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Programming Turning Part Thread on CNC Lathe

Turning Part Thread on CNC Lathe
Four kinds of standard threads can be turned on the CNC lathe: metric, inch, modular thread and diameter controlled thread. No matter what kind of thread is turning, a strict movement relationship must be maintained between the lathe spindle and the tool: That is, every time the spindle rotates (that is, the workpiece rotates once), the tool should move evenly by a lead distance. The following analysis of ordinary threads will strengthen the understanding of ordinary threads in order to better process ordinary threads.
1. Size analysis of ordinary thread
CNC lathes require a series of dimensions for the processing of ordinary threads. The size calculation and analysis required for ordinary thread processing mainly include the following two aspects:
1> The diameter of the workpiece before turning thread processing takes into account the expansion of the thread processing tooth profile. The diameter of the workpiece before threading is D/D-0.1P, that is, the major diameter of the thread is reduced by 0.1 thread pitch. Generally, the deformation capacity of the material is smaller than the diameter of the thread by 0.1 to 0.5.
2> Thread processing feed amount and thread plus feed amount can refer to the bottom diameter of the thread, that is, the final feed position of the thread cutter. The minor diameter of the thread is: the major diameter of the thread-2 times the thread tooth height; Thread tooth height = 0.54P (P is the pitch) The feed amount of thread processing should be continuously reduced. The specific feed amount is selected according to the tool and working material.

2, Tool installation and tool setting of ordinary turning thread tools
If the turning tool is installed too high, when the tool is driven to a certain depth, the flank face of the turning tool will resist the workpiece and increase the friction. The turning tool even bends the workpiece, causing the tool to break;
If the turning tool is installed too low, the chips will not be easily discharged. The direction of the radial force of the turning tool is the center of the workpiece, and the gap between the traverse screw and the nut is too large, which causes the turning depth to continuously and automatically increase. As a result, the workpiece is lifted and the tool appears to be gnawed. At this time, the height of the turning tool should be adjusted in time to make the tip of the tool the same height as the axis of the workpiece (the tip of the tailstock can be used for tool setting). In rough turning and semi-finish turning, the position of the tool tip is about 1% D higher than the center of the workpiece (D represents the diameter of the workpiece).

The workpiece is not firmly clamped, and the rigidity of the workpiece itself cannot withstand the cutting force during turning, resulting in excessive deflection. The center height of the turning tool and the workpiece is changed (the workpiece is raised), resulting in a sudden increase in the cutting depth, and the turning tool is gnawed. At this time, the workpiece should be clamped firmly, and the tailstock center can be used to increase the rigidity of the workpiece. Common thread tool setting methods include: trial cutting tool setting and tool setting tool automatic tool setting. You can directly use the turning tool to test the tool setting, or use G50 to set the workpiece zero point. Use the workpiece shift to set the workpiece zero point for tool setting. The tool setting requirements for thread processing are not very high, especially the Z-direction tool setting has no strict restrictions, which can be determined according to the programming processing requirements.

3. Programming and processing of ordinary threads
In current CNC lathes, thread cutting generally has three processing methods: G32 linear cutting method, G92 linear cutting method and G76 oblique cutting method, due to the different cutting methods and different programming methods, the machining errors are also different. We must carefully analyze the operation and use, and strive to process high-precision parts.
1>. With the G32 straight cutting method, since both sides of the cutting edge work at the same time, the cutting force is large and the cutting is difficult, so the two cutting edges are easy to wear during cutting. When cutting a thread with a larger pitch, the cutting edge wears faster due to the larger cutting depth, which causes an error in the pitch diameter of the thread; However, the precision of its processed tooth profile is high, so it is generally used for small pitch thread processing. Moving a cutting tool are due to be accomplished by the program, so the longer the processing program; Since the blade is easy to wear, frequent measurements must be taken during processing.
2>. The straight cutting method of G92 simplifies programming and improves efficiency compared with G32 commands.
3>. G76 oblique thread cutting method: Because it is a single-sided edge processing, the processing blade is easy to damage and wear, so that the processed thread surface is not straight, the tool tip angle changes, and the thread profile accuracy is poor. But because it is a single-sided edge work, the tool load is small, chip removal is easy, and the cutting depth is decreasing. Therefore, this processing method is generally suitable for large pitch thread processing. Because this processing method is easy to remove chips and the cutting edge processing conditions are better, this processing method is more convenient when the thread accuracy requirements are not high. When processing higher-precision threads, it can be completed by two-cut machining, first using the G76 machining method for rough turning, and then using the G32 machining method for fine turning. But pay attention to the accuracy of the starting point of the tool, otherwise it is easy to buckle the thread randomly, resulting in scrapped parts.
4> After thread processing is completed, measures can be taken in time to judge the thread quality by observing the thread profile. When the thread crest is not sharp, increasing the cutting amount of the knife will increase the thread diameter. The amount of increase depends on the plasticity of the material. When the tooth top has been sharpened, increasing the cutting amount of the knife will reduce the major diameter proportionally. According to this feature, the amount of thread cutting must be treated correctly to prevent scrapping.

Numerical control turning method of general parts thread

4. Detection of ordinary threads
For standard threads of general parts, thread ring gauges or plug gauges are used to measure. When measuring external threads, if the thread "over-end" ring gauge is just screwed in, but the "end-stop" ring gauge does not screw in, it means that the processed thread meets the requirements, otherwise it is unqualified. When measuring internal threads, use threaded plug gauges and measure in the same way. In addition to thread ring gauge or plug gauge measurement, other measuring tools can also be used for measurement. Use a thread micrometer to measure the pitch diameter of the thread. Measure the pitch diameter of the trapezoidal thread and the pitch diameter of the worm with a tooth thickness vernier caliper. The pitch diameter of the thread is measured by the measuring needle according to the three-needle measurement method.
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