Helical surface creation by wire electrical discharge machining for micro tools

In mechanical micromachining, micro tooling is one of the key factors affecting the finished geometrical accuracy and surface quality. To overcome the serious tool wear caused by relatively longer micromachining time, micro tools are usually made of ultra-hard materials such as polycrystalline diamond (PCD) or cubic boron nitride (CBN). Wire Electrical discharge machining (WEDM) is a good choice for efficient fabrications of micro tools made of ultra-hard materials. Considering the traces of wire motions form ruled surfaces, in this paper, typical custom micro milling tools with helical surfaces are generated by ruled surfaces. The simulation shows that the selection of guide lines and generating lines for ruled surface is the key point relating to the final geometrical accuracy and machining efficiency in custom micro tool fabrications by WEDM. Based on the mathematical models built in this paper, overcut can be avoided in the process planning stage for complicated helical surfaces. Furthermore, wire locations can be created conveniently by the introduced mathematical models for post processing in dedicated CAM systems.     

Solid modeling of 4-axis wire EDM cut geometry

NC verification has been widely used in milling and turning machining, but the implementation of NC verification for wire electrical discharge machine (wire EDM) has received little attention, made difficult by the characteristics of geometry cut by the thin wire. For an NC verification system, the speed and the accuracy of are two of the most critical issues. In this paper, a novel solid modeling method, which utilizes a dual quadtree structure and a boundary representation, is applied to modeling the parts cut by a wire EDM. In this implementation, two extended quadtrees on the top and bottom surfaces of the stock are first created. Special quadtree nodes are introduced into the dual quadtree structure to model the features associated with the almost coincident surfaces left by a thin wire. The region swept by a wire in each move is generated based on the diameter of the wire, spark gap size and the paths. The overall geometry of the part is represented by a boundary representation which is updated by means of efficient Boolean operations of extended quadtrees. The free-falling objects which may damage the machine tool can be detected. Dimensional inspection and feature detection can be made with the solid model representing the machined part. The system has demonstrated very promising speed and accuracy.     

Machining accuracy reliability analysis of multi-axis machine tool based on Monte Carlo simulation

Although machine tool can meet the specifications while it is new, after a long period of cutting operations, the abrasion of contact surfaces and deformation of structures will degrade the accuracy of machine tool due to the increase of the geometric errors in six freedoms. Therefore, how to maintain its accuracy for quality control of products is of crucial importance to machine tool. In this paper, machining accuracy reliability is defined as the ability to perform its specified machining accuracy under the stated conditions for a given period of time, and a new method to analyze the sensitivity of geometric errors to the machining accuracy reliability is proposed. By applying Multi-body system theory, a comprehensive volumetric model explains how individual geometric errors affect the machining accuracy (the coupling relationship) was established. Based on Monte Carlo mathematic simulation method, the models of the machining accuracy reliability and sensitivity analysis of machine tools were developed. By taking the machining accuracy reliability as a measure of the ability of machine tool and reliability sensitivity as a reference of optimizing the basic parameters of machine tools, an illustrative example of a three-axis machine tool was selected to demonstrate the effectiveness of the proposed method.     

An integrated framework of tool path planning in 5-axis machining of centrifugal impeller with split blades

Centrifugal impeller is a complex part commonly used in aerospace, energy, and air-conditioning industries. Its manufacture involves multi-axis free form machining, a time consuming and error-prone process. Tool path planning is considered a critical issue in the process but still lacking of systematic solutions. This paper proposes a tool path planning framework for 5-axis machining of centrifugal impeller with split blades. It provides several CAM functions that assist the users to generate collision-free cutter motions with smooth tool orientations. First, the machining process is divided into four operations and the planning tasks of each operation are standardized. Second, the hub surfaces are properly decomposed, re-grouped, and re-parameterized to facilitate calculation of quality tool path with reduced cutter retraction and plunging. Finally, geometric algorithms are developed to automatically detect tool collisions and then correct the erroneous tool orientations. An optimization scheme is applied to minimize the total amount of tool posture changes after the correction. An impeller is machined with the NC codes generated from the framework. The result shows the effectiveness of this work in automating the tool path planning in 5-axis machining of highly intricate impeller.     

Accurate tool position for five-axis ruled surface machining by swept envelope approach

This paper presents a swept envelope approach to determining tool position for five-axis ruled surface machining. The initial tool position is traditionally located to contact with two directrices of a ruled surface. The swept profile of the tool is then determined based on the tool motion. By comparing the swept profile with the ruled surface, the tool position is corrected to avoid machining errors. The cutter's swept envelope is further constructed by integrating the intermediate swept profiles, and applied to NC simulation and verification. This paper presents the explicit solution for the swept profile of a taper-end cutter in five-axis ruled surface machining. The relation of the ruled surface geometry, the tool motion and the machining errors is developed. Therefore, the error sources can be detected early and prevented during tool path planning. The explicit swept envelope indicates that the machined surface is not a ruled surface in five-axis ruled surface machining. Manufacturing industries should take extra care in high precision ruled surface machining. Computer illustrations and example demonstrations are shown in this paper. The results reveal that the developed method can accurately position tool location and reduce machining errors for five-axis ruled surface machining.     

Algorithms for selecting cutters in multi-part milling problems

This paper describes geometric algorithms for automatically selecting an optimal sequence of cutters for machining a set of 2.5-D parts. In milling operations, cutter size affects the machining time significantly. Meanwhile, if the batch size is small, it is also important to shorten the time spent on loading tools into the tool magazine and establishing z-length compensation values. Therefore, in small-batch manufacturing, if we can select a set of milling tools that will produce good machining time on more than one type of parts, then several unnecessary machine-tool reconfiguration operations can be eliminated. In selecting milling cutters we consider both the tool loading time and the machining time and generate solutions that allow us to minimize the total machining time. In this paper we first present algorithms for finding the area that can be cut by a given cutter. Then we describe a graph search formulation for the tool selection problem. Finally, the optimal sequence of cutters is selected by using Dijkstra's shortest path planning algorithm.     

Tool orientation adjustment for improving the kinematics performance of 5-axis ball-end machining via CPM method.

For 5-axis ball-end machining, it is desired to maintain the expected cutting performance of tool orientation when adjusting tool orientation for improving the motions of rotary axes of 5-axis machine. For this purpose, a cutting performance maintained (CPM) method is proposed to adjust the tool orientations, the objective of which is to minimize the sum of the absolute deviations between the initial and adjusted coordinates of the rotary axes while improving the kinematics performance of the rotary axes and ensuring no machining interferences. In order to speed up the solving of the optimization objective, the analytical linear representations for the drive limits of rotary axes and especially irregular geometry feasible domains (GFDs) of tool orientations are first discussed in detail. The nonlinearity of the objective function is then eliminated by introducing two new auxiliary variables for further simplifying the computation of optimal tool orientation. After rewriting the drive limits and GFD constraints with the two auxiliary variables, the linear objective function can be efficiently solved by the simple linear programming method. The tool orientations adjusted by the proposed CPM method can not only improve the interference-free motions of the rotary axes, but also can maintain the expected cutting performance. Finally, the computer simulation and real milling were conducted to validate the proposed method.     

Dealing with feature interactions for prismatic parts in STEP-NC

Determining the precedence of machining features is a critical issue in feature-based process planning. It becomes more complex when geometric interaction occurs between machining features. STEP-NC, the extension of STEP (ISO 10303) standard developed for CNC controllers, is a feature-based data model. It represents all the geometric and topological product data minus feature interactions. In this paper, machining precedence of interactive and non-interactive STEP-NC features is discussed. Local and global precedence of machining features are defined on the basis of geometric constraints, such as geometric interaction of features and feature approach face and technological constraint such as access direction of the cutting tool. A software tool has been developed to visualize the STEP-NC part model and to generate the graphs of feature interaction and feature precedence. The output can be then used to augment the STEP-NC data in order to generate the optimal sequence of operations.     

High aspect ratio meso-scale parts enabled by wire micro-EDM

Micro-electro discharge machining (EDM) is a subtractive meso-scale machining process. The Agie Excellence 2F wire micro EDM is capable of machining with a 25 micron diameter wire electrode and positioning the work piece to within ±1.5 microns. The over-burn gap can be controlled to within 3 microns to obtain a minimum feature radius of about 16 microns while achieving submicron surface finish and an imperceptible recast layer. For example, meso-scale gears that require vertical sidewalls and contour tolerances to within 3 microns can be wire EDMed into a variety of conductive materials. Material instabilities can affect the dimensional precision of machined meso-scale parts by material relaxation during the machining process. A study is done to investigate the machining performance of the wire micro EDM process by machining a high aspect ratio meso-scale part into a variety of metals (e.g. 304L stainless steel, Nitronic 60 Austentic Stainless, Beryllium Copper, and Titanium). Machining performance parameters such as, profile tolerance, perpendicularity, and repeatability are compared for the different materials. Pertinent inspection methods desirable for meso-scale quality assurance tasks are also evaluated. Sandia National Laboratories is developing meso-scale electro-mechanical components and has an interest in the assembly implications of piece parts fabricated by various meso-scale manufacturing processes. Although the wire EDM process is typically used to fabricate 2½ dimensional features, these features can be machined into a 3 dimensional part having other features such as hubs and chamfers to facilitate assembly.     

Towards efficient 5-axis flank CNC machining of free-form surfaces via fitting envelopes of surfaces of revolution

We introduce a new method that approximates free-form surfaces by envelopes of one-parameter motions of surfaces of revolution. In the context of 5-axis computer numerically controlled (CNC) machining, we propose a flank machining methodology which is a preferable scallop-free scenario when the milling tool and the machined free-form surface meet tangentially along a smooth curve. We seek both an optimal shape of the milling tool as well as its optimal path in 3D space and propose an optimization based framework where these entities are the unknowns. We propose two initialization strategies where the first one requires a user’s intervention only by setting the initial position of the milling tool while the second one enables to prescribe a preferable tool-path. We present several examples showing that the proposed method recovers exact envelopes, including semi-envelopes and incomplete data, and for general free-form objects it detects envelope sub-patches.     

One-sided offset approximation of freeform curves for interference-free NURBS machining

Generating valid tool path curves in NURBS form is important in realizing an efficient NURBS machining. In this paper, a method for computing one-sided offset approximations of freeform curves with NURBS format as tool paths is presented. The approach first uses line segments to approximate the progenitor curve with one-sided deviations. Based on the obtained line approximating curve and its offsets, a unilateral tolerance zone (UTZ) is constructed subsequently. Finally, a C¹-continuous and completely interference-free NURBS offset curve is generated within the UTZ to satisfy the required tolerance globally. Since all of the geometric computations involved are linear, the proposed method is efficient and robust. Interference-free tool path generation thus can be achieved in NURBS based NC machining.     

A Study on Error Recovery Search Strategies of Electronic Connector Mating for Robotic Fault-Tolerant Assembly

In this paper, a CAD-based trajectory planning scheme for parallel machining robots is introduced using the parametric Non-uniform rational basis spline (NURBS) curves. First, a trajectory is designed via a NURBS curve then, a motion scheduling architecture consisting of time-dependent and constant feedrate profiles is advised to generate the position commands on the represented NURBS curve as the tool path. Using the generated commands, the inverse kinematics is elaborated to obtain the joints motions of the parallel machining robot. This paper investigates the NURBS trajectory generation for a parallel robot with 4(UPS)-PU mechanism as the case study. In order to evaluate the effectiveness of the proposed method, the inverse kinematic results for the parallel machining robot of 4(UPS)-PU is compared with the simulation results obtained from the CATIA software. The results confirmed that the proposed trajectory planning scheme along with the advised motion planning architecture is not only feasible for the parallel machining robots but also yields a smooth trajectory with a satisfactory performance for all the joints.     

快速刀具伺服系统自抗扰控制的研究与实践

快速刀具伺服系统(fast tool servo, FTS)是实现非圆截面和非轴对称表面零件加工的关键部件. 加工过程中,FTS应克服时变切削力负载和自身参数的非线性, 驱动刀具完成高频高精度跟踪运动. 为了解决FTS的快速精密跟踪控制问题, 根据刀具运动参考轨迹已知的特点, 应用自抗扰控制原理和前馈控制策略, 针对基于剪应力和正应力电磁驱动的两种直线执行机构, 分别设计了采用线性和非线性扩张状态观测器的自抗扰控制器, 并利用传递函数和描述函数方法, 分析了线性控制器的跟踪精度和动态刚度特性, 探讨了非线性控制系统的极限环问题. 两种基于自抗扰控制的快速刀具伺服系统已应用于发动机椭圆截面活塞的精密车削和二维正弦微结构表面的超精密车削,满足了加工需求. 研究与应用结果表明: 自抗扰控制思想独特、算法易于工程实现, 具有很好的工程应用价值.     

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.     

Quick segmentation for complex curved surface with local geometric feature based on IGES and Open CASCADE

Complex curved surface parts with local geometric feature are usually critical parts in high-end equipments. However, the processing for this kind of parts is usually difficult or inefficient due to the adoption of difficult-to-machine material and special structure. Current approaches cannot satisfy the rapid development of high-end equipments. Due to the existence of the local geometric feature for the parts, processing such parts with constant machining parameters is less applicative, restricting the improvement of machining efficiency. By separating the local geometric feature and generating tool path for the local geometric feature and the remaining processing area separately, the more efficient machining with variable machining parameters will be obtained for the complex curved surface with local geometric feature. In this way, the quick segmentation for the complex curved surface with local geometric feature is of great importance to the NC machining with variable machining parameters for this kind of parts, and a quick segmentation system is developed based on Initial Graphics Exchange Specification (IGES) and Open CASCADE (OCC) platform in this study. The complex curved surface model in IGES format is firstly imported into the system and then trimmed into independent surface patches. After computing the feature size of each surface patch, the segmentation for the complex curved surface is achieved by sorting and classifying the surface patches according to their feature sizes. Taking the whole impeller with small splitter blades for an example, the experimental result shows that the segmentation of small splitter blades from the whole impeller is successful and a serialized processing program could be generated, and then the whole impeller could be machined precisely and efficiently with NC equipment. In the machining experiment, it is proved that the machining with various machining parameters can improve the efficiency by 28.18% in the comparison experiment, 20.14% and 12.33% in the estimation. The research provides an important foundation for the high quality and more efficient machining of the complex curved surface with local geometric feature.     

Design of an optimal damper to minimize the vibration of machine tool structures subject to random excitation

The vibration of machine tools during machining adversely affects machining accuracy and tool life, and therefore must be minimized. The cutting forces for stable turning are generally known to be random, and hence excite all the resonance modes. Of all these modes, those that generate relative motions between a cutting tool and a workpiece are of concern.This paper presents a new approach for designing an optimal damper to minimize the relative vibration between the cutting tool and workpiece during stable machining. An approximate normal mode method is employed to calculate the response of a machine tool system with nonproportional damping subject to random excitation. The major advantage of this method is that it reduces the amount of computation greatly for higher-order systems when responses have to be calculated repeatedly in the process of optimization. An optimal design procedure is presented based on a representative lumped parameter model that can be constructed by using existing experimental or analytical techniques. The two-step optimization procedure based on the modified pattern search and univariate search effectively leads the numerical solution to the global minimun irrespectively of initial values even under the existence of many local minima.     

Automatic feedrate adjustment for pocket machining

As high-speed machining and unmanned machining become common, the demand for cutting-load regularization increases, so NC machining can be more efficient. To be presented is a simple cutting-load regularization method for pocket machining. As the conventional off-line approaches where cutting-load is predicted and cutting parameters are adjusted before actual cutting, the proposed method requires a cutting force model, which is quite simplified with the function of two independent variables. One is the geometric measure so called 2D chip-load (cutter-engagement angle or effective cutting depth), and the other is the feedrate. Based on the 2D chip-load analysis for the concave line-line segment of the NC tool path, the adjusted feedrate is calculated by using the simplified-cutting force model (SCFM) obtained by the cutting experiment with a tool dynamometer. The concept of the automatic feedrate adjustment (AFA) method to be proposed is very simple, and the implementation requires little effort. Furthermore, the proposed method does not need much calculation time because there are no complex calculations or cutting simulation.     

Instantaneous stiffness analysis and simulation for hexapod machines

This paper introduces a derivation and simulation technique of instantaneous stiffness for 6–6 Stewart platforms (hexapods) with the interest in increasing machining accuracy of the new type hexapod machines. It clearly expresses that high non-linearity of motions and driving forces of the parallel mechanisms results in the asymmetric stiffness matrix, and the change in geometry and the loading are the significant factors to the asymmetry. Also, based on the theoretical calculation the simulations for the hexapod machining cycles numerically and illustratively present the instantaneous stiffnesses and deflections of the end-effector (tool tip). Therefore, this work can provide a theoretical principle and an application tool for the anticipation and the evaluation for hexapod machining performance, design optimisation and machining accuracy control, etc.     

Computer graphics simulation of a CNC lathe

A software system for simulation of the machining operations of a computerized numerically-controlled lathe is described. The microcomputer-based, BASIC software reads part program instructions from a sequential file, analyzes each instruction for errors and provides printed output describing the machine commands called for by the instruction as well as graphic output for tool motions. In an educational environment, the system can be used as a substitute for the more expensive machine tool or as a diagnostic and preview tool prior to actual machining.     

Open architecture of CNC system research based on CAD graph-driven technology

Based on CAD graph-driven technology, a kind of novel open architecture CNC system is put forward and designed together with the key hardware “PC+PMAC controller”. The intelligent CNC system software including several function modules is developed under Visual C++6.0 environment. Graphic feature identification and geometric parameter extraction from CAD-part-drawing saved as DXF format are performed to control the relative motion between cutting tool and part. The ant-colony algorithm is applied to auto-optimize the cutting tool paths in machining process. The experiment results of a plane engraving machining example show that the proposed method is feasible, and the entire machining process no longer needs NC programming. The efficiency of CNC machining is improved greatly, and the true intelligent CNC machining can be realized when the advanced programming technologies are integrated in one system according to the proposed conception.