A new approach for the geometrical calibration of parallel kinematics machines tools based on the machining of a dedicated part

A main limitation of parallel kinematics machine tools (PKM) in high-speed machining tasks is their low level of accuracy compared with serial kinematics machine tools, which is largely due to geometrical transformation errors. These errors can be reduced by identifying the geometrical parameters of the inverse kinematics model integrated in the controller by exteroceptive calibration. The aim of this paper is to propose a new external measurement method in order to perform the geometrical calibration of PKM, taking into account machining requirements. This method is implemented in three steps: machining of a dedicated part, measurement, and identification of the geometrical parameter values. In this paper, the method is described with a particular emphasis on the machined surface profiles of the dedicated part and on the numerical calibration approach. Measurement errors on the machined surface enable the identification of the PKM geometrical parameters. Thus, calibration is performed with respect to machined surface defects without taking into account the entire tool pose defect, as is the case in usual calibration methods. The study is illustrated using the Verne PKM, which is located at IRCCyN (Nantes, France).     

A hexapod-based machine tool with hybrid structure: Kinematic analysis and trajectory planning

This paper shows kinematic analysis and trajectory planning for a novel machine tool structure consisting of a six degree-of-freedom hexapod machine and a two-degree-of-freedom rotary table. Accordingly, to operate the proposed machine tool, eight coordinates should be defined. Since a conventional part programming can define at the most five axes, the three remaining coordinates should be defined by using appropriate trajectory planning. An analytical model is developed that defines the relationships among the parameters in the proposed structure. A rule-based model is also developed to select the redundant coordinates without going through any complex and time-consuming calculations.     

A direct comparison of the machining performance of a variax 5 axis parallel kinetic machining centre with conventional 3 and 5 axis machine tools

The current trend within the Tool and Die manufacturing sector is to machine components directly from hardened material using high speed 5-axis machining. This has been driven by the increasing requirements for cost competitiveness and lead-time reduction. Significant research effort has been applied to the optimisation of the process with factors such as tooling and machining strategies being considerably improved. However, the underlying structures of the machine tools used have remained unchanged and still consist of a serial kinematic chain. One of the standard justifications for the development of machines designed around parallel kinematic chains is that they should exhibit inherently greater stiffness, have higher axis accelerations and be capable of generating significantly higher cutting forces than conventional serial machines. This suggests that they should be ideally suited to the direct manufacture of tools and dies from hardened material.The comparison of different machine tool types is a complex and difficult process, particularly when their structures are fundamentally different. This paper describes an approach used to compare the performance of three very different types of machines. The technique uses two parameters; surface finish and geometric accuracy to assess the relative performance of different machine tools when cutting hardened material. The method is used to compare a serial kinematic 5-axis machining centre, a serial kinematic 3-axis machining centre and a parallel kinematic 6-axis machining centre.The results of the comparison are presented in this paper and show that all the machine tools performed to an equal standard for materials with a hardness of 54HRc but for very hard materials, 62HRc, the parallel kinematic machine out performed the serial machine tools.     

A systematic optimization approach for the calibration of parallel kinematics machine tools by a laser tracker

Parallel kinematics machine has attracted attention as machine tools because of the outstanding features of high dynamics and high stiffness. Although various calibration methods for parallel kinematics machine have been studied, the influence of inaccurate motion of joints is rarely considered in these studies. This paper presents a high-accuracy and high-effective approach for calibration of parallel kinematics machine. In the approach, a differential error model, an optimized model and a statistical method are combined, and the errors of parallel kinematics machine due to inaccurate motion of joints can be reduced by this approach. Specifically, the workspace is symmetrically divided into four subspaces, and a measurement method is suggested by a laser tracker to require the actual pose of the platform in these subspaces. An optimized model is proposed to solve the kinematic parameters in symmetrical subspaces, and then arithmetical mean method is proposed to calculate the final kinematic parameter. In order to achieve the global optimum quickly and precisely, the initial value of the optimal parameter is directly solved based on the differential error model. The proposed approach has been realized on the developed 5-DOF hexapod machine tool, and the experiment result proves that the presented method is very effective and accurate for the calibration of the hexapod machine tool.     

汽车同步器齿套摆线加工数控机床结构总体方案设计

将传统的汽车同步器齿套倒锥及滑块槽的几种加工方法，与用摆线逼近法加工汽车同步器齿套倒锥及滑块槽的加工方法加以比较，阐明了这种新方法产生的背景和优势；介绍了用摆线加工滑块槽的原理；在此基础上设计了汽车同步器齿套摆线加工数控机床的总体结构；给出了机床的主要动力和运动参数，最后展望了这种方法的应用前景。     

Kinematic and dynamic synthesis of a parallel kinematic high speed drilling machine

Typically, the term “high speed drilling” is related to spindle capability of high cutting speeds. The suggested high speed drilling machine (HSDM) extends this term to include very fast and accurate point-to-point motions. The new HSDM is composed of a planar parallel mechanism with two linear motors as the inputs. The paper is focused on the kinematic and dynamic synthesis of this parallel kinematic machine (PKM). The kinematic synthesis introduces a new methodology of input motion planning for ideal drilling operation and accurate point-to-point positioning. The dynamic synthesis aims at reducing the input power of the PKM using a spring element.     

经济型数控机床常见的加工故障分析与处理

文章针对在采用华中HNC-1型数控系统的经济型数控车床、铣床加工零件过程中,出现的一些常见的硬件方面的加工故障及系统软件方面的不足问题进行了分析,并给出合理的解决方案.     

转向球销球面加工组合机床设计

根据球面形成原理，将工件回转轴线和刀具回转轴线形成交角，利用两个圆运动，合成转向球球销的不完整球面。通过铣剧和滚压，在一台机床上完成球面精加工。     

并联运动机床的误差分析

文章介绍了并联运动机床的误差来源及其解决方法.     

Modeling the machining stability of a vertical milling machine under the influence of the preloaded linear guide

The prediction of machining stability is of great importance for the design of a machine tool capable of high-precision and high-speed machining. The machining performance is determined by the frequency characteristics of the machine tool structure and the dynamics of the cutting process, and can be expressed in terms of a stability lobe diagram. The aim of this study is to develop a finite element model to evaluate the dynamic characteristics and machining stability of a vertical milling system. Rolling interfaces with a contact stiffness defined by Hertz theory were used to couple the linear components and the machine structures in the finite element model. Using the model, the vibration mode that had a dominant influence on the dynamic stiffness and the machining stability was determined. The results of the finite element simulations reveal that linear guides with different preloads greatly affect the dynamic behavior and milling stability of the vertical column spindle head system. These results were validated by performing vibration and machining tests. We conclude that the proposed model can be used to accurately evaluate the dynamic performance of machine tool systems designed with various configurations and with different linear rolling components.     

Analysis and dimensional design of a novel hybrid machine tool

This paper presents a four degrees of freedom (DOF) hybrid machine tool based on a novel planar 3-DOFs parallel manipulator and a long movement of the worktable. Closed-form solutions are developed for both the inverse and direct kinematics about the parallel manipulator. Three kinds of singularities are presented. Based on the formulation of condition number and the definition of the position workspace and orientation workspace, the effects of the design parameters on the dexterity and workspace were discussed. The change ratios of condition number were defined and used as the evaluated indicator to analyze the effects of the design parameters on the performances. A dimensional design approach that could satisfy the requirement of motion platform realizing orientation capability, dexterity and workspace was proposed. A novel 4-DOFs hybrid machine tool was manufactured. The methodology presented in this paper can be of great help in the design, trajectory planning, control and application of such devices.     

Optimal spindle speed determination for vibration reduction during ball-end milling of flexible details

In the paper a method of optimal spindle speed determination for vibration reduction during ball-end milling of flexible details is proposed. In order to reduce vibration level, an original procedure of the spindle speed optimisation, based on the Liao–Young criterion [1], is suggested. As the result, an optimal, constant spindle speed value is determined. For this purpose, non-stationary computational model of machining process is defined. As a result of modelling, a hybrid system is described. This model consists of following subsystems, i.e. stationary model of one-side-supported flexible workpiece (modal subsystem), non-stationary discrete model of ball-end mill (structural subsystem) and conventional contact point between tool and workpiece (connective subsystem). The method requires identification of some natural frequencies of stationary modal subsystem. To determine them, appropriate modal experiments have to be performed on the machine tool, just before machining. Examples of vibration surveillance during cutting process on two high speed milling machines Mikron VCP 600 and Alcera Gambin 120CR are illustrated.     

结合PLC对X5032机床的变频调速改造

本文阐述了X5032的变频调速改造方案,介绍了如何通过PLC来控制变频调速的频率和其他复杂顺序加工程序.重点说明了利用BPTS系列变频器对该机床主轴的调速改造过程,最后给出了改造后机床运行的性能状况.     

A computer simulation approach to machining complicated shapes, on an orthogonal 6-rod machine tool

A computer simulation approach to machining a gear and cam by using a virtual orthogonal 6-rod machine tool is presented. First, based on Stewart's parallel mechanism, a virtual 3D orthogonal 6-rod machine tool with 6 degrees of freedom is created with advanced CAD software. Secondly, a 3D spur gear base and cam base are constituted. Thirdly, the gear base and cam base are assembled into a virtual orthogonal 6-rod machine tool with some auxiliary links and a new virtual 1-degree of freedom mechanism is created. Finally, using reversing motion analysis, the kinematic curves of the 6 rods and the feeding process are simulated and analyzed.     

直线电机在数控机床上的调试

本文主要介绍了直线电机的工作原理及其驱动技术、直线电机进给驱动技术及在数控机床上的调试,直线电机进给驱动技术将是高速数控机床未来发展的方向。     

CNC machine tool surface interpolator for ball-end milling of free-form surfaces

This paper presents a new type of CNC machine tool interpolator that is capable of generating the cutter path for ball-end milling of a free-form surface. The surface interpolator comprises on-line algorithms for cutter-contact (CC) path scheduling, CC path interpolation, and tool offsetting. The interpolator algorithms for iso-parametric, iso-scallop and iso-planar machining methods are developed, respectively. The proposed surface interpolator method gains the advantages for minimizing the data loaded to the CNC machine tool and maintaining the desired feedrate and position accuracy along the CC path.     

A study of back cutting surface finish from tool errors and machine tool deviations during face milling

The surface finish of mechanical components produced by face milling is given by factors such as cutting conditions, workpiece material, cutting geometry, tool errors and machine tool deviations. The contribution of the different tool teeth to imperfections in the machined surface is strongly influenced by tool errors such as radial and axial runouts. The surface profile of milled parts is not only affected by chip removal due to front cutting, but also by back cutting, which must be taken into account when predicting surface roughness. In the present work, the influence of back cutting on the surface finish obtained by face milling operations is studied. Final part surface roughness is modelled from tool runouts and height deviations that affect the surface marks provoked by back cutting. Round insert cutting tools and surface positions defined by cutter axis trajectory are considered, and milling experiments are developed for a spindle speed of 750 rpm, depth of cut of 0.5 mm and feeds from 0.4 to 1.0 mm/rev. Experimental observations are compared with the theoretical predictions provided by the surface roughness model, and good agreement is found between both results. Surface imperfections caused by front and back cutting are analysed, and discrepancies between experiments and numerical predictions are explained by undeformed chip thickness variations along the tool tooth cutting edge, the tearing of the workpiece material, and fluctuations in the feedrate and height deviation during machine tool axis displacement.     

Qualification of parallel kinematics machines in high-speed milling on free form surfaces

Serial milling machines may one day find their limits in high-speed milling due to their limited dynamic characteristics. Indeed, the major drawback of a serial structure is that it consists of a pile of actuated joints; hence the mass on board for the axis underneath can be huge. This is mainly the reason why parallel structures are of interest in milling: in order to go faster.Such structures have been developed since 1980 for robotic tasks, while the first parallel kinematics machine tool appeared only 14 years later. Since then, very few papers have been published that deal with the potential of these structures in milling. The objective of this study is to show the potentialities of parallel structures in milling and especially in high-speed milling of free form surfaces, in comparison to serial structures. To do so, experiments have been realised on four serial kinematics machines (SKMs) and four parallel kinematics machines (PKMs). These experiments were of two kinds: either a real piece has been milled, or the output axis encoder data have been saved (this method of measurement has been previously developed in our team).So far, a comparison between the two structures (serial and parallel) has been possible, which permitted us to show that PKMs can provide interesting results in terms of time and precision. Hence, the goal of this study has been reached, as the PKMs potentialities in milling free form surfaces have been highlighted. With more experiments, a generalisation, by taking into account all the parameters (shape and material of the part, structure of the machine, etc.), will be attempted. In parallel, the development of our simulator for PKMs is necessary in order to be able to predict milling on these machines.     

Stability limits of milling considering the flexibility of the workpiece and the machine

High speed machining of low rigidity structures is a widely used process in the aeronautical industry. Along the machining of this type of structures, the so-called monolithic components, large quantities of material are removed using high removal rate conditions, with the risk of the instability of the process. Very thin walls will also be milled, with the possibility of lateral vibration of them in some cutting conditions and at some stages of machining. Chatter is an undesirable phenomenon in all machining processes, causing a reduction in productivity, low quality of the finished workpieces, and a reduction of the machine-spindle's working life.

In this study, a method for obtaining the instability or stability lobes, applicable when both the machine structure and the machined workpiece have similar dynamic behaviours, is presented. Thus, a 3-dimensional lobe diagram has been developed based on the relative movement of both systems, to cover all the intermediate stages of the machining of the walls. This diagram is different and more exact than the one that arises out of the mere superposition of the machine and the workpiece lobe diagrams. A previous step of rejecting resonance modes that are not involved in the milling at the bottom zones of the thin walls must be previously performed.

Finally, the proposed method has been validated, by machining a series of thin walls, applying cutting conditions contrasted with the limits previously obtained in the three-dimensional lobe diagram.