Common problems and solutions of thread processing

Thread is one of the common geometric features in mechanical engineering and is widely used. There are many processing techniques for threads, such as thread rolling and thread rolling based on plastic deformation, turning, milling, tapping and sleeve threading, thread grinding, thread grinding, etc. based on cutting processing. Among them, stainless steel turned parts is one of the commonly used processing methods for single-piece or small batch production. As a CNC lathe, thread turning is one of its basic functions.

Thread turning features

Thread CNC machining is different from contour machining. Its characteristics are as follows: thread machining is a forming process, and the cutting edges involved are long, which is prone to gnawing and knife stabbing. Generally, multi-knife cutting is required to complete; in order to ensure the lead (or The thread pitch) is accurate, and there must be a suitable cut-in and cut-out length; the thread shape and thread angle of thread processing are basically guaranteed by the shape of the tool, so the shape and correct installation of the tool directly affect the quality of the thread profile; The feed rate and the spindle speed must maintain a strict transmission ratio, that is, F = Ph(mm/r). Therefore, it is forbidden to use constant linear speed control during machining; the cutting speed of thread cutting is generally not high, so as not to appear built-up edge Or the plastic damage of the tool is the principle.

In the thread turning process, it is often necessary to re-install the tool to set the tool due to the wear and tear of the thread tool. The quality of the tool-setting directly affects the accuracy of the turning thread, especially the repair turning of the thread, which requires a second clamping. The second tool setting restricts the processing efficiency of the CNC lathe thread processing. When the thread accuracy is high, such as a trapezoidal thread, both sides need to be finished. It is necessary to change the finishing tool for finishing after roughing. To solve the problem of tool setting in the machining process, CNC thread turning will not be well applied.

The principle of thread processing in CNC lathe

There is a big difference between CNC turning thread and ordinary lathe thread turning. Ordinary lathe is turned through gear mechanical transmission and lead screw linkage, that is, every time the spindle rotates, the tool holder moves a thread lead. During the entire thread processing process This drive chain cannot be disconnected, otherwise it will be buckled randomly.

In CNC turning, a pulse signal is sent into the CNC system through the encoder installed on the spindle, and the CNC system performs arithmetic control, and sends an instruction to control the servo motor to control the movement of the tool through the ball screw to realize thread turning. In order to make thread turning more efficient When the tool is moved, the thread is not buckled randomly, and the starting position of the thread is controlled by detecting the pulse signal. When the program processing starts, the spindle rotates. When the second thread is cut, the tool returns to the starting point of the last turning, or waits for the synchronization signal (zero signal) to be received and then turns again, so that the turning thread is always on the same helix, so there will be no random threading phenomenon.

Problems Existing in Tool Setting in Thread Turning

1) Turning the clamping tool for the first time

When the thread cutter is clamped for the first time, the unequal height of the thread cutter tip and the workpiece rotation center will occur, which is generally common in welding knives. Due to rough manufacturing, the size of the cutter bar is not accurate, and the center height needs to be adjusted by adding a shim, which affects the center height. The actual geometric angle of the tool after turning. When the tool is installed, the corner of the tool tip is misaligned, which is prone to error in the thread profile angle and skewed tooth profile. If the thread cutter protrudes too long, it will generate vibration during machining, which will affect the surface roughness of the thread.

2) Rough and fine turning tool setting

In the process of machining high-precision threads and trapezoidal threads, it is necessary to use two thread cutters to separate the roughing and finishing.

3) Repair workpiece tool setting

Repairing the workpiece to the tool Due to the second clamping of the workpiece, the repaired helix and the encoder's one-turn signal have changed, and there will be random buckles when repairing and processing again.

way of solving the problem

1) The tool tip of the threading tool must be at the same height as the center of rotation of the workpiece. After the tool is sharpened, use the tool setting template against the axis of the workpiece for tool setting to keep the tool tip angle installed correctly. If a CNC machine is used to clamp the tool, due to the high manufacturing precision of the tool holder, it is generally only necessary to place the tool holder against the side of the tool holder.

2) For rough-finishing thread cutter setting, a certain point is set as the reference point, and the usual method is sufficient for setting the tool. In the actual tool setting process, the trial cutting method is used, as long as the tool compensation is slightly adjusted.

3) In thread processing, if the tool is worn or chipped, it is necessary to re-sharpen the tool and then calibrate the tool. If the workpiece is not removed for repair, it is only necessary to overlap the position where the thread cutter is installed and the position before removal. Equivalent to the same turning tool processing.

4) If the removed workpiece is to be repaired, the repairing work can be carried out only after determining the starting point of processing. How to determine the starting point of processing and the position of the one-turn signal? First, use the test bar to perform thread turning with a surface depth of 0.05-0.1mm (all parameters). The same as the thread parameters to be machined), the Z value is the integer thread lead distance from the right end face of the starting point of the thread, a helix is ​​engraved on the surface, the starting point of thread turning is determined, and a mark is marked on the corresponding position of the chuck circular surface (even if the engraved line and the The starting point of the helix on the test rod is in the same axial section).