra-Precision Free-Form Machining

The machining of ultra-precision optical components with form tolerances in the sub-micron range requires a close interaction between the machine tool, the process and the procedure for the NC tool path generation. Especially for the optical free-form machining the choice of a data-format for the surface description as well as the calculation of the tool path is crucial for the overall achievable quality of the work piece. This paper presents the layout of a tool path calculation based on the NURBS data format that has been developed at the Fraunhofer IPT. In addition the interfaces and the hardware and software for the realisation of a NURBS based control unit for Fast Tool Servo turning and local corrective polishing operations are described.

Development of a force controlled orbital polishing head for free form surface finishing

Polishing is the last and the most essential process step to guarantee the required form accuracy and especially the surface roughness in the production of steel molds. This is true for polishing of molds for optics manufacturing as well as mold making in the field of aviation, automotive or consumer products. One of the characteristics of the production is that in most cases the finishing is done by manual labor which is extremely time consuming and cost intensive. Moreover the polishing result is crucially depending on the worker’s skill and education. To overcome these limitations Fraunhofer IPT is developing a fully automated polishing cell which realizes a completely automated and reproducible finishing process. By this means an important competitive advantage for high-wage countries will result. A standard six-axis industrial robot with a novel force controlled orbital polishing head will support the manual processing. In this paper, the mechanical setup of the developed orbital polishing head as well as its control system is described, starting from an analysis of the polishing process and the derivation of the requirement specification.

Simulation of dynamic effects on hydrostatic bearings and membrane restrictors

Hydrostatic bearings have an excellent static and dynamic behavior and are used for different kinds of application. Application of hydrostatic bearings is limited by friction and therewith by velocity. Typical characteristics of the hydrostatic system (load, stiffness, flow) are calculated without a velocity dependency. The geometry of the hydrostatic bearing pockets and their restrictors are optimized by using time continuous pressure distribution at the bearing pocket, laminar flow behavior as well as constant velocity of the bearing. The dynamic effects of the flow at high velocities are not considered. The paper reflects the common design and calculation methods and shows their limitations in regard to the calculation of hydrostatic bearings at high velocities. It analyzes the results of complex dynamic flow simulations of hydrostatic bearings and presents a new design and optimization concept of hydrostatic bearings. This concept analyses the oil flow at high bearing velocities and it optimizes the bearing geometry, the restrictor geometry as well as the geometry of the main mechanical components.

Dynamic long axis for ultra-precision machining of optical linear structures

Currently, fly cutting is the most popular manufacturing technology for the machining of planar groove structures. The disadvantage of this technology is the long machining time. A promising alternative technology for the machining of planar grooves is planing. The main disadvantage of planing in comparison to fly cutting is the limitation of conventional precision axes concerning a high dynamic movement. Regarding to this aspect the Fraunhofer IPT has developed a precise linear axis. It allows high dynamic movements by using an impulse decoupling system (AiF-FV-Nr.: 13,270 N). The paper describes the mechanical setup and the development and optimization of the mechanical main component. The detailed simulation of the drive system (including motor control loop and impulse decoupling system), results of static and dynamic measurements and test machining results are presented.

Interferometrische Messung von Freiform-Schneidkanten auf einer Diamantwerkzeugbearbeitungsmaschine

Zusammenfassung Mit Diamantwerkzeugen lassen sich in einem Bearbeitungsschritt Oberflächen mit optischer Qualität erzeugen. Um insbesondere auch komplexe Oberflächenstrukturen mit einer reduzierten Anzahl an Zerspanungsabläufen fertigen zu können, sollen neue Werkzeuge mit Freiform-Schneidkanten zum Einsatz kommen. Voraussetzung sind jedoch Diamantwerkzeuge mit hoher Maßhaltigkeit, die bei ihrem Herstellungsprozess geprüft werden müssen. Hier werden sowohl das Konzept zur Prüfung dieser Freiform-Diamantwerkzeuge als auch die Ergebnisse eines simulierten messtechnischen Soll-Ist-Vergleichs vorgestellt. Abstract With diamond tools it is possible to achieve high surface qualities with one process step. To produce complex mouldings with a reduced amount of chipping runs new tools with free-form cutting edges will be used. However, this requires a high accuracy of the diamond tools which must be tested consequently and very precisely in real process time. The concept of the used measurement technique for the free-form diamond tools and the simulated results of a set-actual comparison of a cutting edge are presented.

Pneumatic Positioning System for Precision Assembly

Micro assembly is typically characterised by positioning tolerances below a few micrometers. In the case of a hybrid micro system assembly, such as optical glass fibres, micro ball lenses or micro probes for measurement tasks, even positioning accuracies in the sub-micrometer range have to be achieved. Due to the need for highly accurate assembly systems and extensive alignment procedures the assembly of hybrid microsystems is characterised by customised solutions. In this context the Fraunhofer IPT develops a concept on how to realise a highly flexible, fast and cost-efficient hybrid assembly system, consisting of a conventional assembly device and an active assembly head.

Automated tool exchange for ultraprecision diamond milling and turning applications

Active tool wear control is a crucial factor for fulfilling the high shape and surface requirements in the large area ultra-precision diamond surface structuring of optical components. The Fraunhofer IPT has developed optomechanical setups for a fully automated in situ tool characterisation and a reproducible tool exchange. The systems embody a submicron accuracy for both, the machining of planar masters for replication by means of planing and fly-cutting operation as well as for the turning of large rollers for embossing applications.

Model-based adhesive shrinkage compensation for increased bonding repeatability

The assembly process of optical components consists of two phases – the alignment and the bonding phase. Precision - or better process repeatability - is limited by the latter one. The limitation of the alignment precision is given by the measurement equipment and the manipulation technology applied. Today’s micromanipulators in combination with beam imaging setups allow for an alignment in the range of far below 100nm. However, once precisely aligned optics need to be fixed in their position. State o f the art in optics bonding for laser systems is adhesive bonding with UV-curing adhesives. Adhesive bonding is a multi-factorial process and thus subject to statistical process deviations. As a matter of fact, UV-curing adhesives inherit shrinkage effects during their curing process, making offsets for shrinkage compensation mandatory. Enhancing the process control of the adhesive bonding process is the major goal of the activities described in this paper. To improve the precision of shrinkage compensation a dynamic shrinkage prediction is envisioned by Fraunhofer IPT. Intense research activities are being practiced to gather a deeper understanding of the parameters influencing adhesive shrinkage behavior. These effects are of different nature – obviously being the raw adhesive material itself as well as its condition, the bonding geometry, environmental parameters like surrounding temperature and of course process parameters such as curing properties. Understanding the major parameters and linking them in a model-based shrinkage-prediction environment is the basis for improved process control. Results are being deployed by Fraunhofer in prototyping, as well as volume production solutions for laser systems.

Strategies for precision adhesive bonding of micro-optical systems

Today’s piezo-based micromanipulator technology allows for highly precise manipulation of optical components. A crucial question for the quality of optical assemblies is the misalignment after curing. The challenge of statistical deviations in the curing process requires a sophisticated knowledge on the relevant process parameters. An approach to meet these requirements is the empirical analysis such as characterization of shrinkage. Gaining sophisticated knowledge about the statistical process of adhesive bonding advances the quality of related production steps like beam-shaping optics, mounting of turning mirrors for fiber coupling or building resonators evaluating power, mode characteristics and beam shape. Maximizing the precision of these single assembly steps fosters the scope of improving the overall efficiency of the entire laser system. At Fraunhofer IPT research activities on the identification of relevant parameters for improved adhesive bonding precision have been undertaken and are ongoing. The influence of the volumetric repeatability of different automatic and manual dispensing methods play an important role. Also, the evaluation of UV-light sources and the relating illumination properties have a significant influence on the bonding result. Furthermore, common UV-curing adhesives are being examined on their performance and reliability for both highest precision prototyping, as well as their application as robust bonding medium in automated optics assembly cells. This paper sums up the parameters of most influence. Overall goal of these activities is the development of a prediction model for optimized shrinkage compensation and thus improved assembly quality.

Structurally Integrated Sensors

The work described here explores the suitability of metrological applications for direct measurement of thermo-elastic deformations in a machine tool’s basic structure with the goal of correcting positioning errors by means of control engineering. This paper first introduces the measurement principle of the sensors applied. It then presents the outcomes of investigations aimed at the system approach’s validation, demonstrating the concept’s suitability to measure structural deformations. It was also possible to show that the deformation of simple structural elements can be represented with sufficient accuracy by the linear functions of the signals obtained using structurally integrated sensors.