Providing stability maps for milling operations

This article presents a new stability prediction tool. The method is based on the dynamic behaviour of both milling tool and workpiece, computed using finite element method. Dynamic behaviour is expressed under the form of transfer functions and used to predict stability lobes at each tool position. The unconditionally stable depth of cut is then stored and displayed on a graphic representation of the machined surface under the form of colour axis, named stability map.An application of the method on a Renault cylinder block is presented as an illustration.     

The effect of runout on the milling tool vibration and surface quality

When milling with tools of a high length to diameter ratio, there is often a non negligible runout. Since those tools tend towards chatter because of their low stiffness, the effect of runout on the dynamic behavior of the tool must be considered. Runout adds an additional dynamic component to the tool vibration and thus to the dynamicly changing cutting forces. Furthermore runout affects the surface quality even in stable machining. This paper analyzes the effect of runout by simulation of the dynamic milling process and compares the results to experimental data. One aspect is the difference of the vibration patterns with and without runout. Furthermore, a method for the analysis of timeseries is presented in order to distinguish between chatter and runout. Another topic is the expected surface quality resulting from stable processes with runout. This surface is modeled, examined and compared to the one produced by a process without runout.     

Expert spindle design system

This paper presents an expert spindle design system strategy which is based on the efficient utilization of past design experience, the laws of machine design, dynamics and metal cutting mechanics. The configuration of the spindle is decided from the specifications of the workpiece material, desired cutting conditions, and most common tools used on the machine tool. The spindle drive mechanism, drive motor, bearing types, and spindle shaft dimensions are selected based on the target applications. The paper provides a set of fuzzy design rules, which lead to an interactive and automatic design of spindle drive configurations. The structural dynamics of the spindle are automatically optimized by distributing the bearings along the spindle shaft. The proposed strategy is to iteratively predict the Frequency Response Function (FRF) of the spindle at the tool tip using the Finite Element Method (FEM) based on the Timoshenko beam theory. The predicted FRF of the spindle is integrated to the chatter vibration stability law, which indicates whether the design would lead to chatter vibration free cutting operation at the desired speed and depth of cut for different flutes of cutters. The arrangement of bearings is optimized using the Sequential Quadratic Programming (SQP) method.     

Prediction of chatter in high speed milling including gyroscopic effects

Dynamic stability of machine tools during operations is dependent on many parameters including the spindle speed. In high and ultra high speed machining, the gyroscopic effect on the spindle dynamics becomes more pronounced and can affect the borders of stability of the rotating system. In this paper, a finite element based model of spindle, tool holder and cutting tool is presented which uses Timoshenko beam theory to obtain the frequency response of the system when gyroscopic terms are included. Using this response, the stability of a high speed spindle system in the presence of gyroscopic effect is investigated. It is shown that the gyroscopic effects lower the critical depth of cut in high speed milling.     

Modeling and simulation of 5-axis milling processes

5-axis milling is widely used in machining of complex surfaces. Part quality and productivity are extremely important due to the high cost of machine tools and parts involved. Process models can be used for the selection of proper process parameters. Although extensive research has been conducted on milling process modeling, very few are on 5-axis milling. This paper presents models for 5-axis milling process geometry, cutting force and stability. The application of the models in selection of important parameters is also demonstrated. A practical method, developed for the extraction of cutting geometry, is used in simulation of a complete 5-axis cycle.     

Modelling and simulation of chatter in milling using a predictive force model

A predictive time domain chatter model is presented for the simulation and analysis of chatter in milling processes. The model is developed using a predictive milling force model, which represents the action of milling cutter by the simultaneous operations of a number of single-point cutting tools and predicts the milling forces from the fundamental workpiece material properties, tool geometry and cutting conditions. The instantaneous undeformed chip thickness is modelled to include the dynamic modulations caused by the tool vibrations so that the dynamic regeneration effect is taken into account. Runge–Kutta method is employed to solve the differential equations governing the dynamics of the milling system for accurate solutions. A Windows-based simulation system for chatter in milling is developed using the predictive model, which predicts chatter vibrations represented by the tool-work displacements and cutting force variations against cutter revolution in both numerical and graphic formats, from input of tool and workpiece material properties, cutter parameters, machine tool characteristics and cutting conditions. The system is verified with experimental results and good agreement is shown.     

Theoretical-experimental modeling of milling machines for the prediction of chatter vibration

Productivity of high speed milling operations can be seriously limited by chatter occurrence. Chatter vibrations can imprint a poor surface finish on the workpiece and can damage the cutting tool and the machine. Chatter occurrence is strongly affected by the dynamic response of the whole system, i.e. the milling machine, the tool holder, the tool, the workpiece and the workpiece clamping fixture. Tool changes must be taken into account in order to properly predict chatter occurrence. In this study, a model of the milling machine-tool is proposed: the machine frame and the spindle were modeled by an experimentally evaluated modal model, while the tool was modeled by a discrete modal approach, based on the continuous beam shape analytical eigenfunctions. A chatter identification technique, based on this analytical-experimental model, was implemented. Tool changes can be easily taken into account without requiring any experimental tests. A 4 axis numerically controlled (NC) milling machine was instrumented in order to identify and validate the proposed model. The milling machine model was excited by regenerative, time-varying cutting forces, leading to a set of Delay Differential Equations (DDEs) with periodic coefficients. The stability lobe charts were evaluated using the semi-discretization method that was extended to n>2 degrees of freedom (dof) models. The stability predictions obtained by the analytical model are compared to the results of several cutting tests accomplished on the instrumented NC milling machine.     

Uncharted islands of chatter instability in milling

This paper provides conclusive evidence that isolated islands of chatter vibration can exist in milling processes. Investigations show these islands are induced by the tool helix angle and act to separate regions of period-doubling and quasi-periodic behavior. Modeling efforts develop an analytical force model with three piecewise continuous regions of cutting that describe helix angle tools. Theoretical results examine the asymptotic stability trends for several different radial immersions and helix angles. In addition, new results are shown through the implementation of a temporal finite element analysis approach for delay equations written in the form of a state space model. Predictions are validated by a series of experimental tests that confirm the isolated island phenomenon.     

Programming spindle speed variation for machine tool chatter suppression

This paper presents a novel method for programming spindle speed variation for machine tool chatter suppression. This method is based on varying the spindle speed for minimum energy input by the cutting process. The work done by the cutting force during sinusoidal spindle speed variation S³V is solved numerically over a wide range of spindle speeds to study the effect of S³V on stable and unstable systems and to generate charts by which the optimum S³V amplitude ratio can be selected. For on-line application, a simple criterion for computing the optimal S³V amplitude ratio is presented. Also, a heuristic criterion for selecting the frequency of the forcing speed signal is developed so that the resulting signal ensures fast stabilization of the machining process. The proposed criteria are suitable for on-line chatter suppression, since they only require knowledge of the chatter frequency and spindle speed. The effectiveness of the developed S³V programming method is verified experimentally.     

A Modeling Approach for Analysis and Improvement of Spindle-Holder-Tool Assembly Dynamics

The most important information required for chatter stability analysis is the dynamics of the involved structures, i.e. the frequency response functions (FRFs) which are usually determined experimentally. In this study, the tool point FRF of a spindle-holder-tool assembly is analytically determined by using the receptance coupling and structural modification techniques. Timoshenko's beam model is used for increased accuracy. The spindle is also modeled analytically with elastic supports representing the bearings. The mathematical model is used to determine the effects of different parameters on the tool point FRF and to identify contact dynamics from experimental measurements. The applications of the model are demonstrated and the predictions are verified experimentally.     

An expert troubleshooting system for the milling process

Common problems experienced in milling processes include forced and chatter vibrations, tolerance violations, chipping and premature wear of the tools. This paper presents an expert system which attempts to troubleshoot the source of milling problems by utilising dynamics data coupled with the opinion of the operator and acoustic Fourier spectrum data taken from the cutting process. The expert system utilises a fuzzy logic based process to interpret the signals and information, and recommends possible alterations to the process to achieve high-performance milling operations.Specific inference engines were developed to assess the chatter stability, variation in cutting force coefficient, tool run-out and forced vibration characteristics of the system. Lastly, a stability lobe plot interpretation engine to automate the lobe selection process and recommend new, chatter free cutting conditions, was also developed. The chatter stability inference engine was tested with real cutting data, through acoustic measurements taken from various cutting conditions on an aluminium milling process. The chatter inference engine successfully determined the stability of the system for each sampled cutting condition. The robustness of the troubleshooting system depends on the accuracy of acoustic and frequency response measurements.     

Finite element method based machining simulation environment for analyzing part errors induced during milling of thin-walled components

The rigid body motion of the workpieces and their elastic–plastic deformations induced during high speed milling of thin-walled parts are the main root causes of part geometrical and dimensional variabilities; these are governed mainly from the choice of process plan parameters such as fixture layout design, operation sequence, selected tool path strategies and the values of cutting variables. Therefore, it becomes necessary to judge the validity of a given process plan before going into actual machining. This paper presents an overview of a comprehensive finite element method (FEM) based milling process plan verification model and associated tools, which by considering the effects of fixturing, operation sequence, tool path and cutting parameters simulates the milling process in a transient 3D virtual environment and predicts the part thin wall deflections and elastic–plastic deformations during machining. The advantages of the proposed model over previous works are: (i) Performs a computationally efficient transient thermo-mechanical coupled field milling simulation of complex prismatic parts comprising any combination of machining features like steps, slots, pockets, nested features, etc., using a feature based milling simulation approach; (ii) Predicts the workpiece non-linear behavior during machining due to its changing geometry, inelastic material properties and fixture–workpiece flexible contacts; (iii) Allows the modelling of the effects of initial residual stresses (residing inside the raw stock) on part deformations; (iv) Incorporates an integrated analytical machining load (cutting force components and average shear plane temperature) model; and (v) Provides a seamless interface to import an automatic programming tool file (APT file) generated by CAM packages like CATIA V5. The prediction accuracy of the model was validated experimentally and the obtained numerical and experimental results were found in good agreement.     

Stability of milling processes with continuous spindle speed variation: Analysis in the frequency and time domains, and experimental correlation

Up to now, the theory for analysis of continuous spindle speed variation in milling processes was developed for sinusoidal variation only, and for average tooth passing frequency an exact multiple of speed variation frequency. This paper presents the general theory for analysis in the frequency domain and for any speed variation strategy. Results are compared with those obtained by semidiscretization and time integration, as well as with those obtained by experiments. The discrepancies of the results obtained by the different approaches are discussed, and the analysis of the evolution of the stability along the speed variation period is proposed.     

A virtual machine concept for real-time simulation of machine tool dynamics

When designing CNC machine tools it is important to consider the dynamics of the control, the electrical components and the mechanical structure of the machine simultaneously. This paper describes the structure and implementation of a concept for real-time simulation of such machine tools using a water jet cutting machine as an application. The concept includes a real control system, simulation models of the dynamics of the machine and a virtual reality model for visualisation. The real-time capability of the concept, including the simulation of electrical and rather detailed mechanical component models is proofed. The validation process indicates good agreement between simulation and measurement, but suggests further studies on servo motor, connection and flexibility modelling. However, already from the initial simulation results presented in this paper it can be concluded that the influence of structural flexibility on manufacturing accuracy is of importance at desired feeding rates and accelerations. The fully automated implementation developed in this work is a promising base for dealing with this trade-off between productivity and accuracy of the manufacturing process through multidisciplinary optimisation.     

On obtaining machine tool stiffness by CAE techniques

In this paper, a single module method and a newly developed hybrid modeling method for analyzing the stiffness of machine tools are introduced in detail. Techniques include building suitable finite element models, determining equivalent loads, simulating the interface between two modules, considering boundary constraints, and interpreting results. By taking a detailed finite element mesh for one of the five modules (the headstock, the column, the table, the saddle and the bed), together with simplified meshes for the other four modules, a hybrid finite element model is assembled. The elastic modulli of the four simplified meshes are kept several orders higher than that of the detailed one. Therefore, the calculated stiffness of the hybrid model is essentially the stiffness of the softer module with the detailed mesh. The stiffness of the five modules can be obtained one after another in the same manner. By supporting the hybrid model only at the middle of the short edge on the bottom surface of the bed, the machine tool can be properly constrained, and its stiffness can be estimated correctly. The controversial issue as to how to simulate properly the boundary condition of the casters under the bed will not occur in this method. A cumbersome procedure to transform the external loads into the equivalent forces as required in SMM is also avoided. There is no local effect due to unevenly distributed nodal forces. It is shown that the hybrid modeling method is better than the single module method in accuracy and efficiency.     

A rotary flexural bearing for micromanufacturing

This study proposes a design methodology for a novel rotary flexural bearing that is based on the motion principles of elastic flexures. The bearing is capable of providing rotational oscillations of one complete revolution and is characterized by potentially high repeatability, smooth motions, no mechanical wear and no lubrication requirements, no gaps or interfaces, zero maintenance, in addition to its compactness. From the structural characteristics and the basic working principles of the flexural bearings, the study provides a design analysis on the various aspects of the bearing, including material selection, stress analysis and calculations (such as nonlinear finite element analysis, static and fatigue strength designs), motion error analysis and error reduction strategy, parametric design, etc.     

Shrink fit tool holder connection stiffness/damping modeling for frequency response prediction in milling

In this paper we present a finite element modeling approach to determine the stiffness and damping behavior between the tool and holder in thermal shrink fit connections. The continuous contact stiffness/damping profile between the holder and portion of the tool inside the holder is approximated by defining coordinates along the interface contact length and assigning position-dependent stiffness and equivalent viscous damping values between the tool and holder. These values are incorporated into the third generation receptance coupling substructure analysis (RCSA) method, which is used to predict the tool point frequency response for milling applications. Once the holder and inserted tool section are connected using the finite element analysis-based stiffness and damping values, this subassembly is then rigidly coupled to the (measured) spindle–holder base and (modeled) tool. Experimental validation is provided.     

Mechanics and dynamics of thread milling process

This paper presents the mechanics and dynamics of thread milling operations. The tool follows a helical path around the wall of the pre-machined hole in thread milling, which has varying tool-part engagement and cut area during one threading cycle. The variation of cut area that reflects the kinematics of threading as well as structural vibrations is modeled along the helical, threading path. The mechanics of the process are first experimentally proven, followed by the formulation of dynamic thread milling which is periodic in threading cycle, in a semi-discrete time domain. The stability of the operation is predicted as a function of spindle speed, axial depth of cut, cutter path and tool geometry. The mechanics and stability models are experimentally proven in opening M16×2 threads with a five-fluted helical tool on a Steel AISI1045 workpiece.     

Measurement and finite element simulation of micro-cutting temperatures of tool tip and workpiece

Temperature generates in micro-scale cutting process has a great effect on cutting performance due to centralized heat generation. In this study, a set of micro-cutting experiments (8 μm/r≤f≤50 μm/r) were carried out to measure temperatures in micro-cutting process with high accuracy. A fast-response thermocouple with a property of self-renewing was installed in a cylinder workpiece to measure the temperatures of workpiece and tool tip simultaneously. In each test, temperature of the workpiece surface is obtained just before the hot junction of thermocouple is machined. When the hot junction is machined, the tested maximum temperature is recognized as the temperature of tool tip. In parallel, an energy density-based ductile failure material model is developed to simulate the micro-cutting process by finite element method. In simulation, when mesh distribution is changed, the predicted forces with same energy density G_ε are closer to the forces at original mesh distribution than those predicted by same energy G_f. Consequently, the energy density-based ductile failure material model can reduce mesh dependence in different mesh distribution conditions. Under new mesh distribution, Temperatures of the workpiece surface and tool tip are identified in the predicted micro-cutting temperature field. The predicted micro-cutting temperatures of workpiece surface and tool tip are very close to the experimental results. Further, the variation of temperature and its relationship with chip curling are also discussed.     

A critical analysis of effectiveness of acoustic emission signals to detect tool and workpiece malfunctions in milling operations

The industrial demands for automated machining systems to increase process productivity and quality in milling of aerospace critical safety components requires advanced investigations of the monitoring techniques. This is focussed on the detection and prediction of the occurrence of process malfunctions at both of tool (e.g. wear/chipping of cutting edges) and workpiece surface integrity (e.g. material drags, laps, pluckings) levels. Acoustic emission (AE) has been employed predominantly for tool condition monitoring of continuous machining operations (e.g. turning, drilling), but relatively little attention has been paid to monitor interrupted processes such as milling and especially to detect the occurrence of possible surface anomalies.This paper reports for the first time on the possibility of using AE sensory measures for monitoring both tool and workpiece surface integrity to enable milling of “damage-free” surfaces. The research focussed on identifying advanced monitoring techniques to enable the calculation of comprehensive AE sensory measures that can be applied independently and/or in conjunction with other sensory signals (e.g. force) to respond to the following technical requirements: (i) to identify time domain patterns that are independent from the tool path; (ii) ability to “calibrate” AE sensory measures against the gradual increase of tool wear/force signals; (iii) capability to detect workpiece surface defects (anomalies) as result of high energy transfer to the machined surfaces when abusive milling is applied.Although some drawbacks exist due to the amount of data manipulation, the results show good evidence that the proposed AE sensory measures have a great potential to be used in flexible and easily implementable solutions for monitoring tool and/or workpiece surface anomalies in milling operations.