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Feature-Based Mechanical Product Modeling
**Abstract:**
Based on the analysis of the modeling process of mechanical product features, a feature-based product information model was developed. The study focused on the method of constructing product features using parametric modeling techniques. It explored the structure and functions of the feature modeling system and clarified its application. The paper also outlined the methods and steps involved in designing mechanical product features.
**Keywords:** Solid Modeling, Feature Modeling
**1. Feature Modeling Method**
Mechanical product design is a complex decision-making process involving multiple factors and cycles. To meet the demands of social development and market competition, modern product design methods that leverage high-tech platforms are constantly being sought. The feature-based design approach emerged in response to CAD/CAM integration needs. It is built upon solid modeling methods and is more suitable for computer-integrated manufacturing systems. While it shares similarities with physical modeling in principles and methods, it has distinct differences:
(1) While 3D wireframes, surfaces, and solid models focus on geometric descriptions, they often neglect engineering significance, leading to inconsistencies between design and manufacturing information. In contrast, feature modeling captures complete technical and production management data, preserving the original definitions and relationships of functional elements. This allows a unified product model to replace traditional drawings and technical documents, enabling parallel execution of design and preparation tasks.
(2) In feature modeling, reference points, center lines, local coordinate systems, and protruding surfaces (such as mating, supporting, positioning, and datum surfaces) are considered. These surfaces differ from general geometric ones and must be easily identifiable through their shaping and positioning dimensions. This requires the inclusion of isolated points, lines, and faces outside the 3D object, along with non-manifold and non-rule sets, expanding Euler’s operations.
(3) Feature modeling operates at a higher level, where the objects are functional elements like threaded holes, keyways, bosses, and flanges. This requires local operation and size-driven technology, as well as new data structures and feature combination algorithms. Features directly reflect design intent, making the model easier to understand and reducing design time.
(4) Geometric and non-geometric information within features is passed through all subsequent stages, with timely feedback. This strengthens communication between design, analysis, process planning, manufacturing, and inspection departments, promoting standardization and serialization in product and process design.
In conclusion, feature-based modeling is the core of virtual product design, providing support for design, manufacturing, and production management. It lays the groundwork for intelligent CAD and manufacturing systems, making it essential to study feature-based modeling in mechanical products.
**2. Feature Model Information Description**
From the perspective of CIMS and virtual design, feature classification is illustrated in Figure 1.
[Image: Feature Classification]
The generation of part models does not rely on voxel consolidation but emphasizes the role of various surfaces such as datum planes, work planes, and connection planes. It is necessary to process and record the inheritance, adjacency, subordination, and citation links between different features. Based on the connections between features, the instance of the feature class is defined as an object, resulting in a feature-oriented connection diagram shown in Figure 2.
[Image: Feature Contact]
A feature attribute set includes three attributes: parameter attributes, which describe the shape composition and non-geometric information; constraint attributes, which define internal constraints and relationships between features; and association attributes, which describe mutual constraints or references between this feature and others.
According to the definition of features and their connections, a hierarchical structure based on the feature part information model is established, divided into three layers: component layer, feature layer, and geometric layer. The geometric information of the part is expanded in layers to extract information according to different needs. The component layer reflects overall part information, the feature layer contains combinations of submodels and their relationships, and the geometric/topological information of the B-REP structure is the basis of the entire model.
**3. Parametric Feature Modeling Method**
Parametric feature modeling uses variable geometry and generation process-based methods to construct and edit features. A geometric model is defined by a series of feature points, with nonlinear constraint equations formed using these coordinates as variables. Dimensional constraints, such as length, radius, and intersection angle, are determined, along with azimuth or relative position relationships. When constraints change, iterative methods solve the equation system to generate new feature points and thus a new geometric model.
By generating 3D models from simple models, all information during the generation process is recorded, and quantitative data is used as variable parameters. Assigning different values to these parameters updates the model generation process, resulting in different sizes or shapes. This type of model is often used for parametric modeling of 3D solids, with the generation process forming a tree structure where leaves represent basic sub-models and the root represents the final model.
When parametric modeling is based on the model's generation history, the objects that can be parameterized include basic model data and various operation parameters in the history tree. Parameterized dimensions and imposed constraints are retained in the model generation process. The parameterizable basic model data refers to the geometric dimensions of various voxel feature sizes and plane shapes. Intermediate or final models are generated through operations, and the tree contains various types of operation parameters related to Boolean operations, scan conversion, rounding, chamfering, and positioning.
Since processing environments, production scales, degrees of product similarity, standardization, and serialization vary, a parametric design method is used for feature design. For standardized and serialized products, changes in each part family can be controlled by a set of parameters or described using variation laws, allowing geometric features and processing of each part family to be expressed through parameters and variation laws.
**4. Feature Modeling System Structure and Function**
Based on existing geometric modeling systems, interactive feature recognition and feature modeling methods were used to develop and research a feature-based mechanical product modeling system. The system architecture is shown in Figure 3.
[Image: Overall System Architecture]
**4.1 Feature Definition and Editing Subsystem**
This subsystem defines parameterized entities, attribute information patterns, and processing knowledge rules. It includes general design features, dynamically generated compound features, and custom features defined through parametric methods.
A Boolean processor with operations such as union, intersection, and difference is included, ensuring calculation accuracy meets design and manufacturing requirements. It provides feature editing functions, including modification, addition/subtraction, size drive, recognition, replacement, and movement, allowing designers to generate part models efficiently for downstream processes.
**4.2 Feature Library Management Subsystem**
Mechanical parts are categorized into five groups: shafts, discs, curved bodies, cases, and brackets. Feature classification is based on part families, and the feature database is managed using group technology. Each feature library expresses specific component clusters, enabling users to analyze component characteristics and extract relevant features.
**4.3 Database Operation and Management Subsystem**
This subsystem supports effective management and maintenance of the concurrent design engineering database. It includes functions such as initialization, extension, central operations, entity operations, and database restoration. It performs tasks like retrieval, backup, editing, restoration, pointer management, decoding, sorting, and space compression.
**4.4 Product Engineering Drawing Subsystem**
It provides two-dimensional sketch modeling and direct generation of engineering drawings from 3D models. It offers a set of commands and functionalities for graphic editing, including cut, extend, translation, rotation, copy, fillet, hatching, dimensioning, and complex 2D graphics, enabling designers to quickly create 2D drawings.
**5. Application Examples**
A low-speed, high-torque reducer is a mechanical transmission device with large carrying capacity. According to user specifications, the gear box was analyzed to achieve small size, light weight, and high load capacity. A rigid box with a welded structure was proposed, with double-walled front and rear walls and box-shaped stiffeners in areas of maximum stress. Two positioning beams and reinforcing ribs were added to reduce distortion. The feature design method for the gearbox involves constructing the base body, creating the part information model, building the shape feature model, generating the shape feature, creating the geometric model, editing features, and building a digital simulation cabinet.
**6. Conclusion**
Feature modeling is a key technology in product virtual design and a milestone in CAD/CAM technology. The research starting point for feature-based modeling of mechanical products is high, requiring the comprehensive application of modern design theories and advanced technologies. The system function needs continuous improvement to meet the requirements of modern mechanical product design.