Tool-workpiece Engagement 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.

Analytical method to extract tool workpiece engagement from CAM data for five-axis machining stability prediction

Ball end milling cutters are commonly used for precision machining of complex curved parts in five axis CNC systems. To solve the problem of difficulty in adjusting the spindle speed to achieve stable milling during the five axis milling process of ball end cutters, this paper presents an analytical method for extracting the tool-workpiece engagement (TWE) from computer-aided manufacturing (CAM) data. This method extracts the TWE between the end portion of the ball-end mill and the surface of the workpiece during the processing of free-form surfaces, including slotting, first cutting, and following cutting. Further, the TWE extracted by this method is applied for constructing the cutting force model. The entry and exit angles for each of the micro-element cutting edges under different tool postures are obtained through the conversion matrix, and an efficient full discretization method is used for predicting the stability of the five-axis machining in the slotting mode. Finally, the accuracy and efficiency of the analytical method are verified through simulation and experimentation. The experimental results show that the efficiency of the proposed method increases by about 69% when compared to the arc-surface intersection method and that the predicted value of the five-axis machining stability shows good agreement with the experimental results.

Investigation of the low-frequency chatter in robotic milling

In robotic milling with large allowance process, low-frequency chatter (LFC) is an important factor observed in high-speed and low-speed milling, affecting the processing efficiency and quality. Previous research has used the regenerative chatter theory, ignoring modulated tool-workpiece engagement conditions, or mode coupling theory under the assumption of threading operations to explain the LFC mechanism and predict the stability boundary. However, these models overlook or inaccurately characterize the modulation effect, leading to inaccurate modeling of dynamic chip thickness changes during milling, making it difficult to understand the mechanism of LFC. Here, we propose an LFC stability model that considers modulated tool-workpiece engagement conditions and the mode coupling effect of the robotic structure for robotic milling. This approach allows us to reveal the mechanism of LFC and identify the characteristic signal of low-frequency vibration, which is the sideband frequency signal. Initially, the evolution of LFC is analyzed, and its characteristics are summarized. Further, a surface renewal (SR) model is proposed to accurately calculate the dynamic cutting force caused by modulated tool-workpiece engagement conditions in LFC. Furthermore, the LFC stability model, considering the modulated tool-workpiece engagement conditions and mode coupling effect, is established based on impulse response function (IRF) method. Finally, we verify the accuracy of our model through milling experiments and compare it with that of the classical stability prediction model. Our results show that LFC is highly dependent on speed, and our stability model can effectively predict the stability boundary of LFC in robotic milling with large allowance process.

Towards understanding the wear mechanisms of turning with FreeTurn

For many years, tool wear has been a critical issue with respect to productivity, cost, and part quality, particularly for superalloy machining. The underlying wear mechanisms of a new turning tool, FreeTurn, is revealed in this paper towards its potential for a more sustainable way of metal cutting. It is a specially designed turning tool that is held by the tool spindle and has a working plane perpendicular to the centreline of the tool spindle, making it rotatable on its working plane. Owing to the unique feature of FreeTurn compared with conventional turning tools, the focus of this study is on the effect of dynamic tool–workpiece engagement on tool wear. The tool path strategy is first set to achieve a cylindrical surface in longitudinal turning with a rotatable FreeTurn tool. Series of experiments with different tool path strategies are performed on a turn–milling machine, using Inconel 718 as an example of workpiece material. The results indicate that the coupling effect between notch and flank wear is the decisive factor of tool wear in free turning with respect to its rotation and feeding directions. While the tool wear can be significantly reduced if the tool feeds with increasing cutting edge angle, the tool will soon be worn out if it feeds with decreasing cutting edge angle. Tool rotation in free turning plays a key role in reducing adhesion on the cutting tool and increasing the tool rotation speed can further improve this effect. Additionally, a more sustainable solution that uses both sides of the cutting edge becomes possible with FreeTurn as well, improving the tool utilisation. The results demonstrate considerable guiding significance for exploring the potential of this new turning tool towards a longer tool life.

Towards energy efficient milling of variable curved geometries

Development of energy efficient machining is the present focus to industries for reduction of energy consumption and making the manufacturing system more sustainable. The continuous fluctuation in force profile during machining of variable curved components creates a barrier in stable machining and in cutting power consumption. The fluctuation in force profile happens due to alteration in chip load in the presence of workpiece curvature. The current research work intends to develop an energy efficient machining strategy for milling of variable curved components where more uniform cutting force and power profiles are accomplished due to constant chip load along the peripheral length of curved geometry. The proposed strategy is compared with conventional cutting in order to confirm the acceptability of the present algorithm. The proposed algorithm involves mechanics of milling, cutting forces and cutting power consumption. It is formulated based on constant chip load by regulating entry angle of milling cutter according to workpiece curvature along the peripheral length. Thus, the cutting power fluctuation occurred due to variation of workpiece curvature is reduced by regulating tool-workpiece engagement. The energy consumption during machining is reduced almost 22 % using the proposed approach. It results into developing an energy efficient machining strategy for milling of variable curved geometry. It also provides stable machining process and increased tool life by reducing tool wear due to reduction of force fluctuation during metal removal process.

Direct calculation of Johnson-Cook constitutive material parameters for oblique cutting operations

Characteristics and behaviors of materials during manufacturing processes highly depend on their mechanical properties and microstructures. However, these characteristics and behaviors can be greatly influenced by other factors such as strain, strain rate, and temperature during the process. The Johnson-Cook (J-C) constitutive material model has been extensively used to describe the material behavior under such circumstances. The J-C parameters are usually determined using the Split Hopkinson Pressure Bar (SHPB) test which is time consuming and costly. Other methods that have been introduced to numerically determine the J-C parameters are inverse methods that minimize the difference between the experimentally measured and theoretically calculated data. These numerical methods still require a combination of experimentally measured cutting forces and chip thickness. These two items are easy to obtain for simple machining processes like turning in which chip thickness and thus cutting force have almost steady state constant values. Nevertheless, majority of these numerical methods are not applicable to more complex machining processes, e.g., milling, in which tool-workpiece engagement is complex and the chip thickness and consequently cutting forces are varying during the process. To address this limitation, this paper presents a model to identify the J-C strain rate hardening and temperature softening parameters for complex oblique machining processes. The proposed model searches for an optimum set of J-C parameters by matching the calculated cutting forces using Oxley model with those obtained from experimental measurements. In addition, simulation and experiments were performed to compare the results and to validate the proposed model. The results proved the validity of the proposed model in estimating the J-C parameters directly from milling tests as an oblique cutting operation.

Research on the influence of robot structural mode on regenerative chatter in milling and analysis of stability boundary improvement domain

Regenerative chatter is an important factor affecting the efficiency and quality of robot milling of large and complex curved parts, especially when the weak rigid posture is forced to be selected under geometric interference. And the regenerative chatter in robotic milling is not only related to the tool-spindle mode but also affected by the tool point low-frequency vibration caused by robot structure mode. At some specific spindle speed, the tool point low-frequency vibration interrupts the occurrence process of chatter, increases the critical cutting depth, and generates a stability boundary improvement domain (SBID). This phenomenon makes it possible to avoid the adverse effects of the weak rigidity of the robot structure, and ensure and even improve machining efficiency. In order to reveal the mechanism of low-frequency vibration improving the stability boundary, a tool-workpiece engagement state considering the tool point low-frequency vibration caused by robot structure mode is analyzed, and radial and tangential tool-workpiece separation models, time-varying process damping model, and axial cutting depth modulation model are established to describe the mechanism of tool-workpiece engagement state. And then the time delay coefficient dependent on the cutting state is proposed. Finally, the stability prediction model considering the tool point low-frequency vibration caused by robot structure mode is established and verified by experiments with different postures. Through simulation and experiment with different postures, a set of evaluation indexes is proposed to evaluate the affection of the low-frequency vibration, and it is found that different robotic postures will lead to different SBIDs, thus proving the effectiveness of posture optimization on stability improvement. The results show that the proposed robotic milling stability model can effectively characterize the effect of low-frequency vibration on the stability boundary, and provide a theoretical basis for accurate positioning of SBID and efficient robotic milling.

Machinability Analysis of Finish-Turning Operations for Ti6Al4V Tubes Fabricated by Selective Laser Melting

With the advent of additive manufacturing as an advanced technology for the fabrication of titanium components, there is a pressing need to investigate the machinability of parts produced using these techniques compared to components made with conventional wrought methodologies. The motivation for this study was to investigate the influences of machining parameters, especially cutting depth, on the machinability of selective laser melted (SLMed) Ti6Al4V tubes, by analyzing the cutting responses, including cutting forces, machined surface roughness and tool wear at varying cutting parameters. Generally, it can be inferred that specific cutting tools used to machine wrought titanium components can also be used for SLMed parts when carrying out finish-machining operations. Cutting forces in the machining of SLMed workpieces could be up to 70% higher than those in machining the wrought counterparts. In contrast, the tool-wear analysis correspondingly showed larger tool-workpiece engagement area on the tool rake face for tools used for machining wrought parts. Adhesion on the cutting edge in the form of built-up edge and attrition of the tool surface were found to be the two most dominant tool-wear mechanisms, and the oxidation condition of the tool surface in machining SLMed parts was more severe (about 8% and 21%). Vibration analysis was also carried out, but no significant difference between the SLMed and wrought workpieces was observed, and the quality of the machined surface was similar.

Effect of fiber bending induced matrix shear behavior on machined surface quality in carbon fiber reinforced plastic milling

Fiber bending by the cutting tool is one of the dominant causes of fiber pull-out on machined surfaces during CFRP milling. During the bending, the polymer matrix between the fibers is subjected to shear stress resulting in fiber-matrix debonding. The increase in the debonding length causes an increase in the fiber pull-out length. In milling, not all the new surfaces created by tool-workpiece engagement become the machined surface. However, during this process, the strain energy absorbed by the deformed matrix is released into fiber-matrix debonding along the fibers which can cause fiber pull-out on the machined surface. In this study, shear strain energy of polymer matrix due to fiber bending during milling tool-workpiece engagement was calculated with the geometrical relationship between fibers and a cutting edge. We found that the increase in Ra and Rz of the machined surface had clear tendencies to the increase in the calculated energy.

Evolution of hole surface integrity in successive processing using sequential milling and laser shock peening

Part's surfaces are usually produced by successive/integration machining process and the performance of the part is directly tied to the generated surface integrity. Holes are commonly used in many critical engineering structures to connect parts, whose surface primarily characterizes a high risk of fretting or contact fatigue failure. In this paper, the evolution of metallurgical and mechanical surface integrity of Ti-6Al-4V alloy hole surface which machined using multi-step milling and successive laser shock peening (LSP) treatment was investigated. Firstly, a special workpiece with various hole surface pieces was designed and four different sequential milling processes were set to conduct the milling experiments. A comprehensive analysis of the evolution of surface roughness, microstructure, microhardness, and residual stress along the sequential processing steps was carried out. The results showed that the forms of surface integrity evolution could be classified into mono-evolution and coupling evolution, which were correlated to the combined effect of the milling parameters, cutter-workpiece engagement nature, milling sequence, milling mode, and the coolant/lubricant applied strategy. Mono-evolution was predominated in material removal sequences (milling sequences), while the coupling evolution was dependent on the radial depth of cut (DoC, ~50 μm). By comparison, coupling evolution was prominent in non-material removal related sequences (LSP treatment). Due to the conformal contact nature of tool-workpiece engagement and the flood cooling strategy, the microhardness in the subsurface layer was not significantly altered in milling sequences, except that the up-milling normally produced a surface with lower hardness and the down-milling produced almost unchanged surface hardness compared with bulk material. The residual stress was a result of the coupling effect of the residual stress field introduced in the previous and current machining sequences, whose in-depth distribution could be represented as an exponential decay function. The degree of influence of the machining sequences and their interactions on the final residual stresses could be evaluated by utilizing a set of weighting coefficients. This research indicated that understanding the effect of successive processing on the surface integrity evolution could help in planning of appropriate processes to achieve desirable surface integrity.

Anticipatory Online Compensation of Tool Deflection Using a Priori Information from Process Planning

Removing excess material from build-up welding by milling is a critical step in the repair of blades from aircraft engines. This so-called recontouring is a very challenging machining task. Shape deviations often result from the deflection of tool and workpiece due to process forces. Considering the individuality of repair cases, compensation of those deflections by process force measurement and online tool path adaption is a very suitable method. However, there is one caveat to this reactive approach. Due to causality, a corrective movement, following a force variation, is always delayed by a finite reaction time. At this moment, though, the displacement has already manifested itself as a deviation in the machined surface. To overcome those limitations and to improve compensation beyond the reduction of control delays, this study proposes a novel approach of anticipatory online compensation. Flank-milling experiments with abrupt changes in the tool-workpiece engagement conditions are conducted to investigate the limitations of reactive compensation and to explore the potential of the new anticipatory approach.

Kinematics-Based Five-Axis Trochoidal Milling Process Planning for Deep and Curved Three-Dimensional Slots

Abstract Trochoidal (TR) milling is a popular means for slotting operation. Attributing to its unique circular-shaped path pattern, TR milling avoids the full tool–workpiece engagement, which helps reduce the cutting heat accumulation and hence slow down the tool wear. While traditionally TR milling is only used for machining 2.5D cavities, it has now been extended to machining genuine 3D curved cavities under the realm of five-axis machining. However, since for a typical five-axis machine tool the rotary axes have a much larger moment of inertia than the three linear axes, to reduce both the total machining time and the consumed electric energy (for driving the machine tool), it is desirable to minimize the use of the two rotary axes (particularly the one with the larger moment of inertia) when planning a TR tool path for a given 3D cavity. Nevertheless, due to the newness of five-axis TR machining, there has no published reports on this subject. In this paper, we present a five-axis TR tool path planning algorithm for machining an arbitrary 3D curved cavity, which will consider the kinematical characteristics of the five-axis machine tool and try to minimize the use of the rotary axis with the largest moment of inertia, while tending to all the required constraints such as the threshold on the cutting force. Both computer simulation and physical cutting experiments of the proposed method have been conducted, and the results give a preliminary confirmation on the feasibility and advantages of the proposed method.

A variable-depth multi-layer five-axis trochoidal milling method for machining deep freeform 3D slots

Trochoidal milling is a popular machining method for slotting operation, as it avoids a full tool-workpiece engagement and hence helps, often significantly, slow down the tool wear, which is particularly a concern in the machining of hard-to-cut materials like titanium alloy. However, as the traditional trochoidal milling method assumes a fixed tool orientation, it can only apply to 2D shaped slots, while for many industrial parts the slot to cut is typically a genuine 3D freeform slot such as grooves on a blisk. In this paper, we present a multi-layer five-axis trochoidal milling method for machining an arbitrary 3D deep slot that is defined by two freeform side boundary surfaces. Aiming at minimizing the total cutting time, instead of conservatively dividing the slot into equi-depth layers, we strive to minimize the number of layers with variable layer depths while at the same time satisfying the two most critical physical constraints on the tool – the tool deformation and the tool stress. Both computer simulation and physical cutting experiments of the proposed method have been carried out, and the experimental results confirm the feasibility and advantages of the proposed method.

General Cutting Dynamics Model for Five-Axis Ball-End Milling Operations

AbstractFive-axis ball-end milling is used extensively to machine parts with sculptured surfaces. This paper presents the general cutting dynamics model of the ball-end milling process for machine tools with different five-axis configurations. The structural dynamics of both the tool and workpiece are considered for the prediction of chatter stability at each tool location along the tool path. The effects of tool–workpiece engagement and tool axis orientation are included in the model. By sweeping the spindle speeds, the chatter-free spindle speeds are selected followed by the prediction of forced vibrations in five-axis milling of thin-walled, flexible parts. The proposed model has been experimentally illustrated to predict the chatter stability and forced vibrations on a table-tilting five-axis computer numerical control machine tool.

Optimization of Cutting Process Parameters in Inclined Drilling of Inconel 718 Using Finite Element Method and Taguchi Analysis.

Nickel-based superalloys are famous in the demanding applications. Inconel 718 is one of the most commonly used nickel-based superalloys due to its extraordinary inherent properties. Inconel 718 is a suitable material for high temperature applications due to the properties such as anti-oxidization, high hot hardness, high creep, and fatigue strength. Drilling operation is one of the most widely used manufacturing operations in almost all industrial sectors. However, drilling operation is very complex in nature due to the presence of intricate geometry of the drill bit. In conventional drilling, cutting is performed by the combined action of the chisel edge and the two or more cutting lips. In depth analysis of the cutting process shows that chisel edge starts with an indentation at the center of the twist drill. Then away from the center, chisel edge performs orthogonal cutting with negative rake angle. Whereas, cutting action at the cutting lip is oblique in nature, and force analysis involves the use of element formulation due to involvement of radius. It is rarely found in the literature where drilling operation at different inclination angles is conducted and analyzed. The presented study numerically investigates the cutting performance of drilling operation, when operated at different inclination angles. The study revealed cutting force variation at different inclination angles due to the different tool workpiece engagement for each inclination. The magnitude of thrust force increased when inclination angle is changed from 30° to 60°. It can be linked with the higher chip load initially in this case as compared to the 30° inclination angle. The cutting temperature was affected by spindle speed (53.7%), followed by feed rate (33.31%) and inclination angle (3.44%).

Numerical investigation of crack initiation, propagation and suppression in robot-assisted abrasive belt grinding of zirconia ceramics via an improved chip-thickness model

Motivated by the prevailing assisted techniques in wheel grinding of brittle and hard materials, an attempt is made in this paper to identify the feasibility of robot-assisted abrasive belt grinding of zirconia ceramics. Owing to the flexible machining system, the challenge of this attempt resides in achieving the required profile accuracy and surface quality, in which the evolution of grinding-induced micro-cracks is prioritized. The single-grit scratching simulation based on an improved chip-thickness model that incorporates elastic modules of tool-workpiece engagement is employed to explore the damage mechanism in terms of the initiation, propagation and suppression of micro-cracks. The simulation results demonstrate that the critical depth of cut for brittle-to-ductile transition of zirconia ceramics is determined as 0.42 μm according to the tentative maximum undeformed chip thickness (UCT) values. In ductile-regime grinding, the zirconia surface morphologies are independent of the abrasive particle velocity. Lateral cracks begin to initiate especially when the maximum UCT exceeds 0.42 μm, and the brittle removal becomes dominant. In brittle-regime grinding, high abrasive particle velocity could help substantially enhance the workpiece surface integrity by suppressing the median/radial cracks that initiate once the maximum UCT approaches 0.8 μm. Experiment concerning the force-controlled robotic belt grinding of zirconia ceramics is conducted to verify the simulation results via the microscope observation of ground surface morphologies. The findings are likely to provide experimental evidence on the feasibility of belt grinding of brittle and hard materials with a flexible industrial robot.

A prediction model for the milling of thin-wall parts considering thermal-mechanical coupling and tool wear

In thin-wall milling processes, the interactions between cutting loads and the displacement of the thin-wall part lead to varying tool-workpiece engagement boundaries and undesired surface form errors. This unavoidable issue becomes more severe in the machining of titanium alloys due to their poor machinability caused by the low thermal conductivity, high strength and high chemical reactivity. This paper presents a new predictive model to calculate the cutting-induced thermal-mechanical loads and workpiece deflection in milling Ti-6Al-4V thin-wall components. The cutting heat sources and the development of tool flank wear were considered in the modelling process to improve the prediction accuracy. The cutting loads were modelled analytically and calculated using an efficient iterative algorithm, and the deformation of the thin-wall part was simulated through a finite element model. A series of cutting experiments were conducted under various cutting conditions to validate the predicted results. Both the cutting forces and thin-wall displacement were recorded to examine prediction accuracy, and good agreements have been achieved between the measured results and simulated outcomes. The predicted cutting forces in the radial, feed and axial directions are within errors of 14%, 10% and 5%, respectively, concerning the experimental values. Meanwhile, the maximum predicted deformation errors at the initial, middle and end portions of the workpiece are less than 20%.

Analytical modeling of material removal mechanism in dry whirling milling process considering geometry, kinematics and mechanics

Whirling milling is a promising machining process that couples the tools and workpiece motions while both conventional turning and milling are carried out, which is widely used for machining precision screw parts made of hard materials, such as titanium alloy, quenched steel, etc. The challenge is due to the complex kinematics can lead to the varying tool-workpiece engagement and undeformed chip geometries which affect significantly the mechanics, quality and productivity. The existing studies are mainly based on the simplification of process mechanism to study the mechanics and quality however, the material removal mechanism and influences of cutting parameters are still not well understood. This paper presents an analytical approach to investigate the material removal mechanism in whirling milling, thus to predict the undeformed chip geometry, material removal rate (MRR), cutting forces and form errors. The varying tool-workpiece engagement geometry along the cutting trajectory is identified to model the varying undeformed chip geometry including instantaneous chip thickness, cross-section area and tool-workpiece contact length. The form errors including circularity error, scallop height and surface roughness are defined and predicted as a function of tools and workpiece motion, position and dimension parameters. The whirling milling experiments were conducted to validate the analytical modeling approach with the largest error 11.3% and average error 13.0% for force and surface roughness prediction, respectively. The influences of cutting parameters on surface roughness and MRR are finally analyzed to explore the potential of productive cutting conditions for whirling milling.

Five-axis Trochoidal Flank Milling of Deep 3D Cavities

Trochoidal milling is widely used in slotting and pocketing operation owing to its unique cyclic pattern that restricts the tool-workpiece engagement and hence reduces the cutting force load and helps heat dissipation. Especially when cutting extremely hard materials such as super titanium alloy, trochoidal tool path has been a good milling strategy for reducing tool wear and restraining heat generation. However, the conventional trochoidal milling is two-dimensional in nature and thus can only apply to 2.5D machining. Aiming at further extending the application of trochoidal milling, in this paper we propose a novel five-axis trochoidal flank milling strategy applicable to machining more complex 3D shaped cavities. Rather than the traditional circular trochoidal pattern, our proposed method can adaptively generate a spatial cubic curve-based cyclic five-axis tool path according to the given complex 3D cavity, and, subject to the given tool-workpiece engagement threshold, the material removal rate is maximized in the process of tool path generation. In addition, we also present a scheme of adjusting the tool orientation at the boundary surfaces of the cavity to mitigate the overcut in case they are non-developable. Both computer simulation and physical cutting experiments are conducted and the preliminary results have given a definitive confirmation on the correctness and effectiveness of the proposed method.

High-Quality Machining of Edges of Thin-Walled Plates by Tilt Side Milling Based on an Analytical Force-Based Model

The milling of thin-walled workpieces is a common process in many industries. However, the machining defects are easy to occur due to the vibration and/or deformation induced by the poor stiffness of the thin structures, particularly when side milling the edges of plates. To this problem, an attempt by inclining the tool to a proper tilt angle in milling the edges of plates was proposed in this paper, in order to decrease the cutting force component along the direction of the lowest stiffness of the plates, and therefore to mitigate the machining vibration and improve the machined surface quality effectively. First, the milling force model in consideration of the undeformed chip thickness and the tool-workpiece engagement (TWE) was introduced in detail. Then, a new analytical assessment model based on the precisely established cutting force model was developed so as to obtain the optimum tool tilt angle for the minimum force-induced defects after the operation. Finally, the reliability and correctness of the theoretical force model and the proposed assessment model were validated by experiments. The methodology in this paper could provide practical guidance for achieving high-quality machined surface in the milling operation of thin-walled workpieces.