**Introduction** Process planning is a critical component of CNC turning programming. Effective planning can significantly enhance production efficiency and improve the surface quality of machined parts. While conventional part process planning methods have been well-developed, with corresponding CAM software systems available, there remains a category of parts that presents unique challenges due to their complex cross-sectional profiles and varying blank sizes. This issue arises primarily because of the relatively outdated mechanical processing equipment in China, which leads to low precision in rolled blanks. As a result, programming is often based on the maximum blank size, leading to unnecessary air cutting and reduced processing efficiency. The goal of CNC turning process planning is to minimize single-piece man-hours (for high productivity) or reduce the ratio of time to material, while ensuring no geometric or process interference. In CNC systems equipped with cutting force detection capabilities, it is possible to measure machining allowances online for workpieces with large manufacturing errors in their blanks. Using cutting force sensors, the system can determine the amount of material to be removed at different points on the rotating part. This allows for dynamic adjustment of process parameters such as cutting depth and number of passes, enabling efficient processing of various blanks and saving valuable time. In this paper, we will use the CNC turning of train wheels as an example to explain the principles of online measurement, measurement techniques, tool path generation, and feed rate determination during the turning of large-diversity rotating parts. **1. Online Measurement of Blank Machining Allowance** Figure 1 shows a half-section view of a train wheel. Since the blank is produced through hot rolling, its dimensions are not uniform, resulting in uneven machining allowances. By measuring these allowances online, the number of passes and the depth of cut can be determined accordingly.  **1.1 Measurement Principle** Online measurement of machining allowance relies on the cutting force detection function of the CNC system. A force sensor is mounted on the tool holder, converting force signals into voltage signals transmitted to the CNC via serial communication. A threshold value—typically set as the yield limit σs for chromium-manganese steel—is pre-set in the sensor. When the tool contacts the workpiece and begins cutting, the cutting force increases rapidly. Once it reaches the threshold, the CNC system ends the current measurement and records the tool's X and Z coordinates. After calculating the margin, the data is stored in the CNC system. Visually, the tool retracts after contacting the workpiece, meaning only one cut is made per measurement point. **1.2 Measurement Methods** Based on the principle of machining allowance measurement, the tool cuts along the workpiece’s ICI (Intermediate Cutting Index) during measurement. Figure 2 illustrates the online measurement of machining allowance. Here, A(x, y) represents the planned measurement points, and R is the tool radius. During the normal cutting process, the unit normal vector of the cutting allowance may not be directly calculable. The direction of the normal vector is opposite to the direction vector EA in the cutting entry section. Let θ represent the angle between the feed direction and the horizontal line; then n = (cosθ, sinθ).  When the tool tip contacts the workpiece at a radius coordinate of R601, the distance A between the point on the finished contour and the shank O is (R601 - x)/cosθ. The total machining allowance ttotal = A0 - R = (R601 - x)/cosθ - R. If the finishing allowance is set to t, then the roughing allowance t = ttotal - tr = (R601 - x)/cosθ - R - tr. With the known machining allowance ttotal, the infeed path is D→E→O, and the retract path is O→F→G. Let tmax be the maximum depth of cut; the number of roughing cycles N is calculated as: $$ N = \left\lfloor \frac{t_{total}}{t_{max}} \right\rfloor $$ The floor function ensures no rounding up. In actual programming, the depth of cut is treal = t/N. The profile is first translated tr distance in the normal direction, then repeated N times, resulting in the final tool path. **1.3 Tool Path Planning for Margin Measurement** Planning the measurement process involves determining measurement points, designing the cutting-in and cutting-out sections. To ensure accurate machining allowance measurement, the tool must cut into the workpiece along the normal direction of the part’s surface. **2. Process Planning** **2.1 Cutting-In and Cutting-Out Section Planning** Different workpieces may require different cutting approaches. Common directions include normal, tangential, and oblique cutting, which are determined based on the starting position, ending position, and geometry of adjacent surfaces. Proper planning of cutting-in and cutting-out directions helps prevent or reduce tool impact when entering or exiting the workpiece. **2.2 Cutting Section Planning** For blanks with low manufacturing precision (e.g., large machining allowances, eccentricity, curvature, or non-uniform margins), conventional CNC machining may damage the tool or cause failure. Several special methods can be used in such cases: - **Variable Feed Cutting**: Gradually increase the feed rate when entering the cut, suitable for highly eccentric blanks. - **Intersection Cutting**: Jump between steps during cutting, ideal for workpieces with large contour curvatures and daily machining allowances. - **Multi-Pass Cutting**: Change cutting-in and cut-out points to avoid overcutting, especially useful for areas with large machining allowances. - **Constant-Line-Speed Cutting**: Ideal for cutting spokes, where constant rotational speed would otherwise lead to variable cutting speeds. Figure 3 shows the K860B wheel processed using the above planning principles. The dashed lines represent the advance/retreat paths, and numbers like "1010" indicate step IDs.  **2.3 Feed Rate Selection and Cutting Speed Determination** Feed rate selection typically relies on experience. During roughing, it depends on the material, tool shank size, workpiece diameter, and cutting depth. For semi-finishing and finishing, it is based on surface finish requirements, material type, tip arc radius, and cutting speed. Cutting speed is also usually selected based on experience, with lower speeds used for roughing and higher speeds for finishing. However, a reasonable cutting speed must also consider material processing performance, tool cutting performance, and operating conditions. **3. Process Planning Data Storage** Process planning data storage is divided into three stages: storage of individual process data, data storage between processes, and integrated storage of process data. **3.1 Storage of Individual Step Data** The process planning is carried out interactively in a human-machine graphic environment. Each work step serves as the basis for process planning and data access. The data includes two parts: tool data and cutting process data. Tool data consists of tool number and compensation method, while cutting process data includes cutting parameters, cutting-in sections, cutting sections, cutting-out sections, and step intersections. The data structure of any step is as follows:  Due to the large volume of data in each step, using a structured format helps organize internal relationships effectively. **3.2 Data Storage Between Steps** Each process consists of multiple steps. Planned data can be modified at any time, requiring a flexible storage structure that supports easy addition, modification, and deletion. A doubly-linked list is used to store this data, ensuring flexibility in the process planning process. Each node in the doubly-linked list has two pointers: one pointing to the next node and the other to the previous node. The data structure is defined as follows:   **3.3 Integrated Storage of Step Data** The extended data segment is a custom data format provided by the CAD platform, allowing convenient access to different types of data (including graphics and non-graphics) within the entity. Any voxel in the integrated product model can be supplemented with auxiliary data, serving as attribute data or attribute connection data. For example, linestrings, polygons, and curves behave similarly at design time. The head information is followed by the number of vertices, then the coordinates of those vertices. The number of appearances of these types in a single child is limited. The difference between a polygon and a line string lies in the fact that the first and last vertices of the polygon must match. For curves, extra points are added at the beginning and end of the vertex set to determine endpoint curvature. Extending entity data builds upon conventional graph data by adding blocks formed from a series of classification codes. These, combined with conventional data, form a more general entity data format. Different applications require access to different data, so the extended entity data is grouped by application type as follows:  **4. Conclusion** This paper describes the online measurement method, process planning method, and data storage approach for blanks with uneven dimensions in turning process planning. The core idea is to move away from fixed cutting amounts and instead use variable cutting strategies. By dynamically adjusting cutting parameters, different blanks can be processed efficiently, significantly improving overall processing efficiency. The double linked list is used to store step data, and an extended data segment is added to integrate the step data structure into the numerical control processing model. This enhances the simplicity and efficiency of code generation during post-processing in CNC programming.

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