In the field of CNC turning programming, process planning plays a crucial role. Effective and well-structured planning significantly enhances production efficiency and improves the surface quality of machined parts. While conventional parts have been extensively studied and supported by mature CAM software systems, there are still complex components that pose unique challenges. These components often feature irregular cross-sectional profiles and varying blank sizes, which is common in regions where mechanical processing equipment is not highly advanced, and the precision of rolled blanks is limited. In such cases, programming is typically based on the maximum blank size, leading to unnecessary air cutting and reduced efficiency. The primary goal of CNC turning process planning is to minimize single-piece man-hours or achieve the lowest possible material removal ratio while ensuring no geometric or process interference. To address these issues, modern CNC systems equipped with cutting force detection can measure the machining allowance of the blank online using a force sensor. This allows for dynamic adjustment of cutting parameters—such as depth of cut and number of passes—based on the actual material remaining on each part. This approach optimizes processing time and ensures efficient use of resources. In this paper, we focus on train wheel turning as a case study, discussing the principles of online measurement, measurement techniques, tool path generation, and the determination of feed rates and cutting speeds. ### 1. Online Measurement of Blank Machining Allowance Figure 1 shows a half-section view of a train wheel. Due to the hot-rolling process, the blank's dimensions are not uniform, resulting in uneven machining allowances. Accurate online measurement of these allowances helps determine the number of passes and the depth of cut. #### 1.1 Measurement Principle The online measurement system relies on the CNC machine’s cutting force detection capabilities. A force sensor mounted on the tool holder converts force signals into voltage and transmits them via serial communication. A threshold value, usually set to the yield limit of the material (e.g., chromium-manganese steel), is pre-programmed. When the tool engages the workpiece, the cutting force increases rapidly. Upon reaching the threshold, the CNC system stops the current measurement cycle and records the tool’s position. This method ensures accurate margin calculation without overcutting. #### 1.2 Measurement Methods During online measurement, the tool cuts along the workpiece’s surface normal. Figure 2 illustrates the process. Measuring points (A) are planned, and the tool radius (R) is considered. The unit normal vector (n) is calculated based on the feed direction angle (θ). Using this information, the total machining allowance is determined, and the roughing allowance is derived accordingly. #### 1.3 Tool Path Planning for Margin Measurement Proper planning of the measurement process includes determining the measuring points, as well as designing the entry and exit paths. Ensuring that the tool enters the workpiece along the surface normal is critical for accurate measurement. ### 2. Process Planning #### 2.1 Cut-In and Cut-Out Section Planning Different workpieces may require different cutting approaches. Common methods include normal, tangential, and oblique cuts, depending on the starting point, ending point, and adjacent geometry. Proper planning of these sections minimizes tool impact and enhances stability. #### 2.2 Cutting Section Planning For low-precision blanks, conventional cutting methods may damage tools. Several special techniques can be used: - **Variable Feed Cutting**: Gradually increasing feed when dealing with large eccentricity. - **Intersection Cutting**: Jumping between steps to prevent overcutting on curved surfaces. - **Multi-Pass Cutting**: Changing entry and exit points to avoid overcutting in areas with large allowances. - **Constant Line-Speed Cutting**: Maintains consistent speed for complex geometries like spokes. Figure 3 shows the K860B wheel’s tool path, with dashed lines indicating advance/retreat movements. #### 2.3 Feed Rate and Cutting Speed Selection Feed rate selection is often based on experience and depends on factors like material type, tool size, and depth of cut. For finishing, it is influenced by surface finish requirements and tool tip radius. Cutting speed is also experience-based, with lower speeds used for roughing and higher speeds for finishing, while considering material properties and tool performance. ### 3. Data Storage in Process Planning Process planning data is stored in three stages: individual step data, inter-process data, and integrated storage. #### 3.1 Storage of Individual Step Data Each work step contains tool data and cutting process data. The data structure includes tool numbers, compensation methods, cutting parameters, and path details. A structured format ensures clear relationships between data elements. #### 3.2 Data Storage Between Steps Using a doubly-linked list allows for flexible modification and management of process data. Each node contains pointers to both the previous and next steps, enabling efficient navigation and updates. #### 3.3 Integrated Storage of Step Data Extended data segments allow for flexible access to both graphic and non-graphic data within a CAD model. This format supports attribute data and connections, making it suitable for various applications. ### 4. Conclusion This paper outlines an online measurement method, process planning strategy, and data storage technique for turning operations involving irregularly sized blanks. By dynamically adjusting cutting parameters based on real-time measurements, the method enhances productivity and reduces waste. The use of a doubly-linked list for data storage and extended data segments for integration improves the efficiency and flexibility of NC programming post-processing.

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