Papers by Author: Ekkard Brinksmeier

Abstract: Brittle materials like ceramics or glass can be machined by cutting with negative rake angles and by abrasive machining processes. Especially grinding allows for low surface roughness and high shape accuracy. Conventional path-controlled grinding processes may damage functional surfaces if brittle fracture occurs and may thus lead to lateral, radial and axial cracks. High grinding forces can be a reason for brittle fracture when grinding ceramic materials. A solution for this effect may be the application of force controlled grinding processes. In this paper adapted control algorithms were implemented for force controlled grinding and verified in grinding experiments. As an example, cylindrical grooves were ground with an injection moulded spherical grinding tool in alumina and zirconia ceramics. After grinding surface roughness, shape accuracy and process forces are analysed and discussed.

Abstract: Many machining operations e.g. turning, milling or grinding are dependent on the application of water-based metalworking fluids (MWF) which contribute significantly to their high level of performance. MWF in-use are exposed to a microbial contamination, which leads to a deterioration of water-based MWF components and can cause a premature failure of the whole coolant system. Expensive monitoring and the addition of biocides are needed to maintain the MWF quality and to reduce the microbial load, regardless of the potential risk for health and environment. To overcome these limitations, the paradigm shift of using microorganisms as a replacement for conventional MWF is investigated in this paper. Microbial cell components and some microbial inclusions are comparable to conventional MWF components like e.g. fatty acids or sulfur compounds. Due to this fact, it is possible to create a regenerative system on a microbiological basis for the substitution of conventional MWF components. In preliminary tribological investigations the basic lubrication properties of microorganisms and their potential as a replacement for conventional MWF were shown. The presented approach intends to investigate the influence of microbial cell counts, cells size and extracellular polymeric substances (EPS) on the lubrication behavior respectively. The results of the tribological tests show that especially microorganisms with a big cell volume or a high EPS productivity exhibit superior Brugger-values (up to 174%) compared to a highly concentrated conventional MWF (emulsion 10%) and indicate the great potential of microorganisms as a replacement for conventional MWF.

Abstract: In order to generate desired functional properties in the surface layer of machined parts, today several iterative attempts are needed to achieve the specified surface and subsurface properties. Despite progress in simulation and modelling, which is limited to individual processes and investigations, a predictive adjustment of a manufacturing process in terms of a specific state of surface integrity is not possible. In this work, an alternative view regarding manufacturing processes, based on occurring internal physical mechanisms, is presented. It is shown for several processes, that the surface integrity due to material modifications can be correlated with internal material loads occurring during the process. To utilise this method in practical applications, approaches are presented which should enable a reduction in the complexity of the mathematical description of the internal material loading states.

Abstract: This paper presents a diagram of maximum contact zone temperature T_max versus contact time Δt, based on the analysis of workpiece surface layer properties after cylindrical grinding experiments. Apart from resulting surface layer properties, process quantities (T_max, normal and tangential grinding forces F_n, F_t) are investigated with reference to the resulting workpiece surface layer state as well. Ground workpieces are analyzed by performing Barkhausen noise level measurements together with subsequent metallographic and X-ray diffraction investigations. By mapping characteristic values T_max and the contact time Δt to corresponding surface layer properties, a general analysis of workpiece material response to the thermo-mechanical load during grinding is possible.

Abstract: Galvanic retroreflector moulds suffer from fabrication errors and considerable wear due to the manufacturing process and the subsequent use for injection moulding. To remove degradations like flaws and wear marks from the mould’s optical surfaces without tedious manual labour, a partially automated process chain using a 6-axis robot is being developed combining surface metrology and machining technology. The areal limitation of the structured surfaces excludes conventional rotatory polishing processes. Vibration polishing is applied instead to remove critical surface errors and to enhance surface roughness to a near optical value of Sa ≤ 50 nm. The work presented in this paper gives insights into experiments performed on a robot polishing system and illustrates and the suitability for an application in the aforementioned partially automated process chain.

Abstract: In recent years microfluidic devices became of great interest, as they offer a wide range of bio-analytical and fluid processing applications through the utilization of size effects. Especially a mass manufacturing of disposable polymeric microfluidic devices by hot embossing or injection molding is expected to have high economic potential. It is known, that channels and areas showing a localized change in wettability can considerably improve fluid processing tasks like mixing or droplet generation. Chemical approaches, like the polymerization of lauryl acrylate, were successfully shown to achieve hydrophobic coatings for micro channels but are not suitable for a mass manufacturing. Since microstructures are known to provide water repellent properties of surfaces, this paper focuses on the applicability of diamond grooving and Diamond Micro Chiseling (DMC) processes for the manufacture of microstructured areas in brass molds inserts, in order to achieve hydrophobic properties of their replica. Major design features of structures, like a height range of 6 to 16μm or aspect ratios in between 0.5 and 3.2 are derived from the natural example of the lotus leaf. Molding is carried out by using a two component silicone filler. The performance of the replicated hydrophobic surfaces is evaluated by droplet contact angle measurements. After presenting methodology and results, the paper will conclude on how to transfer the investigated microstructuring methods to the manufacture of mold inserts for the replication of polymeric microfluidic chips with localized hydrophobic areas and channels.

Abstract: The machining process of carbon fiber reinforced plastics is very complex due to its inhomogeneous material structure and anisotropic properties. Experimental investigation can be very demanding and time consuming because of the different fiber/matrix compositions and the influence of the fiber volume content which needs to be considered. In this work milling of unidirectional CFRP was simulated using two different material models with implicit and explicit description of fiber and matrix. The objective is to improve the knowledge of the physical mechanism of the cutting process and to predict the cutting forces and surface damage. For this purpose machining of unidirectional CFRP with fiber orientations 90°, 0°, +45° and-45° was simulated, verified by experiments. A significant influence of the fiber orientation on all determined variables was found. The cutting mechanism is dominated by matrix crushing for the fiber orientations 90°, 0° and +45°. Surface damage is caused by either matrix cracking and/or failure of fiber/matrix interface and is occurring in orientations 90° and-45° only. Additionally the evolution of a saw-tooth shaped surface could be identified for the-45° orientation.

Abstract: 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.

Abstract: This paper presents advanced tools for ultra precision grinding which offer a high wear resistance and can be used to generate high-quality parts with an ultraprecise surface finish. The first approach features defined dressed, coarse-grained, single layered, metal bonded diamond grinding wheels. These grinding wheels are called Engineered Grinding Wheels and have been dressed by an adapted conditioning process which leads to uniform abrasive grain protrusion heights and flattened grains. This paper shows the results from grinding optical glasses with such Engineered Grinding Wheels regarding the specific forces and the surface roughness. The results show that the cutting mechanism turns into ductile removal and optical surfaces are achievable. On the other hand, the specific normal force F´_n increases due to increased contact area of the flattened diamond grains. It is shown that the topography of the Engineered Grinding Wheels has a strong beneficial influence on surface roughness. The second new tool for ultra precision grinding is made of a CVD (Chemical Vapour Deposition) poly-crystalline diamond layer with sharp edges of micrometre-sized diamond crystallites as a special type of abrasive. The sharp edges of the crystallites act as cutting edges which can be used for grinding. It is shown that by using CVD-diamond-coated grinding wheels a high material removal rate and a high surface finish with surface roughness in the nanometre range can be achieved. The CVD-diamond layers exhibit higher wear resistance compared to conventional metal and resin bonded diamond wheels. In conclusion, this paper shows that not only conventional fine grained, multi-layered resinoid diamond grinding wheels but also coarse-grained and binderless CVD-coated diamond grinding wheels can be applied to machine brittle and hard materials by ultra precision grinding.

Abstract: The chip removal process in grinding is characterized by intensive friction and plastic deformation leading to the risk of thermal damage of the surface-layer of the machined part. Thus productive and reliable grinding processes need effective monitoring. The difficult to access contact zone between the grinding wheel and the workpiece led to extensive research work on the temperature measurement in the grinding arc. In order to develop a tool integrated temperature monitoring system a new approach was undertaken which makes use of the measurement of infrared-radiation to monitor the temperatures in the grinding arc. The presented research work shows promising results suitable for an industrial applicable system for temperature measurement in grinding. The optical transmission of the infrared temperature information in combination with a fast detecting infrared sensor bares the potential to establish a highly miniaturized measuring system which is easy to integrate in any grinding wheel at comparably low production costs.