CNC Knowledge: Analysis and treatment of X-axis jitter fault of CNC gantry milling machine

PART 1

Preface

The gantry milling machine shown in Figure 1 was purchased in 2019 and is suitable for medium-sized diesel engine bodies, textile machinery, printing machinery, automobile manufacturing, shipbuilding machinery, aerospace manufacturing , machine tool manufacturing and other industries, especially for multi-station machines. , multi-process rectangular boxes It is an indispensable key equipment for the treatment of body parts. Detecting the position of the X, Y and Z feed axes of the machine tool uses metal linear grid rulers produced by well-known German companies. The spindle orientation position detection device uses toothed photoelectric encoders produced by well-known German companies, which have high precision and high standard. reliability. In terms of electrical control, the modular design of electrical devices is adopted to ensure the electrical control unit of CNC gantry series machine tools and facilitate debugging, operation and maintenance. The whole machine is equipped with the 840D sl CNC system. The machine tool is equipped with accessory milling heads, which use semi-automatic and automatic tool changing technology to replace various accessory milling heads. It can realize multi-station and multi-process processing such as milling, boring, drilling, reaming and tapping. the part after a single tightening.

Figure 1 Appearance of the gantry milling machine

PART 2

Fault phenomenon

After installing and debugging the new equipment, the X axis still had loud sounds and slight shaking. The manufacturer also made several optimization adjustments, but the effect was not ideal. As the equipment continues to operate, the jitter phenomenon becomes more and more serious, and sometimes contour error monitoring alarms appear, which seriously affects the processing accuracy of the product.

PART 3

X-axis transmission structure

The worm box is fixed in the middle of the machine bed and is connected with the drive shaft of the workbench feed box. The worm gear engages with the worm band under the workbench to move the workbench (X direction).

The worm in the worm box is a hydrostatic worm. During operation, the constant pressure oil of the hydraulic system is injected into the surface of the meshing teeth of the worm and the worm bar, forming a high-strength hydrostatic oil film of a certain thickness , so that the worm and the worm bar form a frictionless transmission. Therefore, the transmission efficiency and precision are high, there is no wear, and the service life is long.

The oil chamber is located on the worm tooth side. The hydrostatic auger adopts double pump to supply oil, that is, the double pump leads to two oil paths with equal flow rates to supply oil to the oil chambers located respectively on the left and right sides of the worm.

In addition to a static pressure oil feed pump assembly, the static pressure auger also has a static pressure auger pre-flush oil system. When the auger oil chamber and the auger band are out of the mesh area, the static pressure oil circuit. is disconnected and the pre-flushing oil is connected, so that the mesh-free oil chamber is always filled with oil, preventing air from being mixed in the oil line, so as to improve the static pressure rigidity of the worm oil film and. ensure the load-bearing strength. The hydrostatic auger adopts the form of axial oil distribution. The structure of the static pressure auger is shown in Figure 2.

Figure 2 Structure of the hydrostatic screw

PART 4

Analysis of the causes of failure

Based on the analysis of the working principle of the X-axis transmission mechanism, it is believed that there are many reasons for the X-axis jitter, such as low static pressure on the workbench , low static pressure on the worm, the worm brake. is not lit and the adjustment gap of the workbench pressure plate is too small. The servo motor itself is defective, the sensing unit is defective, the servo parameter settings are incorrect, and the gap between the worm and the worm band is incorrect. , etc.

In order to avoid a large number of blind disassembly and improve maintenance efficiency, first check the static pressure of the workbench, the static pressure of the worm screw, the braking pressure of the worm screw end, the X-axis detection unit and servo control parameters. There is no anomaly. Next, disengage the X-axis servo motor and change the X-axis to semi-closed loop operation for testing. The motor operates normally without jitter. This eliminates the problem of the servo motor itself. It has been analyzed that the main influencing factors that can cause this failure are that the adjustment gap of the pressure plate of the workbench is too small or the gap between the worm and the worm band ending is inappropriate.

(1) The adjustment gap of the workbench pressure plate is too small. The workbench adopts a closed static pressure guide rail, with a total of 8 pressure plates underneath. If the pressure plate gap is too small when the machine tool is installed, it will cause insufficient floating quantity of the workbench and cause X-axis jitter failure.

(2) Improper clearance between the worm gear and the worm band. The constant pressure oil forms a static pressure oil film between the worm gear and the worm band, allowing the worm gear and the worm to achieve a frictionless transmission if there is a gap between the worm gear. and the worm band is too small, it will make the static pressure of the oil film unstable and cause jitter failure.[1]。

PART 5

Troubleshooting

5.1 Bench Pressure Plate Clearance Inspection

By consulting the relevant information on the machine tool, the gap between the worktable pressure plate and the guide rail should be controlled within 0.05mm. If the gap is too small, the static pressure float of the workbench is not enough and the machine tool will vibrate during processing. If the gap is too large, the machine tool cannot guarantee its own rigidity requirements.

Remove the protective covers on both sides of the[2]。

Inspection of the workbench pressure plate gap is shown in Figure 3.

Figure 3 Checking the gap between the workbench pressure plates

5.2 Inspection and adjustment of clearance between worm and worm band

By adjusting the thickness of the gasket under the mounting seat of the worm box, the gap between the worm and the worm band can be adjusted. Since the worm box is located in the middle of the bed, the workbench must be moved away during adjustment to expose the worm box, then the gaskets must be removed to adjust the gap. The specific implementation steps are as follows.

1) Set the status point of PLC DB99.DBX1.1 to 1, DB99.DBX1.2 to 0, change the worm gear, modify the PLC. programs and protects its alarm function. The main purpose of turning off the static pressure of the auger is to better detect the actual gap between the auger and the auger band.

2) Prepare a dial indicator to detect the actual gap between the worm and the worm band. First move the handwheel forward and slowly for a certain distance (note that it must be slow, because the static pressure of the auger is cut off and there is no pressure oil film static between the worm and the worm band At the same time, the current of the X axis should be monitored when moving), then move 0.01mm in the opposite direction, and the gap measured. is 0.03 mm. The measured gap between the worm and the worm band is relatively small, and the normal value of the gap should be 0.06mm.[3]。

3) Remove the X-axis grid ruler read head and mounting bracket, protect them and seal them with an airtight bag.

4) Remove the X-axis hard limit switch and cable chain fixed brackets on both sides, then activate the static pressure of the worm to move the X-axis forward until Whether the auger and auger band are about to separate, shield pressure alarm for multi-head pump behind the shaft. Because when the workbench moves backward, there is a period during which static pressure cannot be established.

5) Stop the hydrostatic worm pump, install a hand hoist in the front position of the workbench, use the hand hoist to pull the workbench with light force for the last distance, and at the same time slowly rotate the worm forward. direction until the auger and auger band are completely disengaged.

6) Remove the 8 locating pins and 14 M17 hex socket fixing screws from the worm box, then place a threaded top under each end of the worm box, use the two threaded tops to lift lightly the worm box and remove the worm box. and the bed There are 4 pieces of joints between the body, and they need to be marked for easy installation later.

7) The specific value for adjusting the joint thickness can be determined according to the angle between the worm and the worm band. The end result of the calculation is that the removed joint must be ground by 0.3mm, which may increase the clearance of the worm. gear about 0.03mm.

8) The joint is processed, assembled according to the marks, drive the worm box locating pin and tighten the fixing screw. Run the worm at idle and clean the dirt off the worm with your hands.

9) Connect the worm and worm bar, turn the positive end of the toothless worm half-turn upwards, and slowly push the workbench to the negative side with a jack until ensure that the front end of the worm bar on the workbench contacts the end face. teeth of the worm, at this time, the cylinder is lightly stressed, the machine tool activates the static pressure of the worm and slowly rotates the worm to connect the worm and the screw band endless. Then move the X axis slowly and monitor whether its current is normal and whether there is any abnormal noise.

10) Install the components removed in the steps above in order according to the marks made before. At the same time, according to the above-mentioned detection method of the gap between the worm and the worm band, detect the reverse clearance. the clearance is 0.06 mm and the detection is normal. Then change the X axis to fully closed loop, enable all PLC protected alarms, and modify the cycle program to check the working status of eliminated.

PART 6

Conclusion

As an important mechanical transmission component of large-scale CNC gantry boring and milling machines, the worm transmission mechanism has high requirements for machining precision, assembly precision and control precision, at the same time as the power transmission method of the machine tool. feed axis, the worm and worm mechanism must withstand large cutting force and irregular vibration, so the failure rate is relatively high.

The troubleshooting was lengthy and was a combination of a series of mechanical, hydraulic and electronic control issues. To deal with this kind of breakdown, it is necessary to have a certain level of understanding of the mechanical part of the transmission mechanism, the hydraulic control part and PLC control system Know. After fully analyzing the fault and understanding the structure of the machine tool, detecting and adjusting the gap between the worm and the worm band can effectively eliminate the failure of machine jitter. tool.