I. Problem Statement
When turning threads on a CNC lathe, we often encounter the following problems: (1) The thread cutter is damaged during thread turning. (2) We want to change a thread finishing tool to high-speed thread turning. (3) After the thread is turned, we remove it and measure it again, but find that the depth is not enough. When we encounter these problems, we are often helpless because after changing the tool or re-clamping the workpiece, the thread cutter is difficult to align with the original spiral groove, the starting position is not accurate, and the teeth are always misaligned.
II. The principle of thread turning on a CNC lathe
To solve these problems, we must first understand the principle of thread turning on a CNC lathe. To complete thread cutting, the workpiece must rotate one rotation and the tool must feed one lead. To achieve this, a spindle encoder is set on the CNC lathe. Its synchronous pulse is used as the control signal for the tool feed point and retract point, so that the pulse number of the feed servo motor has a corresponding relationship with the spindle speed. However, there is no direct mechanical connection between the rotation of the CNC lathe spindle and the feed motion. Therefore, it is not easy to make the tool tip cut along the original spiral groove after reinstalling the tool or reclamping the workpiece.
III. Existing preventive measures
Currently, when using CNC lathes to turn threads in enterprises, most companies use a machine clamp tool for turning. The same tool is used for both roughing and finishing, and the speed is the same. If the tool breaks during the process, the tool bar is not removed and the blade is directly replaced to prevent the teeth from being misaligned. However, if the roughing and finishing tools are turned separately or the workpiece is reclamped, the problem of misaligned teeth will arise.
IV. Solution to the problem
If the tool is damaged during the threading process or you want to change a finishing tool for finishing the thread. Then, after the new tool is installed, you only need to re-align the tool. When aligning the tool, the x direction is relatively easy to align, and the key is to align the tool in the z direction. Many people move the tool tip to the end face of the workpiece by feeling, as shown in Figure (1). The tool alignment error is relatively large. It is okay for roughing, but not for finishing. Here, I will introduce a method to everyone. You can use an existing angle template or a template made by yourself, as shown in Figure (2). Accurately measure the distance L in the figure.
When setting the tool, as shown in Figure (3), the template is placed against the workpiece end face. The tool is moved to the inner corner of the template. The Z force offset value L is then input into the system. This tool setting method is highly accurate and is suitable for fine turning.
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2) Repair of single or small batch threaded parts. If the threaded parts are removed after turning and the thread depth is found to be insufficient, they need to be re-clamped for repair. This situation is quite troublesome. First, the problem of part runout after reassembly must be solved. An open sleeve can be made to eliminate the runout error. I believe most people know this method, so I will not explain it here. The remaining problem is to solve the problem of cutting along the original spiral groove to prevent the teeth from being misaligned. This problem is easy to solve on ordinary lathes. But it is more difficult on CNC lathes. The method used by many people is shown in Figure (4).
After aligning the tool, make sure the tool does not touch the part and run the thread processing program at a slow speed. Measure the distance L between the tool tip and the spiral groove, and then correct the position of the thread cutting starting point in the program. Let it offset a distance L from the original point A to point B. Then repeat the above process and modify the distance L. Repeat this process until the tool tip cuts into the thread groove. This method requires constant adjustment of the distance L. It is very time-consuming. In fact, we can solve these problems by slightly modifying the original program. Just change the thread length in the program to about 2mm. Clamp the workpiece. After aligning the turning tool, call out the program. Turn a very short and shallow thread. Find the starting position of the thread on the end face, as shown in Figure (5).
Assume that the starting position of the thread just turned is at point 2, and the original starting position of the thread is at point l. Draw a line AB on the chuck along point 2, and then release the clamping jaws. Rotate the part from point 1 to point 2 along the marked line AB. If the workpiece moves axially during rotation, the thread cutter must be recalibrated in the z-direction. Clamp the workpiece and change the thread length in the program to the original value. Run the program again to test the error. Based on this error, fine-tune the starting point of the thread cutting. As for the thread just cut from point 2, since it is very short, it can be removed by chamfering. This method of repairing threaded parts is much faster and more accurate than the previous one. The key step is to accurately rotate the part from point 1 to point 2. If a small batch of threaded parts needs repair, we can use the original program to create a new thread, find the starting point of the thread, mark it with a line on the chuck, and then use the above method to repair it. 3) Repairing Large Quantities of Threaded Parts If a large number of threaded parts need repair, a quick, simple, and accurate method is required. I have developed a method through practical experience and would like to share it with you.
Post time: Jul-29-2025
