October 17, 2025
Data Interface Technology in Flexible Sheet Manufacturing System
We developed a computer-aided design and manufacturing system tailored for sheet metal products as part of a flexible manufacturing system for a technology company based in Shenyang. The system is composed of three key subsystems: CAD, data interface, and CAPP. The sheet metal CAD subsystem is built on AutoCAD, leveraging its general drawing capabilities for secondary development. The CAD/CAPP data interface subsystem processes the graphical information from the CAD module and transfers it to the CAPP system, enabling internal data integration, real-time information delivery, and efficient processing.
**1. Graphical Data Features of Sheet Metal Products**
The sheet metal CAD subsystem is designed using a parametric approach. By inputting a few key parameters, the system automatically generates both 3D views and flat (unfolded) 2D representations of the sheet metal parts. These unfolded images are two-dimensional graphics that serve as the basis for further processing. The CAD/CAPP data interface subsystem processes the *.DXF files generated from these drawings and sends the relevant data to the CAPP system, ensuring accurate blanking operations.
Sheet metal product design and manufacturing involve unique structural, material, and process characteristics. For instance:
- **Expanding Drawings**: These consist of lines, arcs, and circles, all lying on a single plane. Curves are typically represented through linear or arc interpolations.
- **Process Features**: Parameters such as roughness, positioning accuracy, and straightness are guaranteed by NC machines.
The CAD system uses various modeling techniques, including geometric models, wireframe models, face frame models, and voxel models. Tolerances and technical specifications are handled through label information. However, the CAPP system requires process features and manufacturing information rather than detailed geometric data. This mismatch poses challenges when directly using *.DXF files from the CAD system, as they often contain unnecessary non-processing information such as dimension lines and auxiliary lines, which can lead to confusion in the CAPP subsystem and result in incorrect NC operation instructions.
**Issues with *.DXF Files**:
- **Geometric Intersections**: Many intersections in *.DXF files may not align precisely, causing inaccuracies in automatic process programming.
- **Topological Disorganization**: The lack of clear topological relationships between graphic elements can prevent the formation of closed loops, making the data unsuitable for CNC machining.
- **Lack of Rotation Direction**: The *.DXF file does not provide clear direction for inner or outer contours, requiring additional analysis to determine the correct processing sequence.
**2. Solutions**
**2.1 Eliminate Issues within the CAD Subsystem**
When designing the overall CAD subsystem, we took into account the requirements of the CAPP system to ensure compatibility and reduce errors during data transfer.
**2.2 Establish an Independent Data Interface**
This method leverages the AutoCAD platform for CAD development while focusing on creating a dedicated data interface for the CAPP system. The interface software processes the *.DXF files, removing irrelevant information, aligning points, establishing topological relationships, identifying processing rings, and defining the direction of each ring.
**3. Design Ideas and Algorithm Implementation**
In the sheet metal CAD subsystem, the first step is to use the AutoCAD EXPLODE command to break down the graphics and generate a *.DXF file. From this file, only the solid segment information (CIRCLE, LINE, ARC) related to the workpiece profile is extracted. The algorithm then analyzes the data to identify the outer contour, followed by the inner contours. It checks the loop direction and writes the corresponding data into the output file.
**3.1 System Data Structure**
The system uses a doubly linked list structure, which is ideal for handling sheet metal data due to its uniformity in 2D graphics. Both punching and blanking operations form closed loops, which are essential for CNC machining.
```cpp
class ARC : public CIRCLE, public LINE {
int Direct;
};
```
Here, `ARC` is a subclass of `CIRCLE` and `LINE`. The `Direct` member indicates the arc direction—clockwise or counterclockwise.
**3.2 Geometric Information Extraction Module**
The system breaks down graphics into three types: "LINE", "ARC", and "CIRCLE". The core of the program focuses on these entities. It reads the *.DXF file, identifies the entity type, and extracts the necessary geometric information using group codes.
For example, for a straight line, the starting and ending coordinates can be obtained using group codes 10 and 11.
**3.3 Group Ring Algorithms**
Each class has a `Distant()` function used to find the maximum distance from the origin. By comparing these distances, the outer contour can be identified. If the outermost entity is a circle, it becomes the outer contour ring. If it's a line or arc, the ring is built around it. The algorithm then searches for approximate coincident points to form the next chain of the ring, continuing until the loop is complete.
Inner contours are grouped similarly, and any remaining circles are treated as separate inner rings.
**3.4 Data Output Modules**
During data output, the direction of each loop is determined. For a ring made of three entities, the right-hand rule is applied. For more complex rings, the direction is calculated using cross-product methods. The data is then formatted and transferred to the CAPP subsystem.
The integration of CAD and CAPP systems represents a fusion of design and process modules. The data interface subsystem ensures seamless communication between the two, enabling efficient information exchange, processing, and sharing. This setup allows the system to fully utilize the secondary development capabilities of AutoCAD, achieving parameterization, standardization, and automation in sheet metal product design and manufacturing, ultimately reducing production cycles and increasing productivity.
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