Multi-thread turning steps and precautions
A multi-thread is a thread with two or more helical threads, where the lead is equal to the product of the pitch and the number of threads. It is widely used in applications requiring fast transmission or large lead angles, such as machine tool lead screws and jack screws. Compared to single-thread threads, the turning of multi-threads requires not only ensuring pitch accuracy, but also strict control of the accuracy of the equal division of each helical thread. Otherwise, it will lead to interference and uneven force when the threads are mated. The turning steps of multi-threads include preparation, selection of the division method, rough turning, fine turning, and testing. Each link has special technical requirements, and attention must be paid to key issues such as division errors and tool wear to ensure the transmission performance and service life of the thread.
Preparation before turning multi-start threads is essential for ensuring machining quality. First, the basic thread parameters must be determined: number of threads (n), pitch (P), lead (S = n×P), thread angle, and major, pitch, and minor diameters. Based on these parameters, the appropriate tool and threading method should be selected. Tool geometry for multi-start threads is essentially the same as for single-start threads, but the tool tip width must be slightly smaller than the tooth groove width (generally 0.1-0.2mm smaller) to avoid interference during machining. For high-speed steel tools, the rake angle is 10°-15° and the clearance angle is 6°-8°. For carbide tools, the rake angle is 5°-10° and the clearance angle is 5°-6°. Workpiece clamping must ensure rigidity. Threaded shafts with an aspect ratio exceeding 10 require a steady rest or steady rest to prevent bending and deformation during turning. After clamping, use a dial indicator to correct radial runout of the workpiece’s outer diameter to within 0.02mm, ensuring that the thread axis is aligned with the spindle axis. In addition, it is necessary to adjust the lathe’s pulley or CNC system parameters so that the lathe’s feed lead is consistent with the workpiece’s lead to avoid thread scrapping due to lead error.
The choice of thread division method directly affects the accuracy of multi-start thread division. Commonly used methods include axial thread division, circumferential thread division, and CNC thread division. The axial thread division method achieves thread division by controlling the axial displacement of the tool. It is suitable for lathes with screw pitch that is an integer multiple of the workpiece pitch. During operation, the first thread is first turned, and then the tool is moved axially by one thread pitch using the lathe dial or dial indicator before turning the second thread. This method is simple and easy to use, but the thread division accuracy is affected by the accuracy of the lathe feed system, with a typical error of ≤0.03mm. The circumferential thread division method uses the circumferential indexing of the spindle to achieve thread division. It requires an indexing disk installed on the spindle end or the number of teeth on the lathe’s gear for indexing. For example, when turning a double-start thread, the spindle can be rotated 180° before turning the second thread. This method has high thread division accuracy (error ≤0.01mm) and is suitable for high-precision multi-start threads. The CNC line dividing rule realizes automatic line dividing through the program instructions of the CNC system. It only needs to set the line dividing angle or axial displacement in the program, and the CNC system can accurately control the tool movement. The line dividing accuracy can reach 0.005mm, which is suitable for mass production and complex multi-line threads (such as three-line and four-line threads).
Roughing and finish turning multi-threads should be performed in a step-by-step manner to gradually improve thread accuracy. During rough turning, first determine the thread’s starting point and create the profile for the first spiral. The cutting depth is distributed using a “decreasing method”: the initial cutting depth is 1-1.5mm, decreasing by 0.5-0.8mm each time until the crest diameter approaches the drawing requirements (allowing a 0.3-0.5mm allowance for finish turning). After roughing the first spiral, the tool is moved using the selected step-by-step method to turn the second and third spirals. During this process, ensure that the cutting depth remains consistent across all spirals to avoid uneven thread stress. During finish turning, first adjust the tool position to ensure the cutter head is symmetrical with the thread axis. Then, finish turn each helix line sequentially with a small depth of cut (0.1-0.2mm). The pitch diameter and pitch of the thread must be measured multiple times during the finish turning process. Use a thread micrometer to measure the pitch diameter and a pitch gauge to check the pitch. Ensure that the pitch diameter tolerance is within IT7-IT8 and the pitch error is ≤0.02mm/100mm. For high-precision multi-thread threads (such as machine tool lead screws), grinding or honing is required after finish turning to further reduce surface roughness (to below Ra0.4μm) and improve thread wear resistance.
Precautions for multi-start thread turning must be considered throughout the entire machining process to avoid common defects. Line division error is the most common problem. Use a dial indicator or gauge block to check line division accuracy during machining. If the error exceeds 0.02mm, readjust the tool position. For axial line division, eliminate backlash in the lathe screw. This can be reduced by pre-tightening the tool (first moving the tool in the opposite direction 0.5-1mm, then moving it forward to the desired position). Tool wear can blunt the thread profile. Regularly inspect the tool edge condition. If flank wear exceeds 0.2mm, regrind or replace the tool promptly. After regrinding, ensure the profile angle is consistent with the previous tool to avoid “scratched” threads. Furthermore, the choice of cutting fluid is crucial for multi-start thread machining. Use an emulsion (8%-10% concentration) for cooling during rough turning, and an extreme pressure cutting oil (containing sulfur and phosphorus additives) for lubrication during finish turning to minimize tool wear and scratches on the thread surface. After processing is completed, the thread engagement must be checked with a comprehensive gauge. The gauge should be able to be screwed in smoothly without obvious shaking to ensure that the matching performance of the multi-thread meets the design requirements.
