Surface integrity in precision turning of steel Willert, Melanie; Riemer, Oltmann; Brinksmeier, Ekkard
International journal of advanced manufacturing technology,
2018/1, Volume:
94, Issue:
1-4
Journal Article
Peer reviewed
The functional properties of a technical component depend directly on the geometrical and material properties, in particular the surface and surface layer properties of a component. Since material ...loads are generated in the material during a machining process, which cause alterations in the surface and surface layer properties, so-called material modification, the knowledge of these relationships is of great interest for a material-oriented manufacturing. In order to elucidate this for precision machining, there are two main process-specific challenges: (1) Cutting edge radius and undeformed chip thickness are of the same order of magnitude, so the engagement ratios and cutting conditions vary from cutting to plowing depending on the undeformed chip thickness. This phenomenon is known as the so-called size effect, occurring as a nonlinear increase in the specific cutting force with decreasing undeformed chip thickness. Therefore, the consideration of deformation and chip formation mechanisms is necessary. (2) The layers influenced by a precision machining process are very thin and thus difficult to characterize. The experimental investigation shows that a surface layer zone with a plastic deformation of a few micrometer depth below the surface is generated by precision turning of steel (42CrMoS4, AISI 4140). In addition, there is another size effect with regard to the depth of the plastic deformation, showing a nonlinear increase with decreasing undeformed chip thickness. The plastic deformation is influenced by the process parameters as well as the process kinematics.
Based on the special physical–chemical characteristics of optical crystal in the field of aeronautics, a new anhydrous based shear-thickening polishing (ASTP) method has been proposed to restrain ...deliquescence and to improve physical properties for KDP machining. The ultra-precision polishing of KDP crystal is completed by ASTP. A kind of anhydrous based thickening polishing slurry (ATPS) was proposed in our work, and high-performance rheological properties were determined to achieve the ASTP of KDP crystal. A material removal model of ASTP in KDP machining is established, followed by the verification experiment of the prediction model. The maximum error of the predictive model is only 9.8%, which proves the validity of the material removal model for KDP polishing. The polishing experiments were carried out on the polishing platform developed by ourselves. The results show that the new polishing method can polish 20 mm × 20 mm × 5 mm KDP crystal and obtain a super-smooth surface with a surface roughness of 1.37 nm and high shape accuracy. The surface accuracy of polished KDP crystal reaches up to 0.68λ (RMS). The experimental results show that the ASTP is a potential ultra-precision machining method for KDP crystal.
The loads acting on a workpiece during machining processes determine the modification of the surface of the final workpiece and, thus, its functional properties. In this work, a method that uses ...thermocouples to measure the temperature in precision fly-cutting machining with high spatial and temporal resolution is presented. Experiments were conducted for various materials and machining parameters. We compare experimental measurement data with results from modern and advanced machining process simulation and find a good match between experimental and simulation results. Therefore, the simulation is validated by experimental data and can be used to calculate realistic internal loads of machining processes.
A novel grain-based DEM (Discrete Element Method) model is developed and calibrated to simulate RB-SiC (Reaction-Bonded Silicon Carbide) ceramic and associated scratching process by considering the ...bonded SiC and Si grains and cementitious materials. It is shown that the grain-based DEM model can accurately identify transgranular and intergranular cracks, and ductile and brittle material removal modes. It also shows that by increasing the scratching speed or decreasing the depth of cut, the maximum depth of subsurface damage decreases, because the scratching force is relatively large under the low scratching speed or large depth of cut that facilitates the occurrence of transgranular cracks, large grain spalling from the target surface and the propagation of median cracks into the target subsurface. It has further been found that increasing the cutting-edge radius can enhance the target ductile machinability and reduce the target subsurface damage.
The application of a specific rheological polishing slurry is proposed first for high-efficiency machining of steel materials to achieve high-quality ultraprecision finished surfaces. The rheology of ...the polishing slurry was explored to show that the non-Newtonian medium with certain parameters of content components exhibits shear-thickening behavior. Then the new high-efficiency nano polishing approach is applied to process spherical surfaces of bearing steel. Several controllable parameters such as shear rheology, abrasive data, rotational speed, and processing time are experimentally investigated in this polishing process. A special finding is that the surface roughness and material removal rate can increase simultaneously when a small abrasive size is applied due to the thickening mechanism during the shearing flow of slurries. Excessive abrasives can decrease surface quality due to the uneven agglomeration of particles scratching the surface. Under optimized conditions, a high-accuracy spherical bearing steel surface with a roughness of 12.6 nm and roundness of 5.3 μm was achieved after a processing time of 2.5 h. Thus, a potential ultraprecision machining method for target materials is obtained in this study.
Microinjection moulding has been developed to fulfil the needs of mass production of micro components in different fields. A challenge of this technology lies in the downscaling of micro components, ...which leads to faster solidification of the polymeric material and a narrower process window. Moreover, the small cavity dimensions represent a limit for process monitoring due to the inability to install in-cavity sensors. Therefore, new solutions must be found. In this study, the downscaling effect was investigated by means of three spiral geometries with different cross sections, considering the achievable flow length as a response variable. Process indicators, called "process fingerprints", were defined to monitor the process in-line. In the first stage, a relationship between the achievable flow length and the process parameters, as well as between the process fingerprints and the process parameters, was established. Subsequently, a correlation analysis was carried out to find the process indicators that are mostly related to the achievable flow length.
The functional properties of a workpiece are determined by a modification of the surface and subsurface materials. In this work, the correlation between thermo-mechanical material loads and the ...modification of the residual stresses is presented. While the resulting residual stresses were measured by X-ray diffraction after machining experiments, the material loads were determined using a process simulation. The experimental data (measured process forces and results from previous experiments) are used to validate the simulation, which is then applied to calculate the internal thermo-mechanical loads of the maximal temperature and the equivalent von-Mises-stresses per volume element during the machining experiments. In conclusion, a higher depth impact of mechanical loads compared to a lower depth impact of thermal loads in precision machining is observed. For the sake of novelty, the thermo-mechanical loads were plotted and interpreted in a three-dimensional fashion. Finally, cross sections of this mutual representation at certain constant material loads—thermal and mechanical—result in a process signature, which can prospectively improve the prediction of functional workpiece properties.
Safety-related plastic parts for the automotive industry require sophisticated, retroreflector mould inserts with a surface roughness in the near optical range (Sa ≈ 20 nm). Due to wear, only a ...limited number of plastic parts can be moulded and visual inspection and reconditioning of the structured mould insert by manual polishing is tedious and costly. Current work aims to establish an automated process chain as a remedy but due to the limited access to edges and corners, a vibrational motion with a frequency of 133 Hz is applied. This paper investigates the fundamental influence of polishing parameters in this process like polishing pad, grain size of diamond abrasives and polishing time on the material removal and the Archard wear coefficient, which then is applied to estimate the ratio of 2- and 3-body abrasion and compared to the actual surface topography. It was found that the material removal rate is not constant and decreases with longer polishing times. Generally, larger grain sizes and denser polishing pads with shorter fibres cause a higher material removal rate. For all investigated parameter combinations, a 2-body abrasion was present, which is proven by both Archard wear coefficient and surface topography images.
Metallic implants were the only option for both medical and dental applications for decades. However, it has been reported that patients with metal implants can show allergic reactions. Consequently, ...technical ceramics have become an accessible material alternative due to their combination of biocompatibility and mechanical properties. Despite the recent developments in ductile mode machining, the micro-grinding of bioceramics can cause insufficient surface and subsurface integrity due to the inherent hardness and brittleness of these materials. This work aims to determine the influence on the surface and subsurface damage (SSD) of zirconia-based ceramics ground with diamond wheels of 10 mm diameter with a diamond grain size (d
) of 75 μm within eight grinding operations using a variation of the machining parameters, i.e., peripheral speed (v
), feed speed (v
), and depth of cut (a
). In this regard, dental thread structures were machined on fully sintered zirconia (ZrO
), alumina toughened zirconia (ATZ), and zirconia toughened alumina (ZTA) bioceramics. The ground workpieces were analysed through a scanning electron microscope (SEM), X-ray diffraction (XRD), and white light interferometry (WLI) to evaluate the microstructure, residual stresses, and surface roughness, respectively. Moreover, the grinding processes were monitored through forces measurement. Based on the machining parameters tested, the results showed that low peripheral speed (v
) and low depth of cut (a
) were the main conditions investigated to achieve the optimum surface integrity and the desired low grinding forces. Finally, the methodology proposed to investigate the surface integrity of the ground workpieces was helpful to understand the zirconia-based ceramics response under micro-grinding processes, as well as to set further machining parameters for dental implant threads.
Ultraprecision cutting used for creating highly precise components and particularly micro-shapes requires ultraprecision machine tools. However, these machine tools are operated under restricted ...cutting conditions such as feed rate and depth of cut, resulting in low productivity. Therefore, this study deals with this problem by devising a novel ultraprecision cutting system with automation of the workpiece setting operation. In the devised system, the workpiece is roughly machined by an ordinary machine tool. After that, an industrial robot transfers the workpiece to an ultraprecision machine tool to complete the rest of the machining process. The proposed machining system overcomes two types of errors. The first is the shape error due to rough machining on the ordinary machine tool. It is practically difficult to create the expected shape exactly in rough machining, and therefore the shape error of the roughly machined workpiece is affecting the finish machining directly. The other is the setting errors due to the transfer of the workpiece to the ultraprecision machine tool from the ordinary machine tool. The differences from the ideal workpiece position and orientation are detected to identify the setting errors. Thus, the roughly machined workpiece is scanned to derive an error map by on-machine measurement. Additional tool paths are generated for semi-finish machining, and the error map helps to eliminate the form errors induced by rough machining. The NC program for finish machining is modified to compensate the identified workpiece setting errors. Finally, finish machining can be conducted on the roughly machined workpiece. Hence, the removal volume in finish machining on an ultraprecision machine tool is reduced, and the time required for the whole machining process is expected to be shorter. The experimental results confirm that the developed ultraprecision cutting system contributes to the automated workpiece setting operation to create micro-shapes with high accuracy.
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