Thread turning amount
Thread turning parameters are a key factor in determining thread machining quality, efficiency, and tool life. These parameters primarily include cutting speed, feed rate, and depth of cut. These three parameters are interrelated and mutually influential, requiring proper matching based on thread type, workpiece material, tool material, and machining precision requirements. Cutting speed, the linear velocity of the turning tool’s cutting edge relative to the workpiece’s threaded surface, directly impacts cutting temperature and tool wear. Feed rate, closely related to thread pitch, is crucial for ensuring accurate thread parameters. Depth of cut determines the amount of metal removed with each pass, impacting machining efficiency and surface quality. Properly selecting thread turning parameters maximizes production efficiency and reduces costs while ensuring thread accuracy.
The selection of cutting speed requires a comprehensive consideration of the workpiece material, tool material, and thread accuracy. For plastic materials (such as 45 steel and aluminum alloy), higher cutting speeds increase friction between the chip and the tool rake face, generating higher cutting temperatures that can easily cause tool wear and thread surface burns. Therefore, the cutting speed should be appropriately reduced. For brittle materials (such as cast iron), lower cutting speeds can achieve better machining results. Tool material is a significant factor influencing cutting speed. High-speed steel tools have poor heat resistance, so cutting speeds are typically controlled at 8-15 m/min. Carbide tools offer better heat resistance, allowing cutting speeds to be increased to 30-100 m/min. Coated carbide tools (such as TiAlN coatings) offer superior wear and heat resistance, allowing cutting speeds of up to 100-200 m/min. Furthermore, the higher the thread accuracy required, the lower the cutting speed should be to minimize the effects of vibration and deformation on thread parameters. For example, when machining grade 6 threads, the cutting speed is typically 10%-20% lower than when machining grade 8 threads.
Feed rate is the most unique parameter in thread turning. Its value is equal to the thread pitch (for single-start threads) or lead (for multi-start threads), so the feed rate is determined by the thread design parameters. For example, when machining a single-start thread with a 2mm pitch, the feed rate must be set to 2mm/r; when machining a double-start thread with a 4mm lead, the feed rate is 4mm/r. Although the feed rate is fixed, the feed rate (the product of feed rate and workpiece rotational speed) affects cutting forces and machining stability, so it is necessary to control the feed rate by adjusting the workpiece rotational speed. For high-pitch threads (e.g., pitch greater than 6mm), due to the large feed rate, excessively high feed rates can lead to a sharp increase in cutting forces, which can easily cause workpiece vibration and tool chipping. Therefore, the workpiece rotational speed must be reduced to reduce the feed rate, generally within the range of 50-100mm/min. For fine-pitch threads (e.g., pitch less than 1mm), the workpiece rotational speed can be appropriately increased to improve machining efficiency.
The depth of cut should be selected in a gradual, incremental manner, with multiple passes gradually removing stock to ensure dimensional accuracy and surface quality. The total stock of a thread depends on the thread profile height. For example, the total stock of a triangular thread is approximately 1.3 times the pitch. The first pass should have a large depth of cut, typically 0.5-1mm, to quickly remove the majority of the stock. Subsequent passes should gradually decrease in depth, with the final pass typically not exceeding 0.1-0.2mm to ensure surface roughness and profile accuracy. The depth of cut should be adjusted based on the required thread accuracy and material properties. When machining high-strength alloys, due to their greater cutting resistance, the depth of cut should be reduced with each pass to avoid excessive tool wear. When machining plastic materials, the depth of cut can be increased in the final passes to reduce chip-tool friction and produce a smoother surface. In addition, the selection of cutting depth also needs to consider the strength of the tool. For high-speed steel tools, due to their better toughness, a larger cutting depth can be used; for carbide tools, due to their greater brittleness, the cutting depth should be appropriately reduced to prevent tool chipping.
Optimizing thread turning parameters requires verification and adjustment through trial cutting. Especially before mass production, test cutting of workpieces is necessary to check the thread’s dimensional accuracy, surface quality, and tool wear. Fine-tune the cutting parameters based on the test cutting results. For example, if the thread surface roughness is too high after the test cutting, it may be due to too low a cutting speed or too great a cutting depth on the last pass. In this case, the cutting speed should be increased or the cutting depth on the last pass should be reduced. If the tool is found to be wearing too quickly, it may be due to too high a cutting speed or too great a cutting depth. In this case, the cutting speed or cutting depth should be reduced. Furthermore, the impact of cooling and lubrication conditions on cutting parameters must be considered. When using high-efficiency cutting fluids (such as extreme pressure emulsions), the cutting speed and cutting depth can be increased to improve processing efficiency. Under dry cutting conditions, the cutting parameters should be reduced to minimize tool wear. By continuously optimizing thread turning parameters, high-quality, efficient, and low-cost thread processing can be achieved.
