Tool positioning error (TPE) characterisation in milling

Where the geometrical features so permit, the {workpiece–work-holding fixture} assembly is generally considered to be infinitely rigid. The {tool–tool-holder–spindle} assembly and the machine axes are then deformed under the action of the cutting forces. This deformation leads to a positioning error of the tool in relation to the theoretical position. With the aim of taking this positioning error into account, the inaccuracies obtained during end milling and side milling were experimentally modelled from the cutting conditions used for a given machine/mill/material triplet (TriM). Our “Virtual Worker” then used these models to predict machining errors according to the type of machining and to compensate for them.     

Analysis of the effects of fixture clamping sequence on part location errors

Several fixture-related error sources are known to contribute to part location error, which can lead to poor part quality. In addition to typical error sources, such as fixture geometric error and elastic deformation of the fixture and part due to clamping forces, the clamping sequence used can also influence part position and orientation. In this paper, the effect of clamping sequence on workpiece location error is modeled analytically for a fixture–workpiece system where all major compliance sources and fixture geometric error are considered. Part location error is quantified by the displacement of a response point on the part surface. An algorithmic procedure designed to understand how forces and deformations change as clamps are applied sequentially is presented. The effect of clamping sequence on part location error and locator reaction force is examined through model simulations and experiments via an example involving a 3-2-1 machining fixture.     

A model to predict minimum required clamp pre-loads in light of fixture–workpiece compliance

The determination of minimum required clamp pre-loads is an important process in the design of machining fixtures. This paper presents a linear, clamp pre-load (LCPL) model that can be applied to fixture–workpiece systems whose compliance is load invariant. The model considers the static deformation of the fixture–workpiece system in response to the clamping process and the machining process. Sources of compliance throughout a fixture–workpiece system are considered. The model computes the minimum required pre-loads necessary to prevent workpiece slip at the fixture–workpiece joints throughout the machining process.This paper also describes an experimental study that was used to characterize the accuracy of the LCPL model with regard to the application of a ramping external load to a fixture–workpiece system. Over the contact conditions tested, the LCPL model was observed to overestimate the minimum required clamp pre-loads by an average of 7%. This experimental study also revealed the sensitivity of the computed pre-loads to the relative compliance of the fixture elements as well as the coefficient of friction.     

Design of a six-axis high precision machine tool and its application in machining aspherical optical mirrors

The paper presents the design of a six-axis machining system and its application in fabricating large off-axis aspherical mirrors with sub-aperture lapping techniques. The new system is based on computer-controlled optical surfacing (CCOS), which combines the faculties of grinding, polishing, and on-machine profile measuring, has the features of conventional loose abrasive machining with the characteristics of a tool having multiple degrees of freedom moving in planar model. And a novel dual touch-trigger probe profiler is designed, which is composed of a probe, model METRO-MT60 made by HEIDENHAIN Co., is integrated into the system for measuring the shape accuracy of the tested aspherical surface, another probe modeled METRO-MT12 is designed as a calibrating device for minimizing the cosine error caused by assembly inaccuracy. The new CNC machining system with two kinds of moving coordinate systems, dual tool activities and on-machine measuring is presently developed based on the new concept. The general material removal function during machining is analyzed on the basis of the Preston hypothesis. Further, an alignment test of the measuring profiler is carried out using a leveling rule as a specimen. The accuracy of the optical surfaces measured by the dual probe profiler is found to be within 1 μm PV after removing cosine error and error compensating, achieves to the resolving power of the profiler is about 0.2–0.5 μm, so the developed system can be applied to the shape accuracy measuring of aspheric fabrication with micro precision during fine grinding process according to the calibrating results. Finally, the manufacturing experiments are carried out by virtue of an off-axis oblate ellipsoid mirror with rectangular aperture as 770 mm×210 mm and centered 127 mm. The accuracy of the aspherical mirror improved from the initial form error of 17.648 μm rms to the final one of 0.728 μm rms after grinding for 200 h.     

Digital manufacture of large-grade hydro turbine's blades

The blade is one of the vital components and the most difficulty in manufacturing of large hydro turbines. In order to cost-effectively and productively manufacture these kinds of blades, a series of innovative digital techniques have been developed. It includes the digital design of hydro turbine blades based on manufacturers’ requirements, computer-aided location and the machined error evaluation with three-dimensional digitized measure, tool path generation strategy for enhancing machining efficiency and controlling deviation in NC machining, tool path generation and NC machining simulation by means of a virtual NC machining environment for blades, and feasible strategy and the systematic scheme for manufacturing of large blades with 5-axis simultaneous CNC machining technology. The developed innovative digital manufacture techniques have been successfully applied in the manufacturing of both the large grade Kaplan and Francis hydraulic turbine blades. It has been shown that the higher efficiency and the better surfaces finish accuracy can be achieved in practical engineering.     

Integrated planning for precision machining of complex surfaces—III. Compensation of dimensionai errors

This paper presents an approach that compensates machining errors in ball-end milling processes using the control-surface strategy. In the proposed approach, machining errors induced by cutter deflections are predicted from a surface generation model so that actual machining experiments and dimensional inspections are not required. Therefore, the proposed approach can be integrated into an integrated framework for machining planning in which prediction and compensation of dimensional errors take place in the process development phase rather than in the manufacturing phase of the production cycle. Based on the predictive capability of the surface generation model, a sensitivity function is defined so as to characterize the variations of the machining errors when perturbing the control surface. Using the defined sensitivity function, a compensated control surface can be constructed, based on which new cutting paths can be planned and “near-zero” surface dimensional errors can be accomplished. In order to verify the proposed strategy, actual production of a turbine blade die using a CNC machining center was conducted. By using the control-surface strategy with the sensitivity function approach, the maximum surface dimensional error is reduced from ± 340 to ± 10 μm.     

细长轴加工夹具的应用

介绍在车床上加工细长轴专用夹具的设计制造过程。使用该夹具可切削加工任意长度的细长轴。     

整体构件电解装置的设计

整体构件结构特殊,强度高,体积小,大大简化了零件的装配,提高了发动机的推重比,广泛应用于航空航天等制造领域;但整体构件腔体结构复杂,材料硬度高,采用机械加工刀具的加工难度非常大,甚至刀具无法到达加工区域。而电解加工可一次成型,生产率高,特别适合于加工复杂腔体零件。针对整体构件的形状特点,对电解加工的外装置进行了设计,确保加工过程零件的绝缘、导电、装夹。通过对零件的试加工,针对电解加工过程出现的密封问题、腔体位置偏离问题进行探讨,在设计结构的基础上进行了方案的优化设计。实验证明:合理的工装能够满足整个零件加工过程的需求,从而避免机床的腐蚀,并最终保证零件成形的位置以及尺寸。     

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.     

A unified framework of error evaluation and adjustment in machining

Errors of machine tool, fixture, and datum on workpiece to be machined influence the machining accuracy of the workpiece. The objective of this paper is to provide a framework for abstracting an error model that integrates three types of errors, i.e., machine tool, fixture, and datum errors, into a unified one. Differential motion theory is used to build the evaluation model of three types of errors. The resultant deviation model of the tool with respect to the workpiece is derived by using the model. For the purpose of eliminating the deviation, the resultant geometric variation is mapped into the locator errors on the fixture. Then the position and orientation errors of the tool with respect to the workpiece may be reduced by adjusting the length of locators. Finally, the effectiveness of the resultant deviation model is verified by examples.     

卧式加工中心配置回转夹具回转中心位置误差的检验

在卧式加工中心上配置回转夹具是国内外机床制造技术的发展方向。卧式加工中心配置回转夹具主要是由夹具体、伺服电机、转台和液压系统等装置组成的一种复杂的夹紧结构，适用于汽车行业缸体、缸盖等零件的加工，在卧式加工中心装配调试工作中，经常遇到卧式加工中心配置回转夹具（转台＋夹具）专机，而确定回转夹具回转中心位置成为该专机调试工作的技术关键，该文主要研究回转夹具回转中心的确定，夹具主定位面（合定位销）误差的判断、调整及其机床三个坐标轴运动之间的关系。     

An implementation of drive key cellular manufacturing

The drive key is one of the most important high-precision components of aircraft braking systems. In this paper, a cost effective drive keys manufacturing process is developed. This proposed manufacturing process makes use of near-net-shape manufacturing, computer numerical control (CNC) machining, and cellular manufacturing methods. To make the proposed process a reality, proper engineering with the proposed parts along with group technology are applied to the product design in conjunction with the proposed manufacturing methods. In addition, a general spreadsheet program is developed for each style of fixture modules to assist in part program coding. Moreover, statistical process control (SPC) is implemented to verify product quality and accept the product at the point of manufacture. It is observed that the implementation of the proposed drive key cellular manufacturing process results in a tremendous improvement in quality, throughput and productivity as well as a drastic reduction in manufacturing operations, workholding fixtures, cutting tools, and inspection media.     

高精度复杂型线刀具制造工艺与装备研究

以高精度复杂型线刀具制造工艺与装备为研究对象，以菌形叶片叶根型线加工专用刀具为切入点，通过分析叶根型线的特点及加工工艺的优劣，确认以单边两次成型加工作为刀具加工工艺方案；并在此基础上确定刀具的轮廓度精度、前后刀面表面粗糙度等各项参数及精度要求。通过分析专用刀具的加工要求，确定专用磨床的整体布局及结构方案，并对机床的关键部件磨削装置的结构做了具体设计与阐述。分析磨削点在B轴回转轴线延伸线上的结构优点可知，B轴回转的角度误差未使磨削点位置发生偏差，可有效提升刀具的加工精度。为验证设计的合理性，通过制造与装配机床，利用雷尼绍XK10激光校准仪检测可知，几何精度均达到设计要求。最后，采用设计的刀具加工工艺方案及专用磨床对某菌形型线刀具进行加工，Zoller检测仪检测结果表明：刀具轮廓公差为工作面±0.005 mm,其余面±0.015 mm。     

核电冷却泵泵体五轴数控加工工艺研究

核电冷却泵泵体是主回路系统中三大核心部件之一,泵体的制造加工精度对整体设备的运行安全有直接影响。为提高泵体的加工精度,提出用五轴数控加工工艺加工泵体。分析泵体特征,应用尺寸中差建模,并设计专用夹具。基于遗传算法,研究加工工艺及方法,应用五轴联动数控加工中心完成核电冷却泵泵体加工。解决了圆柱倒扣面、90°弯管管道、薄壁件等加工难题。     

数控立式加工中心夹具控制系统的设计

针对数控加工中使用传统夹具导致生产效率低的问题,以重卡前桥镗孔加工为研究对象,设计了一种以数控系统和PLC控制为基础的夹具控制系统,给出了该系统的详细设计方案,并且对M指令的工作过程,CNC、PMC、PLC信号的流向,PMC程序和PLC流程进行了分析。开发的系统运行稳定,达到了预期效果,节省了加工时间。     

基于夹具规划系统的机床和刀具自动选择研究

针对基于特征的夹具自动规划设计系统，讨论了如何自动确定加工过程中的机床及刀具的方法，以MDT为运行平台，利用VC＋＋、Object ARX、Microsoft Access等软件工具，开发建立了常用的加工中心和机加工刀具数据库和自动选择机床刀具子系统，并用实例验证了其可行性。为夹具自动设计的实现奠定了基础。     

Incorporated tool selection system using object technology

Tool selection is perhaps one of the most important functions in a process planning system because the selection of a tool affects the selection of machining parameters, jig and fixture selection, production rate, cost of the product, and the resulting accuracy. Therefore computer-aided tool selection (CATS) is an essential element within a computer-aided process planning (CAPP) system (C. Koulamas, Tool requirements in multi-level machining systems, International Journal of Production Research, 29 (2) (1991) 417–437.), and in turn in a Computer Integrated Manufacturing (CIM) environment. This paper proposes an object–oriented methodology for selecting the tooling parameters for parts in a CIM environment. As this system can be incorporated into both static and dynamic process planning systems, it is termed as Incorporated Tool Selection Systems (ITSS). The steps of the methodology are discussed in detail, together with an example.     

Improved workpiece location accuracy through fixture layout optimization

Inaccuracies in workpiece location lead to errors in position and orientation of a machined feature on the workpiece. The ability to accurately locate a workpiece in a machining fixture is strongly influenced by rigid body displacements of the workpiece caused by elastic deformation of loaded fixture–workpiece contacts. This paper presents a model for improving workpiece location accuracy in fixturing. A discrete elastic contact model is used to represent each fixture–workpiece contact. Reduction in workpiece locating error due to rigid body displacements is achieved through optimal placement of locators and clamps around the workpiece. The layout optimization model is also shown to improve the overall workpiece deflection and reaction force characteristics.     

Robust fixture layout with the multi-objective non-dominated ACO/GA approach

Fixture design is an important procedure in many manufacturing operations in terms of maintaining product quality, consistency and assisting the assembly, machining and inspection processes. A robust fixture design approach is presented in this study. It treats fixture layout as a multi-objective optimization problem, which considers both the mean and the variation of the mean localization errors of the machining features in a set-up, and the distance of the location points among the locators to ensure the location is stable and reliable. The non-dominated ACO and GA are proposed to solve this multi-objective problem, and their performances are compared.