What Every Engineer Should Know about Practical Cad/cam Applications

John Stark

• Editore: CRC Press
• Anno: 2021
• ISBN:
  9781000447231
• Edizione: 1
• Argomento: Informatica
• Categoria: CAD-CAM

This book presents basic information on CAD/CAM and describes how to select, implement, and run a CAD/CAM system in the mechanical engineering environment. It also describes the overall state of CAD/CAM today in different industrial sectors and for different manufacturing technologies.

Part I
Selection and Implementation

1
INTRODUCTION TO CAD/CAM

Computer-aided design/computer-aided manufacturing (CAD/CAM) is a computer-aided technique for improving the efficiency of design engineering and manufacturing engineering activities. A CAD/CAM system is the tool used for applying the technique. Some of the activities (or application areas) are shown in Table 1.1. There are two apparently conflicting ways of looking at CAD/CAM. One of them sees it as a tool for improving the productivity of a particular application such as numerical control (NC) programming or drafting. The other viewpoint sees it as a way of treating and transferring information in a more efficient, secure,

Table 1.1 Application Areas Assisted by CAD/CAM

Design engineering

Preparation of quotations

Conceptual design

Styling

Finite-element analysis

Simulation

Kinematics

Engineering design

Detailed design

Schematics and wiring diagrams

Quality assurance

Drafting

Parts lists

Technical publications

Manufacturing engineering

Tool and fixture design

NC machine tool programming

Robot programming

Quality control machine programming

Process planning

Preparation for automated testing

Plant layout

Material handling simulation

PLC programming

and useful way than is possible with manual methods. The two viewpoints are not mutually exclusive. One of the problems of CAD/CAM implementation is to combine the requirements for the more easily quantifiable “productivity” viewpoint with the less tangible but often greater benefits that can be derived from improving the quality of information flow.

When considering the overall engineering and production activities of a company it is clear that the use of computers is not limited to CAD/CAM but extends into other areas such as manufacturing planning and control systems, management information systems, structural analysis, robotics, purchasing, testing, marketing, and so on. Computers have been used in these areas for different lengths of time. Often the first activities in a company to be assisted by computer were those in the financial sector. However in some companies, computers have been used since the 1950s both in design engineering, for example in finite-element analysis (FEA), and in manufacturing engineering, for example in programming numerically controlled (NC) machine tools.

1.1 BATCH PROCESSING

In the early days of computing, applications were traditionally carried out in “batch,” that is, a complete task (or job) was first defined by the user and submitted to the computer. The computer processed the complete job without further interaction with the user, and then produced output. Several hours could elapse between the submission of the job and receipt of the output. The user would then take the output, generally a listing, away and study it at the desk. As a function of the results, the input parameters of the job could be modified and the whole batch process repeated. In the batch process, the computer is mainly used as a rapid calculator to solve a particular problem faster than would be possible by manual techniques. Although the calculation process is rapid, the overall process is slow (the user may have to wait several hours for results if the computer is busy running other jobs) and the information transfer techniques lengthy. Typically the user would have entered all input data at a keyboard, and output results would have been transferred by listing to other areas of the overall design engineering and manufacturing engineering process. In these areas, data would be read off the listing and entered into further manual or batch calculations.

1.2 INTERACTIVE COMPUTER GRAPHICS AND CAD/CAM

Two features that distinguish CAD/CAM from traditional engineering computing are its use of interactive graphics techniques rather than batch techniques, and its potential to reuse information (e.g., part data) in the computer both within an application area and across the boundaries of different application areas. The requirement to reuse part data leads to two further distinguishing features. The first of these is the need to model (in the computer) the geometry of the part in such a way that sufficient information is available for reuse later both within the same application and within other applications. The other feature is the need to be able to store, transmit, and retrieve part data. As an example of the reuse of part data, consider the case of turbine blade manufacture. First of all, the geometry of the turbine blade has to be designed. While using the CAD/CAM system for design, a model of the geometry of the blade is built up. Once that model exists it can be made available both to engineers carrying out stress analysis and to those preparing the NC program. Savings will occur because the geometry does not have to be recreated for analysis and NC tool path generation, and also because all three application areas are using exactly the same geometry data, thus reducing the risk of errors. These types of savings can be translated into direct benefits such as a reduction in time cycles, a reduction in costs and an improvement in product quality. However, these direct benefits are just an expression of the fact that information, perhaps a company’s most important asset, is being used more efficiently.

Computer-based interactive graphics techniques involve use of a computer and a television-like screen on which drawings (graphics) and results (text) can be displayed. The user interacts with the computer via an input device such as a mouse or a light pen. The computer carries out the necessary calculations, controls the image on the screen in real time, and can output results in the form of drawings, tapes, listings, etc. In many cases, less than one second will elapse between the time at which the user inputs a command and the time at which the user sees the result at the screen. This almost instantaneous process gives the user the impression of carrying out an interactive dialogue with the system. The visual nature of interactive graphics increases the user’s understanding of a task and reduces the potential for some of the errors and oversights associated with manual procedures. As well as calculating faster than humans, the computer can carry out repetitive and uninteresting work more efficiently than humans. It can also store large quantities of information and retrieve required information very quickly. The computer and interactive graphics can aid the user to substantially increase quality. Many engineering tasks are still run in batch because they require a lot of computer processing time. It would be inefficient and wasteful for a user to sit in front of an empty screen for several minutes waiting for the result of a complex calculation. However, even those tasks that have a high batch content can often be aided by the use of interactive graphics. As an example, consider the case of finite-element analysis. Although the analysis calculations may take several hours and be run in batch, the preparation of the finite-element mesh (the pattern of small elements into which the part is decomposed) can be carried out in an interactive fashion with the help of a CAD/CAM system. Understanding of the results of the calculation can be increased by the use of graphics techniques. Similarly the generation of an NC tool path can be eased by using a CAD/CAM system. Although the actual calculation of the tool path may be carried out in batch, it is very useful to display the path and to simulate the movement of the tool at the screen.

1.3 SOME APPLICATIONS OF CAD/CAM

Apart from finite-element analysis and NC programming, other applications can benefit from the use of CAD/CAM. Engineering drawings can be built up (or “drawn”) on the screen, stored by the computer, and afterwards recalled to the screen and, if necessary, modified. Once the drawing is satisfactory it can be automatically drawn by a plotter. Another application that benefits from the use of CAD/CAM is simulation. The designer may investigate on the screen, and from a variety of viewpoints, how a mechanism moves and whether it collides with another part. As well as aiding the programming of NC machine tools such as lathes and milling machines, CAD/CAM can be used when programming robots and quality control machines. In the manufacturing engineering area it can also aid the design of tools and the preparation of process plans. CAD/CAM can be seen to be a very useful computer-based technique using interactive graphics techniques. It can be used for many applications throughout design engineering and manufacturing engineering. However, it must not be forgotten that the CAD/CAM system is only a tool. It does not design or analyze or manufacture. These tasks continue to be carried out by people (e.g., designers), programs (e.g., finite-element analysis), and machines. There is, though, one new “application” which has come into existence because of CAD/CAM. It is called geometry modelling.

1.4 GEOMETRY MODELLING

It has been seen that among the distinguishing features of CAD/CAM are its reuse of part data and the consequent need for a computer-based model of the part that can be used in several application areas. In the past, with manual techniques, information on part data was transmitted from person to person on drawings. Manually produced drawings of typical mechanical parts often do not exactly reflect what the part is—they tend to be incomplete, ambiguous, and incorrect. Computer systems do not currently have enough intelligence to decide what the person who produced the drawing was really trying to describe. Even if the drawing does give a meaningful description, it may not contain the information required by another application area.

The term “product modelling” covers the process of building up a computer-based model containing all the necessary information on the part or product. This information includes attributes such as geometry, color, material and so on.

Geometry modelling is the process of building a model (in the computer) that contains all the necessary information on the part’s geometry. The model should be unique (i.e., the part will not be mistaken for another) and complete (it contains all the geometry information required in the various application areas). It will be seen later that there are several different methods for modelling geometry. Early attempts at developing CAD/CAM did not always model geometry suitably. Part geometry was not sufficiently described and systems tended to be specific to one application. The variety of terms used to describe these systems [CAD, CADD (computer-aided design drawing), CAD/CAM, CAM, CAE (computer-aided engineering), etc.] gives rise to misunderstandings.

1.5 PROBLEMS WITH ACRONYMS

The term CAD was originally used to mean computer-aided design and is still sometimes used in this sense in that it is the use of the computer in the conceptual design/engineering design part of the process and includes analysis and simulation rather than drafting. The term CAD has also nevertheless been used to mean computer-aided drafting. CAM was originally used to mean computer-aided manufacture and could be applied equally well to programming an NC machine tool as to scheduling use of the tool or as to the use of the tool to manufacture a part. The term “NC programming” is not ambiguous but does not refer to the entire manufacturing engineering activity. The terms CAP (computer-aided production) and CAPP (computer-aided production or process planning) only seem to confuse the issue.

In this book, CAD/CAM is defined as a computer-based technique to aid design engineering and manufacturing engineering activities. Thus CAD/CAM is computer-aided design engineering and computer-aided manufacturing engineering. However, this definition of CAD/CAM does not englobe all computer techniques in these areas. For example finite-element analysis is not part of CAD/CAM, although, as has been shown, CAD/CAM can improve productivity in this particular area.

This book will consider the computerized manufacturing plant as being made up of four major “islands of automation”:

• CAD. Computer-aided design engineering (defining what the product is).
• CAM. Computer-aided manufacturing engineering (defining how to make the product).
• MRP. Manufacturing resource planning (defining when to make the product).
• FA. Factory automation (making the product).

Two other acronyms that describe important concepts but do not currently have agreed meanings are CAE and CIM. In this book they are defined as:

• CAE. Computer-aided engineering. All computer-based techniques used in the design engineering and manufacturing engineering areas (i.e., not only CAD/CAM but also analysis, simulation, etc.).
• CIM. Computer-integrated manufacturing. Integration of all computer-based techniques applied to all functions throughout a manufacturing company. These techniques include CAE, MRP, FA, office automation, etc. The “integration” in CIM implies that all decisions and command and information flows in the company are computer assisted.

1.6 CAD/CAM, COMPUTER-AIDED DRAFTING AND NC PROGRAMMING

Is computer-aided drafting, i.e., the use of a computer-based system for producing drawings, an application area of CAD/CAM? Under the above definition of CAD/CAM, the answer is that sometimes it is and sometimes it is not. If the part’s geometry has been modelled and stored in the data base, and can be used again in manufacturing engineering, then drafting of the part is an application area of CAD/CAM. On the other hand, if there is no part model in the data base and someone is going to use the drawing to manually input geometry data into the manufacturing program, then drafting is not an application area of CAD/CAM—it is just computer-aided drafting. The distinction is very important, because...