计算机辅助设计与制造-外文文献

Modern design and manufacturing CAD/CAMCAD/CAM is a term which means computer-aided design and computer-aided manufacturing. It is the technology concerned with the use of digital computers to perform certain functions in design and production. This technology is moving in the direction of greater integration（一体化）of design and manufacturing, two activities which have traditionally been treated as distinct（清楚的）and separate functions in a production firm. Ultimately, CAD/CAM will provide the technology base for the computer-integrated factory of the future.Computer-aided design (CAD) can be defined as the use of computer systems to assist in the creation, modification, analysis, or optimization（最优化）of a design. The computer systems consist of the hardware and software to perform the specialized design functions required by the particular user firm. The CAD hardware typically includes the computer, one or more graphics display terminals, keyboards, and other peripheral equipment. The CAD software consists of the computer programs to implement（实现，执行） computer graphics to facilitate the engineering functions of the user company. Examples of these application programs include stress-strain（压力-应变）analysis of components（部件）, dynamic（动态的）response of mechanisms, heat-transfer calculations, and numerical control part programming. The collection of application programs will vary from one user firm to the next because their product lines, manufacturing processes, and customer markets are different these factors give rise to differences in CAD system requirements.Computer-aided manufacturing (CAM) can be defined as the use of computer systems to plan, manage, and control the operations of a manufacturing plant through either direct or indirect computer interface with the plants production resources. As indicated by the definition, the applications of computer-aided manufacturing fall into two broad categories: 1.computer monitoring and control. 2.manufacturing support applications.The distinction between the two categories is fundamental to an understanding of computer-aided manufacturing.In addition to the applications involving a direct computer-process interface（界面，接口）for the purpose of process monitoring and control, compute-aided manufacturing also includes indirect applications in which the computer serves a support role in the manufacturing operations of the plant. In these applications, the computer is not linked directly to the manufacturing process. Instead, the computer is used “off-line”（脱机）to provide plans, schedules, forecasts, instructions, and information by which the firms production resources can be managed more effectively. The form of the relationship between the computer and the process is represented symbolically in the figure given below. Dashed lines（虚线）are used to indicate that the communication and control link is an off-line connection, with human beings often required to consummate（使圆满） the interface. However, human beings are presently required in the application either to provide input to the computer programs or to interpret the computer output and implement the required action.Process dataControl signalsCAM for manufacturing supportWhat is CAD/CAM software? Many toolpaths are simply too difficult and expensive to program manually. For these situations, we need the help of a computer to write an NC part program.computer Manufacturing operationsThe fundamental concept of CAD/CAM is that we can use a Computer-Aided Drafting (CAD) system to draw the geometry of a workpiece on a computer. Once the geometry is completed, then we can use a computer-Aided Manufacturing (CAM) system to generate an NC toolpath based on the CAD geometry. The progression（行进，级数 ）from a CAD drawing all the way to the working NC code is illustrated as follows:Step 1: The geometry is defined in a CAD drawing. This workpiece contains a pocket to be machined. It might take several hours to manually write the code for this pocket（凹槽，型腔）. However, we can use a CAM program to create the NC code in a matter of minutes. Step 2: The model is next imported into the CAM module. We can then select the proper geometry and define the style of toolpath to create, which in this case is a pocket. We must also tell the CAM system which tools to use, the type of material, feed, and depth of cut information.Step 3: The CAM model is then verified to ensure that the toolpaths are correct. If any mistakes are found, it is simple to make changes at this point. Step 4: The final product of CAD/CAM process is the NC code. The NC code is produced by post-processing（后处理）the model, the code is customized（定制，用户化）to accommodate the particular variety of CNC control. Another acronym that we may run into is CAPP, which stands for Computer-Aided Part Programming. CAPP is the process of using computers to aid in the programming of NC toolpaths. However, the acronym CAPP never really gained widespread acceptance, and today we seldom hear this term. Instead, the more marketable CAD/CAM is used to express the idea of using computers to help generate NC part programs. This is unfortunate because CAM is an entire group of technologies related to manufacturing design and automation-not just the software that is used to program CNC machine tools.Description of CAD/CAM Components and FunctionsCAD/CAM systems contain both CAD and CAM capabilities each of which has a number of functional elements. It will help to take a short look at some of these elements in order to understand the entire process.1. CAD ModuleThe CAD portion of the system is used to create the geometry as a CAD model. The CAD model is an electronic description of the workpiece geometry that is mathematically precise. The CAD system, whether stand alone or as part of a CAD/CAM package, tends to be available in several different levels of sophistication. （强词夺理，混合）2-D line drawings 两维线条图Geometry is represented in two axes, much like drawing on a sheet of paper. Z-level depths will have to be added on the CAM end.3-D wireframe models 三维线框模型Geometry is represented in three-dimensional space by connecting elements that represent edges and boundaries. Wiregrames can be difficult to visualize（想象，形象化，显现）, but all Z axis information is available for the CAM operations. 3-D surface models 三维表面模型These are similar to wireframes except that a thin skin has been stretched over the wireframe model to aid in visualization. Inside, the model is empty. Complex contoured Surfaces are possible with surface models.3-D solid modeling 三维实体模型This is the current state of the market technology that is used by all high-end software. The geometry is represented as a solid feature that contains mass. Solid models can be sliced（切片，部分 ，片段）open to reveal internal features and not just a thin skin. 2. CAM ModuleThe CAM module is used to create the machining process model based upon the geometry supplied in the CAD model. For example, the CAD model may contain a feature that we recognize as a pocket .We could apply a pocketing routine to the geometry, and then all of the toolpaths would be automatically created to produce the pocket. Likewise, the CAD model（模子，铸型）may contain geometry that should be produced with drilling operations. We can simply select the geometry and instruct the CAM system to drill holes at the selected locations.The CAM system will generate a generic（一般的，普通的 ）intermediate（中间的，媒介） code that describes the machining operations, which can later be used to produce G & M code or conversational programs. Some systems create intermediate code in their own proprietary（所有的，私人拥有的 ） language, which others use open standards such as APT for their intermediate files.The CAM modules also come in several classes and levels of sophistication. First, there is usually a different module available for milling, turning, wire EDM, and fabrication（装配）. Each of the processes is unique enough that the modules are typically sold as add-ins（附加软件）. Each module may also be available with different levels of capability. For example, CAM modules for milling are often broken into stages as follows, starting with very simple capabilities and ending with complex, multi-axis toolpaths : 21/2-axis machining Three-axis machining with fourth-axis positioning Surface machining Simultaneous five-axis machiningEach of these represents a higher level of capability that may not be needed in all manufacturing environments. A job shop might only require 3-axis capability. An aerospace contractor might need a sophisticated 5-axis CAM package that is capable of complex machining. This class of software might start at $5,000 per installation, but the most sophisticated modules can cost $15,000 or more. Therefore, there is no need to buy software at such a high level that we will not be able to use it to its full potential.3.Geometry vs. toolpathOne important concept we must understand is that the geometry represented by the CAD drawing may not be exactly the same geometry that is produced on the CNC machine tool. CNC machine tools are equipped to produce very accurate toolpaths as long as the toolpaths are either straight lines or circular arcs. CAD systems are also capable of producing highly accurate geometry of straight line and circular arcs, but they can also produce a number of other classes of curves. Most often these curves are represented as Non-Uniform（不均匀的，不一致的）Rational Bezier Splines (NURBS) （非均匀有理 B 样条）. NURBS curves can represent virtually any geometry, ranging from a straight line or circular arc to complex surfaces. Take, for example, the geometric entity that we call an ellipse（椭圆形）. An ellipse is a class of curve that is mathematically different from a circular arc. An ellipse is easily produced on a CAD system with the click of the mouse. However, a standard CNC machine tool cannot be use to directly problem an ellipse it can only create lines and circular arcs. The CAM system will reconcile（使和解，使顺从）this problem by estimating the curve with line segments.CNC machine tools usually only understand circular arcs or straight lines. Therefore, the CAM system must estimate curved surfaces with line segments. The curve in this illustration is that of an ellipse, and the toolpath generated consists of tangent line segments that are contained within a tolerance zone.The CAM system will generate a bounding geometry on either side of the true curve to form a tolerance zone. It will then produce a toolpath from the line segment that stays contained within the tolerance zone. The resulting toolpath will not be mathematically correct the CAM system will only be able to estimate the surface. This basic method is used to produce estimated toolpaths for both 2-D curves and 3-D surface curves.Some CAM programs also have the ability to convert the line segments into arc segments. This can reduce the number of blocks in the program and lead to smoother surfaces.The programmer can control the size of the tolerance zone to create a toolpath that is as accurate as is needed. Smaller tolerance zones will produce finer toolpaths and more numerous line segments, while larger tolerance zones will produce fewer line segments and coarser（粗糙的） toolpaths. Each line segment will require a block of code in the NC program, so the NC part program can grow very large when using this technique.We must use caution when machining surfaces. It is easy to rely on the computer to generate the correct tooolpath, but finished surfaces are further estimated during machining with ball end mills. If we do not pay attention to the limitations of these techniques, then the accuracy of the finished workpiece may be compromised（妥协，折衷） .4.Tool and material librariesTo create the machining operations, the CAM system will need to know which cutting tools are available and what material we are machining. CAM systems take care of this by providing customizable （可定制的 ）libraries of cutting tools and materials. Tool libraries contain information about the shape and style of the tool. Material libraries contain information that is used to optimize（使最优化）the cutting speeds and feeds. The CAM system uses this information together to create the correct toolpaths and machining parameters.（参数）The format of these tool and material libraries is often proprietary（专利的，独占的，私有的）and can present some portability issues. Proprietary（轻便，移动）tool and material files cannot be easily modified or used on another system. More progressive （ 改革论者，进步论者，前进的）CAM developers tend to produce their tool and material libraries as database files that can be easily modified and customized for other applications. 5.Verification and post-processorCAM systems usually provide the ability to verify that the proposed toolpaths are correct. This can be via a simple backplot（背景绘制） of the tool centerline or via a sophisticated solid model of the machining operations. The solids verifications（确认，查证）is often a third-party software that the CAD/CAM software company has licensed.（得到许可的 ） However, it may be available as a standalone package.The post-processor is a software program that takes a generic intermediate code and formats the NC code for each particular machine tool control. The post-processor（后置处理器） can often be customized through templates（模板）and variables to provide the required customization. （用户化，专用化，定制）6.Portability 轻便，可带的Portability of electronic data is the Achilles heel(唯一致命的弱点)of CAD/CAM systems and continues to be a time-consuming concern. CAD files are created in a number of formats and have to be shared between many organizations. It is very expensive to create a complex model on a CAD system; therefore, we want to maximize the portability of our models and minimize the need for recreating the geometry on another system. DXF, DWG, IGES, SAT, STL and parasolids are a few of the common formats for CAD data exchange.CAM process models are not nearly as portable as CAD models. We cannot usually take a CAM model developed in one system and transfer it to another platform. The only widely accepted standard for CAM model interchange is a version of Automatically Programmed Tool (APT). APT is a programming language used to describe machining operations. APT is an open standard that is well documented and can be accessed by third-party software developers. A number of CAD/CAM systems can export to this standard, and the CAM file can later be used by post-processors and verification software. There are some circumstances when the proprietary intermediate files created by certain CAD/CAM systems can be fed directly into a machine tool without any additional post-processing. This is an ideal solution, but there is not currently any standard governing this exchange.One other option for XAD/CAM model exchange is to use a reverse post-processor. A reverse post-processor can create a CAD/CAM model from a G &M-code of NC part program. These programs do work; however, the programmer must spend a considerable amount of time determining the design intent of the model and to separate the toolpaths from the geometry. Overall, reverse post-processing has very limited applications.Software issues and trendsThroughout industry, numerous software packages are used for CAD and CAD/CAM. Pure CAD systems are used in all areas of design, and virtually any product today is designed With CAD software-gone are the days of pencil and paper drawings.CAD/CAM software, on the other hand, is more specialized. CAD/CAM is a small but important niche（适当的位置）confined to machining and fabrication organizations, and it is found in much smaller numbers than its CAD big brother.CAD/CAM systems contain both the software for CAD design and the CAM software for creating toolpaths and NC code. However, the CAD portion is often weak and unrefined when compared to much of the leading pure CAD software. This mismatch sets up the classic（第一流的，标准的）argument between the CAD designers and the CAD/CAM programmer on what is the best way to approach CAD/CAM.A great argument can be made for creating all geometry on an industry-leading CAD system and then importing the geometry into a CAD/CAM system. A business is much better off if its engineers only have to create a CAD model one time and in one format. The geometry can then be imported into the CAD/CAM package for process modeling. Furthermore, industry-leading CAD software tends to set an unofficial standard. The greater the acceptance of the standard, the greater the return on investment for the businesses that own the software.The counter argument comes from small organizations that do not have the need or resources to own both an expensive, industry-standard CAD package and an expensive CAD/CAM package. They tend to have to redraw the geometry from the paper engineering drawing or import models with imperfect（有缺点的，未完成的 ） translators. Any original models will end up being stored as highly non-standardized CAD/CAM files. These models will have dubious（可疑的，不确定的）prospects（景色，前景，期望 ） of ever being translated to a more standardized version.Regardless of the path that is chosen, organizations and individuals tend to become entrenched（以壕沟防护） in a particular technology. If they have invested tremendous effort and time into learning and assimilating（吸收）a technology, then it becomes very difficult to change to a new technology, even when presented with overwhelming（压倒性的，无法抵抗的） evidence of a better method. It can be quite painful to change. Of course, if we had a crystal ball and could see into the future, this would never happen; but the fact is that we cannot always predict what the dominant（有统治权的，占优势的）technology will be even a few years down the road.The result is technology entrenchment（堑墩）that can be very difficult and expensive to get out from under. About the only protection we can find is to select the technology that appears to be the most standardized (even if it is imperfect) and stay with it-then, if major changes appear down the road, we will be in a better position to adapt.