Establishing an accurate and efficient parts information model is a core aspect of CAD/CAPP/CAM integration. Currently, the parts information model is primarily based on feature modeling technology. The common approach involves first building a feature library according to feature classification, then invoking basic features as needed for modeling, and finally constructing the part model through Boolean operations between features. However, this method has several limitations. Firstly, to support the creation of various complex part models, the feature library tends to include as many basic features as possible, which is challenging to achieve effectively. Secondly, current feature recognition technology is still not mature, making it difficult to manage and control the feature database efficiently. Thirdly, during actual part modeling, designers often struggle to quickly and accurately select the required features, which significantly slows down the modeling process. Lastly, there is no one-to-one correspondence between existing feature classification methods and machining methods. A single machining method may correspond to multiple basic features, which should be stored as composite features in the library. This is currently impractical and remains a challenge to resolve. In response to these issues, this paper first defines the concept of the parts information model. Based on an analysis of the connecting rod manufacturing process, it conducts feature planning and design, and then uses destructive modeling with features to directly construct the part model. This approach successfully unifies feature design with the machining process, ensuring that each feature corresponds directly to a specific machining operation of the connecting rod. 2 Feature-Based Part Information Model A feature is a unit that fully expresses part information, combining certain shapes, semantics, and abstractions [1]. A complete part model is more than just a collection of data; it reflects how different types of data are expressed and their relationships. Only part models based on clear expressions can be effectively used by various application systems. A comprehensive parts information model should include management features, shape features, precision features, material features, and technical features, as shown in Figure 1. (1) Shape Features: These describe functional geometric information with engineering significance, divided into primary and secondary features. Primary features build the main structure of the part, while secondary features modify or attach to the primary ones. Shape features are central to product design and manufacturing and serve as carriers for other information. (2) Precision Features: Used to describe dimensional tolerances, geometric tolerances, and surface roughness. Dimensions and tolerances are crucial attributes in design and manufacturing. In feature design, they are analyzed and directly incorporated into the part information model, clearly representing non-geometric properties and relationships between features. (3) Material Features: Describe the type code, performance, heat treatment, and surface treatment of the part's material. (4) Technical Features: Describe the performance and functionality of the part. (5) Management Features: Include part name, designer, date, quantity, figure number, version, and other administrative details. The geometry and topology of the part form the basis, while the feature layer is the core. Relationships among various sub-models in the feature layer reflect semantic connections between features. These features have high-level engineering meaning and support CAPP, NC programming, and machining simulation requirements. 3 Establishment of a Three-Dimensional Parts Information Model The key to building a parts information model lies in effective feature planning, as illustrated in Figure 1. Using direct modeling techniques, the structure can be designed hierarchically, with parameterized feature modules established at different levels. Each feature is defined by a set of parameters that uniquely determine its shape. Taking the connecting rod in a diesel engine as an example, the modeling method and design steps of the 3D part information model are explained using Pro/ENGINEER software. 3.1 Link Function and Structure Analysis The connecting rod is a critical component of the engine, as shown in Figure 2. It transmits the pressure from the expanding gas acting on the piston top to the crankshaft, driving it to rotate, while being driven by the crankshaft to compress the gas in the cylinder. The connecting rod has a complex structure, typically divided into two parts: the connecting rod body and the connecting rod cover at the large end. The connecting rod shaft has an I-shaped cross-section, tapering from the large head to the small head. Without proper feature planning, directly building a 3D model of the connecting rod using feature modeling technology can lead to failures and poor results, as the structure is not simply a matter of adding or subtracting features. Figure 1 The overall model of the feature-based part information model Figure 2 The characteristics of the connecting rod 3.2 Analysis of Connecting Rod Machining Process Linkage feature design is closely tied to machining. Each machining method corresponds to a specific feature, which is the foundation of feature planning. The connecting rod blank is a forged part, with the connecting rod body and cover forged together. The main machining processes include: milling both ends, drilling and enlarging the small hole, pulling to ensure size and surface roughness, milling the head positioning boss, cutting the lower cover, drilling bolt holes, machining threads, and finally assembling the connecting rod and cover with bolts and prying the big hole. 3.3 Feature Planning and Design Based on the analysis of the connecting rod’s function, structure, and machining characteristics, the connecting rod model is divided into the feature level shown in Figure 2, composed of independent features. 3.4 Feature Modeling of the Connecting Rod Based on Pro/ENGINEER 3.4.1 Solid Model This paper uses a feature-reduction modeling method to create the solid model of the connecting rod. The feature-reduction method involves first creating the blank model, then gradually removing features to establish the final part model. Below is the detailed modeling process of the connecting rod: 1. Connecting Rod Modeling Process (1) Determine the parting surface and draft angle. Selecting a reasonable parting surface is the first step in forging production, so it must be determined early in the modeling process. (2) Use the "stretch" method to generate the tie-in model. (3) Use the "pull touch" method to create a 7° draft angle. (4) Use the "curved cutting material" method along with the "rounding" function to produce the middle connecting part of the rod body. (5) Use the "cut material" method to shape the connecting rod. (6) Use the "cut material" method to create a punched skin at the large head hole location. The connecting rod blank is shown in Figure 3. 2. Connecting Rod Modeling According to Machining Process (1) Use the "cut material" method to create the large and small end faces to meet dimensional requirements. Machining the large and small end faces is usually the first step in the connecting rod process, as it serves as the main reference surface and greatly affects the overall quality. (2) Use the "co-axial hole" method to generate the small hole and ensure its size and surface roughness. (3) Generate the large head positioning boss using the "rotation cut material" method. (4) Cut the large end of the connecting rod in a “CUT†manner, dividing it into the connecting rod body and cover. This division is necessary for subsequent processing and assembly. (5) Use the "stretch cut material" method to pry the nut bosses on the connecting rod cover, drill the bolt holes using the "hole" method, and use the "helical scan cut material" method to generate the threads. (6) Assemble the connecting rod and cover with bolts and pry the big hole. The big head hole must fit closely with the bearing shell and crankshaft, while the small head hole connects to the piston pin, reducing impact and facilitating heat transfer. Ensuring the shape and tolerance of both holes is critical, so the big head hole must be built in the model alongside the small hole. At this point, the three-dimensional geometric model of the connecting rod has been completed. 3.4.2 Construction of Other Features The accuracy features of the connecting rod were created directly on the geometric model using Pro/ENGINEER. Material features are attached to the model in text form. By selecting “Settings†→ “Material,†users can define, modify, or delete material parameters in a text file. Based on the performance requirements, the connecting rod is made of 45# steel. Technical and management features can be added through external programs. After completing these steps, a full 3D part information model is established, allowing for automatic generation of the parts map. Figure 4 shows the 3D model of the connecting rod generated through this process. 4 NC Program and Machining Simulation Pro/ENGINEER provides powerful Pro/NC modules for designing NC machining and manufacturing programs. It allows for the establishment of a 3D machining simulation environment, automatically generating NC programs, simulating tool paths, observing cutting conditions, verifying over-cutting, interference, and prediction errors, and avoiding processing failures. Pro/NC uses image-based programming techniques, making the programming logic clear and reducing human error caused by manual uncertainties. This method allows each machining process to be treated as a group of shape features. By integrating CAD and CAM via CAPP, it receives part information directly from CAD, generates related documentation, and produces NC code. This enables CNC programmers to work with higher-level features rather than low-level geometric data, improving programming efficiency. Once the NC program is verified, the CL DATA generated by the manufacturing process is converted by Pro/NCPOST to obtain the required NC CODE for actual machining. According to the Pro/NC design process, the plane machining simulation of the connecting rod is shown in Figure 5. Figure 3 Linkage rough model 5 Concluding Remarks This paper presents a reasonable planning and design of the connecting rod's features, constructs its information model, and uses the Pro/NC module to complete the machining simulation and automatic programming. It verifies the correctness of the feature subtraction modeling method and basically realizes the integration of CAD/CAM for connecting rods, thereby improving design efficiency. References 1 Wei Shengmin, Zhu Xilin, Chief Editor. Mechanical CAD/CAM[M]. First Edition. 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Figure 4 Three-dimensional information model of connecting rod
Figure 5 Simulation of connecting rod machining process
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