A brief discussion on a thread rolling method based on CNC lathe

<p>The purpose of thread rolling is to use the rolling tool to apply a certain pressure on the workpiece surface, so that the surface metal of the workpiece will flow plastically and fill into the original residual concave troughs, thereby reducing the surface roughness of the workpiece. Due to the plastic deformation of the rolled surface metal, the surface structure is cold hardened and the grains are refined, forming a dense fiber shape, and a residual stress layer is formed, the hardness and strength are increased, thereby improving the various properties of the workpiece. It is common to use ordinary lathes to roll threads. This article provides a new process solution for thread rolling based on CNC lathes.</p><p>Background technology: After using CNC horizontal lathe equipment to process long shaft threads (such as hydraulic press tie rods), the turning tool cannot meet the processing requirements of the thread bottom or the processing accuracy problem, and there is a right angle or large roughness at the bottom of the thread, which leads to stress concentration and easy fracture when subjected to force. The bottom of the thread needs to be rolled, and the rolled surface metal is plastically deformed.</p><p>After rolling the bottom of the thread, the bottom of the thread is rounded, the roughness is reduced, the stress at the bottom of the thread is reduced, the force of the thread segment is improved, and the hardness and strength of the parts are improved. At present, when rolling the bottom of the thread, ordinary lathes are mostly used for manual rolling. This method is simpler and easy to adjust during the processing process. However, manual operation is prone to damage due to misoperation, and it is more dependent on manual experience, and the production quality is difficult to be consistent.</p><p>Although CNC lathes are automated, they can be processed according to the preset processing program as soon as the processing starts, reducing manual operations and producing more uniform production quality. However, after the CNC lathe executes the processing program, it cannot be stopped midway and needs to be stopped after the processing is completed. In addition, rolling processing is different from cutting processing, so the requirements for thread rolling processing are relatively high. If the rolling method is incorrect, the parts and tools will be damaged, and even the CNC lathe will be damaged, which poses a greater risk.</p><p>Therefore, CNC lathes are rarely used for thread rolling in the industry. Technical personnel in this field hope to have a thread rolling method based on CNC lathes that can realize automatic rolling processing and reduce processing risks.</p><h2>Implementation plan</h2> <p>In order to more clearly illustrate the implementation process of the entire thread rolling plan, the following is a brief introduction using the attached figure.</p><p style="text-align: center;">Figure 1 Structural diagram of thread rolling method</p><p style="text-align: center;">Figure 2 Schematic diagram of the cross-sectional structure when the hob is inserted into the screw groove</p><p style="text-align: center;">Figure 3 Structural diagram of the thread rolling method from another angle</p><p style="text-align: center;">Figure 4 Schematic diagram of the cross section of the screw groove when processing along the extension direction close to the end face</p><p style="text-align: center;">Figure 5 Schematic diagram of the cross section of the screw groove when processing the side close to the back-cut groove along the extension direction</p><p>In the figure: 1-workpiece, 11-thread segment, 111-screw groove, 12-back-cut groove, 13-transition slope, 2-hob</p><h2>Specific implementation method</h2> <p>Before machining on the CNC machine tool, the hob 2 and the thread segment 11 of the workpiece 1 to be rolled are aligned so that the hob 2 can move accurately in the screw groove 111 during machining. The tool alignment step is to first move the hob 2 to one side of the thread segment 11 and set it as the Z-axis coordinate of the tool starting point during machining; then move the hob 2 along the Z axis according to the integer multiple of the pitch, and no longer move the hob 2 along the Z axis during the tool alignment process; then, manually rotate the workpiece 1 and move the hob 2 along the X axis so that the screw groove 111 can face the hob 2, and the hob 2 enters the screw groove 111 and contacts the bottom of the tooth. At this time, the X-axis coordinate of the tool starting point is set, and the current angle of the spindle is set as the machining starting angle, that is, the original starting angle of the spindle for machining the workpiece is changed.</p><p>The tool setting method is to move hob 2 from the tool starting point to a position that is a multiple of the pitch and then stop moving along the Z axis. Manually rotate workpiece 1 and screw groove 111 to adapt to the position of hob 2. When rotating workpiece 1, screw groove 111 moves a shorter distance on the Z axis, which is convenient for control and adjustment. Rotating workpiece 1 is more accurate and convenient than moving hob 2 on the Z axis, which reduces the damage to the thread caused by excessive movement of hob 2 along the Z axis in screw groove 111.</p><p>Since the moving distance of hob 2 from the tool starting point is an integer multiple of the pitch, after the screw groove 111 is aligned at this position, hob 2 can enter the screw groove 111 during processing, and the position requirement for the tool starting point is low. It is not necessary to align the tool with the end of the thread segment 11, so that hob 2 can be easily aligned with the thread on the CNC machine tool to improve the tool alignment accuracy. After accurate tool alignment is achieved, the processing program is executed to start rolling processing, and hob 2 can smoothly roll the bottom of the thread along the thread, so that rolling processing of the thread can be realized on the CNC machine tool, reducing processing risks.</p><p>In some specific embodiments, the tool starting point is located at the back cut groove 12 of the thread segment 11, the diameter of the back cut groove 12 is smaller than the diameter of the thread root, there is a transition bevel 13 between the thread segment 11 and the back cut groove 12, the end of the thread is on the transition bevel 13, so that a bevel transition is formed between the transition bevel 13 and the root of the thread end, and the top of the thread part on the transition bevel 13 will be inclined with the transition bevel 13, that is, the depth from the top of the transition bevel 13 to the root of the thread will gradually become shallower, and the cross-sectional size of the screw groove 111 will gradually become smaller in the direction close to the back cut groove 12, refer to Figures 3 and 5.</p><p>When the hob 2 approaches the thread segment 11 from one side of the undercut groove 12, the hob 2 first passes through the transition slope 13 and gradually approaches the tooth bottom of the screw groove 111, gradually increasing the contact and pressure with the tooth bottom. Moreover, the cross-sectional size of the screw groove 111 gradually increases. When the hob 2 just enters the screw groove 111, the thread heights on both sides of the screw groove 111 are very small, and the hob 2 can easily enter the screw groove 111. Refer to Figure 5, which shows a cross-sectional schematic diagram of the screw groove 111 along the extension direction near the undercut groove 12.</p><p>If rolling is started from the starting side of the cutting process of the thread segment 11, when this side is the end face, refer to Figure 4, which shows a cross-sectional schematic diagram of the side of the screw groove 111 close to the end face along the extension direction, the height of the tooth bottom may be too high, and the hob 2 may directly collide with the tooth bottom, and the pressure is very high when it just contacts. In addition, the end of the thread segment 11 on this side has no chamfer or the chamfer is very small, and the height of the thread teeth on both sides of the screw groove 111 is relatively high.</p><p>The hob 2 needs to directly enter between the thread teeth on both sides. The position of the hob 2 and the screw groove 111 needs to be very precise to enter, otherwise it is easy to collide with the tool and damage the thread and equipment. This solution can reduce or avoid the direct collision of the hob 2 with the tooth bottom or thread teeth when it just enters the screw groove 111, making the processing smoother and further reducing the risk of rolling threads on CNC lathes.</p><p>In some specific embodiments, the hob 2 moves along the Z axis while tilting away from the thread segment 11 along the X axis. During the hobbing process, the bearings connected to the hob 2 and the main shaft are subjected to high pressure and heat, which may burn out the bearings after long-term use, or the main shaft may be overloaded and stop due to excessive force, affecting the processing or even causing equipment failure.</p><p>Therefore, the hob 2 is set to move away from the thread segment 11 along the X axis during the processing, that is, the hob 2 is tilted outward during the movement of the Z axis, forming an angle α as shown in Figure 1, gradually reducing the rolling depth, and finally moving away from the bottom of the tooth, so that the rolling process of the thread segment 11 by the hob 2 is a section close to the retreat groove 12, not the entire thread segment 11. Referring to Figure 1, the length M is the actual rolling length, which can shorten the processing time.</p><p>The rolling pressure also shows an overall decreasing trend during the machining process. When the threaded segment 11 of the workpiece 1 is subjected to force in cooperation with the nut, the nut is mainly connected to the side of the threaded segment 11 close to the back cutter groove 12, that is, the main force position of the threaded segment 11 is in the section close to the back cutter groove 12. Therefore, rolling only the section close to the back cutter groove 12 does not affect the normal use of the workpiece 1, thereby reducing the pressure-bearing time and pressure of the hob 2 and the spindle, and further reducing the risk of rolling threads on the CNC lathe.</p><p>In some specific embodiments, the hob 2 can be rolled to the required depth in multiple times, reducing the pressure of each rolling and the load of the CNC lathe. The multiple feed processing can be: when rolling the first to fourth cuts, each time increase 0.1mm, gradually make the hob go deeper to the required depth; when rolling the fifth cut or more, such as the fifth to the tenth cut, the hob depth is not increased, the hob moves idly according to the depth of the fourth cut, and the root of the entire thread is trimmed at this depth.</p><p>In some specific embodiments, the hob 2 includes a rolling seat and a rolling wheel rotatably connected to the rolling seat, and there is a swing gap between the rolling wheel and the rolling seat, so that the rolling wheel can swing to adapt to the screw groove 111 during the rolling process, thereby reducing the damage to the screw thread caused by processing errors.</p><p>Since there is a swing gap between the rolling wheel and the rolling seat, the matching accuracy of the rolling wheel entering the screw groove 111 after the CNC lathe starts processing is required to be higher. Through this tool setting method and starting from one side of the backing groove 12 as the starting point to reduce the position accuracy requirement when entering, the hob 2 can enter the screw groove 111 more accurately.</p><h2>Conclusion</h2> <p>The application of CNC lathes has become more and more popular, and ordinary machine tools will eventually slowly withdraw from the stage of history. This article details a thread rolling method based on CNC lathes to solve the application occasions where there is no suitable ordinary lathe in real applications and thread rolling is needed; solve the problem of easy misoperation and difficult to consistent production quality when ordinary machine tools are used for manual operation in existing applications; solve the technical problem of high risk of CNC lathe processing in existing applications, and finally provide a more efficient and quality guaranteed thread rolling processing solution to promote the progress of mechanical processing technology.</p>