Research and implementation of CAPP development en

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Research and practice of CAPP development environment for welding plate feature assembly

Abstract: the development and principle of CAPP system are briefly introduced. Taking the development of CAPP system for welding parts such as support and box under integrated environment as an example, the development and practice of CAPP system for welding parts under integrated environment are emphatically discussed

key words: CAPP; Integrated environment; Welded parts; Knowledge base


CAPP (Computer Aided Process Planning) technology has attracted more and more attention, and has developed rapidly in recent years. CAPP is the bridge and link between CAD and cam, and it is an important link in the integrated production environment. As the main content of production preparation of machinery manufacturing enterprises, process design plays an important role in ensuring product quality, shortening production cycle, reducing costs and improving economic benefits

1 CAPP principle

capp is the basic principle of using computer to generate process. It is to store the standardized or optimized process or the logical idea of compiling process (the knowledge and experience accumulated by process personnel for a long time) into the computer through the CAPP system. When generating process by computer, the CAPP system first reads the information about parts, and then identifies and retrieves the composite process and related processes of a part family, After deletion and (derivative); Or automatically generate the process of specific parts by reasoning (generative) according to the process decision logic. The computer can only generate the technological process in the way specified by CAPP software, but cannot create new technological methods and new processing parameters

2 CAPP development strategy

(1) the description of product information should meet the requirements of different enterprises. Product information description can be summarized into two aspects: (1) product drawing information description method; (2) CAD information description method. Because there are still many difficulties and quite a long time to realize the integration between CAD and CAPP in many enterprises, it is necessary to use the product drawing information description method for such enterprises in a few years. In addition, some enterprises with good computer application conditions need to realize the integration of cad/capp/cam. When designing and developing tools, they should be based on step files and equipped with information conversion tools to identify and use all kinds of information described by CAD on parts and generate process procedures. Based on the advanced CAPP architecture, according to the functional requirements of different types of CAPP, each task function module is divided, and the corresponding construction tools of each module are developed, such as part information construction tools, process design construction tools, etc

(2) new technologies such as expert system, feature modeling technology, concurrent engineering technology of object-oriented program and graphic design and information integration technology should be introduced in the development and design, so that the CAPP system generated by this tool can adapt to the development of manufacturing technology, facilitate the integration of CIMS, and pay attention to the coexistence of universality, progressiveness and flexibility of the system at the same time

(3) CAPP development should have the diversity of process design methods. According to the research and practice of CAPP for many years, to meet the requirements of process design of various parts, CAPP system must have a variety of control strategies

(4) principle of ca1pp development tool the purpose of developing CAPP development tool is to make the development tool universal. It provides a design environment for establishing CAPP systems for different types of products in different production environments. Through this tool, users can quickly generate a practical CAPP system for a certain type of products through necessary input preparation and simple and standardized secondary; At the same time, the CAPP system constructed by the development tools has different knowledge base contents. The knowledge in each knowledge base is simple and advanced, represents the development level of current process and manufacturing technology, and tracks the development of high technology [1]. Abstract the problem and turn it into a component. Users can construct a prototype of practical system engineering by using the CAPP development tool. Under the guidance of friendly interface, users can input relevant process design knowledge and data according to specific regulations, and then a specific CAPP system facing specific processing objects, specific manufacturing environment and specific process habits can be formed. The construction principle is shown in Figure 1

3 development of CAPP system in integrated environment

CAPP overcomes the shortcomings of low design efficiency, poor consistency and low standardization in manual process design. Computer integrated manufacturing system (CIMS) is a new mode of modern production organized by factories. It integrates decentralized automation into an optimized whole through related technologies. Cad/capp/cam is recognized as the key of CIMS, and CAPP is the link between CAD and CAPP. The CAPP system of welding and assembling parts, which has a large demand and good practicability, is in urgent need

3.1 CAPP and composition of welded plate feature assembled parts

Figure 2 welded plate feature unit

the following characteristics can be obtained by analyzing the welded assembled support and box parts: the shape and size of the parts are greatly different and changeable, but the local structure is basically stereotyped or similar, which can be divided into standard structure or non-standard structure (similar structure or typical structure), Then the non-standard structure is summarized into typical structure. It is difficult to divide the part family according to the similarity of part shape or process by derivation; If we give consideration to the manufacturability of the outline shape of the support display, it can be divided according to the functional characteristics of the parts. The principle of dividing each unfolded part is based on the idea of group technology, that is, it is divided into several units according to the structure, function and auxiliary characteristics of the shape. Each unit is generated interactively and gradually assembled into the whole part. Figure 2 lists the contour feature units of some unfolded parts of support parts. While completing the graphic assembly, the system uses feature codes to distinguish in order to reflect the sequence of various features selected by the designer. Based on the needs of the above design ideas and functions, the system adopts the structure of knowledge integration, modular layered parallel structure, and the integration of data management and graphics processing with the growth of small household appliance market, so that it can achieve the process of graphics parameterization and automatic assembly synchronously under the control of the system. The overall structure of the system is shown in Figure 3. The whole system is mainly composed of two relatively independent modules: CAD design module; CAPP design module

Figure 3 data flow chart of the system

3.2 decision making mechanism of CAPP system

the process design module of this system is a generative process design mode

(1) the traditional retrieval CAPP system of process decision knowledge base stores typical process procedures for a group code, and the workpiece needs to match all code points; At the same time, group codes correspond to process procedures one by one, which cannot express the various feasible processes of parts. The parts of this system use generative method to make decisions, and previous research on knowledge-based expert system has tried to improve the conductivity of structural adhesive bonding by adding metal particles or carbon nanotubes to the bonding material. The process knowledge acquisition of the function creation module in the system is based on the actual process decision-making rules formulated for the welding parts. For example, the cutting or blanking process of welded support parts is often relatively consistent. As shown in Figure 2, the bottom plate, support plate, reinforcing plate, rib plate and other parts of welded plates often do not need bending, deep drawing and other processes, and the final shape can be obtained only through the blanking and blanking processes. Therefore, the process design task can be divided into three sub modules, and the corresponding knowledge base can be established for each sub module, that is, ① select the stamping (cutting) process necessary for processing according to different types of sheet parts; ② Carry out detailed design for the determined part processing sequence; ③ For the part processing sequence formed by the first two modules, select the corresponding processing equipment, process equipment, etc. one by one

after the decomposition of sub modules, the original large-scale design tasks are divided into smaller design tasks according to different stages of the design process. This not only reduces the scale of solution and makes the design goal more clear, but also makes it easy to establish the corresponding knowledge base of each sub module. At the same time, the rule expression is simple and the reasoning speed is faster

(2) formulation of process decision-making rules

① process selection knowledge base due to the different shape, size, accuracy requirements, batch size and raw material properties of stamping or cutting sheet metal parts, we can see that the installation natural environment and operation methods of steel zigzag experimental machine are the same, and the stamping process methods used are also different, but generally speaking, stamping process can be divided into separation process and forming process. The separation process includes cutting, punching, cutting, trimming, etc; The forming process includes bending, drawing, local forming, etc. Operation selection rules are used to determine which of the above stamping hardness operations are required for the currently designed parts in processing

② process design knowledge base process design is to make a detailed decision design for each process that has been determined to be used, analyze whether these processes are decentralized or centralized, and finally form a processing process sequence. In order to reduce the search space of knowledge rule set and clarify the scope of each process design, the process design rule set is divided into rule subsets. The rules in each rule subset are formulated for a certain stamping process object. In this way, the structure of the rule set is clearer, the decision-making speed is faster, and the fault tolerance is good

③ auxiliary can realize complex multi-step nested loop control Functions such as automatic return, automatic fracture judgment and automatic zeroing can be set to design the knowledge base. In order to make the CAPP system practical, it is not enough to only infer the above process sequence, but also need to be able to form process documents that conform to the actual production. In this way, CAPP system has the decision-making knowledge of process content design, including: machine tool selection rule set, tooling selection rule set, etc., and finally can generate detailed process procedures

3.3 data storage and exchange

in the integrated environment, the information selected or entered by the user needs to be saved in the database before it can be used. The system adopts the document view structure to save and exchange information, that is, the information required by CAD and CAPP and the dynamic data directly input by users are saved in the document class, that is, they are defined as the public data members of the document class, so that the view class can also be called [2]

in the integrated environment, as shown in Figure 3, the document view class is used to store the initial information, intermediate information and final design results of parts. The system divides the knowledge base into three parts: process selection knowledge base, process design knowledge base and rule set. The first two parts store narrative knowledge, such as operation set, work step set and header information set; The latter stores process knowledge, such as processing method selection, process route arrangement, tooling, equipment selection and other process decision-making knowledge. Because the system adopts visualc++ object-oriented programming language, the system pays more attention to the relationship between various class objects and the cohesion within class objects, which makes the expression of knowledge quite free, and the search and matching convenient and fast

3.4 implementation of CAPP inference engine

the system reasoning module is divided into three steps: (1) process selection reasoning; (2) Process design reasoning; (3) Select equipment, tooling and auxiliary design reasoning. The process design of welded plate support parts generally follows the following steps: first, expand the welded plate support, such as base plate, support plate, reinforcing plate, rib plate and other non flat stamping parts into a plane, calculate the size of the unfolded parts, and determine the blanking size; Then it is classified into feature graphics menu and stamping process according to part features

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