Runout effects in milling: Surface finish, surface location error, and stability

This paper investigates the effect of milling cutter teeth runout on surface topography, surface location error, and stability in end milling. Runout remains an important issue in machining because commercially-available cutter bodies often exhibit significant variation in the teeth/insert radial locations; therefore, the chip load on the individual cutting teeth varies periodically. This varying chip load influences the machining process and can lead to premature failure of the cutting edges. The effect of runout on cutting force and surface finish for proportional and non-proportional tooth spacing is isolated here by completing experiments on a precision milling machine with 0.1 μm positioning repeatability and 0.02 μm spindle error motion. Experimental tests are completed with different amounts of radial runout and the results are compared with a comprehensive time-domain simulation. After verification, the simulation is used to explore the relationships between runout, surface finish, stability, and surface location error. A new instability that occurs when harmonics of the runout frequency coincide with the dominant system natural frequency is identified.     

Enhancement of dynamic stability of cantilever tooling structures

The least rigid components of machining systems are cantilever tools and cantilever structural units of machine tools (rams, spindle sleeves, etc.). These components limit machining regimes due to the development of chatter vibrations, limit tool life due to extensive wear of cutting inserts, and limit geometric accuracy due to large deflections under cutting forces. Use of high Young's modulus materials (such as sintered carbides) to enhance the dynamic quality of cantilever components has only a limited effect and is very expensive. This paper describes a systems approach to the development of cantilever tooling structures (using the example of boring bars) which combine exceptionally high dynamic stability and performance characteristics with cost effectiveness. Resultant success was due to: (1) a thorough survey of the state of the art; (2) creating a “combination structure” concept with rigid (e.g. sintered carbide) root segments combined with light (e.g. aluminum) overhang segments, thus retaining high stiffness and at the same time achieving low effective mass (thus, high mass ratios for dynamic vibration absorbers, or DVAs) and high natural frequencies; (3) using the concept of “saturation of contact deformations” for efficient joining of constituent parts with minimum processing requirements; (4) suggesting optimized tuning of DVAs for machining process requirements; (5) development of DVAs with the possibility of broad-range tuning; (6) structural optimization of the system; and (7) using a novel concept of a “Torsional Compliant Head”, or TCH, which enhances dynamic stability at high cutting speeds and is suitable for high rev/min applications since it does not disturb balancing conditions. The optimal performance and interaction of these concepts were determined analytically, and then the analytical results were validated by extensive cutting tests with both stationary and rotating boring bars, machining steel and aluminum parts. Stable performance with length-to-diameter ratios up to L/D = 15 was demonstrated, with surface finish 20–30 μm with both steel and aluminum at L/D = 7–11. Comparative tests with commercially available bars demonstrated the advantages of our system.     

Enhanced virtual machining for sculptured surfaces by integrating machine tool error models into NC machining simulation

Sculptured surface machining is a time-consuming and costly process. It requires simultaneously controlled motion of the machine axes. However, positioning inaccuracies or errors exist in machine tools. The combination of error motions of the machine axes will result in a complicated pattern of part geometry errors. In order to quantitatively predict these part geometry errors, a new application framework ‘enhanced virtual machining’ is developed. It integrates machine tool error models into NC machining simulation. The ideal cutter path in the NC program for surface machining is discretized into sub-paths. For each interpolated cutter location, the machine geometric errors are predicted from the machine tool error model. Both the solid modeling approach and the surface modeling approach are used to translate machine geometric errors into part geometry errors for sculptured surface machining. The solid modeling approach obtains the final part geometry by subtracting the tool swept volume from the stock geometric model. The surface modeling approach approximates the actual cutter contact points by calculating the cutting tool motion and geometry. The simulation results show that the machine tool error model can be effectively integrated into sculptured surface machining to predict part geometry errors before the real cutting begins.     

Geometric adaptive straightness control system for the peripheral end milling process with large error sources

An off-line Geometric Adaptive Control (GAC) scheme is proposed to compensate for machining straightness errors due to the machine tool's inaccuracy and those arising as a result of the metal cutting process during the finish peripheral end milling process. In the milling process, the workpiece travels along the guideway while the spindle system remains fixed. The scheme is based on the exponential smoothing of post-process measurements of relative machining errors due to the tool and bed deflections. Without a priori knowledge of the variations of the cutting parameters, the time-varying parameters are estimated by an Exponentially Weighted Recursive Least Squares (EWRLS) method. This method is able to incorporate a straightedge which is not necessarily accurate to identify the guideway errors. To reduce the drift of the cutting parameters, a single parameter adaptation method is introduced. Experimental results show that the location error is controlled within the range of the fixing error of the milling bed on the guideway. Further, the waviness error is reduced to less than 10 μm in the machining of a 508-mm long prismatic workpiece regardless of the machining conditions.     

NC end milling optimization using evolutionary computation

Typically, NC programmers generate tool paths for end milling using a computer-aided process planner but manually schedule “conservative” cutting conditions. In this paper, a new evolutionary computation technique, particle swarm optimization (PSO), is proposed and implemented to efficiently and robustly optimize multiple machining parameters simultaneously for the case of milling. An artificial neural networks (ANN) predictive model for critical process parameters is used to predict the cutting forces which in turn are used by the PSO developed algorithm to optimize the cutting conditions subject to a comprehensive set of constraints. Next, the algorithm is used to optimize both feed and speed for a typical case found in industry, namely, pocket-milling. Machining time reductions of up to 35% are observed. In addition, the new technique is found to be efficient and robust.     

Design and control of a dual-stage feed drive

High precision positioning over a large workspace is a fundamental feature of a precision machine. Connecting coarse (large stroke) and fine (high resolution) drive stages, in series, to form a dual-stage feed drive (DSFD) system can provide the desired performance. The DSFD concept has applications that include fast tool servos for the creation of asymmetric surfaces or online chatter suppression, and micro–macro robots for high precision assembly. This paper studies the design of DSFDs for machine tools. The design issues are discussed with special considerations for the dynamics and control of the two drive stages. Two DSFDs, single-axis and two-axis, are designed with piezoelectric actuators (PAs) for the fine stages and linear motors (LMs) for the coarse stages. Both feature flexures for frictionless precision motion that are designed to meet the static and dynamic requirements of a milling process. A model-based control algorithm ensures that the stages work together in a complementary fashion. The single-axis DSFD reduced the tracking error by about 75% in comparison to a similarly controlled LM drive. A second DSFD was built for milling experiments. In sinusoidal profile cutting the maximum tracking error was reduced by 83% and the average magnitude of the error was reduced by 63%. In sharp corner cutting the DSFD reduced the maximum tracking error by 38% and the average magnitude of the error by 39%.     

Preferentially oriented diamond micro-arrays: A laser patterning technique and preliminary evaluation of their cutting forces and wear characteristics

The conventional manufacturing methods of superabrasive grinding wheels generally result in random crystallographic orientations of the abrasive grits with their inherent positioning/spacing inconsistencies on the wheel's working surface. To strategically address these variances, the paper reports on a novel concept of robust generation of preferentially orientated and feature-controlled diamond micro-arrays. Firstly, an Nd:YAG Q-switched pulsed laser was used to accurately produce innovative patterns of micro-crystallite abrasive features in thick-film chemical vapour deposition (CVD) diamond where the size, spacing and orientation of the crystallites can be accurately controlled using carefully selected laser path and operating parameters. Geometrical characterisation of diamond micro-arrays have then been evaluated to enable reproducibility of the laser patterning technique as well as to have a robust basis for comparison of the wear evolution of the crystallites when tested in cutting conditions. Secondly, the performance of both polycrystalline and preferentially orientated ({1 0 0} and {1 1 0} faces) monocrystalline CVD diamond micro-arrays were evaluated in simulated surface grinding trials against a Ti–6Al–4V alloy workpiece where levels/mechanisms of wear of the crystallites as well as the main cutting forces have been analysed. It was found that the diamond micro-arrays of {1 0 0} orientation in the 1 1 0 direction had a higher wear than the diamond arrays of {1 1 0} orientation in the 1 0 0 direction, while both array types resulted in similar levels of cutting forces and workpiece surface roughness. However, the use of monocrystalline CVD diamond micro-arrays yielded considerable lower level cutting forces and wear of the crystallites when compared with the values obtained with micro-arrays made of polycrystalline diamond. Although this novel idea of exploiting these diamond micro-crystallites/arrays as customised cutting tools is at a preliminary testing stage, the paper concludes by giving directions in developing highly engineered (micro) tooling solutions for niche applications.     

Evaluation of the stiffness chain on the deflection of end-mills under cutting forces

This study presents the investigation of the stiffness of the system formed by the machine-tool, shank and toolholder, collet and tool. Cutting forces induce the deflection of the system, and consequently an error appears on the machined surface.Comparing values obtained from cantilever beam models applied to the cutting tool, analytical or FEM, with those experimentally obtained, large differences have been observed, which in some cases are more than 50%. For this reason, we have proceeded to evaluate the stiffness of each of the existing elements between the machine bed and the tool tip. Thus, deflections of the machine-tool, toolholder and toolholder clamping in the spindle, tool clamping in the toolholder, and tool itself, were measured experimentally under the effects of known forces.The final application is the ball-end milling of complex surfaces, an operation commonly performed in the finishing of moulds or forging dies, where errors of more than 70 μm are not unusual. A great part of this error comes from the deflection of the machine-tool assembly, spindle, shank and tool, due to the high cutting forces of the high speed machining of tempered steels. Cutting forces can be estimated using a semi-empirical approach, and from here some values of probable errors may be taken into account to check if the CNC programs are sufficiently adapted. However, a previous study of the deflection chain in the cutting process is needed, as is presented in this work.Results show that stiffness of the slender and flexible tools is 15 times lower than that of the machine and toolholder system. But this correlation is only 5–7 times lower for shorter and thicker tools.     

Modelling machine tool dynamics using a distributed parameter tool–holder joint interface

Increasing productivity in machining process demands high material removal rate in stable cutting conditions and depends strongly on dynamic properties of machine tool structure. Combined analytical–experimental procedures based on receptance coupling substructure analysis (RCSA) are employed to determine the stability of machine operating conditions at different tool configurations. The RCSA employs holder–spindle experimental mobility measurements in conjunction with an analytical model for the tool to predict the dynamics of different sets of tool and holder–spindle combinations without the need for repeated mobility measurements. In this paper an alternative approach using the concept of tool on resilient support is adopted to predict the machine tool dynamics in various tool configurations. In the proposed model the tool, represented by an analytical model, is partly resting on a resilient support provided by the holder–spindle assembly. The support dynamic flexibility is measured by performing vibration tests on the holder–spindle assembly. Tool–holder joint interface characteristics are included in the model by considering a distributed elastic interface layer between the holder–spindle and the tool shank part. The distributed interface layer takes into account the change in normal contact pressure along the joint interface and comparing with the lumped joint model used in RCSA it allows more detailed representation of the joint interface flexibility and damping which have crucial roles in machine dynamics. Experiments are conducted to demonstrate the efficiency of proposed model in prediction of milling operation dynamics and it is shown that the model is capable of accurately predicting the dynamic absorber effect of spindle in a tool tuning practice.     

A PC–NC milling machine with new simultaneous 3-axis control algorithm

Increasing demands on precision machining have necessitated that the tool move not only with a position error as small as possible, but also with smoothly varying feed rates. In this paper, a 3-axis PC–NC milling system, which is capable of synchronized simultaneous 3-dimensional (3D) machining, is developed. To achieve the synchronous 3D linear and circular motions, new interpolation algorithms based on the intersection criteria are presented. A real-time reference-pulse 3D linear and circular interpolator is developed using a PC to implement in the framework of the PC–NC milling machine reconfigured in this research. The performance test via computer simulation and actual machining have shown that the developed PC–NC milling system is useful for the machining of arbitrary lines and circles in 3D space.     

MVC850B型立式数控铣床误差分析与补偿试验研究

针对企业实际生产中铣床加工精度波动的问题,应用Renishaw XL-30激光干涉仪对MVC850B数控铣床的定位误差进行精密检测与补偿试验.利用环境参数对比试验,得出影响定位误差测量的因素;通过三因素双指标正交试验判断进给速度、加工时间以及测距等输入变量对反向间隙与螺距累积误差影响的主次关系;通过单因素对比试验获得反...     

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.     

Dynamic calibration of the positioning accuracy of machine tools and coordinate measuring machines using a laser interferometer

This paper describes a method for evaluating the positioning accuracy of machine tools and coordinate measuring machines (CMM) under dynamic conditions. It is based on the Hewlett Packard 5519A laser interferometer which is capable of performing dynamic calibration. Such method uses the A-quad-B pulses from the machine encoder as the position trigger signals, thus enabling to make measurements “on-the-fly”. A software package has been developed so as to permit the data acquisition and presentation of the positional errors in accordance with ISO 230–2 standard. Thereby, the accuracy and repeatability of positioning of the machine axis on test are assessed considering the ISO parameters. This technique provides a more realistic and detailed picture of the errors of the machine. Such errors are measured in high resolution and dynamically. Application of this positional error calibrator on a moving bridge type CMM is undertaken. The results are presented and discussed.     

Servo design of a vertical axis drive using dual linear motors for high speed electric discharge machining

This paper proposes a synchronous control scheme for a linear servo system applied to the vertical axis drive of a die-sinking electric discharge machine (EDM) tool. The investigated vertical axis drive is constructed with dual parallel linear motors, which are arranged to jointly drive the feed axis for improvement of the overall thrust and structural stiffness. A pneumatic cylinder is employed to compensate the gravitational effect of the feed axis with its electrode and holder. A mechanical coupling is designed to firmly bridge the two linear motors and carry the feed axis. Therefore, synchronous control for the motors is critical for not only position accuracy, but also machine safety. Moreover, by controlling the thrust outputs and positions of the motors to be as equal as possible, the potential “pull and drag” effect between the motors can be reduced and loads can also be equally shared. The proposed “position/thrust hybrid synchronous control” scheme is applied to the EDM to achieve high-speed, accurate machining, and the experimental results show that the synchronization error between the two parallel motors and the positioning accuracy are both satisfactory when operated under high-speed conditions.     

An experimental technique for the measurement of temperature on CBN tool face in end milling

An infrared radiation pyrometer with two optical fibers connected by a fiber coupler was developed and applied to the measurement of tool–chip interface temperature in end milling with a binderless CBN tool. The infrared rays radiated from the tool–chip interface and transmitted through the binderless CBN are accepted by the optical fiber inserted in the tool and are then sent to the pyrometer. A combination of the two fibers and the fiber coupler makes it possible to transmit the accepted rays to the pyrometer, which is set up outside of the machine tool. This method is very practical in end milling for measuring the temperature history at tool–chip interface during chip formation. The maximum tool–chip interface temperature in up milling of a 0.55% carbon steel is 480 °C when the cutting speed is 2.2 m/s and 560 °C at 4.4 m/s, and in the down milling, 500 °C at 2.2 m/s and 600 °C at 4.4 m/s.     

An analytical model to predict specific shear energy in high-speed turning of Inconel 718

Machining of Inconel 718 at higher cutting speeds is expected to provide some relief from the machining difficulties. Therefore, to understand the material behavior at higher cutting speeds, this paper presents an analytical model that predicts specific shearing energy of the work material in shear zone. It considers formation of shear bands that occur at higher cutting speeds during machining, along with the elaborate evaluation of the effect of strain, strain rate, and temperature dependence of the shear flow stress using Johnson–Cook equation. The model also considers the ‘size-effect’ in machining in terms of occurrence of ‘ploughing forces’ during machining. The theoretical results show that the shear band spacing in chip formation increases linearly with an increase in the feedrate and is of the order of 0.2–0.9 mm depending upon the processing conditions. The model shows excellent agreement with the experimental values with an error between 0.5% and 7% for various parametric conditions.     

Modelling the machining dynamics of peripheral milling

The machining dynamics involves the dynamic cutting forces, the structural modal analysis of a cutting system, the vibrations of the cutter and workpiece, and their correlation. This paper presents a new approach modelling and predicting the machining dynamics for peripheral milling. First, a machining dynamics model is developed based on the regenerative vibrations of the cutter and workpiece excited by the dynamic cutting forces, which are mathematically modelled and experimentally verified by the authors [Liu, X., Cheng, K., Webb, D., Luo, X.-C. Improved dynamic cutting force model in peripheral milling—Part 1: Theoretical model and simulation. Int. J. Adv Manufact Tech, 2002, 20, 631–638; Liu, X., Cheng, K., Webb, D., Longstaff, A. P., Widiyarto, H. M., Jiang, X.-Q., Blunt, L., Ford, D. Improved dynamic cutting force model in peripheral milling—Part 2: Experimental verification and prediction. Int. J. Adv Manufact Tech, 2004, 24, 794–805]. Then, the mechanism of surface generation is analysed and formulated based on the geometry and kinematics of the cutter. Thereafter a simulation model of the machining dynamics is implemented using Simulink. In order to verify the effectiveness of the approach, the transfer functions of a typical cutting system in a vertical CNC machine centre were measured in both normal and feed directions by an instrumented hammer and accelerometers. Then a set of well-designed cutting trials was carried out to record and analyse the dynamic cutting forces, the vibrations of the spindle head and workpiece, and the surface roughness and waviness. Corresponding simulations of the machining processes of these cutting trials based on the machining dynamics model are investigated and the simulation results are analysed and compared to the measurements. It is shown that the proposed machining dynamics model can well predict the dynamic cutting forces, the vibrations of the cutter and workpiece. There is a reasonable agreement between the measured and predicted roughness/waviness of the machined surface. Therefore the proposed approach is proven to be a feasible and practical approach analysing machining dynamics and surface roughness/waviness for shop floor applications.     

基于Delcam Vortex旋风铣的不锈钢深腔零件的应用研究

旋风铣Vortex是Delcam最新的高速区域清除加工策略,可用于2轴、3轴、定位5轴以及残留加工,和传统高速加工方法相比,可节省多达40%的加工时间。旋风铣Vortex与Machine DNA的结合使粗加工效率提升的同时,还能最大限度地发挥机床潜能,从而可在不锈钢材料的深孔(腔)类零件合理安全的切削条件下实现加工效率最大化。     

基于模糊故障树的数控龙门铣床加工误差分析与试验

针对数控龙门铣床加工中出现的平面度误差问题进行研究与改进,分析工艺系统因素产生的误差,建立加工平面误差的模糊故障树,通过下行法对其进行简化分析,定性分析影响加工平面度误差的最小割集,运用模糊隶属函数定量分析顶事件的发生概率及底事件的发生概率重要度,确定主要因素为工作台形位误差、定位元件误差、热变形等。使用激光测量技术对安装的工作台平面进行矩形布点测量,根据采集数据建立基于最小二乘法的数学模型,运用Matlab软件定性分析平面度的评定,并对超差区域进行改进,最终实现了机床装配与使用要求。     

A fundamental study on Ti–6Al–4V's thermal and electrical properties and their relation to EDM productivity

There are strong needs for productive/quality machining strategies of notoriously “difficult-to-machine” aerospace materials. The current means of machining these materials is dominated by mechanical cutting methods, which are costly due to high tooling costs, poor surface quality and limitations in the workpiece features and operations that can be machined. The newest EDM technology may be able to circumvent problems encountered in mechanical machining methods. In this paper, the EDM technology has been used to machine titanium alloy Ti–6Al–4V to investigate the effect of Ti–6Al–4V's thermal and electrical properties on the EDM productivity. In the study, temperature measurements have been made for Ti–6Al–4V workpieces with various duty factors to clarify the essential causes of difficulty in machining titanium alloys and observe the optimal duty factor in terms of productivity and quality.