The method of dividing the threads of multi-thread turning
Multi-thread turning is a challenging lathe process. The key lies in ensuring precise and equal division between the threads, i.e., thread division accuracy. This accuracy directly impacts the fit and performance of the multi-thread. Common thread division methods include axial, circumferential, and combined methods, each with its own applicable scenarios and key operational points.
The axial thread splitting method achieves thread splitting by controlling the axial movement of the workpiece. It is suitable for machining multi-threads with large pitches. The key to this method is to use the lathe’s feed scale or the lead screw’s pitch to determine the starting position of each thread. After turning the first thread, the chuck is loosened, the workpiece is moved axially by one thread pitch, then the workpiece is re-clamped and the second thread is turned. This process repeats until all threads are machined. For example, when machining a two-thread thread with a 4mm pitch, after turning the first thread, the workpiece is moved axially by 4mm to ensure even axial spacing between the two threads. The axial thread splitting method has the advantage of being simple to operate and requiring no complex auxiliary tools. However, it requires a high level of operator experience and precise control of the workpiece’s axial movement distance, otherwise thread splitting errors can occur. To improve thread splitting accuracy, a dial indicator or vernier caliper can be used to measure the workpiece’s axial movement to ensure the accuracy of each movement.
The circular thread division method achieves thread division by controlling the workpiece’s rotation angle. It is suitable for multi-thread machining with a small pitch or high precision requirements. This method requires an indexing device, such as an indexing head or chuck, to precisely determine the starting position of each thread run by dividing the workpiece’s circumferential angle. For example, when machining a three-thread run, the 360° circumferential angle is divided into three equal parts of 120° each. After turning the first thread, the workpiece is rotated 120° to turn the second thread, and then rotated 240° to turn the third thread. The circular thread division method offers high precision, especially when using an indexing head. The line division error can be controlled within a few seconds to a few minutes, meeting the requirements of high-precision multi-thread machining. However, this method requires a specialized indexing device, is relatively complex to operate, and has certain requirements for equipment accuracy. It is therefore suitable for mass production or applications requiring high precision. When using the circular thread division method, it is important to ensure accurate positioning of the indexing device and to regularly calibrate the device to prevent line division errors caused by wear or loosening.
The comprehensive line division method is a line division method that combines the advantages of the axial line division method and the circumferential line division method. It is suitable for multi-thread thread processing with medium precision and medium pitch. This method usually first uses the circumferential line division method to determine the approximate position of each thread line, and then uses the axial line division method to make precise adjustments to make up for the shortcomings of a single line division method. For example, when processing a double-thread thread, the workpiece is first rotated 180° using the chuck to roughly determine the position of the second thread line, and then fine-tuned through axial movement to ensure that the pitch of the two threads is consistent. The comprehensive line division method can not only ensure a certain level of line division accuracy, but also simplify the operation process. It is widely used in actual production. When using the comprehensive line division method, it is necessary to pay attention to the coordination of the two line division methods to avoid affecting the overall line division accuracy due to the superposition of errors of the two methods.
Using the lathe’s own transmission system to divide the lines is also a common method. This method achieves line division by changing the transmission ratio between the lathe’s lead screw and the main shaft. For lathes with interchangeable gears, the transmission ratio can be changed by replacing the number of teeth on the interchangeable gear, so that the distance the lead screw drives the tool holder to move matches the angle of rotation of the workpiece, achieving accurate line division. For example, when processing double-thread threads, the transmission ratio of the interchangeable gear is adjusted to 1:2. When the workpiece rotates one circle, the tool holder moves half a pitch, thereby ensuring equal division of the two thread lines. The accuracy of this line division method depends on the accuracy of the lathe’s transmission system. It is suitable for mass production and can improve processing efficiency. However, this method requires the calculation of the number of teeth of the interchangeable gear based on the number of threads and the pitch, which requires a high level of computing power from the operator. At the same time, the interchangeable gear needs to be replaced frequently, which is a cumbersome operation.
Error control during the line division process is the key to ensuring the quality of multi-line thread processing. Common line division errors include axial displacement error and angular indexing error. Axial displacement error is mainly caused by factors such as the gap in the lathe feed system and the clamping deformation of the workpiece. It can be reduced by eliminating the gap in the feed system and adopting a reasonable clamping method. Angular indexing error is mainly caused by factors such as the accuracy of the indexing device and the installation eccentricity of the workpiece. It can be controlled by selecting a high-precision indexing device and accurately aligning the workpiece. In addition, during the line division process, attention should also be paid to the wear of the tool, and the tool should be replaced in time to avoid inconsistent thread sizes due to tool wear. At the same time, the trial cutting method can be used for line division. First, a small amount of thread is processed, the line division accuracy is measured, and batch processing is carried out after adjustment to ensure that the line division accuracy of the multi-line thread meets the requirements.
