Maximum Undeformed Chip Thickness Research Articles

Ductile–brittle transition and ductile-regime removal mechanisms in micro- and nanoscale machining of ZnS crystals

The ductile–brittle transition and ductile-regime removal mechanisms of ZnS crystal, a typical soft-brittle infrared optical material, have remained unclear at the micro- and nanoscale. In this context, this study modeled the energies consumed in two machining modes, namely ductile deformation and brittle fracture, during the ultraprecision cutting of ZnS crystal. Subsequently, based on the transition point of the energy mode, the critical undeformed chip thickness (CUCT) for the ductile–brittle transition of the material was determined. Additionally, the effect of tool parameters on CUCT was quantitatively analyzed, and the correctness of the model was verified by scratch experiments. Moreover, the cutting parameters that affect the maximum undeformed chip thickness (MUCT) were analyzed. By comparing the MUCT and CUCT, the cutting conditions to realize ductile-regime machining were predicted. The correctness of the ductile-regime machining conditions was also verified through surface quality characterization after face-cutting experiments. The MUCT can be controlled to be less than the CUCT by using a cutting tool with a sufficiently large tool rake angle, a nose radius and a reduced feed rate, thus realizing the ductile-regime removal of the material. The surface morphology, chip morphology, and surface roughness of machined ZnS crystal confirmed the validity of the model. Using a diamond cutting tool with a large cutting edge angle (–25°) and nose radius (1.2 mm) as well as a low feed rate (0.5 μm/rev) and depth of cut (3 μm), the generation of cracks and pits was effectively inhibited, resulting in a smooth surface with a surface roughness of 1.60 nm. Overall, the findings of this study provide important insights into the superfinishing of soft–brittle materials such as ZnS crystal.

Evaluation of grinding parameters on the surface and sub-surface characteristics of flat optical elements by cup wheel

Optical glass BK7 is an amorphous crown glass widely used in the aviation industry and high-power laser equipment due to its mechanical and optical properties. With the hard surface and brittle nature, the grinding process is one of the standard machining methods of BK7. Fracture is a dominant mechanism in removing brittle materials, which causes surface and sub-surface defects in the workpiece. Prediction of the depth of sub-surface damage (SSD) that has occurred allows the optimal reduction or even elimination of sub-surface defects in the finish grinding stage. This research aimed to comprehensively investigate the multifaceted influence of seven pertinent parameters on the depth of sub-surface damage (SSD) and surface roughness (SR) resulting from the grinding process. More specifically, it delved into the effects of the grinding wheel's movement direction under varying contact conditions, including angular grinding, vertical grinding, and face grinding, focusing on three critical aspects: the maximum undeformed chip thickness, depth of sub-surface damage, and resulting surface roughness. The contact conditions between the grinding wheel and the workpiece were systematically classified into four groups based on predetermined input parameters to facilitate in-depth analysis. Subsequently, the resulting outputs were evaluated and compared across these diverse contact conditions. The comparative examination between theoretical equations and experimental findings unveiled a maximum deviation of 10 % in SSD depths and 15.61 % in SR estimations. Furthermore, this study's experimental assessments identified nonlinear and monotonic relationships between the depth of SSD, surface roughness, and the maximum undeformed chip thickness (hm). The experimental relationships and the measured results notably highlighted observed discrepancies of 4.27 %, 10 %, and 10 % for the respective parameters (SSD, surface roughness, and hm).

Surface damage characteristics and identification based on acoustic emission in diamond point grinding carbon/carbon ceramic matrix composites

This study mainly investigated the effect of machining parameters on acoustic emission (AE) characteristics in grinding three-dimensional carbon/carbon ceramic matrix composites with superabrasive diamond grinding points. Signal processing techniques including fast Fourier transform (FFT) and short-time Fourier transform (SWFT) were applied to evaluate frequency features of AE signals. The RMS values were used as an indicator to detect grinding energy releasing characteristics. Results indicated that grinding speed and grinding depth were the main influencing factors on RMS values, while feed speed presented limited effects. The maximum undeformed chip thickness hmax and active grits number were two key factors affecting RMS variation of the AE signals, which corresponded to the intensity of single AE source and the number of total AE ones, respectively. The material removal mechanisms were studied via detailed SEM micrographs of machined surfaces, revealing that fiber fracture and debonding were the main material removal modes in point grinding carbon/carbon composites. The material removal mode can be possibly identified from the frequency of AE signal, as the band of 12–35 kHz mainly corresponded to fiber fracture, 35–70 kHz referred to debonding between the fiber and matrix, 70–90 kHz indicated the frictions, and the fourth band (90–120 kHz) was related to the matrix cracks.

Investigation on tearing damage of CFRP circular cell honeycomb in end-face grinding

CFRP circular cell honeycomb has promising applications in the aerospace field due to its excellent mechanical properties. However, a large number of periodically distributed tearing damages severely restrict the high-quality machining of CFRP circular cell honeycomb. This paper investigates the formation mechanism of tearing damages during end-face grinding of CFRP circular cell honeycomb and proposes suppression strategies. In this paper, a maximum undeformed chip thickness model considering tearing damage distribution is developed, and the evolution behavior of tearing damage is studied by combining the material removal process and grinding force. The grinding force and maximum tearing damage depth in different grinding conditions are analyzed. The results show that the formation of tearing damage in the cut-in stage is related to the weak stiffness of thin walls and the radial grinding force. The radial grinding force and maximum tearing depth can be altered by changing the maximum undeformed chip thickness which can be adjusted for different grinding speeds, feed rates, and offset ratios. The low-damage machining of CFRP circular cell honeycomb can be achieved by using a large offset ratio and variable feed rate.

Experimental investigation on the feasibility of high-efficiency and precision grinding of silicon carbide ceramics with monolayer brazed diamond wheel

To explore the feasibility of high efficiency and precision grinding of brittle ceramics, the experimental grinding tests were conducted on SiC ceramics with monolayer brazed diamond wheels. The monolayer diamond grinding wheel with 80/100 mesh size diamond grains was brazed using high-frequency induction techniques. The binarization method was proposed to quantitatively characterize the brittle removed areas from the whole ground surface and oblique polishing method was used to assess the subsurface damage depth of the ground SiC samples. The results show that the maximum undeformed chip thickness (agmax) plays a crucial role in the material removal mode. When agmax increases from 0.08 µm to 1 µm, the brittle-removal ratio increases rapidly from 8.20 % to 48.24 % and the subsurface damage depth increase from 3.48 µm to 9.14 µm by 162.6 %. The grinding wheel speed also has a significant influence on the brittle-removal ratio. With the gradual increase of grinding wheel speed (vs) from 20 m/s to 160 m/s, the brittle-removal ratio of the ground surface decreases from 34.24 % to 25.56 %, and the depth of subsurface damage decreases from 5.68 µm to 3.58 µm by 36.97 %. The effect of the grinding depth on the ground surface and subsurface is comparatively not remarkable. The influence of grinding parameters on the grinding process of SiC ceramics indicate that high speed, creep infeed and deep depth grinding method makes it possible to realize high efficiency and low damage machining for SiC ceramics with monolayer diamond wheel.

SiO2f/SiO2 composite grinding characteristics based on maximum undistorted chip thickness

ABSTRACT The thickness of the maximum undistorted chip (a gmax ) is a crucial component of the machining arguments and significantly affects the quality of the grind. For the purpose of investigating the impact of the a gmax on the grinding removal properties of SiO2f/SiO2 composites, grinding experiments of SiO2f/SiO2 composites were conducted. The findings indicate that crack propagation, fiber cracking, matrix breaking, and interface debonding are the primary removal modes of SiO2f/SiO2 composites. Grinding force, surface roughness, and chip size all have a positive correlation with the maximum undeformed chip thickness. Surface brittle fracture brought on by SiO2 fiber microcrack results in little surface roughness. The surface quality degrades when bending fracture and compression fracture take place. Controlling the a gmax can enhance the grinding surface quality of SiO2f/SiO2 composites.

Hemispherical resonator with low subsurface damage machined by small ball-end fine diamond grinding wheel: A novel grinding technique

As for the ultra-precision grinding of the hemispherical fused silica resonator, due to the hard and brittle nature of fused silica, subsurface damage (SSD) is easily generated, which enormously influences the performance of such components. Hence, ultra-precision grinding experiments are carried out to investigate the surface/subsurface quality of the hemispherical resonator machined by the small ball-end fine diamond grinding wheel. The influence of grinding parameters on the surface roughness (SR) and SSD depth of fused silica samples is then analyzed. The experimental results indicate that the SR and SSD depth decreased with the increase of grinding speed and the decrease of feed rate and grinding depth. In addition, based on the material strain rate and the maximum undeformed chip thickness, the effect of grinding parameters on the subsurface damage mechanism of fused silica samples is analyzed. Furthermore, a multi-step ultra-precision grinding technique of the hemispherical resonator is proposed based on the interaction influence between grinding depth and feed rate. Finally, the hemispherical resonator is processed by the proposed grinding technique, and the SR is improved from 454.328 nm to 110.449 nm while the SSD depth is reduced by 94 % from 40 μm to 2.379 μm. The multi-step grinding technique proposed in this paper can guide the fabrication of the hemispherical resonator.

A theoretical and deep learning hybrid model for predicting surface roughness of diamond-turned polycrystalline materials

Polycrystalline materials are extensively employed in industry. Its surface roughness significantly affects the working performance. Material defects, particularly grain boundaries, have a great impact on the achieved surface roughness of polycrystalline materials. However, it is difficult to establish a purely theoretical model for surface roughness with consideration of the grain boundary effect using conventional analytical methods. In this work, a theoretical and deep learning hybrid model for predicting the surface roughness of diamond-turned polycrystalline materials is proposed. The kinematic–dynamic roughness component in relation to the tool profile duplication effect, work material plastic side flow, relative vibration between the diamond tool and workpiece, etc, is theoretically calculated. The material-defect roughness component is modeled with a cascade forward neural network. In the neural network, the ratio of maximum undeformed chip thickness to cutting edge radius R TS, work material properties (misorientation angle θ g and grain size d g), and spindle rotation speed n s are configured as input variables. The material-defect roughness component is set as the output variable. To validate the developed model, polycrystalline copper with a gradient distribution of grains prepared by friction stir processing is machined with various processing parameters and different diamond tools. Compared with the previously developed model, obvious improvement in the prediction accuracy is observed with this hybrid prediction model. Based on this model, the influences of different factors on the surface roughness of polycrystalline materials are discussed. The influencing mechanism of the misorientation angle and grain size is quantitatively analyzed. Two fracture modes, including transcrystalline and intercrystalline fractures at different R TS values, are observed. Meanwhile, optimal processing parameters are obtained with a simulated annealing algorithm. Cutting experiments are performed with the optimal parameters, and a flat surface finish with Sa 1.314 nm is finally achieved. The developed model and corresponding new findings in this work are beneficial for accurately predicting the surface roughness of polycrystalline materials and understanding the impacting mechanism of material defects in diamond turning.

Study on Grinding Force of Two-Dimensional Ultrasonic Vibration Grinding 2.5D-C/SiC Composite Material

The grinding force is an important index during the grinding process, which affects the surface quality and other aspects after machining. However, the research on the grinding force of ceramic matrix composites assisted by two-dimensional ultrasonic vibration-assisted grinding is very weak. In this paper, the impact of the relationship between the critical cutting depth and the maximum undeformed chip thickness on the removal mode of ceramic matrix composites was analyzed. On this basis, the grinding force model of two-dimensional ultrasonic vibration-assisted grinding were developed for ductile removal and brittle removal, respectively. Finally, the correctness of the model was verified, and the impact of grinding parameters on the grinding force was analyzed. The experimental results show that compared with the conventional grinding force, the two-dimensional ultrasonic vibration assisted grinding force decreases obviously. When the feed rate and grinding depth increase, the grinding force increases. When the grinding velocity and ultrasonic amplitude increase, the grinding force decreases. Compared with the experimental value, the average relative error of normal grinding force is 8.49%, and the average relative error of tangential grinding force is 13.59%. The experimental and theoretical values of the grinding force have a good fitting relationship.

Material removal mechanism and subsurface characteristics of silicon 3D nanomilling

Atomic force microscope (AFM) tip-based nanomilling has drawn significant attention because it can greatly improve the material removal rate. Previous studies have mainly focused on the machining process of two-dimensional (2D) nanomilling. However, the material removal mechanism and subsurface damage in three-dimensional (3D) ultrasonic vibration-assisted nanomilling (UVAN) are still unclear. Thus, by considering single-crystal silicon as the sample material, we investigate 3D-UVAN using experiments and molecular dynamics (MD) simulations. The material removal mechanisms for 3D-UVAN are uncovered first. We find that the material removal is dominated by shearing and extrusion, respectively, when the maximum undeformed chip thickness (UCT) is larger and smaller than a critical value. The strain rates generated by 3D-UVAN and 2D nanomilling are calculated by considering the machining velocity. It is higher for 3D-UVAN than for 2D nanomilling, which induces a fracture chip. The effects of vertical vibration amplitude and frequency in 3D-UVAN on the subsurface damage are investigated by MD simulation. Results show that the induced immobile dislocations lead to embrittlement of the material. Furthermore, a high strain rate occurs at a large vertical vibration amplitude and high frequency, which contributes to a shallow subsurface damage. This is also verified by TEM experiments. Thus, our findings enrich understanding of the material removal mechanism in 3D-UVAN and suggest a new route for suppressing subsurface damage.

Modeling for prediction of sawing force based on the maximum undeformed chip distribution in the granite sawing

Sawing by circular saw blades is predominant in the mechanized treatment of natural stone. The predictive sawing force is crucial for optimizing, controlling, and monitoring the sawing process. In this work, a novel model for predicting sawing force, which is based on the distribution of the undeformed chip thickness at the sawing contact arc, was proposed. Based on the analysis of the undeformed chip geometry of the circular saw blade, the segment surface was divided into the front end and rear end according to the contact pattern between the segments and granite, and the models of the maximum undeformed chip thickness of diamond particles on the front end and rear end were established. Results showed that the new proposed force model fully considered the distribution of undeformed chips compared with the current model. The novel model has higher prediction accuracy, the mean maximum absolute error is within 3.77%, and the maximum absolute error is within 7.83%. Through theoretical analysis, the ratio of the maximum undeformed chip thickness of the front segment to the rear segment is 1.67, which can fully explain the wear non-uniformity of the segment. The proposed model is of great significance to the optimization of the saw blade structure and process parameters.

Ductile machining of single-crystal germanium for freeform surfaces diamond turning based on a long-stroke fast tool servo

Freeform surfaces of single-crystal germanium are required increasingly in advanced infrared optics, but there are difficulties in processing due to the high hardness, brittleness and low fracture toughness of germanium itself. In the present study, the machining mechanisms and parameters of single-crystal germanium were investigated via ultra-precision diamond turning based on a long-stroke fast tool servo. To realize ductile regime machining, a ductile machining model in ultra-precision turning of germanium considering both feed rate, cutting depth and surface slope of freeform surface was proposed and discussed. A novel taper cutting test was adopted to determine the critical ductile–brittle transition depth. Subsequently, XYPFS were fabricated under different cutting parameters to demonstrate the effectiveness of proposed model. The morphologies of machined surfaces and cutting chips were identified and compared. Finally, a smaller feed rate and cutting depth should be used to reduce its maximum undeformed chip thickness and realize the ductile machining for a freeform surface having a large surface slope. The freeform surface of single-crystal germanium with a form error of ~1258 nm PV and surface roughness of ~1.92 nm Ra was successfully fabricated. In summary, this research provides a comprehensive understanding of ductile machining mechanisms of diamond turning based on a LFTS as well as is meaningful in the manufacturing of freeform surfaces on germanium or other brittle materials with optically qualified surfaces and high-precision.

Theoretical Analysis of Grinding Wheel Deflection Angle on Peripheral Grinding Parameters and Grinding Force

The peripheral surface of the grinding wheel can grind the rail according to the envelope of the contour of the rail surface, thus a fuller and smoother rail surface can be obtained. Specifically, a better grinding effect can be obtained in that the end face of the grinding wheel deviates from the longitudinal section of the rail at a certain angle. Based on the traditional grinding technology theory, the mathematical models of the peripheral grinding parameters (kinematic contact arc length, wheel-rail grinding contact area, and maximum undeformed chip thickness) and the grinding force are established, in which the angle exists between the grinding wheel end face and the rail longitudinal section. The main influence of grinding wheel circumferential speed, grinding wheel kinematic speed, and the deflection angle of the grinding wheel end face on the grinding parameters and the force are analyzed. The result shows that: when there is angle θ in the models, the ratios of peripheral grinding parameters between up-grinding and down-grinding varies monotonically with the increase in vm, and their maximum variation range is about 12%, vs has the greatest influence on the peripheral grinding parameters, and the maximum variation range of the ratios is about 20% when the vs is 10 m/s. With the increase in the grinding width, Fa’ cannot be ignored and will increase gradually with the increase in angle θ. The analysis and conclusion have guiding significance for the structural design, grinding control strategy, and experimental research regarding rail curved surface grinding equipment.

Experimental study on ultrasonic vibration-assisted grinding of hardened steel using white corundum wheel

The combination of ultrasonic vibration-assisted grinding (UVAG), the ultrasonic vibrating method, and the conventional grinding (CG) process is proposed to improve the grindability of hardened steel in the CG process. Comparative investigations on grinding force, temperature, and specific grinding energy for both grinding processes were conducted. In addition, the morphologies of the wheel wear and ground surfaces were also studied. Findings show that compared with the CG processes, the normal and tangential grinding forces in UVAG are reduced greatly by 16.44% and 17.44%, respectively. The UVAG process decreases the specific grinding energy by 8.30% owing to the increase of the maximum undeformed chip thickness and the grinding temperature by 19.01% due to the improvement of the coolant heat transfer capacity in the grinding arc zone. Meanwhile, the promising grain sharpness of wheels can be guaranteed from the enhanced cooling function on the grinding arc zone and the grain’s micro-fracture caused by ultrasonic impacting actions.

Modeling of strain gradient-dependent strengthening effect in grinding of boron-diffusion-hardened titanium alloy

In grinding of the boron-diffusion-hardened titanium alloy, with the grinding speed increasing, the strain gradient-dependent strengthening (SGS) effect of TiB whiskers would become obvious, resulting in an improvement of the surface quality. To explore such SGS effect of TiB whiskers, a novel model of the strain gradient-dependent strengthening effect in the primary deformation zone was developed, and then a novel specific grinding energy model was further proposed to quantitatively estimate such SGS effect of TiB whiskers. It indicates that the above two models can adequately demonstrate the SGS effect of TiB whiskers in grinding the borided titanium alloy. The results demonstrate that the grinding performance of the borided titanium alloy was dominated not only by the grinding speed but also by the TiB whiskers. Due to high grinding speed (about 120 m/s) inclining to cause the maximum undeformed chip thickness reduced, when it was closed to the size of TiB whiskers, the SGS effect of TiB whiskers would become obvious, leading to an improvement of surface quality. Furthermore, when the size of TiB whiskers was closed to the sub-micron-scale, the enhanced SGS effect of the sub-micro-sized TiB whiskers tends to generate, causing the surface quality further improved.

Parallel axis precision grinding of micro-tooth internal thread with the coarse-grains CBN wheels

As an important part of precision transmission system, internal threads with high precision, high hardness and small pitch are widely used in the fields of aerospace, numerical control machine tools, medical equipment and construction machinery. Aiming at micro-tooth internal thread, the research on parallel axis precision grinding of micro-tooth internal thread with the coarse-grains CBN wheels is carried out. Firstly, the grinding wheel section is designed based on the principle of parallel axis grinding of micro-tooth thread, and the theoretical error is systematically analyzed. Secondly, the maximum undeformed chip thickness of parallel axis thread grinding based on coarse-grains is established. Finally, the precision grinding experiment of internal thread is carried out. The optimal parameters can be obtained: the grinding wheel diameter is 17.08 mm, the grinding wheel angle is 90.13°, the lead is 3 mm, the pitch diameter is 21.06 mm. In this case, the maximum axial error is about 0.1 μm, the tooth profile angle error is 0.01° and the rounded radius error is about 3.5 μm. Considering the machining effect and efficiency, the truing depth is 150 μm, the feed rate is 0.2 mm/s, the grinding speed is 17,000 r/min and the grinding depth is 5 μm.

Feasibility study of single-crystal silicon ductile-regime turning via fast tool servo

Due to the material properties of single-crystal silicon, it is difficult to achieve high-efficiency processing in conventional turning. Fast tool servo (FTS) turning has emerged as a favorable technology for processing brittle materials. This paper investigates the tool radius, feed speed, and cutting depth effects on the maximum undeformed chip thickness (MUCT). A high-frequency low-coupling FTS system is designed for processing experiments. Effects of the tool rake angle, tool radius, cutting speed, and vibration frequency on the critical undeformed chip thickness (CUCT) are explored in this paper. Experimental results indicate FTS technology can increase the CUCT from 80 nm to 580 nm. Surface and subsurface morphology tests of conventional and FTS single-crystal silicon cutting verify that FTS technology can obtain an optical surface roughness of 5 nm and lower subsurface damage. Moreover, FTS turning can reduce tool wear. Lastly, through Raman spectroscopy analysis, the amorphous phase remaining on the machined surface is lower when FTS turning is employed.

Ultra-precision diamond turning of ZnSe ceramics: Surface integrity and ductile regime machining mechanism

ZnSe ceramics has extensive applications in infrared imaging and laser systems. However, as a soft–brittle optical material, the mechanism of material removal and surface defects formation in ultra-precision diamond turning have not been clarified. In the present study, novel taper cutting tests were conducted to determine the ductile–brittle transition depth. Furthermore, the precondition for achieving ductile mode machining was revealed by comparing the maximum undeformed chip thickness and ductile–brittle transition depth. The effect of feed rate on surface roughness, surface quality, phase transition, and subsurface damage were systematically investigated. With the aid of cross-sectional TEM, the material removal mechanism in the ductile regime was revealed first, and the results indicated the formation of high-density dislocations, stacking faults, and deformation twins coordinating to promote ductile deformation. The study has provided insight into ductile regime machining mechanisms of ZnSe ceramics in the ultra-precision diamond turning process.

Wear characteristics of diamond segments on multi-blade combination saw with different diameters in granite sawing

In this paper, the wear characteristics of diamond segments adopted to multi-blade combination circular saw with different diameters were compared and investigated in the process of granite through an experiment. Load model with a diameter as the variable of combination saw was established based on the maximum undeformed chip thickness of a single particle. Combined with the load model, scanning electron microscope, laser microscope and vernier calipers were used to comprehensively analyze the wear characteristics of segment form the aspects of the wear morphology of diamond and metal matrix, the particle protrusion height, and the abrasion loss of segment. The results indicated that load nonuniformity of diamond generated by diameters change of saw blade was the main reason for segment wear discrepancy of large and small diameter saw blade when rocks were sawed by combination saw, and abrasion loss of segment adopted to small diameter saw blade was significantly higher than that of large-diameter saw blade. Diameter variation of saw blade in combination saw caused the different wear mechanism of diamond in segment, pulled-out and macro-fractured crystals were dominant in small-diameter saw blade, then the proportion of whole particles increased in segment used to the saw blade of large diameter. Particle protrusion height was related to the diameter in the combination saw. The high protrusion height of particles existed in the large-diameter saw blade.

Study on the material removal mechanism of ZnSe crystal via ultra-precision diamond turning

ZnSe crystal has been widely used in infrared imaging and laser systems. However, as a typical soft-brittle material, the material removal mechanism in ultra-precision diamond turning process has not been clarified, it is still challenging to obtain nano-smoothed surface. In the study, the effect of tool ranke angle on ductile-brittle transition depth of ZnSe crystal have been investigated through novel plunge-cutting tests. The ductile regime machining model was revealed by comparing the maximum undeformed chip thickness and ductile-brittle transition depth. With the aid of FESEM, white light interferometer, and Raman spectrometer, the effect of feed rate on surface roughness, surface quality, phase transition and subsurface damage were systematically investigated. The surface defects formation mechanism was proposed. Furthermore, the material removal mechanism of ZnSe crystal in ultra-precision diamond turning process have been revealed.