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Maintenance of Spindle Noise of CNC Milling Machine

Posted by: Chris Na 2021-05-26 Comments Off on Maintenance of Spindle Noise of CNC Milling Machine

1. Analysis of the shortcomings of spindle noise

In many CNC milling machines, because the transmission system of the main shaft still uses several transmission shafts, gears and bearings, vibration noise, interference noise and impact noise will inevitably occur during work. The speed change of the main transmission system of the Cnc Milling Machine is controlled by the computer under the condition that the machine does not stop working, so it is more continuous and representative than the noise generated by the general machine tool.

When the main drive system of the Cnc Milling Machine is soiled, it is precisely because the gears, bearings and other parts have been stimulated and transmitted and radiated inside the system. Increase and increase the noise.
CNC Milling Machine
(1) Gear noise analysis. The main transmission system of CNC milling machine mainly depends on gears to end the speed change and transmission. Therefore, the meshing transmission of gears is one of the main noise sources.

The noise generated by gears in the main drive system of the machine tool mainly includes:

① In the gear meshing, continuous impact occurs between the teeth and the gear is forced to vibrate at the meshing frequency and cause impact noise.

②Because the gear is subjected to external excitation force, transient free vibration of the gear’s natural frequency occurs and causes noise.

③The unbalanced inertial force of rotation is caused by the eccentricity of the installation of the gear, the transmission shaft and the bearing, and a low-frequency vibration consistent with the rotation speed occurs. Following the rotation of the shaft, a resonance noise is announced for each rotation.

④ The self-excited vibration of the gear caused by the interference between the teeth and the interference noise. Assuming that the tooth surface is uneven, it will cause rapid periodic impact noise.
(2) Bearing noise analysis. The spindle speed change system of the CNC milling machine has a total of 38 roll bearings. The installation of bearings, journals and supporting holes, preloading force, concentricity, lubrication conditions, the size of the load acting on the bearing, and radial open space all have a great influence on noise. And the manufacturing fault of the bearing itself determines the noise of the bearing to a large extent. The most easily deformed part of a tumbling bearing is its inner and outer ring. Under the influence of external elements and its own accuracy, the inner and outer rings may have shaking vibration, axial vibration, radial vibration, radial vibration and axial bending vibration of the bearing ring itself.
CNC milling machine
2. Dealing with the shortcomings of spindle noise

1. Gear noise control

Because the gear noise is caused by multiple factors, some of them are the characteristics of gear noise in the spindle motion system of the milling machine determined by the gear planning parameters. The original gear is repaired and improved on the basis of not changing the original plan. To reduce noise.

(1) Trimming the tooth tip. Due to the influence of tooth profile fault and tooth pitch, after the gear tooth load is elastically deformed, it forms an instantaneous collision and impact when the gear meshes. Therefore, in order to reduce the meshing impact caused by the unevenness of the tooth top when the gear is meshing, the tooth top trimming can be performed. The purpose of tooth tip trimming is to correct the bending deformation of the teeth and compensate for gear errors, and then reduce gear noise. The amount of trimming depends on the tooth pitch error and the amount of bending deformation of the gear after load, as well as the bending direction. When trimming, focus on the pairs of gears with the highest meshing frequency of the machine tool and the different trimming amounts adopted by these gears when the modulus is 3, 4, and 5mm. When trimming, you must pay attention to the control of the trimming amount, and adopt the method of repeated testing, so as to avoid the excessive trimming amount and destroy the useful working tooth profile, or the trimming amount is too small to have the effect of trimming. When trimming the edge, you can trim only the tooth tip or only the tooth root according to the specific conditions of these pairs of gears. It is only necessary to trim the tooth tip and the tooth root together when the tip of the tooth is trimmed alone or the root of the tooth does not achieve an excellent effect. The radial and axial values ​​of the quantity can be assigned to one gear or to two gears according to the situation.

(2) Control tooth profile fault. Tooth profile fault is composed of many elements. Investigating the gears in the transmission system of the milling machine, found that the tooth profile fault is mainly caused by the processing process, and secondly caused by the poor operating conditions for a long time. Tooth shape errors are common when gears are engaged. Under normal circumstances, the greater the tooth profile error, the greater the noise. For the concave tooth profile, the gear teeth are impacted twice in one meshing, and the noise is very loud, and the more concave the tooth profile, the greater the noise. Therefore, the gear teeth are modified to make them properly convex in order to achieve the purpose of reducing noise.

(3) Control the change of the center distance of the meshing gear. The change of the actual center distance of the meshing gear will cause the pressure angle to change. If the center distance of the meshing gear is periodically changed, the pressure angle will also be changed periodically, and the noise will also increase periodically. The analysis of the meshing center distance shows that the noise effect is not obvious when the center distance is too large, while the noise will increase significantly when the center distance is too small. When the center distance of the meshing gear is controlled, the outer diameter of the gear, the deformation of the transmission shaft, and the The cooperation of the transmission shaft, gears and bearings should be controlled in an ideal state. This can eliminate the noise caused by the change of the meshing center distance as much as possible.

(4) Pay attention to the effect of lubricating oil on noise control. While lubricating and cooling, the lubricating oil also plays a certain damping role, and the noise becomes smaller as the oil volume and viscosity increase. If a certain oil film thickness can be maintained on the tooth surface, it can prevent the meshing tooth surface from directly contacting, which can attenuate the vibration energy and then reduce the noise. Therefore, the use of oil with high viscosity is beneficial to reduce noise. The main transmission system of this shortcoming milling machine chooses splash lubrication, and splash lubrication will increase the disturbance noise of the oil. In practice, gear lubrication requires very little oil, and its primary purpose is to form a pressure oil film to facilitate lubrication. The test proves that the gear lubrication is the best lubricating oil on the engaging side. In this way, it not only plays a cooling role, but also forms an oil film on the tooth surface before entering the meshing area. Assuming that a small amount of splashed oil can be controlled to enter the meshing area, the noise reduction effect is better. According to this, the oil pipes are arranged from the beginning, so that the lubricating oil is splashed into each pair of gears according to the ideal condition to control the noise caused by the bad lubrication.
CNC milling machine
2. Bearing noise control

(1) Control the quality of the inner and outer rings. Disadvantages In the main transmission system of the milling machine, all bearings are rolled on the inner ring and the outer ring is fixed. At this time, if the inner ring has radial deflection, it will cause unbalance during rotation, and then vibration and noise will appear. Assuming that the outer ring of the bearing, the shape of the coordination hole and the azimuth tolerance are not good, radial swing will occur, which will destroy the concentricity of the bearing components. Assuming that the end faces of the inner ring and the outer ring have a large lateral runout, it will also cause the inner ring of the bearing to tilt relative to the outer ring. The higher the accuracy of the bearing, the smaller the aforementioned deflection and the smaller the noise. In addition to controlling the slight shape errors of the inner and outer rings of the bearing, the waviness of the inner and outer ring raceways should be controlled, the surface roughness should be reduced, and the surface of the raceway should be strictly controlled during the installation process. Otherwise, it is impossible to reduce the vibration and noise of the bearing. . After investigation, it is found that when the waviness of the raceway is dense wave or sparse wave, the contact point of the tumbling body is obviously different when rolling, and the vibration frequency caused by this is very different.
(2) Control the coordination accuracy of the bearing, the hole and the shaft. In the main transmission system of this shortcoming milling machine, the cooperation of the bearing, the shaft and the hole should ensure that the bearing has the necessary radial clearance. The optimal value of the radial working space is determined by the cooperation of the inner ring on the shaft and the outer ring in the hole, and the temperature difference between the inner ring and the outer ring under moving conditions. Therefore, the selection of the initial open space in the bearing is of great significance to control the noise of the bearing. An excessively large radial void will increase the noise in the low-frequency part, while a smaller radial void will increase the noise in the high-frequency part.

In addition, the shape and position tolerances and surface roughness of the cooperative parts should meet the requirements of the selected bearing accuracy class. Assuming that the bearing is tightly installed on a shaft that is not precisely machined, the fault of the shaft will be transmitted to the inner ring raceway of the bearing, and will be manifested in a higher waviness method, and the noise will increase accordingly.

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