Variation Of Chip Thickness Research Articles

Analytical modeling of cutting force in vibration-assisted helical milling of Al 7075 alloy

Vibration-assisted helical milling is considered a promising hole-making process in the aerospace industry. The prediction of cutting force magnitude and behavior is very important to determine the cutting power, produce tight tolerance, eliminate tool wear, and improve the hole quality. Therefore, the main goal of this work is to establish a cutting force model and evaluate the cutting force coefficients, based on a mechanistic approach, during vibration-assisted helical milling of aerospace aluminum alloy (Al 7075). The process kinematics, the chip geometry including the chip width and chip thickness variations, and the tool-workpiece engagement are theoretically analyzed throughout the whole machining time. A set of experimental tests is implemented at different conditions of tangential feed and helical pitch. Additionally, model validation is performed to discuss the quality and accuracy of the presented model. Consequently, the behavior of the cutting force components against the application of ultrasonic vibration at different cutting conditions is demonstrated. The results show that the analytical and the experimental results are well agreed with a relatively small error percentage. The cutting force components generally increase as tangential feed and helical pitch increase. Moreover, the superimposed ultrasonic vibration causes a rapid tool-workpiece separation increasing the analytical and the experimental oscillation of the cutting force components, thus decreasing the average calculated cutting force.

A simulation-based analysis on the cooling effect and the tool temperature distribution in modulated turning (MT)

Modulation assistance is widely used for breaking chips of highly ductile materials in turning. This process is also known as the “modulated turning” (MT) or the “modulation assisted machining (MAM)”. The process transforms continuous single point turning into an intermittent cutting process where the tool tip briefly disengages from the workpiece generating discrete short chips. Resembling the milling process mechanics, periodic tool disengagements in modulated turning introduces cyclic cool-down periods and thus it can potentially attenuate tool-tip temperatures. It has been hypothesized that this effect can greatly lower maximum and the average tool-tip temperatures improving wear resistance of the process. This paper presents a thermal model to predict the tool-tip temperature distribution in modulated turning. A generalized chip formation formulation is first developed to predict the uncut chip thickness variation, cutting forces and the tool engagement/disengagement phase durations. Thermomechanical behavior of tool-chip contact is then modeled to predict the rake face temperature distribution. Simulation studies are presented to analyze the temperature distribution and understand peak temperature (hot) spots on the rake face. It has shown that the phase angle parameter – the frequency ratio between workpiece rotation and tool modulation– of MT plays a key role in controlling the hot spots on the rake and can be used as a tool to influence the wear regime.

Investigation on surface quality in micro milling of additive manufactured Ti6Al4V titanium alloy

The additive manufactured technology is widely applied to produce titanium alloy parts with complex structures. The subsequent machining operations are necessary for additive manufactured parts to achieve the required machining quality. In this paper, the theoretical surface roughness model in micro milling was established. The model indicated the theoretical milled roughness mainly depends on the feed per tooth, tool nose radius and bottom edge inclination angle. Then, micro milling experiments are conducted on additive manufactured titanium alloy using PCD micro end mill. The surface roughness on milled surface was measured and investigated. At the middle position of micro milled surface, the distribution curves of surface roughness Ra and Sa show the same variation trend under different milling parameters. Moreover, the measured surface roughness results on different transverse positions of milled surface showed that the difference value between the actual and theoretical surface roughness of milled surface is mostly induced by the unstable cutting behaviors, which is also mainly responsible for the surface roughness size effect. The transverse distribution of surface roughness on milled surface exhibited a W shape because of the variation of the instantaneous undeformed chip thickness and material removal mode at different transverse positions.

Analytical modelling of cutting forces in ultra-precision fly grooving considering effects of trans-scale chip thickness variation and material microstructure

Analytical modelling of cutting forces in ultra-precision fly grooving considering effects of trans-scale chip thickness variation and material microstructure

Effects of temperature and moisture content of logs on size distribution of black spruce chips produced by a chipper-canter at two cutting widths

Four matched groups of black spruce logs were processed with a chipper-canter at temperatures of 20, 0, -10, and -20 °C. Each log was transformed at two moisture contents (MC, green and air-dried) using two cutting widths (CW, 12.7 and 25.4 mm). Mean MC for each CW was assessed from a sample of the obtained chips. Knot characteristics were measured on the cant surfaces after log processing. Chip size was assessed by thickness (Domtar classifier) and width/length (Williams classifier). The results showed that the chip size was significantly affected by the CW and temperature, and in a lesser degree by the chip MC. The weighted mean chip thickness (WCT) increased with the CW. As temperature decreased below 0 °C, WCT and accepts decreased, while proportions of fines and pin chips increased. Chips obtained from green logs were thinner compared to air-dried logs when processed at the coldest temperature (minus 20 °C). The number and size of knots had an important impact on chip size, particularly on WCT. Multiple regressions were developed to predict WCT. Results showed the potential benefits of measuring log temperature and knot features to reduce chip thickness variation during fragmentation and thus improving chip size uniformity.

Analytical modelling of the trans-scale cutting forces in diamond cutting of polycrystalline metals considering material microstructure and size effect

In diamond cutting of polycrystalline metals, the influence of size effect and microstructure on the cutting force is prominent due to the trans-scale variation of undeformed chip thickness (UDCT) from microscale to nanoscale. This study proposes a trans-scale cutting force model for diamond cutting of polycrystalline metals with the full consideration of microstructure, material elastic recovery, size effect and round-tool-edge effect. Specifically, by corelating micro-forming theory and crystal plastic theory, a hybrid slip-line model (HSLM) is developed to determine the flow stress in the primary deformation zone, which can quantify the influence of size effect and microstructure, such as grain size, grain boundary and crystal anisotropy, on flow stress. Then, the normal cutting force and frictional cutting force are determined by analysing the stress distribution and frictional states at tool-chip interface using a tool-chip contact model. The rubbing force induced by material elastic recovery at very small UDCT is determined based on indentation theory. Through diamond cutting of polycrystalline copper with different grain sizes, it is experimentally demonstrated that the proposed HSLM can capture the cutting mechanism transformation phenomenon from shearing (tensile stress) to ploughing (compressive stress) with increasing size factor. Besides, the proposed force model has the improved estimation accuracy compared with the conventional force models developed based on Johnson-Cook constitutive equation.

Machining of Hybrid (Conventionally and Additively) built 316L, IN718 and Ti-6Al-4V Specimen*

Abstract Mounting on a body during the powder bed process is a way to save manufacturing time and costs while still producing individual and complex components. However, components in the powder bed process are always manufactured with an allowance in order to compensate for inaccuracies in the process by means of subsequent production steps. Accordingly, hybrid components must also be reworked. This publication presents the results of investigations into the influence of the transition from conventional to an additive microstructure during the machining of hybrid samples. For this purpose, samples of 316L, IN718 and Ti-6Al-4V were printed on the corresponding conventionally produced samples. The first half of the samples were machined directly after being manufactured in the LPBF process, while the second half passed a heat treatment step before being machined. The machining tests were performed in orthogonal cutting with variation of uncut chip thickness and cutting speed. The chip formation, the cutting and passive forces, the surface roughness, the microstructure and the hardness are analyzed and compared. The measurement of the process forces showed a difference between the conventional and the additive areas for all materials examined, even after heat treatment. In contrast, the roughness profiles as well as the microstructure and the measured hardness of the conventional and the additive area could be approximated by heat treatment.

Mechanistic Cutting Force Model and Specific Cutting Energy Prediction for Modulation Assisted Machining

This paper presents a mechanistic model for analytical cutting force and specific energy prediction for modulation assisted turning. Kinematics of the process is used to analyze the cut surface topography and derive the uncut chip thickness variation for various modulation parameters. It is found that uncut chip thickness is governed by successive spindle rotations and can be represented in the form of trigonometric functions. The cut and no-cut phase durations are controlled by the modulation frequency and amplitude. Cutting (machining) forces are predicted based on the orthogonal cutting mechanics. The thin shear plane angle is predicted considering oscillations in the cutting velocity, resultant effective rake angle and by employing the well-known Minimum Energy Principle (MEP). In order to accurately predict the shear forces, length of the shear plane is estimated directly from the uncut and cut part surface geometries, which incorporates waviness of the surface. Analytical chip thickness predictions and proposed cutting force models are combined used to predict the specific cutting energy in MAM. Experimental results verify accuracy of developed force models in estimating cutting forces and energy in orthogonal modulated assisted turning of Al6061 T6511.

Prediction of cutting forces during micro end milling considering chip thickness accumulation

This study focuses on the prediction of cutting forces during micro end milling using a novel approach that takes into account the chip thickness accumulation phenomenon. The proposed original force model considers the micro end milling kinematics, geometric errors of the machine tool–toolholder–mill system, elastic and plastic deformations of workpiece correlated with the minimum uncut chip thickness, and flexibility of the slender micro end mill. It also includes a novel analytical approach for the instantaneous area of cut. The chip thickness accumulation phenomenon can be manifested as chip thickness variations in the current tool rotation, resulting from material burnishing and elastic recovery in all previous tool rotations. The predicted forces consider the minimum uncut chip thickness value, which has been estimated directly from the micromilling process of AISI 1045 steel based on an original analytic–experimental approach that applies the identification of a stagnant point in the milling process. The results obtained show that the instantaneous and average micromilling forces determined using the proposed model have considerably better conformity with the experimental forces than those predicted using the commonly used rigid micro end milling model. Moreover, the non-linearity of the cutting forces as a function of feed per tooth is strongly affected by multiple cutting mechanism transitions observed during micromilling with uncut chip thicknesses close to the minimum uncut chip thickness value.

Experimental investigation of dynamic chip formation in orthogonal cutting

Orthogonal planing experiments are conducted with high-speed camera recordings and simultaneous cutting force measurements in order to analyze chip formation during machine tool vibrations. By means of carefully designed periodic forcing of the cutting tool, non-stationary cutting experiments are performed including wave generation, wave removal and wave-on-wave cutting, which are then compared to stationary cutting tests. The experiments are performed to address how the cutting force is affected by the chip thickness variation, the surface waviness and the fluctuation of the cutting direction. The results are used to assess some theoretical models that involve shear angle variation and chip segmentation.

Modelling the unidirectional fibre composite milling force oscillations through capturing the influence of the stochastic fibre distributions

Although high frequency variation of cutting forces is an inherent characteristic when milling composites, studies and models on the explanation and time-domain simulation of such processing forces seem to be missing in the literature. This paper first claims that the variation of the composite milling forces comes from three aspects: i) the variation of chip thickness caused by the cycloid trajectories of the cutting edge; ii) the random fibre placements within the composite and iii) the continuous variation of the cutting direction relative to the fibre orientation due to the rotational motion of the milling cutter. Moreover, the cutter’s helix angle leads to each section of the cutting edge engaging different fibre orientations simultaneously, adding additional challenges in understanding and predicting the resultant forces. Thus, this paper develops an analytical approach which can be utilised to accurately simulate the milling forces in time domain for unidirectional fibre composites. The approach calculates the chip thickness, estimates the stochastic fibre placements and defines variable cutting coefficients for different fibre orientations to integrate their effects on the force variability. Besides, the helical cutter is discretized into several slices to simulate the force acting on each engaged part at its relevant fibre cutting angle, resulting in a procedure of simulating the composite cutting force with helical milling cutter. Both straight and helical milling cutters are taken into account to validate the model, the effects from the concluded three aspects are separately investigated to provide an in-depth understanding of the force variation and support the developed model. It is observed that the variation of milling forces follows a Gaussian distribution when the cutting direction of the fibre is fixed. Furthermore, the simulated milling forces show a satisfactory agreement with the experimental results, especially their oscillations illustrate a high degree of consistency.

Simulated and experimental analysis on serrated chip formation for hard milling process

Due to the intermittent cutting processes and variation of undeformed chip thickness, hard milling process displays evident differences regarding thermo-mechancial loads compared to turning process; therefore, a comprehensive understanding of chip formation mechanism in hard milling operation is urgent. In this study, the effect of cutting speed on chip characteristics was experimentally investigated concurently with finite element simulation. The main objective of this study concerns the serrated chip formation accompanying micro-cracks and physical phenomena which was achieved using finite element method (FEM). First, hard milling simulation model was proposed based on Johnson-Cook visco-plastic constitutive material model combined with Johnson-Cook damage accounting for shear localization. Second, the proposed FE model was validated using acquired experimental results. The predicted results are in a good agreement with experimental results regrading chip morphology and cutting forces. In addition, the physical phenomena governing the formation of serrated segment and micro-cracks were discussed and clarified deeply through FE analysis. Finally, the localized plastic strain state on the machined surface associated with serrated segment formation was observed and indentified. This research, on one hand, can deepen the understanding of the formation mechanism of serrated segment; on the other hand, the results also provide guidance for cutting parameters optimization and desired machined surface integrity control.

Experimental investigations and kinematic simulation of single grit scratched surfaces considering pile-up behaviour: grinding perspective

Scratch tests are useful techniques to gain insight into the material removal mechanism of abrasive machining processes. In most of the scratch tests, uncut chip thickness value is either constant or varies from zero to maximum. However, in abrasive machining processes, uncut chip thickness value ranges from either zero to maximum or vice versa. Moreover, regular scratch tests are conducted at very low speeds, in which either the indenter or the workpiece is stationary. Because of these limitations, the knowledge obtained from the existing scratch test results is not valid for most of the abrasive processes. Hence, in this paper, the influence of chip thickness variation, speed ratio, and depth of cut on the pile-up behaviour of AISI 1015 steel and 2017A-T4 aluminium alloy surfaces were investigated. The workpiece having the comparable thermal diffusivity value with the grit has shown a significant difference in its pile-up behaviour. Through a better understanding of chip thickness influence on pile-up ratio, a mathematical model was developed for kinematic simulations. Using the developed model, kinematic simulations were done to visualise the scratch surface topography and material pile-up by considering the grit trajectory path and chip thickness variation. Finally, simulated surfaces were compared with the experimental results to show the proposed method applicability.

Radial Throw in Micromilling: A Simulation-Based Study to Analyze the Effects on Surface Quality and Uncut Chip Thickness

This paper presents a simulation study toward analyzing the effect of radial throw in micromilling on quality metrics and on the deviation in tool-tip trajectory from its prescribed pattern. Both the surface location error (SLE) and the sidewall (peripheral) surface roughness are analyzed. The deviation in tool-tip trajectory is evaluated considering the flute-to-flute variations in the uncut chip thickness and changes in the tooth spacing angle. Radial throw indicates the instantaneous radial location of the tool axis, thereby capturing all salient features of tool-tip trajectory deviations, such as the general elliptical form of the radial motions. This is in contrast to the concept of run-out, which is a scalar quantity (total indicator reading) indicating the total displacement or change in the radial throw measured from a perfect cylindrical surface for one complete rotation of the axis. As such, measurement and analysis of radial throw is essential to understanding micromachining processes. In our previous work, we described an experimental approach for accurate determination of radial throw when using ultra-high-speed micromachining spindles. In this work, we present a simulation-based study to relate radial throw parameters and form to SLE, sidewall surface roughness, flute-to-flute variations of uncut chip thickness, and changes in tooth spacing angle for a two fluted micro-endmill. As expected, our study concludes that the magnitude, orientation, and form of radial throw all significantly affect the studied quality metrics, tooth spacing angle, and the flute-to-flute chip thickness variations. Specifically, the presence of radial throw with an elliptical form induces up to 50% variation in SLE, up to 20% variation in sidewall surface roughness, up to 60% variation in tooth spacing angle deviations, and up to 50% variation in flute-to-flute chip thickness. As such, the presented simulation approach can be used to assess the direct (kinematic) effects of the radial throw parameters on the quality metrics and chip thickness variations.

Notch wear prediction model in high speed milling of AerMet100 steel with bull-nose tool considering the influence of stress concentration

Notch wear is one of the predominant types of tool failure in high speed milling of difficult-to-cut materials. This paper aims to reveal the formation and impact mechanisms of notch wear in bull-nose milling. Tool wear on rake face during dry milling of AerMet100 steel is investigated, severe notch wear is observed in lots of experiments. Based on the comparison of uncut chip thickness variations under two different cutting types, an attempt is made to explain the occurrence of notch wear from the view point of stress concentration. In order to quantitatively characterize the degree of stress concentration at any position on tool edge, a stress concentration factor is defined in this paper. The non-uniform and instantaneous variable characteristics of stress distribution along the cutting edge during bull-nose milling are taken into consideration. Then a predictive model of notch wear depth considering the influence of stress concentration is developed. Series of cutting tests are performed to validate the notch wear depth model, and the results indicate that the proposed model is feasible. In addition, the effects of different cutting parameters on notch wear are discussed. Further, this work can be applied to optimize process parameters for controlling notch wear, improving machining efficiency and reducing production cost.

Radial throw in micromachining: Measurement and analysis

This paper presents a comprehensive approach for measurement and analysis of radial throw and the associated tool-tip trajectory in micromachining when using ultra-high-speed (UHS) spindles. The effect of radial throw on micromachining accuracy could be significant due to the strict absolute-tolerance requirements. However, accurately determining radial throw in micromachining poses challenges due to the micron-scale tool dimensions and high rotational speeds. In contrast to run-out, radial throw depends on the tool-rotation angle and dictates the instantaneous position (trajectory) of the cutting edges. This work first presents a mathematical framework to obtain the radial throw at the cutting edges by measuring radial throw at two locations on the tool shank. A laser Doppler vibrometry-based experimental approach is then described to accurately measure the radial throw from the tool shank in two mutually-perpendicular directions. Next, the variations on radial throw measurements are evaluated, the effect of spindle speed on the radial throw is analyzed, and the predictions of tool-tip radial throw are compared with those directly measured from the tip of a micro-tool blank. A study is then performed to assess the effect of tool attachment/detachment cycles on radial throw. Lastly, a set of simulations is conducted to demonstrate how the radial throw magnitude and orientation affect the surface location error, sidewall surface roughness, and flute-to-flute chip-thickness variations. It is concluded that the presented mathematical framework and measurement approach can be used to determine the tool-tip radial throw magnitude with better than 3.5% ± 2% accuracy. Increased spindle speed considerably increases the radial throw, e.g., from 8 μm at 60,000 rpm to 16 μm at 130,000 rpm. The tool attachment/detachment induces less than ±6.9% variation to the mean radial throw magnitude at 60,000 rpm. The main contributor to the variation of the radial throw orientation is identified as the inherent inaccuracies of the spindle.

Material removal and chip formation mechanisms of UHC-steel during grinding

The grinding process is still an important manufacturing process for the machining of automotive components. For power train components, ultra-high carbon steel (UHC-steel) is a promising new innovative alloy because of its low specific density. Results from turning of UHC-steel showed that the texture of UHC-steel significantly differs from conventional steels. Furthermore, extremely hard carbides, which are embedded into a soft ferrite matrix, result in a UHC-steel specific machining behavior and a high tool wear rate. Therefore, UHC-steel is marked as a difficult-to-cut material. So far, there are no research results available for the grinding of UHC-steel. Therefore, fundamental investigations were conducted in order to analyze the material removal and chip formation mechanisms. Scratching tests with a geometrically defined cubic boron nitride cutting edge showed ductile material removal mechanisms for a single grain chip thickness variation from hcu = 1.5 up to 14 μm. Analysis of the contact zone by means of an innovative quick stop device confirms these results.

An Analytical Approach for Machining Thin-walled Workpieces

An analytical approach is developed to predict the dynamic chip thickness variation in ending milling of thin-walled workpieces. The proposed model considers the mechanism of calculating the stiffness of removed material by the end- mill from the overall stiffness of the workpiece to work out the changing stiffness of the workpiece which in turn is used to determine the displacement occurring. The displacement of the workpiece influences the radial depth of cut and thus yields a dynamic variation. The generalized model of the volume of removed material is computed by considering the discrete axial depth of cut, the radial depth of cut and the circumferential section of the tool radius contact for each tool step taken. The features of this model cater for the helical tool geometry used. The stiffness of removed material is updated by subtraction from the overall stiffness of workpiece. The overall stiffness of the wall is updated by considering the removed material at each time step. The feedback of the displacement amplitude is linked with a cutting forces model to address the influence of the dynamic displacement of the workpiece from the acting chip load. The cutting forces is modeled based on the Oxley variable flow stress machining theory. The predictive capabilities of the proposed model are verified with the experimental results. The comparison of the results is encouraging and reasonable correlation is achieved.

Machining slight burr formed micro-channels with different moving trajectories of a pyramidal diamond tip

In this study, a tip-based channel fabricating method on aluminum alloy material is studied using different tip moving trajectories. A three-sided pyramidal tip is driven by a three-axis piezoelectric actuator to do the movements of the circle, the sharp ellipse (major axis along the feeding direction), and the flat ellipse (minor axis along the feeding direction) revolutions are the three kinds of moving trajectories used for machining. In each revolution, due to the shapes of tip and moving trajectory, there would be two cutting edges participating in machining process. Under these three trajectories, the variations of uncut chip thickness and cutting rake angle with tip revolving and the cutting length of every edge in each tip revolution are different, which results in different sizes of chips and burrs formed. Effects of these three moving trajectories on the material removal are studied in detail. Through processing experiments, it was found that without any deburring method, the channel machined using the sharp ellipse trajectory has two edges with no burr formed and could obtain better bottom surface quality with a smaller feed rate, while channels machined using the other two both have burrs at the right side, which results from the accumulation of chips. Finally, to eliminate or reduce this chip accumulation further, a three-dimensional moving trajectory is designed as a deburring method and used to process channels. It was found that this deburring method plays a significant role on the circle trajectory, but it is ineffective on the flat ellipse trajectory.

Continuous and ultra-fine grained chip production with large strain machining

In this study, orthogonal cutting technique as a severe plastic deformation (SPD) method for producing chips with an ultra-fine grained microstructure and fair mechanical properties is investigated; further, it has been suggested that by controlling the cutting velocity and contact length between tool and material, it is possible to produce a severely deformed and continuous chip with unrestored microstructure even at high cutting velocities. Solution treated Al-6061 samples in plane strain condition were severely deformed through applying various cutting velocities (from 50 to 2230mm/s) for three different rake angles (−5°, −10° and −20°) in fixed cutting parameters. Chip thickness, contact length, shear strain, and Vickers microhardness variations were examined for different samples and chip formation mechanism was discussed for different processing conditions. In addition, the microstructure of especial produced chips was studied using transmission electron microscopy (TEM). The results showed that during the dominance of seizure mechanism at the contact surface, microhardness and shear strain (as well as contact length) have inverse dependency upon the variation of the cutting velocity. The results are discussed by considering the heat-time effect contribution in the final microstructure and mechanical properties.