C.R. Liu

Review of dimensioning and tolerancing: representation and processing

The paper surveys the current state of knowledge of techniques for representing, manipulating and analysing dimensioning and tolerancing data in computer-aided design and manufacturing. The use of solid models and variational geometry, and its implications for the successful integration of CAD and CAM, are discussed. The topics explored so far can be grouped into four categories: (a) the representation ot dimensioning and tolerancing (D& T), (b) the synthesis and analysis of D& T, (() tolerance control, and (d) the implications of D& T in CAM. The paper describes in detail the recent work in each group, and concludes with speculation on a general framework k)r future research.

Feature-based representational scheme of a solid modeler for providing dimensioning and tolerancing information

Abstract Presently available CAD data models have no problem handling the basic shape design of an object as required but they are not properly manufacturing-oriented. It is not possible to represent the dimensioning and tolerancing information or other technological information (such as material data, the surface roughness and accuracy data, feature information, etc.). This paper discusses the requirements of a new CAD data model and proposes a feature-based representation scheme based on the hybrid CSG/B-Rep data structure. This hybrid structure exploits the advantages of both CSG and B-Rep models in representing tolerance information. A relational graph structure (face-based data model of B-Rep) of the object is maintained at each hierarchical level of object construction for associating tolerance and other attributes.

A Contribution to the Analysis and Compensation of the Geometric Error of a Machining Center

Summary This paper deals with the modeling and compensation of the geometric errors or a three-axes machining center. Rigid body kinematics is used to develop a model relating the observed error vector at a point in the machines workspace to Its coordinates. The machine constants (e.g. lengths or the members or the machines kinematic scheme) and the errors in the form and dimensions or each of these members become the model coefficients. The model is then analyzed to show that, if the positioning error or each or the axes is zero, the coefficients of the models (the Individual errors or the members or the machines kinematic scheme) can be evaluated by observing the error vector at a few points, forming a particular pattern. In the machines workspace. Closed-form equations ire derived for the coefficients under these conditions and a heuristic self-organizing modeling procedure, the GMDH algorithm. Is used for modeling the positioning error under varying thermal conditions. This makes It Is possible to estimate the geometric error, usually an expensive task in time, effort and skill, from a few observations or error in the machines workspace. Also, with periodic updates or the model, it is possible to compensate for thermal Influences.

Analysis on machined feature recognition techniques based on B-rep

Abstract Solving the machine feature recognition problem has been widely recognized as a cornerstone for developing an automated process planning system directly linked to a cad system. Various recognition techniques have been developed; however, they are in general deficient in robustness. That is, valid machined features may not be recognized and features which are recognized may not be valid in practice. This paper is intended to analyse the existing machined feature recognition techniques, which are based on the B-rep solid modelling scheme, in order to give the reasons why the robustness problem would occur. The pros and cons for recognizing machined features are also analysed. Finally, a cutter selection methodology, known as process requirement modelling, is introduced; this methodology seems to provide a promising way to solve the machined feature recognition problem.

Single-step superfinish hard machining: Feasibility and feasible cutting conditions

Abstract A major inefficiency in the manufacturing of hard, high precision parts is the number of processes, or steps, currently needed. For instance, the current method of producing the surfaces of bearing races involves annealing, rough turning, hardening, several types of grinding, and, finally, abrasive based superfinishing. There is always an additional set-up for each additional process, and an occasional inspection between processes. Therefore, there has been an economic motivation to study methods of extending one process or one machine tools capability so that other processes in the production sequence can be eliminated, thus, reducing set-up time and the complexity of manufacturing scheduling, and gaining significant benefits for flexibility and system efficiency. The concept and feasibility of using a single step for machining hardened steel to a surface comparable to that of lapped bearing races were presented briefly with limited data (Liu and Mittal: J. Mfg Syst. 14(2): 129–133, 1995). In this paper, extensive literature review and considerable data on surface waviness, profile, finish, microstructure and residual stress confirm the feasibility of the proposed new processing concept. The cutting conditions for producing a specific surface finish ranging from 2 to 8 μin. are also presented.

Establishment of functional relationships between product components in assembly database

Abstract In an assembly environment, explicit interface information between the major part components is necessary to determine proper assembly plans, avoid undesirable interferences between parts, and achieve the required product function. A semantic association model (SAM∗)-based assembly database is proposed to organize this assembly-specific semantic information in a hierarchical structure in its subassembly module. A hybrid constructive solid geometry/boundary representation (as a structured face adjacency graph) is used to represent each component part of the assembly. These are stored in single part modules. The representational scheme is feature based and a functional relationship graph is developed to establish the required mating conditions between these features of different components in an assembly.

Extraction of manufacturing details from geometric models

Abstract The potentials of geometric models as source representations for the generation of manufacturing details in discrete production processes have not been fully exploited. The objective of this paper is to utilize the geometric data base in the CAD system in production and manufacturing applications such as group technology, process planning, etc. The workpiece is represented as a syntactic pattern made up of geometric primitives. The applications presented in this paper are: (1) shape classification for group technology and process selection and (2) procedure for representation and machining of excess material. The procedures described offer a direct link between geometric models and computer-aided manufacturing.

Generation of workpiece orientations for machining using a rule-based system

Abstract This paper deals with the automatic generation of workpiece orientations on a machine for machining operations. A system capable of this has a wide spectrum of applications in many areas of manufacturing planning and is essential to the development of dynamic process planning systems. However, such a system requires the ability to reason about geometrical and spatial relationships, a topic of great difficulty for automatic reasoning. A study of the problem characteristics is made and its results are implemented in a rule-based system. This system, when given a workpiece along with operation instructions and machine specifications is capable of generating a good orientation for the workpiece. The functioning of the system is demonstrated by an example.

Design of an automated assembly environment

Abstract A design environment for acquisition, analysis and validation of information for automated mechanical assembly is proposed in this paper. The environment acts as a preprocessing step before the actual assembly operation. The working memory — consisting mainly of instances of component and subassembly data objects and a B-rep based structured face adjacency graph — is used to establish the assembly precedence relationship. The environment has been implemented in the object-oriented programming language Smalltalk-80. An object-based approach has been preferred to a procedural language because of increased modularity, data abstraction, and efficiency of the code. The hierarchical class (data type) structure of object-oriented programming has been used to define a hierarchical mating requirement module permitting knowledge inheritance from parent classes.

Conceptual design, manufacturability evaluation and preliminary process planning using function-form relationships in stamped metal parts

Abstract A large number of design decisions are made during the conceptual design of a part. However, there are few representation and reasoning tools for decision support during conceptual design. The conceptual design stage is characterized by a lack of complete geometric information. Existing geometric modelers require complete geometric information, while a functional reasoning methodology using a verb, noun > representation is typically too terse. In this paper, we present a new representation called sketching abstraction for conceptual design, using the function-form relations in a design. The functionally critical part of the geometry is presented using a set of functional features, while the rest of the geometry is abstracted as a set of linkages. Part functionality is correlated with the sketching abstraction using data structures called function-form matrices. The sketching abstraction is annotated using a set of primitives, and a set of grammar rules are used to extract canonical relationships between the functional features. The sketching abstraction can be used for extracting designs that are geometrically dissimilar but functionally similar, thus providing the designer with ideas for design alternatives. The sketching abstraction can also be used to carry out domain-dependent manufacturability evaluation checks. A further use of sketching abstractions is to initiate the development of a process plan for manufacturing. Sketching abstractions are related to the solid model of a part. Thus, this representation provides a link between pure functional and pure geometric representations. The domain of application is stamped metal parts. We present the part functionality and the features used in this domain. We also illustrate the use of sketching abstractions for conceptual design, manufacturability evaluation and preliminary process planning.