visits： 36 Date：2024-01-30
When it comes to CNC lathes, they can turn four types of standard threads: metric, imperial, module, and diametral pitch threads. Regardless of the type of thread being turned, a strict motion relationship must be maintained between the lathe spindle and the CNC tool: for each rotation of the spindle (i.e., the rotation of the workpiece), the CNC tool should move uniformly by a distance equal to the lead of the workpiece. The following analysis of various threads will enhance our understanding of them, facilitating better machining of all types of threads.
Analysis of Ordinary Thread Specifications
Machining ordinary threads on a CNC lathe involves multiple specifications. The measurement and analysis of ordinary thread machining specifications mainly include the following two aspects:
1. Diameter of the Workpiece Before Thread Machining
According to the shrinkage rate of the thread tooth profile, the diameter of the workpiece before thread machining is D - 0.1P (where D is the major diameter of the thread and P is the pitch). Specifically, it is determined based on material deformation, generally 0.1 to 0.5mm smaller than the major diameter of the thread.
2. Number of Feeding Passes for Thread Machining
The final tool feeding position for the thread tool can be determined with reference to the thread diameter.
The diameter at this position is: major diameter - 2 × tooth height; tooth height = 0.54P (P is the pitch).
The cutting parameters during thread cutting should be reduced gradually, and the actual cutting parameters are selected based on the tool material.
Tool Selection and Tool Setting for Ordinary Threads
If the tool is installed too high or too low: when feeding the tool to a certain depth, the flank face of the tool will come into contact with the workpiece, increasing sliding friction, or even bending the workpiece, causing tool digging. During feeding, chips are difficult to evacuate, and the direction of the axial feeding force points to the center of the workpiece. Additionally, if the clearance between the lead screw and nut is too large, the feeding depth will automatically and continuously increase, further exacerbating the issue and leading to tool digging. In such cases, the tool height should be adjusted immediately to align the cutting edge with the workpiece centerline (tool setting can be done using the tailstock center). For rough and semi-finish turning, the cutting edge position should be 1% of the workpiece diameter (D, where D is the diameter of the workpiece being machined) higher than the workpiece center.
If the workpiece is not clamped firmly: the rigidity of the workpiece itself cannot withstand the cutting force during machining, resulting in excessive deflection. This changes the center height between the tool and the workpiece (the workpiece is lifted), causing a sudden increase in cutting depth and tool digging. In this case, the workpiece should be clamped firmly, and devices such as the tailstock center can be used to improve workpiece rigidity.
There are two tool setting methods for ordinary threads: the trial cutting method and the automatic tool setter. Direct trial cutting with the tool can be used, or G50 can be used to set the workpiece zero point, and the workpiece zero point can be set by moving the workpiece for tool setting. The tool setting standards for thread machining are not strict, especially for Z-axis tool setting, which can be carried out according to the standards of the programmed machining.
Programmed Machining of Ordinary Threads
Currently, in CNC lathe machining, there are generally three thread machining methods: G32 straight feed machining, G92 straight feed machining, and G76 angular feed machining. Due to differences in turning and programming, machining errors vary. We need to carefully analyze their applications in production to strive for machining high-precision parts.
For the G32 straight feed turning method, since both cutting edges work together, the cutting force is high and chip evacuation is difficult, so both edges are prone to wear during turning. When turning large-pitch threads, due to the large cutting depth, the cutting edges wear quickly, leading to deviations in the thread pitch diameter. However, it offers high accuracy for tooth profiles, so it is generally used for machining small-pitch threads. Since all tool movements are completed through programming, the machining program is relatively long. Considering that the cutting edges are prone to wear, frequent measurements should be performed during machining.
The G92 straight feed turning method optimizes programming and is faster than the G32 command.
The G76 angular feed cutting method: when considering the cutting side edge, the machining side edge is prone to damage or wear, resulting in an uneven thread surface and significant changes in the tool tip angle, thus leading to poor tooth profile accuracy. However, since it is single-edge cutting, the tool load is small, chip evacuation is easy, and the cutting depth decreases gradually. Therefore, this machining method is generally suitable for machining large-pitch threads. Due to its easy chip evacuation and favorable cutting conditions for the cutting edge, it is more convenient when thread precision requirements are not high. For machining high-precision threads, two-tool machining can be adopted: first, rough machining with G76, then finish turning with G32. However, it is crucial to ensure the accurate starting point of the cutting tool; otherwise, thread misalignment may occur, resulting in part scrapping.
After successful thread machining, the thread tooth profile can also be carefully observed to determine if there are measures to improve quality. If the thread crest is not sharpened, increasing the depth of cut will instead increase the thread major diameter, with the increase depending on the material ductility. If the thread crest is already sharpened, increasing the depth of cut will reduce the major diameter. Based on this characteristic, the depth of cut should be adjusted correctly to avoid part scrapping.
