Search

Grinding hard-brittle materials like silicon nitride is faced with some challenges, including sub-surface damage, high tool wear, and low grinding efficiency. Ultrashort-pulse laser structuring of hard materials prior to the grinding process significantly reduces the cutting forces and temperature and increases the achievable material removal rate of the grinding process. These effects are partially due to controllable induced damages into the subsurface of the structured workpieces. However, the impacts of this surface structuring technique on the material removal mechanism of advanced ceramics, such as Si3N4, are not yet thoroughly investigated. The dominant material removal mechanism in grinding hard and brittle materials, such as silicon nitride (Si3N4), defines the surface integrity of the workpiece. For the first time, in-depth single diamond grit scratching experiments are carried out to investigate the changes in the dominant material removal mechanisms at various chip thicknesses by laser structuring of Si3N4. Two different structuring ratios (25% and 50%) were generated on sample surfaces by a femtosecond laser. The effects of laser structuring on material removal mechanism, pile-up area, area and width of the groove, grit path, normal and tangential forces, and specific cutting energy have been investigated. The results indicate that laser structuring considerably affects the reduction of depth ratio, normal (up to 89%) and tangential (up to 82%) forces, and specific cutting energy. The specific cutting energy of laser-structured Si3N4 workpieces converged to about 5 J/mm3, much lower than that of unstructured workpieces.

High flexibility of the micro-milling process compared to nontraditional methods has led to its growing application in manufacturing complex micro-parts with tight tolerances and high accuracies. However, difficulties such as tool deflection, size effect, and tool wear limit the application of micro-milling. In this regard, the role of laser-assisted machining (LAM) is highlighted to prevent mentioned issues through reduction of machining forces and providing the possibility for using higher feeds. Ti6Al4V as a hard-to-machine material is chosen as the workpiece material. Unlike traditional LAM, Ti6Al4V parts were structured using a picosecond laser before micro-milling. The influence of laser structuring at different structure densities on the reduction of machining forces was analyzed at two feeds of 10 and 50 µm/tooth at a constant cutting speed of 35 m/min. A remarkable reduction in cutting forces was observed at both feeds. Additionally, the role of structure density in cutting force reduction is highlighted.

This study addresses the influence of material variations on the grindability of crankshaft steel. Most previous studies on the effect of material microstructure on grindability involve comparisons of significantly different steel grades. This study, in contrast, is focused on batch-to-batch grindability variations for one steel grade, a scenario frequently occurring in industry where batches from different steel makers are fed into a production line. For this purpose, a batch made of recycled steel and a batch made of ore-based steel were compared with regards to microstructure and grindability under identical grinding and dressing conditions. Although both batches met the same material specifications, microstructural variations were identified in terms of grain size and micro-constituents (inclusions, carbonitrides). While specific grinding energy, residual stress and full-width at half-maximum profiles of ground surfaces were the same for both batches, the recycled batch showed different and unfavorable variation in wheel wear and Barkhausen noise (BN) response. Larger fractions of oxide inclusions and larger grain sizes (affected by carbonitrides) were present in the recycled batch, which were the likely reasons for the differences in wheel wear and BN response, respectively. These findings may aid grindability improvement by steel-grade adjustments, e.g. modification of the distribution and type of inclusions and/or amount of elements forming carbonitrides. Furthermore, the results highlight the importance of understanding and controlling material microstructure, as existing in-line quality by BN control may not always be able to correctly indicate surface integrity, which could lead to misinterpretations (e.g. false part-rejection on the assumption of grinding burn).

This paper is concerned with understanding the role of specific energy in micro-grinding of conventional and additively manufactured Ti6Al4V. The effects of grinding and dressing parameters, cooling-lubrication conditions, and the directions of material build-up are studied. It is demonstrated that the minimum specific energy in single grain tests is independent of the material-fabrication method. The lowest measured specific energy obtained is 11.5 J/mm3 for both workpiece materials. The direction of material build-up influenced the process only when grinding with low aggressiveness, where 20% higher specific energy was observed. Similar specific energies were obtained for oil-lubricated and dry conditions, indicating that lubrication had minimal effect. The effects of the diamond concentration in the wheel and the dressing parameters were also investigated. Comparable specific energies were observed for wheels with C150 and C200 concentrations. The specific energy was found being predominantly influenced by dressing. Coarse dressing conditions produced 18% lower specific energy and, therefore, a more efficient micro-grinding process.

Hydrophobic surfaces have gained a vast attention in different fields of biomedical applications over the last few years. Laser treatment has been shown as a promising technology for the generation of functional, inter alia, superhydrophobic surfaces. In this study a picosecond Yb-YAG laser was assigned for the generation of superhydrophobic characteristics on a steel alloy with application in surgical instrumentation. Regarding the ablation energy threshold of about 6 μJ for the given pulse width and laser beam characteristics and by assigning a suitable combination of microstructure kinematics and laser processing conditions a persistent hierarchical pattern is mapped over the laser-radiated surfaces. The average measured contact angle on the laserradiated surfaces was about 150°, which indicates their superhydrophobic properties.

In this study, an additive manufacturing process based on digital light processing was employed for a quick, flexible, and economical fabrication of resin bonded SiC grinding tools. The grinding wheel has been fabricated using laboratory manufacturing processes that utilize ultraviolet-curable resins and conventional abrasives. Also, desirable geometries and features like integrated coolant holes, which are difficult or even almost impossible to manufacture by conventional processes, are easily achievable. Grinding experiments were carried out by different process parameters, and with two different grinding wheels, i.e., with and without cooling channels with different concentrations (25 wt.% and 50 wt.% grains) to evaluate the grinding efficiency of the produced tools. Grinding forces, tool wear, tool loading, and ground surface quality were measured and analyzed. The wear rates of the grinding wheels with cooling channels were generally less than those without cooling channels, particularly in the deep grinding processes with large contact areas. Grinding tests on a hardened steel have shown that the integration of cooling lubricant channels almost prevents the wheel loading. In addition, by increasing the cutting speed (from 15 to 30 m/s) and decreasing the feed rate (from 10 to 2 m/min), the grinding wheel wear was significantly reduced. Furthermore, surface grinding of aluminum resulted in surface roughness values (Ra) in the range of 1 μm to 2.5 μm, while a Ra of about 0.2 μm was achieved by grinding hardened steel (100Cr6) with the same grinding conditions. Using the higher SiC-grain concentration (50 wt.%), it was determined that the surface roughness was 50% finer. Additionally the tool wear was significantly reduced (up to 30 times depending on the process parameters). The wear characteristics of the grinding wheel were analyzed through a novel image processing system. Significant correlations were found between the wear flat of grains and the increase in grinding forces due to the tool wear.

The physical or empirical modeling of the grinding process and the effects of its parameters on the workpiece quality is sophisticated. This is due to the extreme complexity of the process. So far, no remarkable success could be made by the proposed models to achieve a reliable and effective design and control of the process. This article introduces an expert system to enhance the design of the grinding process. A pilot system was built considering three main grinding outputs, including surface roughness, material removal rate and normal grinding force. Based on the primary experimental results, the system suggests the proper grinding and dressing parameters to obtain the desired surface roughness with the highest possible material removal rate and least normal force. The analysis is based on the regression correlation development and the “Non dominated Sorting Genetic Algorithm II” optimization method. The validation tests conducted in three different surface roughness and normal force ranges proved the reliability and effectiveness of the proposed expert system. The surface roughness was predicted with less than 7% deviations from the experiments. The predicted normal grinding forces, in most cases, were in good agreement with the experiments with higher than 75% accuracy. Although the wheel balancing issues at the highest cutting speed (vc = 35 m/s) caused large deviations between the predicted and measured forces.

Polyether ether ketone (PEEK) has been widely used in the medical engineering due to its high strength to weight ratio, creep and wear-resistance, and anti-allergically properties. Grinding is generally used to produce PEEK parts with high accuracy and surface quality requirements. In this research, the tool loading and the effect of cryogenic cooling in the grinding of PEEK are studied for the first time. It is shown that the generated heat in the grinding process, which is mainly influenced by the tool micro-topography, process parameter, and coolant lubricant has an important role in the surface integrity of PEEK. Additionally, the influence of specific material removal rate and the dressing speed ratio on the specific grinding energy of PEEK was studied. The input parameters of the grinding process that are investigated in this study include cutting speed (vs), depth of cut (ae), and feed rate (vft). To investigate the grinding wheel topography, the effects of dressing overlap ratio (Ud) and the dressing speed ratio (qd) were also investigated. Grinding force, surface roughness, and loading of the grinding wheel were considered as output parameters. The experiments were designed based on response surface methodology and the optimum cutting condition was obtained based on this method. The depth of cut and the dressing overlap ratio had respectively the maximum and minimum impact on the surface roughness and cutting forces. Additionally, the tool loading was mainly influenced by the cutting speed.

Super abrasive diamond grinding wheels are the most promising tools for the precision machining of advanced ceramics and carbide materials. However, the efficiency of conventional conditioning of these tools is limited owing to high dressing tool wear, long process time, low form flexibility, and induced damage to the abrasive grains. Wire electrical discharge machining (WEDM) is an alternative method for conditioning of superabrasive grinding wheels with electrically conductive bonding materials. In this study, cylindrical plunge grinding of an alumina ceramic with a resin-bonded diamond grinding wheel is investigated. The assigned type of resin bond contains copper particles and is accordingly electrically conductive for wire electrical discharge conditioning (WEDC). Conventional (mechanical) and WEDC methods are used for generating the same profile on two similar diamond grinding wheels. As a result, the specific grinding energy was reduced up to 26% and 29% during rough and finish plunge grinding, respectively. Reduced specific grinding energy and forces, along with more effective grain protrusion and sharpness by using WEDC for profiling of grinding wheels, have contributed positively to the ground surface conditions despite the relatively rougher wheel surface topography in comparison to the conventional profiling. The more considerable reduction in the mean roughness depth (Rz) than in the arithmetical mean roughness value (Ra) (11% smaller Rz values in WEDC versus mechanical conditioning) verifies that the workpiece surface underwent less surface degradation in case of WEDC because of smaller grinding forces. Furthermore, the profile wear behavior of the workpiece ground with the WED conditioned grinding wheel was superior to the conventionally conditioned one.

Carbon fiber-reinforced Polyether Ether Ketone is widely used in aerospace and medical industries due to its unique mechanical and physical properties. The use of CFR-PEEK in the industry is growing by replacing materials such as aluminum (in aerospace and mechanical industry), titanium, and stainless steel (in the medical industry). In this research, the grinding process of this material is studied through single grit scratch test, which is a common method to study the fundamentals of chip formation in the grinding process. In this method, one single grit as a representative of whole grinding wheel grits engages with the workpiece under the conditions identical to grinding. Investigating the corresponding phenomena during the abrasive-material engagement gives a better understanding of the grinding process. The experimental results showed that by increasing the cutting speed, vc, the specific energy, ec, increases. It is worth to mention that the minimum specific energy in maximum depth is converged to ec = 1.2 J/mm3. The portion of plowing and rubbing are reduced by a rise in the depth of cut. In low cutting depths, tangential force to normal force ratio is low and by an increment in depth, this ratio increases as well.

Understanding the laser ablation mechanism is highly essential to find the effect of different laser parameters on the quality of the laser ablation. A mathematical model was developed in the current investigation to calculate the material removal rate and ablation depth. Laser cuts were created on the workpiece with different laser scan speeds from 1 to 10 mm s−1 by an ultrashort pulse laser with a wavelength of about 1000 nm. The calculated depths of laser cuts were validated via practical experiments. The variation of the laser power intensity on the workpiece’s surface during laser radiation was also calculated. The mathematical model has determined the laser-material interaction mechanism for different laser intensities. The practical sublimation temperature and ablated material temperature during laser processing are other data that the model calculates. The results show that in laser power intensities (IL) higher than 1.5 × 109 W cm−2, the laser-material interaction is multiphoton ionisation with no effects of thermal reaction, while in lower values of IL, there are effects of thermal damages and HAZ adjacent to the laser cut. The angle of incidence is an essential factor in altering incident IL on the surface of the workpiece during laser processing, which changes with increasing depth of the laser cut.

The prediction of cutting forces is critical for the control and optimization of machining processes. This paper is concerned with developing prediction model for cutting forces in micro-grinding. The approach is based on the probabilistic distribution of undeformed chip thickness. This distribution is a function of the process kinematics, properties of the workpiece, and micro-topography of the grinding tool. A Rayleigh probability density function is used to determine the distribution of the maximum chip thickness as an independent parameter. The prediction model further includes the effect of dressing parameters. The integration of the dressing model enables the prediction of static grain density of the grinding tool at various radial dressing depths. The tool deflection is also considered in order to account for the actual depth of cut in the modeling process. The dynamic cutting-edge density as a function of the static grain density, the local tool deflection, elastic deformation, and process kinematics can hence be calculated. Once the chip thickness is calculated, the single-grain forces for individual abrasive grains are predicted and the specific tangential and normal grinding forces simulated. The simulation results are experimentally validated via cutting-force measurements in micro-grinding of Ti6Al4V. The results show that the model can predict the tangential and normal grinding forces with a mean accuracy of 10% and 30%, respectively. The observed cutting forces further imply that the flow stress of the material did not change with changing the cutting speed and the cutting strain rate. Moreover, it was observed that the depth of cut and grinding feed rate had the same neutral effect on the resultant grinding forces.

The production of special high capability polymers and improvement of their physical and mechanical properties have led researchers to study the material removal mechanism of novel polymers. Fast loading of the grinding wheel is one of the main problems of polymers grinding which affect the whole process. Cryogenic cooling in the form of carbon dioxide snow, which is used in this research, is a promising method for the reduction of tool loading. The method may induce low cutting temperatures and has almost no negative environmental aspects. Surface topography, surface roughness, machining forces, and grinding wheel loading have been analyzed in this study. The final results were compared to compact air, which reveals that cryogenic cooling is a powerful method in order to optimize the grinding process. The influence of different input parameters on Polyether ether ketone (biopolymer) grinding was studied and analyzed as well. The results show that the tool loading can be reduced significantly by utilizing the cryogenic cooling, which also led to an improved ground surface quality.

Capability of Additive Manufacturing (AM) technology in the production of complex parts with high flexibility has led to the growing interest in their application as an alternative for conventional manufacturing processes. Despite the outstanding benefits of the AM process, due to their poor surface quality, the precision parts produced by this method generally need to be machined, ground, or polished. This paper addresses the machinability of AM Ti6Al4V titanium alloy parts in the micro-milling process with a specific focus on cutting forces, specific cutting energy, burr formation, and surface quality. Additive parts were produced by Electron Beam Melting (EBM) technique and were compared with the extruded Ti6Al4V parts in the micro-milling process. No significant difference could be observed in the cutting forces of both materials at chip thicknesses between 7.4 and 37.3 μm, despite the higher hardness of the EBM Ti6Al4V compared to the extruded Ti6Al4V. However, micro-milling of the EBM parts produced finer surfaces. Cutting forces and specific cutting energies of EBM parts were less than those of extruded parts at minimal chip thicknesses (lower than 7.4 μm). Continuous wavy-type burrs were formed in micro-milling of the EBM Ti6Al4V and were larger than those of extruded Ti6Al4V.

Micro-grooves have various applications in different industries. Circular laser grooving is a new method for manufacturing micro-grooves on the circumference of cylindrical parts. In this process, the selection of appropriate parameters to reach experimentally the desired groove aspect ratio along with minimum dross formation requires tremendous efforts. Here, the role of a decision-making technique, similarly machine learning is highlighted to evaluate the significance of process inputs and provide the appropriate model for prediction concerning the input parameters. The experiments were conducted with the various inputs such as workpiece rotational speed, laser beam position, duty cycle, power and frequency. The outputs are the groove geometry in terms of width and depth of the circular groove as well as the groove quality considering the dross formation. Then, Random Forest (RF) technique was utilized to derive the most influential inputs on the outputs. The RF analysis revealed that the rotational speed, laser position and duty cycle are the most decisive process parameters in the groove geometry and groove quality. Also, the enhancement of the assist gas pressure does not improve the process outputs according to the results of RF analysis.

Surface integrity is one of the most significant quality aspects of micro-grinding of difficult-to-cut materials. On the other hand, specific grinding energy is a fundamental parameter for describing the micro-grinding process. This paper addresses the surface integrity of the micro-ground surface of a titanium alloy under different cutting speeds and feed-rate-to-depth-of-cut (vw/ae) ratios at the same chip thickness. Three different cutting speeds and vw/ae ratios have been chosen and the residual stress of the workpiece, as well as the specific micro-grinding energy, have been investigated. The results showed that almost the same minimum specific grinding energy was obtained at tested cutting speed and vw/aeratio. The results of the XRD analysis showed that contrary to the specific micro-grinding energy, the residual stresses of the ground surface changed by varying the cutting speed and vw/ae ratio. Higher cutting speeds resulted in lower compressive residual stress, and higher vw/ae ratios resulted in higher compressive stresses. This can be attributed to higher temperatures in the chip-formation process compared to the plastic deformation in micro-grinding at higher cutting speeds and lower vw/ae ratios which was proved via SEM micrographs.

Single point laser ablation of Si3N4 under dry and water medium is investigated and analysed. The experiments were conducted based on the one-factor-at-a-time strategy considering various input parameters such as piercing time, focus point position and water layer thickness. The effects of process inputs on the ablated hole’s geometrical features including the ablation depth and diameter, hole taper, and hole edge roundness under presence and absence of water as an intermediate medium are studied. The laser focus point position was determined and varied during the investigation. It was found that, compared to the dry condition, the maximum ablation depth under wet condition is much more sensitive to the position of the laser focus point. It was also observed that the ablation depth is directly influenced by the piercing time in both dry and under water conditions. Picosecond laser single point ablation under a water layer with a thickness of 0.6 mm could increase the ablation depth up to 23%, compared to the dry condition. However, the ablation depth was significantly decreased by increasing the water layer thickness. Additionally, the hole entrance edge was rounded with increasing the water layer thickness. The hole taper was significantly influenced by the water layer thickness.