The fault diagnosis of mechanical motion of CNC machine tools is to monitor the state of the CNC equipment system, determine whether it is normal, analyze the cause and location of the abnormality, and forecast its development trend. Mechanical failure is the easiest and most frequent failure of CNC machine tools. Mechanical failures mainly occur in the movement actuators of the machine mainframe, such as the spindle of the main motion system, the coordinate axes of the feed motion system, the tool magazines and robots in the automatic tool changer, and the actuators that drive these motions. Power source (servo motor). In addition, there are malfunctions such as lubrication, hydraulics, and pneumatics.
The fault diagnosis of CNC machine tools includes the three aspects of the recognition, prediction and monitoring of the running status of the machine tool. Through the determination of certain characteristic parameters of CNC machine tools such as vibration, noise and temperature, the measured values are compared with the specified normal values to judge whether the working status of the mechanical device is normal. If the mechanical device is monitored periodically or continuously, the trend law of the state change of the mechanical device can be obtained, thereby predicting and forecasting the operating state of the mechanical device.
1) Ask is to ask about the failure of the machine tool, to find out whether the failure is sudden or gradual. The operator is generally familiar with the performance of the machine tool, and at the scene of the fault, he or she has heard and witnessed the scene. The analysis provided by the situation is very helpful. Usually you should ask the following:
1 What are the abnormal phenomena when the machine is started?
2 Compare the accuracy and surface roughness of the workpiece before and after the failure in order to analyze the cause of the failure.
3 Whether the transmission system is normal, whether the output is even, and whether the amount of the back knife and the amount of the knife are reduced.
4 Whether the brand name of the lubricant is in compliance with the regulations and whether the amount is appropriate.
5 When the machine tool was overhauled and repaired.
2) Look at 1 to see the speed. Observe the change of the main transmission speed. For example, if the belt speed of the belt drive is slow, the belt may be too loose or have too much load; for the gear in the main drive system, it mainly depends on whether it is beating or swinging; Or shake.
2 see the color. If the machine rotates, especially if the spindle and bearings are not operating properly, they will generate heat. Warming the machine for a long time will change the appearance of the machine, mostly yellow. The oil in the tank will also become thin due to excessive temperature rise, and the color will change; sometimes it will become dark ink due to a long time without oil change, excessive impurities, or deterioration of the oil.
3 see the scars. It is easy to find the parts of the machine tool that are damaged by bruising. If a crack is found, a mark should be made. After a certain period of time, the change should be compared so that a comprehensive analysis can be performed.
4 Look at the workpiece. From the workpiece to determine the quality of the machine. If the workpiece surface roughness R after turning. The large value is mainly due to the fact that the clearance between the spindle and the bearing is too large, and that the slide plate, knife holder and other pressure plate irons are loosened, and the ball screw is loosened. If the surface roughness after grinding. The value is large, which is mainly due to the poor dynamic balance of the spindle or the grinding wheel, the resonance of the machine tool, and the crawling of the table. If the surface of the workpiece is corrugated, see if the number of corrugations is equal to the meshing frequency of the drive gear of the machine tool spindle. If they are equal, it indicates that the poor meshing of the main shaft gear is the main cause of the failure.
5 see deformation. The main observation is whether the drive shaft and ball screw of the machine tool are deformed; whether the end face of the pulley with large diameter and gear is beating or not.
6 Look at the tank and cutting fluid tank. The main observation is whether the oil or cutting fluid is deteriorating, and determine whether it can continue to be used.
3) Listen to determine if the machine is operating normally. Normally operating machine tools have sounds and rhythms that are consistent with the sound and maintain constant stability. The normal sound emitted by mechanical movement can be roughly classified into the following types:
(1) Rotary motion components are generally used to generate a quiet “buzz” sound when the operating range is small or in a closed system; larger beeps are generated when a non-closed system or a large operating area occurs. All kinds of large-scale machine tools produce low and loud rumbling sounds.
2 The normal operation of the gear pair, generally no sound at low speed; sprocket and rack drive generally send a smooth "beep" sound; linear reciprocating motion of the device, generally issued a periodic "creak" sound; The common cam top rod mechanism, crank linkage mechanism and swing rocker mechanism usually emit periodic “click” sound; most bearing pairs generally do not have obvious sound, and sensors (usually metal rods or screw drivers) are used. A clearer "click" can be heard.
3 The transmission sound produced by various media transmission devices generally varies with the characteristics of the transmission media. For example, the gas medium is mostly "whistling"; the fluid medium is "squeaky"; the solid medium emits "rustle" sound or "Nakhon Ratchasima" sound.
The key to "auditory diagnosis" is to grasp the normal sound and its changes, and to compare it with the sound at the time of failure. Here are a few common noises that can easily occur.
1 friction sound. The sound is sharp and short, often the relative movement of the two contact surfaces. Such noise may occur if there is a lack of lubricant between the belt skid or the spindle bearing and the driving screw pair.
2 leak sound. The sound is small and long, continuous, such as air leakage, leakage and leakage.
3 impact sound. The sound is low and boring, such as the intermittent impact sound in the cylinder, generally due to loose bolts or other foreign objects inside.
4 contrast sound. Lightly tap with a hand hammer to identify if the part is defective. The sound produced by a cracked part is less crisp.
4) Touch the hand to determine the fault of the machine, usually the following aspects:
1 temperature rise. Human finger touch is very sensitive and can reliably judge various abnormal temperature rises. The error can be accurate to 3 to 5°C. According to experience, when the machine temperature is around 0 °C, the fingers feel cold, a long touch will produce a biting pain; lOoC or so, feel cool, but can be tolerated; 20 °C, the hands feel cooler, with contact Extend the time, feel the tidal temperature; when the temperature is around 300C, feel warm and comfortable; when the temperature is around 40°C, feel is like touching a patient with high fever; More than C, the hand feels hot, such as the palm of your hand for a long time can have a sense of sweat; when about 60 °C, the hand is very hot, but can withstand about 10s; around 70C, the hand has a burning sensation, and the hand's contact area is very Red color appears quickly. When the temperature exceeds 800C, the touch of the instant touch is “sparkling” and the burn time is too long. In order to prevent the fingers from being burned, you should pay attention to the method of touching your hand. Generally, you first touch the surface of the device with your right hand and the index finger, middle finger, and ring finger of the middle finger and ring finger to gently touch the surface of the machine. After determining that there is no damage to the skin, you can use the finger belly or palm touch.
2 vibration. The slight vibration can be identified by hand. As for the size of the vibration, a fixed base point can be found. When one hand touches it, the vibration can be compared.
3 scars and ripples. Unable to see the wounds and ripples of the naked eye, you can feel it easily with your fingers. The method of touch is to touch the circular parts tangentially and axially, respectively; When you touch, you can't use too much force. Just touch your finger on the surface to be checked.
4 crawls. Touch can be intuitively sensed, causing many reasons for crawling, common is the lack of oil or improper selection; piston seal too tight or wear caused by increased mechanical friction; hydraulic system into the air or pressure and so on.
5 tightness. For rotating the spindle or shaking the handwheel, you can feel the tightness of the contact area is even and appropriate, which can determine whether the tightness of these parts is appropriate.
5) Smell The olfactory diagnosis method can be used to obtain good results because of severe friction or short circuiting of the insulation of the electrical components, oxidative evaporation or combustion of the attached grease or other flammable substances, and the generation of odors such as fume gas and char gas.
The main diagnosis methods of the above-mentioned practical diagnostic techniques are simple and practical and quite effective, but have certain limitations.
Before a mechanical failure occurs, the life of the parts can be extended through careful maintenance and maintenance. When a fault occurs, generally minor faults can be resolved through careful adjustment, such as adjustment of the clearance, fuel supply, liquid (gas) pressure, flow, bearing and ball screw pre-tightening force, taking plugging and other measures. For worn, damaged, or inoperative parts, troubleshoot them by repair or replacement.
