Cold extrusion of internal thread is a metal plastic processing method that uses extrusion tap to process internal thread. It has the characteristics of high material utilization, high processing accuracy, low surface roughness, high mechanical strength and strong fatigue resistance. Xu Jiuhua et al. carried out experimental research on extrusion strengthening of 300M high-strength steel internal thread. The results show that the fatigue performance of cold extrusion internal thread is significantly improved compared with that of cutting thread. Under the same conditions, the life of cold extrusion thread is 4-30 times that of cutting thread; Zhao Qingrong et al. studied the influence of process parameters such as the diameter of the bottom hole of the workpiece, extrusion speed, cooling and lubricating fluid on the cold extrusion forming of 35CrMo steel internal thread, providing a reference for the selection of process parameters of cold extrusion internal thread; Ma Ping studied the technological parameters in the cold extrusion process of 300M high-strength steel internal thread. The results show that the calculation of theoretical bottom hole diameter and tap is the key to realize the cold extrusion process, and the selection of extrusion speed and cooling lubricant is an important factor to ensure the quality of thread processing; Miao Hong et al. studied the influence of the diameter of the bottom hole of the workpiece on the internal thread of Q460 high-strength steel cold extrusion and the metal flow rule of the cold extrusion forming through experiments, found the best processing bottom hole diameter parameter, and analyzed the change rule of the metal structure during the cold extrusion process from the microscopic perspective. At present, the research of internal thread cold extrusion mainly focuses on the theoretical and experimental research of internal thread cold extrusion under the conditions of different materials, sizes, process parameters, etc., and the relevant research on tap failure is less.
In this paper, the influence of extrusion torque and extrusion temperature on the failure of internal thread cold extrusion tap is studied by using the experimental method of online monitoring. The total energy ratio and low and high frequency energy ratio of extrusion tap vibration and machine tool noise under different processing conditions are tested.
1. Test method
M22 on C6250 machine tool × Cold extrusion processing test of 1.5mm internal thread. The tap adopts hexagonal pyramid extrusion tap, material M35, workpiece Q460, machine speed 25r/min, thread processing length 20mm, workpiece bottom hole diameter Φ 21.25mm, polydimethylsiloxane (PDMS) is selected as lubricating fluid.
① Torque and temperature test method
As shown in Figure 1, a circular shaft type torque sensor is set on the shaft of the tap clamp. Four strain gauges are uniformly distributed on the outer circular surface of the tap clamp shaft. Each strain gauge forms a 45 ° angle with the axis to eliminate the impact of tension and bending, and the torque during cold extrusion is tested. Drill a hole on the outer surface of the workpiece, insert the K-type thermocouple into the hole, and then fix the workpiece on the chuck. The two stages of the thermocouple are connected with the XMT-3000 industrial control/regulator through the machine tool collector, and the average temperature of the extrusion area can be measured.

② Vibration and noise test method
During the vibration test, the TS15200 piezoelectric acceleration sensor is set on the top of the front of the torque clamping device, and the vibration signal is sent to the computer through the adapter and data acquisition card to output the vibration acceleration value. During the noise measurement, the AWA5661 precision pulse sound level meter is installed on the machine tool support plate, and the noise signal outputs the noise sound level through the data acquisition card and the computer.
2. Test results
(1) Failure form of internal thread cold extrusion tap
After the test, the wear condition of the extrusion tap is measured through the microscope to obtain the total width of the working area of all the edge teeth after processing, and the wear amount of the tap is obtained by comparing with the test before, and the wear condition of the tap is determined. According to the wear and damage degree of the tap, the processing condition of the tap is divided into five stages: normal wear, slight wear, severe wear, workpiece slipping and tap fracture. The first two processing states indicate that the wear of the tap is small and does not affect the processing quality of the internal thread, which is normal working state; The last three stages indicate that the tap wear is very large, and even has been damaged, which seriously affects the processing of internal thread. See Figure 2 for the failure state of the tap corresponding to the three stages. The severe wear stage of the tap is mainly manifested by the wear and damage of the tap teeth; The slipping stage of the workpiece is mainly characterized by the slipping of the screw tap and the workpiece relative to the chuck; The tap fracture stage is mainly represented by the tap shaft fracture.
In the process of processing, the processing conditions of cold extrusion internal thread, such as lubricant, spindle speed, workpiece bottom hole diameter, extrusion torque, extrusion temperature and extrusion times, have a great impact on the processing status of tap and the processing quality of internal thread. In order to reduce the wear of the extrusion tap, avoid the breakage and fracture of the tap, and improve the processing quality of the internal thread, the measured values of the processing signals such as extrusion torque, extrusion temperature, tap vibration and machine tool noise are selected as the important characteristic parameters reflecting the normal processing and failure of the tap.

(2) Effect of extrusion torque on tap failure
It can be seen from Figure 3 that when the tap extrusion torque is 210N · m, the tap works normally. With the increase of extrusion torque, the contact area and contact strength between the tap and the workpiece increase continuously, and the tap wear increases; When the extrusion torque reaches 248N · m, the tap enters the stage of slight wear. With the further increase of the extrusion torque, the tap wear increases; When the extrusion torque reaches 509N · m, the tap enters the stage of severe wear; When the extrusion torque reaches 703N · m, the tap is pressed with the workpiece and drives the workpiece to rotate together, causing the workpiece to slip on the chuck; When the extrusion torque increases to 993N · m, the tap shaft breaks.

(3) Effect of extrusion temperature on tap failure
It can be seen from Figure 4 that when the extrusion temperature is 84 ℃, the tap works normally. With the increase of extrusion torque, the contact area and contact strength between the tap and the workpiece increase, the friction on the tap surface increases, the heat generated on the contact surface increases, and the extrusion temperature increases. When the extrusion temperature is 94 ℃, the tap enters into a slight wear stage; With the further increase of extrusion temperature, when the extrusion temperature is 105 ℃, the tap enters into the stage of severe wear; When the extrusion temperature reaches 131 ℃, the tap drives the workpiece to rotate together, and the workpiece slips on the chuck; When the extrusion temperature increases to 145 ℃, the tap shaft breaks.
(4) Vibration signal characteristics of extrusion tap
It can be seen from Figure 5 that the total vibration energy ratio of the tap in five stages is 100%, 112%, 123%, 117% and 129% respectively. The total vibration energy ratio of the tap tends to increase, and the vibration energy ratio decreases when the workpiece slips. When the total energy ratio of tap vibration is below 112%, the tap can work normally. When the total vibration energy ratio of the tap is 117% or more, the tap cannot work normally and the processing shall be stopped. Among them, the total energy ratio is the ratio of the processing conditions to the total energy of normal wear.

It can be seen from Figure 6 that the vibration energy ratio of tap in five stages is 21.6%, 22%, 27.4%, 25.5% and 25.5% respectively. The vibration energy ratio of tap first increases and then decreases, and the energy ratio of the first two stages is basically the same. The vibration energy ratio of the tap at the stage of severe wear is the largest and the machining is the most unstable; The latter two stages have large energy and cannot be processed. The energy ratio is the low and high frequency energy ratio of the signal.

(5) Machine tool noise signal characteristics
It can be seen from Figure 7 that when the total noise energy ratio of the machine tool is below 100%, the tap can work normally. When the total noise energy ratio of the machine tool is 828% or above, the tap can not work normally and the processing should be stopped. When the workpiece slips, the total noise energy of the machine tool reaches the maximum value. Among them, the total energy ratio is the ratio of the processing conditions to the total energy of normal wear.
It can be seen from Figure 8 that when the noise energy ratio of the machine tool is above 7.6%, the tap works normally. When the noise energy ratio of the machine tool is below 3%, the tap can not work normally, and the processing should be stopped. When the workpiece slips, the total noise energy of the machine tool reaches the minimum value. Among them, the energy ratio is the low and high frequency energy ratio of the signal.

Summary
(1) The main failure modes of internal thread cold extrusion tap are severe wear of tap tooth and fracture of tap shaft. The processing state of tap is divided into five stages: normal wear, slight wear, severe wear, workpiece slipping and tap fracture.
(2) Extrusion torque and extrusion temperature are important factors that affect the cold extrusion processing of internal thread. The correct extrusion torque (210-248N · m) and extrusion temperature (84 ℃ - 94 ℃) can effectively reduce tap wear, avoid tap damage and fracture, and ensure the thread processing quality.
(3) The vibration signal of the extrusion tap and the noise signal of the machine tool are the important basis for detecting the cold extrusion processing of the internal thread. When the total vibration energy ratio of the extrusion tap is 100% - 112%, the vibration energy ratio of the extrusion tap is 21.6% - 22%, the total noise energy ratio of the machine tool is 85% - 100%, and the noise energy ratio of the machine tool is 7.6% - 14.3%, the tap can work normally and the internal thread processing is stable and reliable.