1. Numerical control programming technology and its development
CNC programming is one of the most effective aspects of CAD/CAPP/CAM systems. It plays an important role in achieving design automation, improving machining accuracy and processing quality, and shortening product development cycle. There are a large number of applications in areas such as the aerospace industry and the automotive industry. Due to the strong demand for actual production, CNC programming technology has been extensively studied at home and abroad, and has achieved fruitful results. Here are some introductions to NC programming and its development.
1.1 The basic concept of CNC programming
CNC programming is the whole process from part drawing to obtaining CNC machining program. Its main task is to calculate the cutter location point (CL point). The tool point is generally taken as the intersection of the tool axis and the tool surface, and the tool axis vector is also given in multi-axis machining.
1.2 Overview of the development of CNC programming technology
In order to solve the programming problem in CNC machining, in the 1950s, MIT designed a language specifically for CNC machining programming of mechanical parts, called APT (Automatically Programmed Tool). Since then, APT has evolved to form such as APTII, APTIII (for three-dimensional cutting), APT (algorithm improvement, increase multi-coordinate surface programming function), APT-AC (Advanced contouring) (additional cutting database management system) and APT- /SS (Sculptured Surface) (added to the sculpture surface processing programming function) and other advanced versions.
The use of APT language to program NC programs has the advantages of simple program and flexible tool control, which makes CNC machining programming from the "assembly language" level for machine tool instructions to the geometric element APT. There are still many inconveniences: using language to define parts Geometric shape, it is difficult to describe complex geometric shapes, lack of geometric intuitiveness; lack of visual representation of part shape, tool motion trajectory and tool path verification; difficult to effectively connect with CAD database and CAPP system; not easy to make height Automation, integration.
In response to the shortcomings of the APT language, in 1978, Dassault Aircraft Corporation of France began to develop a system integrating 3D design, analysis and NC machining, called CATIA. Subsequent systems such as EUCLID, UGII, INTERGRAPH, Pro/Engineering, MasterCAM and NPU/GNCP have emerged. These systems effectively solve the geometric modeling, the display of part geometry, interactive design, modification and tool path generation. The simulation display and verification of the knife process promoted the development of CAD and CAM in the direction of integration. In the 1980s, based on the CAD/CAM integration concept, the concepts of Computer Integrated Manufacturing System (CIMS) and Concurrent Engineering (CE) were gradually formed. At present, in order to meet the needs of CIMS and CE development, CNC programming systems are developing in the direction of integration and intelligence.
In terms of integration, the development of parametric feature modeling system conforming to the standard of STEP (Standard for the Exchange of Product Model Data) has been carried out with a lot of fruitful work. It is a hot spot for development at home and abroad; in terms of intelligence, The work has just begun, and we have to work hard.
2. Research and development status of NC tool path generation method
The core work of NC programming is to generate the tool path, then discretize it into tool positions, and generate the NC machining program through post processing. Here are some introductions to the tool path generation method.
2.1 NC tool path generation method based on point, line, surface and body
Starting from 2D drawing, CAD technology has experienced the development stage of 3D wireframe, surface and solid modeling, and has been parameterized feature modeling up to now. In the two-dimensional drawing and three-dimensional wireframe stage, CNC machining mainly uses points and lines as the driving object, such as hole machining, contour machining, and plane machining. This type of processing requires a high level of operator and complex interaction. Entity-based machining occurs during the development of surfaces and solid modeling. The processing object of solid processing is an entity (generally represented by a mixture of CSG and B-REP), which is obtained by some basic voxels through set operations (combination, intersection, and difference operations). Solid machining can be used not only for roughing and semi-finishing of parts, but also for cutting out large margins and improving machining efficiency. It can also be used for the research and development of feature-based CNC programming systems, which is the basis of feature processing.
Solid machining generally has two types of solid contour processing and solid area processing. The solid machining method is realized by the layer cutting method (SLICE), that is, a set of horizontal planes is used to cut the processed body, and then the equidistant line is generated as the path of the path.
In this paper, from the perspective of system requirements, this kind of numerical control machining based on point, line, surface and solid is realized on ACIS geometric modeling platform.
2.2 Feature-based NC tool path generation method
Parametric feature modeling has a certain development period, but the research on feature-based tool path generation method has just begun. Feature processing makes NC programmers not operate on low-level geometric information (such as points, lines, faces, and entities), and transforms them into NC programming directly for features that are suitable for engineers and technicians, greatly improving programming efficiency.
In their research, WRMail and AJMcleod presented a feature-based NC code generation subsystem that works by treating each part of a part as a sum of the shape feature sets that make up the part. . Then, the machining of the parts is completed after the entire shape feature or the shape feature group is processed separately. The NC code for each shape feature or shape feature set is automatically generated. The currently developed system is only suitable for the processing of 2.5D parts.
Lee and Chang developed a system that automatically generates a convex free-form feature tool path using a virtual boundary method. The system works by embedding a minimal long square in a convex freeform surface so that the convex freeform feature is converted into a concave feature. The combination of the smallest long square and the final product model constitutes an indirect product model called a virtual model. The tool path generation method is divided into three steps: (1) cutting polyhedral features; (2) cutting free-form surface features; (3) cutting intersecting features.
Jong-Yun Jung studied the feature-based non-cutting tool path generation problem. The article divides the feature-based machining path into two types: contour machining and inner region machining, and defines the cutting direction of these two types of machining, and achieves the goal of optimizing the tool path by reducing the cutting tool path. The article focuses on several basic features (holes, recesses, steps, grooves), discusses the typical path of these basic features, tool selection and processing order, and avoids repeated passes through IP (Inter Programming) technology. Optimize the non-cutting tool path. In addition, Jong-Yun Jong also studied manufacturing feature extraction and feature-based tool and toolpaths in his 1991 PhD thesis.
The basis of feature processing is solid machining, which of course can be considered as more advanced solid machining. However, feature processing is different from physical processing, and physical processing has its own limitations.
Feature processing and physical processing have the following main differences:
Conceptually, the feature is the functional element of the component, which is in accordance with the operating habits of engineers and technicians, and is well known to engineers and technicians; the entity is a low-level geometric object, which is a geometry obtained through a series of Boolean operations, without Any functional semantic information; solid processing is often a one-time processing of the entire part (entity). However, in fact, a part is unlikely to be processed in one time with only one knife. It often has to undergo a series of steps such as roughing, semi-finishing, finishing, etc. Different parts of the part are usually processed with different tools; sometimes one Parts are used for both turning and milling. Therefore, solid machining is mainly used for roughing and semi-finishing of parts. The feature processing solves the above problems in essence;
Feature processing has more intelligence. Certain fixed processing methods can be specified for a particular feature, especially those that have been specified in the STEP standard. If we have a specific machining method for all standard features, then the convenience of machining parts that are made up of standard features can be imagined. If the CAPP system can provide the corresponding process features, the NCP system can greatly reduce the interactive input and have more intelligence. And these physical processing is impossible to achieve;
Feature processing facilitates the full integration from CAD, CAPP, NCP and CNC systems, and realizes the two-way flow of information, laying a good foundation for CIMS and even parallel engineering (CE); physical processing is powerless for these.
2.3 Analysis of NC Tool Path Generation Methods in Several Major CAD/CAM Systems in Active Service
The composition and main functions of active CAM
At present, the more mature CAM system mainly realizes CAD/CAM system integration in two forms: integrated CAD/CAM system (such as UGII, Euclid, Pro/ENGINEER, etc.) and relatively independent CAM system (such as Mastercam, Surfcam, etc.). ). The former obtains the product geometry model directly from the CAD system in an internal unified data format, while the latter acquires the product geometry model from other CAD systems mainly through neutral files. However, no matter which form of CAM system, it consists of five modules, namely, interactive process parameter input module, tool path generation module, tool path editing module, 3D machining dynamic simulation module and post processing module. The following is only a discussion of some of the well-known CAD/CAM system NC processing methods.
Analysis of UGII processing methods
UGII is generally considered to be the best and most representative CNC software in the industry. Its most characteristic is its powerful tool path generation method. Including cutting, milling, wire cutting and other perfect processing methods. Among them, milling mainly has the following functions:
Point to Point: complete a variety of hole processing;
Panar Mill: Face milling. Including one-way row cutting, two-way row cutting, loop cutting and contour processing;
Fixed Contour: Fixed multi-axis projection machining. Use the projection method to control the movement of the tool on a single surface or multiple surfaces. The tool movement can be generated by a generated tool path, a series of points or a set of curves;
Variable Contour: variable axis projection processing;
Parameter line: Equal parameter line processing. Continuous processing of single or multiple surfaces;
Zig-Zag Surface: cutting surface processing;
Rough to Depth: Roughing. Roughing the blank to a specified depth;
Cavity Mill: Multi-stage deep cavity machining. Particularly suitable for roughing of punches and dies;
Sequential Surface: Surface intersection processing. Provide maximum control of tool movement in accordance with the idea of ​​the part face, the guide face and the inspection face.
EDS Unigraphics also includes a host of other features that are not listed here.
STRATA processing method analysis
STRATA is a CNC programming system development environment built on the ACIS geometric modeling platform.
It provides users with two programming development environments, namely the NC command language interface and the NC operating C++ class library. It supports three-axis milling, turning and wire-cut NC machining, and supports wireframe, surface and solid geometry modeling. The NC tool path generation method is based on the solid model. The processing methods provided by STRATA's entity-based NC tool path generation class library include:
Profile Toolpath: contour processing;
AreaClear Toolpath: plane area processing;
SolidProfile Toolpath: physical contour processing;
SolidAreaClear Toolpath: physical flat area processing;
SolidFace ToolPath: solid surface processing;
SolidSlice ToolPath: solid section plane processing;
Language-based Toolpath: Language-based tool path generation.
Other CAD/CAM software, such as Euclid, Cimitron, CV, CATIA, etc. have their own functions, but the basic content is similar, there is no essential difference.
2.4 Main problems of the method of generating the tool path of the active CAM system
According to the traditional CAD/CAM system and the working mode of the CNC system, the CAM system obtains the geometric data model of the product from the CAD system directly or indirectly (through neutral files). The CAM system generates the machining tool path by using points, lines, faces, or solids in the 3D geometric model, and processes the tool path in the form of a tool positioning file, and provides it to the CNC machine in the form of NC code. The following problems exist in the operation of /CAM and CNC systems:
The CAM system can only obtain the low-level geometric information of the product from the CAD system, and cannot automatically capture the geometric information of the product and the functional and semantic information of the high-level product. Therefore, the entire CAM process must be done through graphical interaction with the participation of experienced manufacturing engineers. For example, the manufacturing engineer must select the machining object (point, line, surface or solid), constraints (clamping, interference and collision), tool, machining parameters (cutting direction, depth of cut, feed rate, feed rate, etc.) . The entire system is less automated.
In the tool path generated by the CAM system, only the lower layer geometry information (the geometric positioning information of the line and the arc) and a small amount of process control information (such as feed rate, spindle speed, tool change, etc.) are also included. As a result, downstream CNC systems are unable to obtain higher-level design requirements (such as tolerances, surface finishes, etc.) or process parameters associated with generating tool paths.
The product data between the modules of the CAM system is not uniform, and the modules are relatively independent. For example, the tool positioning file only records the tool path without recording the corresponding machining process parameters. The 3D dynamic simulation only records the interference and collision of the tool path, and does not record the machining object and related machining process parameters that interfere with and collide with it.
The CAM system is a standalone system. There is no unified product data model between the CAD system and the CAM system. Even in an integrated integrated CAD/CAM system, information sharing is only one-way and single. The CAM system cannot fully understand and utilize all the information about the CAD system related products, especially the processing related feature information. Similarly, the CAD system cannot obtain the processing data information generated by the CAM system. This has brought difficulties to the implementation of concurrent engineering.
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