Multi-start thread
A multi-start thread is a thread with two or more helical threads. Compared to a single-start thread, at the same pitch, a multi-start thread has a greater lead (i.e., the distance the thread moves axially per revolution), enabling faster axial movement. It is widely used in applications requiring fast transmission, such as machine tool feed mechanisms, jacks, and screw presses. However, multi-start threads are more difficult to machine than single-start threads, requiring precise and equal division between the helical threads, a process known as line division accuracy. This accuracy directly impacts the fit and performance of the multi-start thread.
The main parameters of multi-start threads include pitch, lead, thread count, and thread angle. These parameters are related to each other. Pitch p is the axial distance between corresponding points on the mid-diameter line of two adjacent threads. Lead S is the axial distance the thread moves per revolution. Thread count n is the number of helical turns in the thread. The relationship between the three is S = n × p. For example, a multi-start thread with thread count n = 2 and a pitch p = 2mm has a lead S = 2 × 2 = 4mm. The thread angle is the angle between adjacent flanks on the thread profile. The thread angle of standard threads is typically 60° (for ordinary threads) or 55° (for imperial threads). The thread angle of multi-start threads is the same as that of single-start threads and determines the thread’s meshing performance and strength. In addition, the definitions of parameters such as the pitch diameter, major diameter and minor diameter of multi-start threads are the same as those of single-start threads. The pitch diameter is the cylindrical diameter at the point where the thread thickness is equal to the width between the teeth, the major diameter is the maximum diameter of the thread, and the minor diameter is the minimum diameter of the thread. The dimensional accuracy and form and position tolerances of these parameters have an important influence on the fitting performance of the thread.
The thread division method for multi-start threads is crucial in the machining process. Common methods include axial thread division, circumferential thread division, and combined thread division. The axial thread division method achieves thread division by controlling the axial movement distance of the workpiece. After turning the first thread, the workpiece is axially moved by one pitch before turning the second thread, and so on. This method is simple to operate, but has lower thread division accuracy and is suitable for multi-start threads with larger pitches and lower precision requirements. The circumferential thread division method achieves thread division by controlling the workpiece rotation angle. For a multi-start thread with n threads, the workpiece needs to be rotated 360°/n. For example, a two-start thread requires 180° rotation, and a three-start thread requires 120° rotation. The circumferential thread division method offers higher thread division accuracy and is suitable for multi-start threads with higher precision requirements. It usually requires an indexing head or specialized thread division tools. The combined thread division method combines the advantages of the axial and circumferential thread division methods. Circumferential thread division is used to determine the approximate position, followed by axial thread division for precise adjustment to improve thread division accuracy.
The processing technology of multi-start threads is similar to that of single-start threads, but attention should be paid to the control of the accuracy of the dividing lines. The steps for processing multi-start threads generally include: turning the outer circle, turning the end face, drilling the center hole, rough turning the threads, fine turning the threads, etc. When rough turning the threads, a certain amount of fine turning allowance should be reserved, and at the same time, the pitch and lead of each thread should be consistent. Fine turning the threads is the key to ensuring the accuracy of multi-start threads, and high-precision tools and measuring tools are required to ensure that the dimensional accuracy and form and position tolerances of each thread meet the requirements. During the processing, the parameters of the thread, such as pitch, lead, and mean diameter, should be measured multiple times, and the cutting parameters and dividing line positions should be adjusted in time to avoid waste due to error accumulation. For multi-start threads with higher precision requirements, grinding or rolling processing is also required to improve surface quality and dimensional accuracy.
The inspection methods for multi-start threads mainly include inspection of pitch and lead, inspection of line division accuracy, and inspection of mean diameter. Pitch and lead can be inspected with a pitch gauge or a universal tool microscope. The pitch gauge is suitable for quickly inspecting whether the pitch is qualified, while the universal tool microscope can accurately measure the values of pitch and lead. Line division accuracy can be inspected with a dial indicator or a gauge. The probe of the dial indicator is brought into contact with the side of the thread. The workpiece is rotated one circle and the swing of the dial indicator pointer is observed. Half of the swing of the pointer is the line division error. The mean diameter can be inspected with a three-needle measurement method or a thread micrometer. The three-needle measurement method is a high-precision measurement method. Three measuring needles of equal diameter are placed in the thread groove. The distance between the measuring needles is measured with a micrometer, and the mean diameter is calculated according to the formula. Through strict inspection, it can be ensured that the quality of multi-start threads meets the design requirements and its stability and reliability during the transmission process are guaranteed.
