Development of an Integrated, Adaptable CNC System

In order to prepare manufacturing companies to face increasingly frequent and unpredictable market changes with confidence, there is a recognized need for CNC machine tools to be further advanced so that they become more integrated with design models and adaptable to uncertain machining conditions. For a CNC system to be able to access any design information, this design information has to be at the task-level, that is what-to-do. For a CNC system to produce the final part, it has to turn the task-level information into method-level information which effectively is the machine control data. These topics are discussed at the beginning of this chapter. The rest of the chapter discusses a CNC native database used for converting the task-level data to method-level data, the methodology of converting the task-level data to methodlevel data, and implementation of the methodology to a conventional CNC machine that employs G-codes. Again both STEP-NC (ISO 14649-1, 2003) and function blocks (IEC 61499, 2005) are used.

CNC Machine Tools

The introduction of CNC machines has radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action has dramatically reduced. With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality can be achieved. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required to change the machine to produce different components. In a production environment, a series of CNC machines may be combined into one station, commonly called a “cell”, to progressively machine a part requiring several operations. CNC controller is the “brain” of a CNC machine, whereas the physical configuration of the machine tool is the “skeleton”. A thorough understanding of the physical configuration of a machine tool is always a priority for a CNC programmer as well as the CNC machine tool manufacturers. This chapter starts with a historical perspective of CNC machine tools. Two typical types of CNC machine tools (i.e. vertical and horizontal machining centres) are first discussed. Tooling systems for a CNC machine tool are integral part of a CNC system and are therefore elaborated. Also discussed are the four principal elements of a CNC machine tool. They are machine base, machine spindle, spindle drive, and slide drive. What letter should be assigned to a linear or rotary axis and what if a machine tool has two sets of linear axes? These questions are answered later in the chapter. In order for readers to better comprehend the axis and motion designations, a number of machine tool schematics are given.

Geometric Modelling and Computer-Aided Design

One of the key activities in any product design process is to develop a geometric model of the product from the conceptual ideas, which can then be augmented with further engineering information pertaining to the application area. For example, the geometric model of a design may be developed to include material and manufacturing information that can later be used in computer-aided process planning and manufacturing (CAPP/CAM) activities. A geometric model is also a must for any engineering analysis, such as finite elopement analysis (FEA). In mathematic terms, geometric modelling is concerned with defining geometric objects using computational geometry, which is often, represented through computer software or rather a geometric modelling kernel. Geometry may be defined with the help of a wire-frame model, surface model, or solid model. Geometric modelling has now become an integral part of any computer-aided design (CAD) system. In this chapter, various geometric modelling approaches, such as wire-frame, surface, and solid modelling will be discussed. Basic computational geometric methods for defining simple entities such as curves, surfaces, and solids are given. Concepts of parametric, variational, history-based, and history-free CAD systems are explained. These topics are discussed in this opening chapter because (a) CAD was the very first computer-aided technologies developed and (b) its related techniques and methods have been pervasive in the other related subjects like computer-aided manufacturing. This chapter only discusses CAD systems from the application point of view; CAD data formats and data exchange issues are covered in the second chapter.

From CAD/CAPP/CAM/CNC to PDM, PLM and Beyond

Companies that have been practicing CAD, CAPP, CAM, and CNC integration have now realized that there is a need to operate in a much broader scope with wider boundaries and more functionality. To foster innovation in a product development lifecycle, change in the early stage is good, and, in fact, should be encouraged. The more iteration a product design can experience at this stage when change is inexpensive, the lower cost our final product will become. At a later stage when hardware set-up is committed against a design, change becomes expensive and should be discouraged. Therefore, there is a need for an effective way of managing product-related information as well as the product development action flow, which captures actions that need to be done, have been done, and what other parts are affected. Engineers that subscribe to a portion of a design also need to be working with other collaborators and then automatically be notified when changes occur. This leads to increased implementation of Product Data Management (PDM) and Product Lifecycle Management (PLM). PDM systems are used to control information, files, documents, and work processes required to design, build, support, distribute, and maintain products. Using PDM, people can contribute at the early stages of product design and development. In addition, PDM can be seen as an integration tool connecting many different areas, which ensures that the right information is available to the right person at the right time and in the right form throughout the enterprise. In this way, PDM improves communication and cooperation be tween diverse groups in an organization, and between organizations and clients (Peltonen, Pitkanen & Sulonen, 1996, Liu & Xu, 2001). PDM is strongly rooted in the world of CAD, CAPP, CAM, and CNC in a more specific sense as well as in the world of engineering and design in a more general sense. In recent years, more focus has also been on the improvement of the entire product lifecycles. The major concern here is time-to-market, as it reflects the competitiveness of a company. In response to the new area of focus, new generation PDM systems are developed to support the entire product lifecycle; from the initial concept to the finishing product. This has subsequently led to the birth to PLM systems. From the information context, PLM should cater for the management of the information throughout the lifecycle of a product, including multiple domain views, different business processes scattered across enterprises and different representations of a multitude of native product-, resource- and process-models (Stark, 2004, Rosén, 2006). This chapter starts with introduction to and discussions about product data management systems. Topics covered include PDM’s capabilities, its benefits, Web-based PDM and PDM standardization. The concept of integrated and extended PDM is also introduced. This is followed by discussions on product lifecycle management, for example definitions of PLM, its solution model, benefits, and implementation are among the topics covered. Like PDM, issues regarding PLM standardisation are also addressed. Share-A-space™ is a practical case of PLM. The core features and its architecture are discussed. Toward the end, the concept and some of the techniques of “grand” integration are introduced.

Function Block-Enabled Integration

Function blocks are an IEC (International Electro-technical Commission) standard for distributed industrial processes and control systems (IEC 61499, 2005). It is based on an explicit event driven model and provides for data flow and finite state automata-based control. Based on previous research, function blocks can be used as the enabler to encapsulate process plans, integrate with a third-party dynamic scheduling system, monitor process plan during execution, and control machining jobs under normal and abnormal conditions. They are also considered to be suitable for machine-level monitoring, shop-floor execution control, and CNC control. Combination of STEP-NC and Function Blocks can be seen as a “natural marriage”. This is because the former provides an informationally complete data model but with no functionality, whereas the latter can embed intelligence and provide functionality in the data model for a more capable CNC regime. This chapter introduces the function block architecture which has been implemented in two types of integrations. The first brings together CAD, CAPP, and CAM. The key is to embed machining information in a function block system that is based on the concept of machining features. The second integration connects CAM with CNC. This is in fact an open CNC architecture that is function block driven, instead of G-code driven.

Program CNCs

A CNC machine can be programmed in different ways to machine a workpiece. In addition to creating the cutting program, many other factors also need to be considered or programmed. These include workholding devices, cutting tools, machining conditions as well as the machining strategy. The first generation CNCs were programmed manually and punched tapes were used as a medium for transferring the machine control data (MCD), that is, G-codes into a controller. Tapes were later replaced by RS232 cables, floppy disks, and finally standard computer network cables. Today’s CNC machines are controlled directly from files created by CAD/CAM or CAM software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the component. This means that for the first time, bringing design and manufacturing under the same automation regime becomes a reachable target. Error detection features give CNC machines the ability to alert the operator in different ways including giving a ring to the operation’s mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts that are already loaded up to that tool and wait for the operator. The focus of this chapter is on a detailed account of the basics of CNC programming, and the emphasis is on G-code and Automatic Programming Tool (APT). G-code is still the dominant manual programming language for CNC machine tools. It is also the main form of control commands many CAD/CAM (or CAM) systems output. APT was developed soon after G-codes and CNC machine tools were developed to alleviate the drudgery work of straight G-code programming. Modern CAD/CAM systems these days are now becoming the main-stream tools for CNC programming.

Feature Interactions

Feature interaction tends to have a wide range of consequences and effects on a feature model and its applications. While these may often be intended, it is also true that feature validity can be violated, one way or another, by feature interactions (Shah & Mäntylä, 1995, Gao & Shah, 1998, Lee & Kim, 1998). They may affect the semantics of a feature, ranging from slight changes in actual parameter values, to some substantial alterations to both geometry and topology or even complete suppression of its contribution to the model shape. To certain extent, successful applications of feature recognition and feature-based techniques have been hindered by interactions among the features. Feature interaction was first studied in relation to feature recognition systems. As an alternative to feature recognition, feature-based design methodology has also become prevalent in recent years. Although a number of successful and commercially available feature-based design systems have been reported, current CAD technology is still unable to provide an effective solution for fully handling the complexity of feature interactions. Very often in a feature-based design system, the interaction between two features gives rise to an unintended feature, nullifying the one-to-one mapping from design features to manufacturing features. The resulting manufacturing feature is usually of a form that the system cannot handle or represent. Thus feature interaction resolution is equally essential for a feature-based design system (Dereli & Baykasoglu, 2004). As discussed in Chapter IV, features can be represented either as a set of faces or as a volume. The interactions between surface features are different from those occurring between volumetric features. This chapter discusses different types of interactions that arise from these two feature representation schemes and uses the interacting entities to classify them. There are two types of surface feature interactions, basic feature interaction and complex feature interaction. Three types of basic feature interactions are discussed. They are nested, overlapping, and intersecting types. Interacting patches are used to classify volumetric feature interactions. These interacting patches can be of a containing, contained, or overlapping type. The significance of feature interactions lies in their effect on the machining sequence of the features involved. This is also discussed in this chapter. When features are close to each other but do not share any geometric entities, interactions may also happen for structural reasons. This type of feature interaction can be called interaction by vicinity. The main aim of this chapter is to take a holistic approach toward feature interaction solutions. The example parts used are from the “Catalogue of the NIST (National Institute of Standards and Technology) Design, Planning and Assembly Repository” (Regli & Gaines, 1996). A case study is provided in the end of the chapter.

Feature Recognition

Conventional CAD models only provide pure geometry and topology for mechanical designs such as vertices, edges, faces, simple primitives, and the relationship among them. Feature recognition is then required to interpret this low-level part information into high-level and domain-specific features such as machining features. Over the years, CAD has been undergoing fundamental changes toward the direction of feature-based design or design by features. Commercial implementations of FBD technique became available in the late 1980’s. One of the main benefits of adopting feature- based approach is the fact that features can convey and encapsulate designers’ intents in a natural way. In other words, the initial design can be synthesized quickly from the high-level entities and their relations, which a conventional CAD modeller is incapable of doing. However, such a feature-based design system, though capable of generating feature models as its end result, lacks the necessary link to a CAPP system, simply because the design features do not always carry the manufacturing information which is essential for process planning activities. This type of domain-dependent nature has been elaborated on in the previous chapter. In essence, feature recognition has become the first task of a CAPP system. It serves as an automatic and intelligent interpreter to link CAD with CAM, regardless of the CAD output being a pure geometric model or a feature model from a FBD system. To be specific, the goal of feature recognition systems is to bridge the gap between a CAD database and a CAPP system by automatically recognizing features of a part from the data stored in the CAD system, and based on the recognized features, to drive the CAPP system which produces process plans for manufacturing the part. Human interpretation of translating CAD data into technological information required by a CAPP system is thus minimized if not eliminated.

CAD Data Exhange and CAD Standards

Today, more companies than ever before are involved in manufacturing various parts of their end products using different subcontractors, many of whom are often geographically diverse. The rise of such global efforts has created the need for sharing information among vendors involved in multi-disciplinary projects. Transfer of data is necessary so that, for example, one organization can be developing a CAD model, while another performs analysis work on the same model; at the same time a third organization is responsible for manufacturing the product. Data transfer fills the need to satisfy each of these functions in a specific way. Accurate transmission is of paramount importance. Thus, a mechanism for good data transfer is needed. The CAD interoperability issue - using one CAD system in-house, yet needing to deliver designs to, or receive designs from, another system, poses a challenge to industries such as automotive, aerospace, shipbuilding, heavy equipment, and high-tech original equipment manufacturers and their suppliers. It is worth studying the issue and determining how engineering model data is delivered today to manufacturers and suppliers, how CAD conversion, geometric translation, and/or feature-based CAD interoperability are handled, at what expense, and under whose authority. This chapter explores the various ways to make this vital transfer possible. The attention will be directed towards data exchange and standards for 3-D CAD systems. Since CAD data formats have a lot to do with CAD kernels that govern the data structure and therefore the data formats, some popular CAD kernels are discussed. The data interoperability section covers different types of data translations and conversions. The use of neutral or standardized data exchange protocols is one of the natural methods for data exchange and sharing. This topic is covered at the end of this chapter.

Integration of CAD/CAPP/CAM/CNC

Technologies concerning computer-aided design, process planning, manufacturing and numerical control, have matured to a point that commercialized software solutions and industrial systems can be acquired readily. These solutions or systems are, however, not necessarily connected in a seamless way, that is they are not fiintegrated. The term “islands of automation” has been used to describe these disconnected groups of systems with no obvious integration points other than the end user. As the engineering businesses are increasingly being run in a more globalized fashion, these islands of automation need to be connected to better suit and serve the collaborative and distributed environment. It is evident that the businesses are struggling with this integration strategy at a number of levels other than the underlying technology, including CAD, CAPP, CAM, and CNC for example. In some cases, where integration does not exist among these computer-aided solutions, promising product technologies may come to a sudden halt against these barriers. The previous chapters have focused on these individual computer-aided solutions, e.g. CAD, CAPP, CAM, CNC, and feature technologies. Some localized integration such as integrated feature technology has been studied. The following chapters, will in particular, look at the integration issues, technologies, and solutions. This chapter starts with a general description of traditional CAD, CAPP, CAM, and CNC integration models. This is followed by an industry case study showcasing how a proprietary CAD/CAM can be used to achieve centralized integration. To illustrate CAM/CNC integration, three different efforts are mentioned. They are APT, BCL (Binary Cutter Location, (EIA/ANSI, 1992)), BNCL (Base Numerical Control Language, (Fortin, Chatelain & Rivest, 2004)) and use of Haskell language for CNC programming (Arroyo, Ochoa, Silva & Vidal, 2004).