Geometric reasoning for mill-turn machining process planning

An integrated system to support both product and manufacturing process design should be such that (1) the part design can be evaluated and redesigned based on manufacturability analysis and (2) the manufacturing processes can be selected efficiently and flexibly exploiting product information provided by part design representations.

In this paper, we describe the feature-based geometric reasoning system for part modeling and process planning as applied to mill-turn machined parts. The feature recognition system based on convex decomposition and the mapping method to relate the negative feature volumes to machining process classes are applied to mill-turn parts. Also, the geometry-based machining precedence relations have been generated for various alternative machining feature decompositions. The above geometric information is input to mill-turn machining process planning to determine machining process sequences and assignment to multiple spindles and turrets.     

Method for automating digital fixture-setups that are optimal for machining castings to minimize scrap

The motivation for this paper is to describe a method for lowering the cost of finishing large castings that have machined surfaces for attaching other components. Considerable time is required to set-up each cast part on a machine-tool, sometimes taking longer than the machining itself, and errors in set-up can result in scrapping expensive parts or attempts to salvage them by rework. Although the focus of the paper is to demonstrate a new technology and software for set-up prior to the machining of iron/aluminum/steel sand castings, the same technology also is applicable to large welded assemblies on which finished machining occurs. In this paper, we outline a method, currently being implemented, that can predictively, and off-line, identify the adjustments needed to position and orient each part in its fixture before machining operations begin so that, after machining, all finished features will lie in their tolerance zones. Computer models first simulate all the to-be-machined (TBM) surfaces and any contact points with the fixture by feature-fitting point clouds taken from selective scanning of the raw casting. The locations of these features are compared with their locations on the CAD model of the part. Then, by using the T-Map model for tolerances, all possible locations of the part in its machining fixture are identified so that all TBM faces lie in their tolerance-zones. An optimum location may then be chosen.     

Handling interacting positive components in machining feature reasoning using convex decomposition

Form features to the product shape can be recognized using a convex decomposition called alternating sum of volumes with partitioning (ASVP). Since the form feature decomposition is compact and faithful to the product shape, it includes both positive and negative components. For machining applications, the positive components are converted into corresponding negative components to represent the removal volume. The positive to negative conversion is done in a top-down manner by abstracting the positive components using halfspaces determined by the original faces and combining with the parent negative component.

In this paper, the handling of interacting sibling positive components which have a common parent component in the positive to negative conversion is described. Interacting sibling positive components are sequentially ordered based on face dependency and extremality. Coplanarity of some faces is exploited to obtain a concise negative feature decomposition.     

Fuzzy-set-based approach for concurrent constraint set-up planning

Material removal processes are integral parts of many manufacturing systems. They are either primary machining processes or an important part of preparing toolings for subsequent forming and moulding processes. Manufacturing process planning identifies the type of material removal processes and the machining parameters, cutting tools and fixtures needed to generate the features on a part. Previous research in manufacturing process planning has concentrated mainly on the role of features, in the derivation of the associated process and fixture plans. Many computer-aided process planning (CAPP) and computer-aided fixture planning (CAFP) systems derive process and fixture plans from the features on a part, on the basis that these features are context-free. However, manufacturing operations are interdependent processes. In the author's computer-aided set-up planning (CASP) system, a different perspective is adopted. Feature relations form the core of the conceptual structure. These features relations, which are often imprecise, are used in deriving set-up plans. The feature relations, which may be due to geometrical constraints, tolerance requirements, etc., are modelled using fuzzy sets and fuzzy relations. This paper presents the various feature relations considered in the present system and proposes a practical planning algorithm for set-up planning.     

Machining feature-based similarity assessment algorithms for prismatic machined parts

This paper presents algorithms for identifying machined parts in a database that are similar to a given query part based on machining features. In this paper we only consider parts that are machined on 3-axis machining centers. We utilize reduced feature vectors consisting of machining feature access directions, feature types, feature volumes, feature dimensional tolerances and feature group cardinality as a basis for assessing shape similarity. We have defined a distance function between two sets of reduced feature vectors to assess the similarity between them from the machining effort point of view. To assess the similarity between the two parts, one set of reduced feature vectors is transformed in space using rigid body transformations with respect to the other set such that the distance between them is minimized. The distance between the two sets of aligned reduced feature vectors is used as a measure of similarity between the two parts. The existing machined parts are rank ordered based on the value of the distance with respect to the query part. The cost of previously machined parts that have a very small distance from the query part can be used as a basis for estimating the cost of machining the new part.     

三角片离散法实现数控铣床加工仿真

三角片离散法是根据三轴数控铣床加工中的特点提出的加工仿真方法。文中结合笔者在微机上制作仿真软件的经验,使用三角片离散法实现数控铣床加工仿真;并详细地介绍了三角片离散法的原理、简化模型、计算方法以及提高效率的途径。该方法简单易行,而且有很好的真实感效果。     

基于B-rep的工件表面动态几何模型

在NC（Numerical Control）仿真中,工件三维图形处理是一项关键技术。既要实现几何造型的显示功能,又要求三维图形具有可加工性、加工成品的信息具有可记录性。为此,采用B-rep（边界表示法）模型和面向对象技术相结合的方法,设计一种记录NC仿真中工件形状数据变化信息的数据结构,构建NC仿真系统中工件表面动态几何模型。实现NC仿真中每个数控刀位工件加工表面的生成与判断。NC仿真完成后,可以判断零件的加工表面与非加工表面,进而分析零件表面加工质量（表面粗糙度）。     

An accurate and efficient approach to geometric modeling of undeformed chips in five-axis CNC milling

Many important and complex parts, such as aero-engine compressors and automotive punch dies, are often machined in five-axis computer numerically controlled (CNC) milling. To machine the parts with accurate dimensions and shapes and low machining costs it is necessary to construct 3D models of the finished parts in the geometric simulation and in-process workpiece models of the parts in the physical simulation of their five-axis milling. A kernel technique of the geometric and the physical simulations is to accurately and efficiently model the geometry of the workpiece material removed at every moment of the machining, which is the instantaneous, undeformed chip geometry. Although in the past decades much research has been conducted on modeling cutter swept volumes in CNC milling to represent the finished part geometry in the geometric simulation, it is very time consuming to calculate the instantaneous, undeformed chip geometries using the cutter swept volumes. Besides, the existing method of modeling undeformed chip geometry in three-axis milling cannot be used for that in five-axis milling. To address this problem, our work proposes an accurate and efficient approach. In this article, a generic theory about the boundary of the area covered by the instantaneous cutting edges on a workpiece layer at any moment is established, which is called the boundary theory. A simple diagram of determining the boundary is invented, which is called boundary construction diagram. This approach lays a theoretical foundation for the geometric and the physical simulations of five-axis milling and will significantly promote them for high performance machining in industry.     

On the role of complexity in machining time estimation

Early cost estimation of machined parts is difficult as it requires detailed process information that is not usually available during product design. Parametric methods address this issue by estimating machining time from predictors related to design choices. One of them is complexity, defined as a function of dimensions and tolerances from an analogy with information theory. However, complexity has only a limited correlation with machining time unless restrictive assumptions are made on part types and machining processes. The objective of the paper is to improve the estimation of machining time by combining complexity with additional parameters. For this purpose, it is first shown that three factors that influence machining time (part size, area of machined features, work material) are not fully captured by complexity alone. Then an optimal set of predictors is selected by regression analysis of time estimates made on sample parts using an existing feature-based method. The proposed parametric model is shown to predict machining time with an average percentage error of 25% compared to the baseline method, over a wide range of part geometries and machining processes. Therefore, the model is accurate enough to support comparison of design alternatives as well as bidding and make-or-buy decisions.

     

Automated process planning method to machine A B-Spline free-form feature on a mill–turn center

In this paper, we present a methodology for automating the process planning and NC code generation for a widely encountered class of free-form features that can be machined on a 3-axis mill–turn center. The free-form feature family that is considered is that of extruded protrusions whose cross-section is a closed, periodic B-Spline curve. In this methodology, for machining a part with B-Spline protrusion located at the free end, the part is first rough turned to the maximum profile diameter of the B-Spline, followed by rough profile cutting and finish profiling with axially mounted end mill tools. The identification and sequencing of machining volumes is completely automated, as is the generation of actual NC code. The approach supports both convex and non-convex profiles. In the case of non-convex profiles, the process planning algorithm ensures that there is no gouging of the work piece by the tool. The algorithm also identifies when sections of the tool path lie outside the work piece and utilizes rapid traverses in these regions to reduce cutting time. This methodology presents an integrated turn–mill process planning where by making the process fully automated from design with no user intervention making the overall process planning efficient. The algorithm was tested on several examples and test parts using the unmodified NC code obtained from the implementation were run on a Moriseiki mill–turn center. The parts that were produced met the dimensional specifications of the desired part.     

A time-varying geometry modeling method for parts with deformation during machining process

Deformation due to residual stress is a significant issue during the machining of thin-walled parts with low rigidity. If there are multiple processes with deformation during machining, some process suitability issues will appear. On this occasion, the actual geometric state of the deformed workpiece is needed for process adjustment. However, it is still a challenge to obtain the complete geometry information of deformed workpiece accurately and efficiently. In order to address this issue, a time-varying geometry modeling method, combining cutting simulation and in-process measurement, is proposed in this paper. The deformed workpiece model can be reconstructed via transforming the deformed workpiece with only a small amount of the measurement points by superimposing material removal and workpiece deformation simulation according to a time sequence, which takes advantage of the proposed Curved Surface Mapping based Geometric Representation Model (CSMGRM). Machining experiment of a typical structural part has shown that the deformed geometry model of the whole workpiece can be reconstructed within the error of 0.05mm, which is less than one tenth of the finish machining allowance in general cases, and it is sufficient to meet the accuracy requirements for interference or overcut/undercut analysis and process adjustment.     

A Data-drivenParameter Planning Method for Structural Parts NC Machining

Structural parts are generallyused to compose the main load-bearing components in various mechanical products, and are usuallyproduced by NC machining where the machining parameters heavily determine the final production quality, efficiency and cost. Due to the complex structures and high precision requirements, a large amount of human interactions are usually required to modify the machining parameters generated by existing optimisation model-based or expert system-based methods, which will induce unstable machining quality and low efficiency. This paper proposes a data-driven methodfor machining parameter planning by learningthe parameter planning knowledge from thehigh-qualityhistorical processing files. An attribute graph is first defined to represent the part model. Then for each of the machining operations in the historical processing files, the machining parameters are correlated to a sub-graph that refers to the faces to be machined in this operation. By this way, a graph dataset of machining parameters could beconstructed from the historical processing files, and graph neural networks (GNN) are established to learn the planning models for machining parameters. The proposed method provides an end-to-end strategy for constructing machining parameter planning models thus human interactions can be greatly reduced and the performance of the models are able to be improved as the increase in historical processing files. In the case study, the historical processing files of aircraft structural parts machining are used to train the GNN models for planning cutting width, cutting depth and machining feedrate, and the prediction accuracies reach 95.50%, 94.79%, 95.02% respectively.     

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.     

A : A tool for identifying and repairing un-machinable shapes in designs

Providing early feedback on the manufacturability of a part design can greatly improve the quality of the product while reducing the time and cost of production. However, the necessary manufacturing knowledge is not always available. Computer tools that can provide this manufacturing knowledge by analyzing a design suggesting changes to improve its manufacturability would be a valuable asset to a designer. To this end, we present an approach to automatically generate redesign suggestions to improve the manufacturability of machined parts. Novel aspects of this approach include the ability to identify un-machinable shapes in a part and transform them into machinable features and to automatically identify the possible shape transformations based on properties of the machining equipment. This increases the scope of redesign generation tools by allowing them to be applied to parts that are not already machinable. We have developed a system called automated redesign for machined parts ( ) that assists users in repairing parts that contain un-machinable shapes.     

Building MRSEV models for CAM applications

In integrating CAD and CAM applications, one major problem is how to interpret CAD information in a manner that makes sense for CAM. The goal is to develop a general approach that can be used with a variety of CAD and CAM applications for the manufacture of machined parts.

In particular, a methodology is presented for taking a CAD model, extracting alternative interpretations of the model as collections of MRSEVs (material removal shape element volumes, a STEP-based library of machining features), and evaluating these interpretations to determine which one is optimal. The evaluation criteria may be defined by the user, in order to select the best interpretation for the particular application at hand.     

Stability of Workpiece in Feature－Based Fixture Planning System

Analyzed the issues of stability of workpiece during clamping and machining,Discussed classification of the stability problem according to the forces acting on the part in the commonly used machining processes;gave the methods of calculating clamping force and measures to guarantee the stability of workpiece in the feature-based fixture planning design system.     

Stability ofWorkpkce Setup in Feature-Based Fixture Planning System

Analyzed the issues of stability of workpiece during clamping and machining. Discussed classification of the stability problem according to the forces acting on the part in the commonly used machining processes; gave the methods of calculating clamping force and measures to guarantee the stability of workpiece in the feature-based fixture planning design system.     

A general model for machinable features and its application to machinability evaluation of mechanical parts

This paper is intended to reveal the rationale in the machinability evaluation and to present an effective, systematic approach to the assessment of the machined parts in the early stage of design. By examining the inherent shaping mechanism of machining processes, a geometric feature model, termed ‘machining volume’, resulting from the cutter's movement that mimics the cutter's real motion trajectory in machining, is proposed with which a set of feature derivatives can be affiliated. The geometric and topological patterns of machining volume permit to capture and convey machinability constraints on a part shape, leading to a new, simple machinability evaluation method centered on machining volume. This method is dedicated to two kinds of inspection on the geometry of a part in design: one is to search for unmachinable surfaces that are beyond the capability of machining processes, and the other to detect machining interference between a cutter and a part. It shows a more useful means to characterize machinable features that result in the machinability evaluation with ease and efficiency.     

Rule and branch-and-bound algorithm based sequencing of machining features for process planning of complex parts

The machining sequence of machining features is vital to achieve efficient and high quality manufacturing of complex NC machining parts. In most feature-based process planning system, the machining features are sequenced as the lowest level unit. However, a single machining feature of complex parts such as aircraft structural parts is usually machined by multiple machining operations. The one-to-many mappings between the machining features and the machining operations cause the increase of the non-cutting tool path. In order to solve this problem, some types of machining features of complex parts are decomposed into several sub-machining features that are associated with a single machining operation individually according to the rules which are abstracted from the machining process of complex parts. Benefitting from the decomposition, the sub-machining features from different machining feature can be assembled into a sub-machining feature in order to avoid the cutting tool marks. The different types of sub-machining features are sequenced in the light of some rules which are also extracted from the machining process of complex parts. And the branch-and-bound algorithm are employed to sequence the same type sub-machining features to minimum the non-cutting tool path. A pilot feature-based process planning system has been developed based on this research, and has been used in some aircraft manufacturers in China.