FISBA OPTIK (Switzerland)

Direct metal deposition (DMD) and plasma transfer arc welding (PTA) are two metal deposition techniques, which are well-known for high-quality and high-productivity level of fabrication, respectively. In the field of additive manufacturing (AM) of large-scale metallic parts, combined technologies of these methods can offer advantageous solutions to manufacture complex parts with the industry’s economical requirements of productivity and energy efficiency. To study the feasibility of a combined DMD–PTA technique, a preliminary analysis in the specification of both techniques is conducted. Hybrid layers are fabricated using stainless steel EN X3CrNiMo13-4. Joining strategy of dissimilar layers, as well as microstructure and tensile strength of the hybrid layers, are examined. A comparison of the PTA and DMD process specifications shows both PTA and DMD processes are capable of being integrated into one operating system to enhance productivity. Layer-wise deposition of both processes presents a dense microstructure between dissimilar layers. However, side-by-side deposition of PTA and DMD layers requires proper joint-strategy due to higher heat input and wider and thicker deposited track in the regular current PTA compared to the DMD. The DMD layers exhibit higher hardness values compared to the PTA layer (300–315 HV and 320–350 HV, respectively) due to the smaller grain size. The tensile properties of the hybrid PTA-DMD layers are more comparable with PTA layer. The mean yield strengths of samples fabricated with the hybrid PTA-DMD layers are 800–850 MPa, while these properties are 794 MPa, and 984 MPa in samples made with PTA and DMD, respectively.

In previous publications the inhomogeneous pressure transfer through the polishing tool onto a glass surface could be shown. This experiment shows polishing trials with different polishing materials and the differences in the homogeneity of the pressure transfer through them. Only the properties of the material will be discussed explicitly, the change of the tool constitution through the process will be part of further publications.

Ductile mode grinding is usually applied for finishing of e.g. tungsten carbide molds used for precision glass molding (PGM) by controlling depth of cut on feed controlled machines. 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.

Subsurface Damages (SSDs) can cause a wide variety of defects to optical lenses and other components. In addition to the adhesion and quality of coatings, the mechanical stability, the transmission quality and the laser-induced damage threshold (LIDT) of the products, is also affected. It is, therefore, attempted to get components as SSD-free as possible at the end of the production chain. Already during the individual production steps, it is important to know the depth of the SSDs in order to remove them in the following manufacturing steps. To design the manufacturing processes efficiently and avoid damage, it is important to be able to measure the depth and characteristics of SSDs as precisely as possible. 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.

As power densities of laser diodes continuously increase, the effects of absorption losses in fast axis collimation lenses become exceedingly important. We report our analysis of two drivers of these absorption losses, coating absorption and glass bulk absorption, and how these absorption losses cause a thermal impact and have an influence on the performance of the laser beam quality emitted from a laser diode equipped with a fast axis collimation lens on a bottom tab. The presented results are derived from finite element method (FEM) simulations and the FEM model used is based on material data from data sheets and a heat transfer coefficient derived from cooling curves of components observed by a thermal infrared camera.

The current demand of even more powerful and efficient microprocessors, for e.g. deep learning, has led to an ongoing trend of reducing the feature size of the integrated circuits. These processors are patterned with EUV-lithography which enables 7 nm chips [1]. To produce mirrors which satisfy the needed requirements is a challenging task. Not only increasing requirements on the imaging properties, but also new lens shapes, such as aspheres or lenses with free-form surfaces, require innovative production processes. However, these lenses need new deterministic sub-aperture polishing methods that have been established in the past few years. These polishing methods are characterized, by an empirically determined TIF and local stock removal. Such a deterministic polishing method is ion-beam-figuring (IBF). The beam profile of an ion beam is adjusted to a nearly ideal Gaussian shape by various parameters. With the known removal function, a dwell time profile can be generated for each measured error profile. Such a profile is always generated pixel-accurately to the predetermined error profile, with the aim always of minimizing the existing surface structures up to the cut-off frequency of the tool used [2]. The processing success of a correction-polishing run depends decisively on the accuracy of the previously computed dwell-time profile. So the used algorithm to calculate the dwell time has to accurately reflect the reality. But furthermore the machine operator should have no influence on the dwell-time calculation. Conclusively there mustn’t be any parameters which have an influence on the calculation result. And lastly it should take a minimum of machining time to get a minimum of remaining error structures. Unfortunately current dwell time algorithm calculations are divergent, user-dependent, tending to create high processing times and need several parameters to bet set. This paper describes an, realistic, convergent and user independent dwell time algorithm. The typical processing times are reduced to about 80 % up to 50 % compared to conventional algorithms (Lucy-Richardson, Van-Cittert …) as used in established machines. To verify its effectiveness a plane surface was machined on an IBF.

While setting up grinding processes for optics fabrication, the optimum tool and process parameters have to be chosen. Unfortunately, datasheets of commercial grinding tools contain only information about grit type and size, bond material, tool shape etc., leaving out any information about process parameters to be applied or tool lifetime or performance. For this reason, tools and machining parameters are commonly adjusted during iterative experimental test runs using actual workpieces from production batches which is time consuming, expensive and without guarantee that the optimum set of machining parameters are achieved. In this paper we present a method to determine grinding tools process window for a given workpiece material. That way, ideal machining parameters can be determined offline for maximum reliability and productivity. In addition, the gained data provides the basis to identify the most suitable grinding tool for the desired application.

This paper presents a load-controlled gPVA (grinding Process Validation Approach) for setting up feed-controlled CNC (Computer Numerical Control) grinding processes for the generation of optical elements. The gPVA enables the quantitative determination of process windows for CNC grinding processes as well as the identification of optimized sets of process parameters without the need to run tests on actual production parts in the industrial workshop occupying expensive CNC machining time.

Paper 1000905

Molds made of tungsten carbide are typically used for the replicative mass production of glass lenses by precision glass molding. Consequently an ultra-precision grinding process with a subsequent fresh-feed polishing operation is conventionally applied. These processes are time consuming and have a relatively low reproducibility. An alternative manufacturing technology, with a high predictability and efficiency, which additionally allows a higher geometrical flexibility, is the single point diamond turning technique (SPDT). However, the extreme hardness and the chemical properties of tungsten carbide lead to significant tool wear and therefore the impossibility of machining the work pieces in an economical way. One approach to enlarge the tool life is to affect the contact zone between tool and work piece by the use of special cutting fluids. This publication emphasizes on the most recent investigations and results in direct machining of nano-grained tungsten carbide with mono crystal diamonds under the influence of various kinds of cutting fluids. Therefore basic ruling experiments on binderless nano grained tungsten carbide were performed, where the tool performed a linear movement with a steadily increasing depth of cut. As the ductile cutting mechanism is a prerequisite for the optical manufacturing of tungsten carbide these experiments serve the purpose for establish the influence of different cutting fluid characteristics on the cutting performance of mono crystal diamonds. Eventually it is shown that by adjusting the coolant fluid it is possible to significantly shift the transition point from ductile to brittle removal to larger depths of cut eventually enabling a SPDT of binderless tungsten carbide molds.