Multi-start thread turning is a complex machining process. Maintaining the precision between each thread run is crucial, and the calculation and line division method for the interchanging gear is crucial to ensuring this accuracy. The lead of a multi-start thread is the product of the pitch and the number of runs. Therefore, in the lathe transmission system, interchanging gears are required to accurately transmit this lead. The calculation of the interchanging gear depends primarily on the lathe’s leadscrew pitch, the lead of the thread being machined, and the transmission ratio of the lathe’s transmission system. The basic formula is: Interchanging gear transmission ratio i = Lead of the thread being machined / Lathe leadscrew pitch. For example, if the lathe’s leadscrew pitch is 6mm and the pitch of the three-start thread being machined is 2mm, the lead is 6mm, and the interchanging gear transmission ratio i = 6/6 = 1. In this case, an interchanging gear with the same number of teeth can be selected. However, in actual calculations, the inherent transmission ratio of the lathe transmission system must also be considered. This may vary between lathe models, so refer to the lathe manual for adjustments to ensure accurate calculation results.
The calculation of the exchange gears also needs to consider the tooth count of the gears to ensure that the transmission ratio can be achieved through a reasonable gear combination. Typically, the exchange gear consists of a driving gear and a driven gear, with the transmission ratio i = number of teeth on the driving gear / number of teeth on the driven gear. After calculating the transmission ratio, it needs to be simplified to the tooth ratio of two or three gears. For example, for a transmission ratio of 2/3, you can choose a driving gear with 40 teeth and a driven gear with 60 teeth, or a driving gear with 20 and 40 teeth and a driven gear with 30 and 60 teeth (transmitted through an intermediate gear). It is also important to ensure that the module of the gears matches the module of the lathe’s exchange gear shaft to avoid module mismatches that could result in gear installation failure or unstable transmission. Furthermore, the exchange gears must be installed in the correct order: the driving gear is installed on the lathe’s input shaft, and the driven gear is installed on the output shaft. Intermediate gears only change the transmission direction and do not affect the transmission ratio. They can be added as needed.
The thread splitting method is key to ensuring uniform distribution of threads when turning multiple threads. Common thread splitting methods include axial and circumferential thread splitting. The axial thread splitting method is suitable for multi-thread threads with a larger pitch. Its principle is to use the lathe’s feed dial or the pitch of the lead screw to control the axial movement distance of the workpiece, so that the tool cuts each thread line at a different axial position. For example, when machining three threads, after each thread line is turned, the workpiece is moved axially by a distance of one thread pitch before the next thread line is turned. This method is simple to operate, but its accuracy is greatly affected by the accuracy of the lathe’s feed system, making it suitable for applications where precision is not a high priority. The circumferential thread splitting method achieves thread splitting by changing the workpiece’s rotation angle. It is suitable for multi-thread threads with a smaller pitch. Commonly used methods include chuck thread splitting, dividing head thread splitting, and thread splitting with a dedicated thread splitting tool. 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° each rotation. 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-line threads.
In actual operation, the selection of the line dividing method needs to be combined with the accuracy requirements of the thread, the pitch size and the equipment conditions. For multi-line threads with higher precision requirements, such as machine tool lead screws, precision transmission threads, etc., the dividing head line dividing method or the special line dividing tool line dividing method in the circular line dividing method should be used first to ensure that the angular error between each line of threads does not exceed ±5′. For multi-line threads with general precision, the axial line dividing method or the chuck line dividing method can be used. During operation, the line dividing accuracy needs to be repeatedly measured, and the line dividing position is adjusted by trial cutting to ensure that the pitch and tooth profile of each line of threads are consistent. During the line dividing process, the position of the tool should remain unchanged, and the line dividing is only achieved by moving or rotating the workpiece to avoid tooth profile deviation of each line of threads due to changes in the tool position. In addition, the first thread line needs to be accurately turned before line dividing, and it is used as a reference for line dividing to ensure the accuracy of subsequent lines of threads.
When turning multi-start threads, the calculation of the interchanging gear and the correct application of the thread splitting method must be combined with a sound machining process to ensure final quality. Before machining, carefully check the interchanging gear for correct installation, transmission flexibility, and the required thread splitting tool accuracy. During the turning process, each thread should be rough-turned first, leaving a sufficient margin for finishing. After rough-turning, the thread splitting accuracy and pitch of each thread should be measured. If any deviation is detected, the thread splitting position should be adjusted or the gear should be replaced. During finish turning, a small amount of cutting should be used to avoid workpiece deformation caused by excessive cutting forces, which could affect thread splitting accuracy. Tool wear should also be monitored, and timely replacement or sharpening should be performed to ensure consistent profile quality across all threads. After machining, the dimensional accuracy and thread splitting accuracy of each thread should be checked using a thread gauge or the three-needle measurement method to ensure compliance with design requirements. Only through scientific calculation, sound thread splitting methods, and rigorous process control can high-quality multi-start threads be successfully turned.
