Turning threads and threaded fasteners
Thread turning is a critical process in the production of threaded fasteners, directly determining the fastener’s connection performance and reliability. Threaded fasteners, including bolts, nuts, and screws, are the most widely used connectors in mechanical equipment. The thread accuracy and surface quality of threaded fasteners, including bolts, nuts, and screws, significantly impact assembly precision, load-bearing capacity, and service life. The thread turning process is suitable for processing fasteners made from a wide range of materials, from common materials like low-carbon and medium-carbon steel to specialized materials like stainless steel and high-temperature alloys. Thread turning can produce threads that meet the required specifications. In bolt production, the shank is typically turned first, followed by thread turning on the end. Large-diameter bolts also require tempering before threading to enhance their strength and toughness. A wind power equipment manufacturer uses a thread turning process to ensure thread consistency for M48 high-strength bolts, keeping the tightening torque deviation for each bolt within 5%, thus ensuring stable connections on wind turbine towers.
The turning process for threaded fasteners requires selecting an appropriate process route based on their structural characteristics. For bolts, the typical process is: bar stock cutting → rough turning of the outer diameter → quenching and tempering → fine turning of the outer diameter → thread turning → chamfering → surface treatment; for nuts, the process is: bar stock cutting → drilling → rough turning of the inner hole → fine turning of the inner hole → thread turning → chamfering → surface treatment. The number of passes for thread turning is determined by the fastener’s precision grade. Standard-grade fasteners can be completed in 2-3 passes, while precision-grade fasteners require 4-6 passes. The back-cut depth of the last pass is controlled at 0.05-0.1mm to ensure the surface roughness and dimensional accuracy of the thread. When producing 8.8-grade bolts, a standard parts factory reduced thread turning processing time by 15% by optimizing the process route, while also increasing the thread pass rate to 99.5%.
Different types of threaded fasteners have different requirements for thread turning accuracy. Ordinary connection threaded fasteners often use a 6g/6H tolerance band combination. During turning, the major, pitch, and minor diameters of the threads must be within the specified range. The pitch diameter is a key parameter that influences the fit and requires specific control. Transmission threaded fasteners, such as trapezoidal thread screws and nuts, require high transmission accuracy and wear resistance. They typically use a 7h/7H tolerance band, and during turning, the cumulative error and half-angle deviation of the threads must be strictly controlled. For threaded fasteners used in high-temperature and high-pressure environments, such as pressure vessel flange bolts, in addition to dimensional accuracy, the thread surface must be free of defects such as cracks and folds. Magnetic particle inspection or penetrant testing is required when necessary. Pressure vessel bolts produced by a chemical equipment factory undergo high-precision turning and non-destructive testing of thread fits to ensure that the thread quality of each batch meets ASME standards.
The material selection for turning threaded fasteners is closely related to cutting performance. Low-carbon steels such as Q235 steel offer excellent plasticity and cutting properties, but are prone to built-up edge when turning threads, necessitating higher cutting speeds or the use of high-cooling cutting fluids. Medium-carbon steels such as 45 steel, after quenching and tempering, offer moderate strength and hardness, offering excellent cutting performance and are the most widely used material for threaded fasteners. Stainless steels such as 304 steel exhibit high toughness and viscosity, but are prone to chip entanglement and tool wear when turning threads, necessitating the use of specialized stainless steel tools and extreme-pressure cutting fluids. Threaded fasteners made of high-temperature alloys such as Inconel 718 present greater challenges, requiring the use of ceramic or cubic boron nitride tools, along with lower cutting speeds and higher cooling flow rates. An aerospace company uses SiAlON ceramic tools to process high-temperature alloy bolt threads, maintaining a cutting speed of 15-20 m/min, successfully addressing the issue of excessive tool wear.
The turning process of threaded fasteners is trending towards automation and intelligent manufacturing. With the application of CNC lathes and automatic loading and unloading systems, the turning process of threaded fasteners has achieved automated mass production, ensuring consistent machining of each product through program control. The introduction of intelligent detection technology allows real-time measurement of thread dimensions during the thread turning process, automatically adjusting cutting parameters when deviations are detected, and avoiding batch scrapping. A fully automated threaded fastener production line built by a certain automotive standard parts manufacturer, equipped with an online thread detection device and an adaptive control system, has increased production efficiency by 30% and reduced the scrap rate to below 0.1%. Furthermore, green manufacturing technologies are increasingly being used in thread turning. By employing dry cutting or minimal lubrication techniques, they reduce the use and emissions of cutting fluids and minimize environmental impact. It is foreseeable that with the continuous advancement of manufacturing technology, the processing quality and production efficiency of turned threads and threaded fasteners will further improve, providing strong support for the high-quality development of mechanical equipment.
