Verification of blade NC machining program
**Foreword**
The blade is one of the most critical components in a steam turbine, with its steam section directly influencing the power output and overall performance of the turbine. As China's steam turbine industry continues to grow, blade design has become increasingly advanced, particularly in the development of variable-section twisted blades. The air path of the blade consists of complex spatial coordinate data, requiring high-precision machining that is challenging to achieve. To address this, Harbin Turbine Co., Ltd. has collaborated with 3D technology companies to enhance the efficiency of steam turbines and reduce heat consumption.
The inlet and outlet steam fins on the blade are thin, and the blade’s tip and root feature small fillets. From machining to inspection, a full-line projection perspective is required, and due to the variety of shapes, multi-axis CNC machines are necessary for processing. This makes the technical requirements extremely high. Therefore, verifying the NC machining program before actual machining becomes an essential step in the process.
**1. Characteristics of Blade NC Machining Programs for Steam Turbines**
Blade airway profiles are highly complex in 3D space. First, B-Spline surfaces are used to fit the shape data points of the blade. Then, factors such as the width of the cutting band, tool step length, and tool radius are interpolated and calculated from horizontal and vertical directions to determine the data points the NC program will pass through. This prepares the NC machining program accordingly.
In preparing the NC program, the principle of unity and minimizing the number of passes is applied, allowing for the simultaneous preparation of programs for the blade’s steam guide surface, tip, root fillet, and inlet and outlet flanges. Medium-length blades can have tens of thousands of procedures. These programs are characterized by long segments, large coordinate spans, complex four- or five-axis data, and a higher risk of errors.
**2. Common Errors in Blade NC Machining Programs**
As blade airway designs become more intricate and precision demands increase, NC machining programs have grown more complex, raising the likelihood of errors. Improperly prepared programs may result in tool collisions, excessive or insufficient material removal, incorrect tool radius selection, improper feed speed or cooling conditions, inefficient processing plans, machine control system incompatibility, incorrect part dimensions, and poor zero-point selection.
These issues often cause significant problems during actual machining, such as reprogramming, post-machining grinding, repairs, scrap, and delivery delays. This undermines the reliability of CNC machining and limits its application. Thus, verifying NC machining programs is both theoretically and practically important.
**3. Common Verification Methods for NC Machining Programs**
Manual inspection is still used but is time-consuming and prone to error due to the complexity of blade airways. Test processing involves using test pieces or non-metallic materials to verify the program, which provides an accurate simulation but is costly and time-consuming.
Computer simulation verification has become widely adopted due to advancements in software and hardware. It allows visualization of the blade model, tool path, and tool shape, helping to detect overcutting, tool interference, and other issues. This method is efficient and reduces the need for physical testing.
**4. Display Verification**
With the development of computer simulation technology, display verification has become a popular method. It involves showing the tool path trajectory on a screen to check for continuity, correctness, and smoothness. For example, in a four-axis NC program, the X, Y, Z coordinates are adjusted based on the rotation angle A. By performing reverse post-processing, the actual tool position can be calculated and plotted.
Display verification is intuitive and effective, especially for checking the blade tip and root. It allows for real-time observation of the tool path relative to the machined surface, ensuring that the tool moves correctly without interference. This method is ideal for verifying complex blade geometries and is widely used in modern CNC machining processes.
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