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How to solve spindle noise on CNC milling machine

1. Spindle noise failure analysis
  In many CNC milling machines, because the transmission system of the main shaft still uses several transmission shafts, gears and bearings, vibration noise, friction noise and impact noise are inevitably generated during work. The speed change of the main transmission system of the CNC milling machine is controlled by the computer when the machine does not stop working, so it is more continuous and representative than the noise generated by the ordinary machine tool.
The noise of a certain CNC milling machine is louder when it is used for the first time, and the noise source mainly comes from the main transmission system. With the extension of the use time, the noise is getting louder and louder. The sound level meter is measured at the spindle speed of 2000r/min. The noise is 85.2dB.
   Failure inspection and analysis
   When the mechanical system is subjected to any external excitation force, the system will vibrate in response to the excitation force. This vibration energy spreads throughout the system. When it reaches the radiating surface, this energy is converted into a pressure wave, and then transmitted through the air, that is, acoustic radiation. Therefore, the three steps of excitation response, system internal transmission and acoustic radiation are the formation process of vibration noise, friction noise and impact noise.
When the main drive system of the CNC milling machine is working, it is precisely because the gears, bearings and other parts are excited to respond, and are transmitted and radiated inside the system, noise appears, and these parts have abnormal conditions that increase the excitation force. Large to increase the noise.
   (1) Gear noise analysis. The main transmission system of the CNC milling machine mainly relies on gears to complete the speed change and transmission. Therefore, the gear meshing transmission is one of the main noise sources.
   The noise generated by the gears in the main drive system of the machine tool during operation mainly includes:
  ① In the gear meshing, continuous shocks occur between the teeth and the gears will generate forced vibration at the meshing frequency and cause shock noise.
  ②Because the gear is subjected to external excitation force, transient free vibration of the gear's natural frequency and noise are generated.
  ③The unbalanced inertial force of rotation is caused by the eccentricity of the assembly of the gear, the transmission shaft and the bearing, which produces a low-frequency vibration consistent with the speed. As the shaft rotates, a resonance noise is emitted every time it rotates.
  ④The self-excited vibration of the gear and friction noise are caused by the friction between the teeth. If the tooth surface is uneven, it will cause rapid periodic impact noise.
   (2) Bearing noise analysis. The spindle speed change system of this CNC milling machine has 38 rolling bearings in total. The assembly, preload, concentricity, lubrication conditions, the size of the load acting on the bearing, and the radial clearance of the bearing, the journal and the supporting hole all have a great influence on the noise. Moreover, the manufacturing deviation of the bearing itself determines the noise of the bearing to a large extent. The most easily deformed part of a rolling bearing is its inner and outer ring. Under the influence of external factors and its own accuracy, the inner and outer rings may produce rocking vibration, axial vibration, radial vibration, radial vibration and axial bending vibration of the bearing ring itself.
Similar to gears, the higher the rotation speed of the bearing, the greater the fundamental frequency of rotation, and the noise will increase accordingly. If the accuracy of the inner and outer rings of the bearing is not high, it will become the main factor affecting the noise of the bearing. There are many depressions on the inner and outer rings, and the higher harmonics of these frequencies are displayed on the frequency spectrum, and the noise caused is higher.
2. Spindle noise troubleshooting
   1. Gear noise control
Because gear noise is caused by many factors, some of them are the characteristics of gear noise in the spindle motion system of a faulty milling machine determined by gear design parameters. On the basis of not changing the original design, the original gear is repaired and improved. To reduce noise.
   (1) Trimming the tooth tip. Due to the influence of the tooth profile error and the tooth pitch, after the tooth load is elastically deformed, it causes 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 can be trimmed. The purpose of tooth tip trimming is to correct the bending deformation of the teeth and compensate for gear errors, thereby reducing gear noise. The amount of trimming depends on the tooth pitch error and the amount of bending deformation of the gear after loading, as well as the bending direction. The trimming mainly focuses on the pairs of gears with the highest meshing frequency of the machine tool and the different trimming amounts of these gears when the modulus is 3, 4, and 5mm. When trimming, we must pay attention to the control of the trimming amount, and adopt the method of repeated testing, so as not to damage the effective working tooth profile due to the excessive trimming amount, 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. Only when the tooth tip or root trimming alone does not achieve good results, the tooth tip and the tooth root can be trimmed together. The radial and axial values ​​of can be assigned to one gear or two gears according to the situation.
   (2) Control the tooth profile error. The tooth profile error is caused by many factors. Observing the gears in the transmission system of the faulty milling machine, it is found that the tooth profile error mainly occurs during the machining process, and secondly, it is caused by poor long-term operating conditions. Tooth profile error is more common when the gear meshes. In general, 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 large, 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 changes periodically, the pressure angle will also change 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 gear outer diameter, the deformation of the transmission shaft The cooperation of the transmission shaft, gears and bearings should be controlled in an ideal state. This can eliminate as much noise as possible due to changes in the meshing center distance.
   (4) Pay attention to the effect of lubricating oil on noise control. While lubricating and cooling, the lubricating oil also has a certain damping effect, and the noise becomes smaller with the increase of oil volume and viscosity. If a certain thickness of the oil film can be maintained on the tooth surface, direct contact with the tooth surface can be prevented, vibration energy can be attenuated, and noise can be reduced. Therefore, the use of oil with high viscosity is beneficial to noise reduction. The main transmission system of this faulty milling machine uses splash lubrication, and splash lubrication will increase the disturbance noise of the oil. Actually, gear lubrication requires very little oil, and its main purpose is to form a pressure oil film to facilitate lubrication. Experiments have proved that the gear lubrication is the best for the lubrication side of the gear. 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. If a small amount of splashed oil can be controlled to enter the meshing area, the noise reduction effect will be better. Accordingly, the various oil pipes are re-arranged so that the lubricating oil is splashed into each pair of gears in an ideal state to control the noise caused by unfavorable lubrication.
   2. Bearing noise control
   (1) Control the quality of the inner and outer rings. In the main drive system of the malfunctioning milling machine, all the bearings rotate in the inner ring and the outer ring is fixed. At this time, if radial deflection of the inner ring occurs, it will cause unbalance during rotation, resulting in vibration and noise. If the outer ring of the bearing, the matching hole shape and position tolerance are not good, radial oscillation will occur, which will destroy the concentricity of the bearing components. If 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 be skewed 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 geometric deviation of the inner and outer rings of the bearing, the waviness of the inner and outer ring raceways should also be controlled to reduce the surface roughness. Strictly control the surface knocks and scratches of the raceways during the assembly process, otherwise it is impossible to reduce the vibration and noise of the bearing. . It is found through observation that when the waviness of the raceway is dense wave or sparse wave, the contact points of the rolling elements during rolling are obviously different, and the vibration frequency caused by this is very different.
   (2) Control the matching accuracy of the bearing, the hole and the shaft. In the main drive system of the malfunctioning milling machine, the bearing should be matched with the shaft and hole to ensure that the bearing has the necessary radial clearance. The optimal value of the radial working clearance is determined by the fit 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 in motion. Therefore, the selection of the initial clearance in the bearing is of great significance to control the noise of the bearing. An excessively large radial gap will increase the noise in the low-frequency part, while a small radial gap will increase the noise in the high-frequency part. Generally, it is best when the gap is controlled at 0.01mm. The matching form of the outer ring in the hole will affect the propagation of noise. A tighter fit will improve the sound transmission, thereby increasing the noise. A tight fit will force the raceway to deform, thereby increasing the shape error of the bearing raceway, reducing the radial clearance, and also leading to an increase in noise. The loose fit of the outer ring of the bearing will also cause a lot of noise. Only a proper fit can dampen the vibration of the outer ring by the oil film at the contact between the journal and the hole, thereby reducing noise. In addition, the shape and position tolerances and surface roughness of the mating parts should meet the requirements of the selected bearing accuracy class. If the bearing is tightly installed on a shaft with inaccurate machining, the shaft error will be transmitted to the inner ring raceway of the bearing and manifested in the form of higher waviness, and the noise will increase accordingly.

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