Multi-start thread turning
A multi-start thread refers to a thread with two or more spiral lines along the axial direction. Its lead is the product of the pitch and the number of lines. It has the characteristics of high transmission efficiency and strong load-bearing capacity, and is widely used in machine tools, mining machinery, aerospace and other fields. The key to multi-start thread turning is to ensure the accuracy between each thread line, including uniform pitch, consistent tooth profile and accurate line division. The turning difficulty is much greater than that of single-start thread. The line division accuracy of multi-start thread directly affects its transmission performance. Excessive line division error will cause uneven force when the threads are engaged, resulting in impact and vibration, and reduced service life. Therefore, multi-start thread turning requires precise line division methods and reasonable processing technology to ensure the dimensional accuracy and positional accuracy of each line of thread.
There are two main methods for thread division in multi-start thread turning: axial and circumferential. Different methods are suitable for different machining applications. The axial method achieves thread division by controlling the axial movement of the workpiece and is suitable for multi-start threads with larger pitches. The principle is that after turning one thread, the tool is moved axially by the pitch of the thread before turning the next thread. The movement distance can be controlled by the lathe’s feed dial or the lead screw pitch. For example, when machining a double-start thread with a 3mm pitch, after turning the first thread, the tool is moved axially by 3mm to start the second thread. The axial method is simple to operate, but its accuracy is significantly affected by the accuracy of the lathe’s feed system. It is suitable for applications with lower precision requirements. The circumferential method achieves thread division by varying the workpiece’s rotation angle and is suitable for multi-start threads with smaller pitches. Common methods include chuck thread division, indexing head thread division, and dedicated thread division tools. The chuck dividing line method is to make marks on the three-jaw chuck or four-jaw chuck and determine the angle of each rotation based on the number of lines. For example, a three-line thread requires 120° rotation each time. This method is simple and easy, but the accuracy is low. The dividing head dividing line method has higher accuracy. The workpiece rotation angle is accurately controlled by the dividing head dial, and it is suitable for the processing of high-precision multi-start threads.
Tool selection and geometric parameter design for multi-start thread turning must meet the requirements of multi-thread cutting. Tool material selection should be based on the workpiece material. High-speed steel or carbide tools can be used for machining ordinary steel; carbide or ceramic tools should be used for machining high-strength alloys or stainless steel. Regarding tool geometry, the rake angle should be 10°–15° to reduce cutting forces; the clearance angle should be 6°–10° to minimize friction between the flank and the workpiece; the lead angle should be 60°–90° to ensure thread profile accuracy; and the tool tip angle should be consistent with the thread profile angle (e.g., 60° for ordinary threads and 55° for pipe threads). Furthermore, the tool edge should be sharp and properly ground to ensure a clear thread profile and reduce cutting forces and vibration. For multi-start threads with larger pitches, high-speed steel thread cutters can be used for low-speed cutting; for multi-start threads with smaller pitches, carbide thread cutters can be used for high-speed cutting to improve processing efficiency.
The selection of cutting parameters for multi-start thread turning requires a balance between machining efficiency and line separation accuracy. The cutting speed, feed rate, and depth of cut all have specific requirements. Regarding cutting speed, high-speed steel tools for machining ordinary steel should maintain a cutting speed of 10-30 m/min; carbide tools can achieve a speed of 50-100 m/min. High-speed cutting requires ensuring the rigidity and stability of the lathe to avoid vibration. The feed rate is equal to the lead of the thread: feed rate f = lead P = number of threads n × pitch p. For example, for a two-start thread with a pitch of 2 mm, feed rate f = 4 mm/r. The depth of cut should be determined based on the thread height. For rough turning, the cutting depth should be 0.3-0.5 mm per cut, while for finish turning, the cutting depth should be 0.1-0.2 mm, gradually reaching the required thread height. During the process, adequate cooling and lubrication are required, using extreme pressure emulsions or sulfurized cutting oils to reduce tool wear and workpiece thermal deformation, while also facilitating chip evacuation and preventing chip accumulation that could affect machining quality.
The process measures and operating techniques for multi-start thread turning are crucial to ensuring machining quality. Before machining, carefully check the lathe’s accuracy, such as the lead screw pitch accuracy and spindle rotation accuracy, to ensure the equipment is in good condition. When clamping the workpiece, ensure that the workpiece axis is aligned with the lathe spindle axis to avoid thread splitting errors caused by eccentric clamping. During turning, each thread should be rough-turned first, leaving a 0.2-0.5mm allowance for finish turning. After rough turning, measure the thread splitting accuracy and pitch. Adjust the thread splitting position promptly if any deviation is detected. During finish turning, use trial cutting to measure the actual thread dimensions and gradually adjust the tool position to ensure consistent pitch and profile across all threads. During thread splitting, the tool position should remain unchanged, and only the workpiece should be moved or rotated to achieve splitting. This avoids thread profile deviations caused by tool position changes. After machining, check the dimensional accuracy and thread splitting accuracy of each thread using a thread gauge or three-needle measurement method to ensure compliance with design requirements. High-quality turning of multi-start threads can be achieved through reasonable thread division method, tool selection, cutting parameter setting and process control.
