On the CNC lathe, four standard threads can be turned: metric, inch, modulus, and diameter control. No matter which type of thread is turned, the spindle must have a strict relationship with the tool: that is, each revolution of the spindle (that is, the workpiece One revolution), the tool should move a distance (of the workpiece) evenly. The following through the analysis of ordinary threads to strengthen the understanding of ordinary threads in order to better process ordinary threads.
Size analysis of ordinary threads
CNC lathes need a series of dimensions for the machining of ordinary threads. The calculation and analysis of the dimensions required for ordinary thread machining mainly include the following two aspects:
1.Workpiece diameter before thread machining
Considering the expansion amount of the thread machining tooth profile, the diameter of the workpiece D / D-0.1P before the thread machining, that is, the major diameter of the thread minus 0.1 pitch, is generally 0.1 to 0.5 smaller than the major diameter of the thread according to the small deformation capacity of the material.
2.Thread feed amount
The thread feed amount can refer to the thread base diameter, that is, the final feed position of the thread knife.
The small diameter of the thread is: large diameter-2 times the tooth height; tooth height = 0.54P (P is the pitch)
The feed amount for thread machining should be continuously reduced, and the specific feed amount should be selected according to the tool and the work material.
Loading and setting of common thread cutters
If the turning tool is installed too high or too low or too high, when the knife is eaten to a certain depth, the flank of the turning tool bears on the workpiece, increasing the friction force, and even bending the workpiece, causing a trowel phenomenon; if it is too low, Chips are not easy to discharge. The direction of the radial force of the turning tool is the center of the workpiece. In addition, the clearance between the traverse screw and the nut is too large, which causes the depth of the knife to continue to automatically deepen, which raises the workpiece and causes a trowel. At this time, the height of the turning tool should be adjusted in time so that the tip of the tool is equal to the axis of the workpiece (the tip of the tailstock can be used for tool setting). In rough turning and semi-finishing turning, the position of the tool tip is about 1% D higher than the exit center of the workpiece (D represents the diameter of the workpiece to be processed).
Insufficient clamping of the workpiece. The rigidity of the workpiece cannot bear the cutting force during turning, which results in excessive deflection, which changes the center height of the turning tool and the workpiece (the workpiece is raised), resulting in a sudden increase in cutting depth and a trowel. At this time, the workpiece should be clamped firmly, and the tailstock tip can be used to increase the rigidity of the workpiece.
Common thread setting methods include trial cutting and automatic setting of the tool setting tool. You can directly use the tool for cutting test setting, or use G50 to set the workpiece zero, and use workpiece shift to set the workpiece zero for tool setting. Thread machining is not very demanding for tool setting, especially Z-direction tool setting has no strict restrictions, which can be determined according to programming machining requirements.
Programming and machining of ordinary threads
In current CNC lathes, there are generally three machining methods for thread cutting: G32 straight-cut cutting method, G92 straight-cut cutting method and G76 oblique cutting method. Due to different cutting methods, different programming methods cause machining errors. different. We must carefully analyze the operation and use, and strive to process high-precision parts.
1. G32 straight-forward cutting method, because both sides of the cutting edge work at the same time, the cutting force is large, and the cutting is difficult, so when cutting, the two cutting edges are easy to wear. When cutting a thread with a large pitch, due to the large cutting depth and rapid wear of the cutting edge, an error occurs in the middle diameter of the thread; however, the accuracy of the tooth profile it processes is generally used for small-pitch threads. Because its tool movement cutting is completed by programming, the machining program is longer; because the cutting edge is easy to wear, it must be measured frequently during machining.
2. G92 straight-forward cutting method simplifies programming and improves efficiency compared to G32 instructions.
3, G76 oblique cutting method, because it is a single-sided cutting, the cutting edge is easy to damage and wear, making the thread surface is not straight, the tip angle changes, resulting in poor tooth shape accuracy. But because it works with a single edge, the tool load is small, chip removal is easy, and the cutting depth is decreasing. Therefore, this machining method is generally suitable for large-pitch thread machining. Because the chip removal is easy with this machining method, the cutting edge machining conditions are better, and the machining method is more convenient when the thread accuracy is not high. When machining high-precision threads, two-tool machining can be used to complete the rough turning with the G76 machining method and the fine turning with the G32 machining method. However, it should be noted that the starting point of the tool must be accurate, otherwise it is easy to buckle and cause parts to be scrapped.
4. After the thread machining is completed, you can judge the quality of the thread by observing the thread profile. When the tip of the thread is not sharp, increasing the cutting amount of the knife will increase the thread diameter. The increase depends on the plasticity of the material. When the tip of the tooth has been sharpened, increasing the cutting amount of the knife will reduce the large diameter proportionally. According to this feature, the cutting amount of the thread should be properly treated to prevent scrapping.
Detection of ordinary threads
For general standard threads, thread ring gauges or plug gauges are used to measure. When measuring the external thread, if the “over-end” ring gauge is screwed in, but the “stop” ring gauge is not screwed in, it means that the processed thread meets the requirements, otherwise it is unqualified. When measuring internal threads, use a thread plug gauge to measure in the same way. In addition to the thread ring gauge or plug gauge measurement, you can also use other measuring tools to measure. Use a thread micrometer to measure the middle diameter of the thread. Use a tooth thickness vernier caliper to measure the thickness of the trapezoidal thread and the worm pitch diameter. Measure the diameter of the thread.
