Maximum Chip Thickness Research Articles

Brittle-ductile transition mechanism during grinding 4H-SiC wafer considering laminated structure

4H-SiC wafer with alloy backside layer is gradually applied in power devices. However, the laminated structure presents various challenges in manufacturing. In this study, a model for brittle-ductile transition in grinding of laminated materials is established and verified by grinding experiment to ensure the complete removal of the alloy backside layer while achieving ductile removal of the 4H-SiC layer. In the modeling process, the maximum unreformed chip thickness and brittle-ductile transition critical depth of each-layer in the laminated material is deriving, taking into account the laminated structure. Consider the variability in proportion of dynamic active grits during grinding, set operation is introduced to analyze the relationship between sets maximum unreformed chip thickness and brittle-ductile transition critical depth, and to predict the removal mechanism of the 4H-SiC layer. Comparing the predicted results with experimental grinding data, found that under the conditions of grinding wheel with average size of abrasive 10 μm, grinding wheel speed vs of 74 m/s, grinding depth ap of 10 μm, and feeding speed vw of 2 mm/s, the alloy backside layer can complete removal while achieving ductile removal of the 4H-SiC layer. This study provides a new method for predicting removal mechanism in grinding of laminated material and theoretical guidance for optimizing machining parameters of 4H-SiC wafer with alloy backside layer.

Material removal mechanisms affected by milling modes for defective KDP surfaces

Surface defects which are caused by diamond fly-cutting can degrade the optical characteristics of KDP crystals by inducing the laser damage and resultantly shortening the service life when working in the condition of extremely high-power laser irradiations. The ball-end milling repair can totally remove the defects and correspondingly enhance the ability of the KDP crystals to resist laser damage. However, because of the existence of pull-milling, push-milling, down-milling, and up-milling in the complex machining trajectory, it is of great challenge for soft-brittle KDP crystals to achieve full plastic domain milling of repair points for obtaining ultra-smooth repaired surface. In this study, theoretical models of the maximum uncut chip thickness (UCT) considering the defect are constructed in the various milling modes. The cutting force and surface morphology affected by milling modes on the defective surface are simulated using a three-dimensional (3D) finite element model. Combined with the simulations, micro-milling tests are employed to confirm the influence of milling modes on the material removal mechanisms for defective KDP crystals by cutting force, surface morphology, surface roughness and specific cutting energy (SCE). It is found that defects could reduce the maximum UCT to distinct degrees in the various milling modes. Defects have a greater influence on the maximum UCT in the push-milling mode than that in the pull-milling mode, while the influence is the same in the up-milling and down-milling modes. Interestingly, the ductile removal mode occurs on the both defect-free and defective surfaces in the pull-milling mode when the feed per tooth (fz) is 0.5 µm/z. In contrast, the brittle-ductile transition caused by surface defects occurs in the other three milling modes. Furthermore, defects would lead to severer size effect in the all milling modes. When the fz is 0.1 µm/z, for the defective surface, the SCE is larger and the ploughing effect is more significant in the push-milling mode compared to the pull-milling mode. More importantly, as a result of the difficulty of chip discharge, the SCE is greater in the up-milling mode than in the down-milling mode on the defective surface. This work reveals the influence mechanism of milling modes on the material removal process for defective KDP crystals and offers a theoretical foundation and technical value for improving the repaired surface quality of functional KDP crystals served as the optical devices.

Analysis and development of elliptical tool path in trochoidal milling

Trochoidal milling has been developed to extend the cutting tool life as compared to conventional milling. It is widely used in the machining of slots and corners of molds and dies. There are several tool paths typically used in trochoidal milling such as true, circular, variable-feed, and variable-step trochoidal tool paths. However, these paths suffer from low material removal rates, low surface qualities, high CPU processing times, and/or the requirement of high-dynamic machine tools. Elliptical trochoidal tool path showed its capability to improve the material removal rate, but the expected low surface quality of the produced slot. In this paper, the elliptical tool path will be analyzed analytically and experimentally. The effect of elliptical tool path on material removal rate, cutting forces, walls waviness, and surface roughness have been investigated. A novel linear elliptical-based tool path has been proposed to enhance the elliptical tool path performance by improving both material removal rate and surface quality. A Performance index has been utilized to evaluate the performance of these processes. An analytical model for the local maximum chip thickness in elliptical tool path has been conducted to correlate this path with the cutting forces. The slot walls waviness has been modeled based on the cutting tool path geometry and validated experimentally with error less than 11%. The experimental results showed that the elliptical tool path improved the material removal rate by 11%, with a slight reduction in the maximum resultant cutting force by 3%. On the other hand, the walls waviness and surface roughness have been increased by 45% and 38% respectively as compared to the typical true trochoidal tool path. The introduction of linear paths at the elliptical path sides in the linear-elliptical tool path improved the performance index about 18 times by improving the material removal rate, waviness, and surface quality by 5%, 300%, and 74% respectively, without a significant effect on cutting forces. Therefore, the novel linear-elliptical paths are promising tool paths in increasing the performance of trochoidal milling due to the improved material removal rate, walls waviness, and surface quality, without a significant effect on cutting forces, which will improve the process machinability and cost-effectiveness.

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.

Model for tool wear prediction in face hobbing plunging of bevel gears

This paper deals with tool wear investigations for face hobbing plunging of bevel gears. Initially, the influence of process parameters and tool geometry on tool wear is analyzed both in cutting trials and with the help of the manufacturing simulation BevelCut. Subsequently, a tool wear model is presented. Input parameters into the model are tool and workpiece data as well as process parameters and chip characteristics. Through the manufacturing simulation BevelCut, which is based on a planar penetration algorithm, chip characteristics such as the maximum chip thickness hcu,max are calculated resolved in time and location along the blade's cutting edge. Combined with the local cutting speed, the chip characteristics are used to determine the thrust force, which is required to calculate the elastic workpiece deformation. The model coefficients are calibrated by multi-variable regression analysis using the results of the cutting trials and simulation results. The quality of the regression is determined with the help of equivalence tests. For verification, the process parameters and tool geometry are varied in series production and the tool wear is assessed. Finally, the modeled tool wear is compared to the measured tool wear.

DEFINITION OF THE GEOMETRIC PARAMETERS OF THE UNDEFORMED CHIP AT THE CUT-IN STAGE WHEN MACHINING AN EXTERNAL GEAR USING THE POWER SKIVING METHOD

In this study, the process of tool plunging into the workpiece during external tooth cutting will be modelled. Cut-in is one of the most dangerous stages not only in gear turning, but also in any cutting process. The gear cutting process is analytically studied in terms of maximum chip thickness and, on this basis, recommendations for process design are offered. The developed simulation is able to calculate the appropriate cutting geometry for each revolution of the tool cut-in to the workpiece. Using the designed cutter profile and transition surface, the depth of penetration of the tool into the workpiece can be calculated for discrete time and space intervals. Finally, the developed simulation was tested on a gear with an external gear crown. This article describes the kinematics of gear cutting at the plunging stage and the algorithm for its modelling. In the description of the gear cutting kinematics, a method for creating a gear cutting simulation model was presented. In addition, an analytical method for calculating the maximum chip thickness was applied and confirmed by simulation.

Morphological characterization of chip segmentation in Ti-6Al-7Nb machining: A novel method based on digital image processing

α + β titanium alloys, namely Ti-6Al-7Nb, have found growing importance in the biomedical industry, in which CNC machining plays a decisive role in the production of high-quality components. Metal chip analysis is a consolidated method for machining optimization, particularly in difficult-to-cut materials that tend to form saw-tooth/segmented chips. A novel automatic method for quantification of classical geometric aspects of Ti-6Al-7Nb alloy saw-tooth chips was developed. Geometric features, including the minimum and maximum chip thickness, as well as the saw-tooth width, were evaluated. These aspects were obtained by association of optical microscopy and digital image processing. The advantage of this methodology is the fully automated analysis of the geometric aspects of the chip, which are generally obtained manually. The robustness of the measurements was ensured by the selection of shape descriptors tested on a range of cutting speeds. This work also delivers a set of valuable experimental results about Ti-6Al-7Nb machining.

Ductile-brittle transition mechanisms in micro-grinding of silicon nitride

Ductile grinding of brittle materials is essential for high precision applications and to maintain the strength and lifetime of the parts. The critical chip thickness of a brittle material defines a threshold for the lateral cracks' initiation in the workpiece due to the grains’ penetration. For ductile grinding, keeping the uncut chip thickness below the critical chip thickness of the brittle material is necessary. This study focuses on the ductile micro-grinding of Si3N4 as an advanced ceramic and brittle material. The critical chip thickness and the maximum uncut chip thickness were first calculated based on the material properties, the grinding parameters and the microtopography of the utilized grinding pin and then validated by both diamond grit scratches and micro-grinding experiments. Micro-grinding experiments on an inclined workpiece were conducted to investigate the material removal regimes of Si3N4. Grinding forces and surface integrity (surface roughness, surface topography and subsurface damages) induced by different micro grinding parameters and micro-grinding of an inclined workpiece are analyzed in detail. The estimated critical chip thickness and measured maximum uncut chip thickness could be used as an exact guide for achieving the ductile micro-grinding mode. The experiments revealed that the material removal mechanism mainly affects the micro-ground surface integrity. The ductile material removal mode induced no detectable subsurface damage and the surface quality deteriorated by the brittle removal mode.

A parametric modeling for fast radial infeed planning process in gear skiving

In gear skiving, the infeed planning process for the multiple radial infeed strategy affects cutting efficiency, local cutting features, and cutter life. The maximum uncut chip thickness nonlinearly changes with infeed combinations, making the infeed planning process challenging. Here, we propose a parametric model to achieve fast infeed planning based on the uncut chip thickness in parametric space. First, the relation between the infeed and chip boundary evolution is elucidated; then, the concept of initial uncut chip geometry (iUCG) is established to perform fast infeed planning according to the chip boundary and uncut chip thickness distribution. An industrial case study is presented to validate the method and demonstrate the implementation. As a result, a decision-making principle that includes infeed planning starting from the finishing to the rough stages enables fast planning owning to the invariance of iUCG and lower simulation capacity requirement on the chip boundary. In such case, the infeed of the “previous” skiving pass can be efficiently determined according to the maximum uncut chip thickness criterion with a constraint condition. The proposed parametric model advances the infeed planning process, further contributing to the multivariable optimization of gear skiving.

Coupled thermo-mechanical analysis and optimization of the grinding process for Inconel 718 superalloy using single grit approach

The current research aims to enhance the grinding efficiency of Inconel 718 by thermo-mechanical assessment of the effects of main grinding parameters. Three optimization criteria were proposed to minimize cutting forces and temperature and maximize removal rate. The overall optimization solution predicted the optimal grinding parameters as: maximum uncut chip thickness of 0.81 µm, cutting speed of 43.5 m/s, grain-edge radius of 0.5 µm, and grain rake angle of − 30°. Combined analysis of those major parameters and correlating them with grinding forces and temperature without doing experiments was a research gap covered by this study. Also, it was found that the incorporation of 0.3 wt% graphene nanoplatelets within a vegetable-based MQL resulted in the decrease of grinding forces and energy.

Surface generation in high-speed grinding of brittle and tough ceramics

Grinding induced sub-surface defects substantially impair the functionality of ceramic components. Effective control of such defects is possible through identification and understanding the governing factors. The present work investigates the interaction of abrasive grits over the ground surface to identify the governing factors for effective control of such defects during high-speed grinding of a brittle (alumina) and a tough (yttria-stabilized zirconia (YSZ)) ceramic. High-speed grinding was carried out using single-layer electroplated diamond grinding wheels in the speed range of 40–200 m/s. The important grinding characteristics, such as material removal mode, residual stress, surface roughness, etc., were correlated with the maximum uncut chip thickness (hm) and the scallop height (Sg). The hm represents the interaction of active grits at the exit of the grinding zone, whereas the interaction of active grits while entering the grinding zone (at ground surface) leads to the formation of scallops. Instead of hm, material removal mode and other related grinding characteristics are found to be governed by the Sg. An efficient reduction of approximately 90 % in Sg is realized by increasing the grinding speed from 40 m/s to 200 m/s. For a brittle ceramic like alumina, such a drastic reduction in Sg reduced grinding induced defects by more than 75 %. For a tougher ceramic like YSZ, a reduction of more than 30 % in the magnitude of compressive residual stress is observed with a reduction in Sg at a higher grinding speed of 200 m/s. This study manifests Sg as a predominant factor in governing the ground surface characteristics during ceramic grinding, which can be effectively utilized to control grinding induced defects in ground ceramic parts.

Multi-variant hybrid techniques coupled with Taguchi in multi-response parameter optimisation for better machinability of turning alloy steel

ABSTRACT Despite having poor cooling property, minimum quantity lubrication (MQL) is very commonly utilised in machining processes because of its superb lubrication, cleanliness, fewer costs and ease of handling. However, the optimum process parameters and the cutting fluid selection are very important for efficient machining. Moreover, an easy technique for optimisation is required so that the technicians can easily find their optimum parameter combinations for any manufacturing process. Considering multiple objectives synchronously for selecting optimum conditions helps to enhance the overall machinability. In this work, optimisation of cooling/lubrication environment, speed and feed were accomplished for minimum cutting temperature, roughness, cutting force along with maximum MRR and chip thickness ratio in turning 42CrMo4 alloy steel. Three powerful multi-variant hybrid techniques, such as Taguchi-based PCA-GRA, PCA-MOORA and DEAR, were implemented for multi-response optimisation, which provided identical results. The optimal process parameters were: MQL with cutting oil with the speed of 352 m/min and feed of 0.10 mm/rev. But among these methods, compared to the complex PCA-GRA, the DEAR is the computationally simplest because it did not require weight calculation before optimisation and then PCA-MOORA. However, the limitation of the DEAR method is that it can work for the problem having conflicting objectives.

Calculation of the maximum chip thickness for a radial-axial infeed in gear hobbing

A numerical penetration calculation was used to simulate the radial-axial infeed in gear hobbing for different tool/workpiece combinations. Taking into account 20,000 simulations, a nonlinear regression analysis was performed to establish a mathematical equation, describing the maximum chip thickness as a function of the infeed angle and relevant gear, tool and process parameters. The equation was validated using a new set of simulation results. The calculated values are in good agreement with the simulated values. The developed equation allows the design of the radial-axial infeed according to the maximum chip thickness without the use of a simulation program.

Study on surface integrity and material removal mechanism in eco-friendly grinding of Inconel 718 using numerical and experimental investigations

Knowledge of material removal behavior and generated surface characteristics are crucial to improve grinding performance. A finite element model was developed using a single grit approach with the consideration of uncut chip thick variations along the grain path. It was found that the increase in cutting speed and the decrease in rake angle led to the generation of discontinuous chips in adiabatic shearing conditions. Besides, the larger shear stresses induced by friction in the grain-chip interface contributed to the formation of chips with larger discontinuities. The enlargement of subsurface damage (SSD) depth by 23.86% and 129.42% was caused by the increase in grain edge radius from 0.5 to 2.5 μm and the rise in maximum uncut chip thickness from 0.5 to 1.5 μm, respectively. However, the SSD depth firstly decreased, and then increased with the approach to more negative rake angles and larger cutting speeds. A multi-response optimization based on analysis of variance (ANOVA) and SSD predictive model calculated the optimum combination of influential parameters as maximum uncut chip thickness of 0.719 μm, cutting speed of 80 m/s, cutting edge radius of 0.5 μm, and rake angle of − 30°. This led to the minimum SSD depth of 0.406 μm and the maximum removal rate of 57.493 mm3/mm s. The experimental results confirmed the positive contribution of an eco-friendly nanofluid minimum quantity lubrication (NMQL), containing graphene nanoplatelets with 0.3 wt% concentration within the green nanofluid, to the significant improvement of surface roughness (by 41.09%, relative to dry grinding) and morphology.

Model for Predicting the Micro-Grinding Force of K9 Glass Based on Material Removal Mechanisms.

K9 optical glass has superb material properties used for various industrial applications. However, the high hardness and low fracture toughness greatly fluctuate the cutting force generated during the grinding process, which are the main factors affecting machining accuracy and surface integrity. With a view to further understand the grinding mechanism of K9 glass and improve the machining quality, a new arithmetical force model and parameter optimization for grinding the K9 glass are introduced in this study. Originally, the grinding force components and the grinding path were analyzed according to the critical depth of plowing, rubbing, and brittle tear. Thereafter, the arithmetical model of grinding force was established based on the geometrical model of a single abrasive grain, taking into account the random distribution of grinding grains, and this fact was considered when establishing the number of active grains participating in cutting Nd-Tot. It should be noted that the tool diameter changed with machining, therefore this change was taking into account when building the arithmetical force model during processing as well as the variable value of the maximum chip thickness amax accordingly. Besides, the force analysis recommends how to control the processing parameters to achieve high surface and subsurface quality. Finally, the force model was evaluated by comparing theoretical results with experimental ones. The experimental values of surface grinding forces are in good conformity with the predicted results with changes in the grinding parameters, which proves that the mathematical model is reliable.

Modeling of micro-grinding forces considering dressing parameters and tool deflection

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 effect of cutting tool material on chatter vibrations and statistical optimization in turning operations

Machine tool vibrations consist of a self-stimulating mechanism during chip removal by machining operations. The system, which is a structural mode of the tool/workpiece, is initially stimulated by force. In turning operations, a wavy surface is formed on the workpiece because of both the previous revolution and structural vibrations. The maximum chip thickness may exponentially increase due to the phase shift between two consecutive waves, while the system oscillates at a chatter frequency very close to its structural mode. Variable growth in chip thickness increases vibrations, cutting forces and tool wear and causes a wavy surface. The purpose of the study is to compare different cutting tool inserts in terms of stable cutting depths. First, an experimental study was carried out and the stable cutting depths without chatter vibrations were determined in turning operations using various materials (AISI-1010, AISI-1050, Al-7075). Alumina inserts (Al2O3) were used in the study. Stable cutting depths were compared with the literature study that was performed using titanium carbide (TiC) inserts. A paired t test was used for the comparison. After the experimental study, a statistical/optimization study was performed to optimize stable cutting depths. It was observed that the chatter frequency was generally higher than the natural frequency of the cutting tool. Also, it was observed that the decrease in the number of revolutions, tool overhang lengths and yield strength of workpiece results in higher stable cutting depths. When the number of revolution is 125 rpm, the overhang length is 70 mm, and the yield strength is 124 MPa, the stable cutting depths maximize. In addition, stable cutting depths were higher when using Al2O3 cutting tool inserts and chatter vibrations were prevented. There was a significant difference between the two inserts (p < 0.05). It was found that the Al2O3 cutting tool inserts had better performances than TiC cutting inserts for the hard materials.

Development of self-tuned diamond milling system for fabricating infrared micro-optics arrays with enhanced surface uniformity and machining efficiency.

Infrared micro-optics arrays (MOAs) featuring large numbers of micro-freeform lenslet are required increasingly in advanced infrared optical systems. Ultra-precision diamond cutting technologies have been widely used to fabricate MOAs with high form accuracy. However, the existing technologies can easily cause the non-uniformly fractured surface of infrared MOAs, due to the inherent low fracture toughness and high anisotropy of infrared materials as well as the time-varying chip thickness induced by ever-changing height and slope of the desired MOAs. In this study, a novel self-tuned diamond milling (STDM) system is proposed to achieve the ductile cutting of infrared MOAs with enhanced the surface uniformity and machining efficiency, and the corresponding toolpath planning algorithm is developed. In STDM system, a dual-axial fast servo motion platform is integrated into a raster milling system to self-adaptively match the maximum chip thickness for each tool rotational cycle with the critical depth of cut of the infrared material according to the local surface topography, thereby obtaining crack-free lenslet with high surface uniformity. Practically, micro-aspheric MOAs free from fractures are successfully machined on single-crystal silicon, a typical infrared material, to validate the proposed cutting concept. Compared with the conventional diamond milling, the proposed STDM is demonstrated to be able to avoid the non-uniform fractures without needing to reduce feed rate, and a smaller surface roughness of 4 nm and nearly double machining efficiency are achieved.

Performance evaluation method for cutting fluids using cutting force in micro-feed end milling

A simplified evaluation method for cutting fluids has not been established to accurately estimate cutting performance. In this study, the friction coefficients of cutting fluids were calculated from the cutting force in micro feed end milling, defined as μ-MFM for the cutting performance index. For cutting conditions in which the finished surface roughness exceeded a largely theoretical value and the feed marks were hardly recognized, there was a high correlation between μ-MFM and the finished surface roughness in end milling of AISI 1045 compared with results from conventional friction tests. Moreover, the finished surface roughness in turning AISI 1045 and AISI 440C also increased with μ-MFM.The finished surface roughness in turning AISI 1045 and AISI 440C showed nearly theoretical values regardless of the cutting fluids when the cutting speed exceeded a certain speed. However, even if the cutting speed was high, the cutting edge transcription error on the finished surface in low-feed turning conditions increased with decreasing μ-MFM. It was possible that the difference in cutting edge biting performance for work material in cutting fluids had a substantial influence on the cutting edge transcription error for small uncut chip thicknesses on the finished surface formation area, such as the low-feed rate in turning. Therefore, cutting edge biting performance was evaluated using the cutting force instability rate CFIR, which was the quantitative value of the drifting cutting force in one end milling cutting with the feed rate based on the maximum uncut chip thickness on the finished surface formation area of the low-feed turning. As a result, the cutting edge transcription error in turning of AISI 1045 and AISI 440C under low-feed cutting conditions increased with CFIR. Therefore, there was a correlation between the CFIR and the cutting edge transcription error in turning at low-feed conditions.

Ductility-oriented high-speed grinding of silicon carbide and process design for quality and damage control with higher efficiency

Grinding of brittle materials is always a removal process of coexisting ductile and brittle removal modes. Ductility-oriented grinding has been regarded as a precision machining pursuit for grinding quality and efficiency. This paper is devoted to investigating ductility-oriented grinding mechanism and process design for quality promotion with a higher efficiency in high-speed grinding of silicon carbide ceramics. The Rayleigh chip thickness model and critical chip thickness model are given to quantitatively calculate the ductile removal proportion. Moreover, the grinding forces and specific removal energy are discussed to reflect the high-speed grinding removal mode. The results show that the increase of wheel speed or decrease of maximum chip thickness could enhance the percentage to a more ductile-oriented removal mode, which will cause a smaller surface roughness with fewer fracture cracks and more plastic removal stripes. Finally, the grinding process conditions for surface roughness below 0.2 μm and ductile removal area higher than 50% are suggested to obtain better surface quality at higher ductile removal and material removal rates.