I. Introduction
The machining accuracy of parts depends on the control precision of CNC machine tools and CNC systems, and the control accuracy is affected by the mechanical motion accuracy, numerical control system performance and electrical control accuracy. Usually, under the premise of correct selection of CNC machine tools and machine tools, the probability of waste generated during machine tool processing is very low. When the machine tool has a functional failure in terms of accuracy, the reject rate increases.
There are two kinds of general accuracy tolerances, one is regular, such as tool wear, misalignment, zero offset value, size unit conversion, tool compensation value input error, coordinate point calculation error, programming error, and This type of fault resolution is easier with the tolerance caused by incorrect tool selection. There is no regularity, such as the change of size and shape error. The cause of such failure is concealed and difficult to solve. If the cause is unclear or the error is judged, it will cause the machine to malfunction or waste. The rate is higher. When CNC machine tools become the key bottleneck equipment on the production line or are in the process that cannot be replaced by general-purpose machine tools, such as shutdown means stop production. Therefore, it is the main task of the current service enterprise to study the causes of the randomness of the CNC machine tool and to propose preventive measures to guide the enterprise in how to quickly find the cause and quickly eliminate the randomness of the equipment.
Second, the search and judgment method of precision fault
The search for the accuracy fault must be judged according to the condition of the processed product before the continuous occurrence of the tolerance. Generally, the index for measuring the machining accuracy of the machine tool is the process capability index C pk . The large index value indicates that the size fluctuation range is small, and the overshoot probability is smaller. The normal scrap rate is generally related to the tolerance zone width of the processed product, the accuracy of the machine tool, and the stability of the process system. Normal machine tool process capacity C pk >1, ideal process capability C pk >1.33.
When the equipment has some mechanical or electrical faults that affect the accuracy, it is reflected in the product size, the fluctuation range is large, and the scrap rate will deviate from the normal value. For a machine tool, this normal value is the value of the scrap rate obtained by statistical methods after long-term operation under normal processing conditions. Once the statistical scrap rate value is deviated, it indicates that the machine tool is faulty. The effect of solving the machine tool's accuracy failure can only be judged by the frequency trend of the failure.
Third, the treatment of random precision faults
When the randomness accuracy is too bad, don't rush to do it. The operator or quality inspector should be informed of the processing conditions before the equipment appears to be waste, and the inspection data of the processed products of the equipment should be collected. Some key process equipments also have product quality control charts, carefully analyze the change in processing size trends, and see if the fluctuation range is normal. There are two situations here: one is that the fluctuation range is normal, and only the size is out of tolerance is abnormal. Generally, don't pay attention to it, but pay attention to the trend of size. The other is that the range of fluctuations is not normal. The size difference is the inevitable development result of the abnormal fluctuation range, which is to find out the cause and give it exclusion.
Fourth, the randomness accuracy is worse than the cause of the fault
In order to clarify the cause of the accuracy failure, we must first understand the control method of the accuracy of the CNC machine. CNC machine tools control the machining accuracy by ensuring its displacement accuracy and positioning accuracy. The displacement accuracy of the feed servo system means that the command pulse requires the displacement of the machine tool feed and the degree of compliance between the command pulse and the actual displacement of the table through the servo system. The smaller the error between the two, the higher the displacement accuracy of the servo system. The positioning accuracy of the feed servo system means that the output can reproduce the accuracy of the input amount. In order to ensure high displacement accuracy and positioning accuracy, CNC machine tools use closed-loop servo system and semi-closed loop servo system. They are controlled by the difference between the position command obtained by the interpolation operation and the actual position of the machine axis returned by the position detecting device until the difference is eliminated. The difference is the closed-loop servo system. The position detecting device is installed at the end of the actuator. The error of each component in the ring, the error of the transmission chain and the error caused by the motion can be compensated in advance, so that higher displacement accuracy and positioning accuracy can be obtained. . The accuracy of the closed-loop servo system depends only on the manufacturing accuracy and mounting accuracy of the measuring device. In the semi-closed-loop servo system, the position detecting device is mounted on the servo motor shaft or the end of the screw rod, and the mechanical member is enclosed outside the feedback control ring, and the resulting transmission chain error and the error caused by the motion cannot be compensated, so Its control accuracy is lower than the closed-loop servo system, but the system stability is good. The semi-closed loop system and the open loop system have a relatively high probability of fault tolerance.
The randomness and accuracy faults that occur in the machining process of CNC machine tools are summarized as follows:
1. The uneven wear of key parts such as the spindle, guide rail, ball screw nut, table and tool holder of the machine tool leads to a decrease in motion accuracy;
2. Pre-tightening of the machine screw nut, bearing, improper adjustment of the guide rail clearance, etc.;
3. There is a gap or looseness between any one of the transmission components from the feed motor to the nut connected to the tool post or the table. This may cause randomness accuracy, such as connecting the pin to the motor and the lead screw and preventing the axial direction of the lead screw. The loose nut is loose;
4. The guide rail is poorly lubricated, the moving parts of the guide rail are not cleaned in time, the scraping board is broken, the iron filings enter between the moving pairs of the guide rails, and the guide rails are seriously worn;
5. The machine tool parameter setting is unreasonable. If the parameter backlash and clamping tolerance related to the position control are unreasonable, the accuracy will be poor.
6. The foundation and level of the machine tool do not meet the requirements;
7. The grounding requirements of the machine tool do not meet the specifications, no shielding measures, and poor anti-interference ability;
8. Overload cutting, rapid movement beyond the specified speed causes the stepper motor to lose steps;
9. The machine tool repair is not properly adjusted.
V. Case
1. Jiangling Chassis Company has a number of semi-closed-loop CNC lathes CAK6150D. During the processing, there are irregular radial (X-direction) dimensions, and occasionally there is an out-of-tolerance phenomenon.
According to the investigation, the tolerances of the products processed by these CNC lathes are relatively wide, ranging from 0.035 to 0.06 mm. In normal processing, the size fluctuation range is generally between 0.005 and 0.025 mm, so there is very little out of tolerance. There is an out-of-tolerance, which means that the machine tool may have faulty functional faults. Through the follow-up inspection of the whole product, it is found that the size fluctuation range of the other qualified products is larger than normal when the waste products are out of tolerance. By tracking the elimination effects of these precision faults, it is concluded that the machine randomness fault is out of tolerance for the following reasons.
(1) X-axis and servo motor connection end bearing ZRKN2052T is not in place due to automatic lubrication. It is sometimes not used during manual lubrication. The use of coolant can easily damage the lubricating oil film, causing bearing ball deformation, damage to the cage or raceway, and severe wear. Failure mechanism: When the ball is deformed, the cage or the raceway is seriously damaged, when the cutting force is transmitted to the bearing through the screw nut pair, the lateral turbulence of the tool holder is different due to irregular wear or deformation, and the positioning accuracy is too large. . Remedy: Replace the bearing. Preventive measures: Improve bearing life by improving bearing lubrication conditions. Do not use equipment beyond the load.
(2) The ball screw nut pair is unevenly worn. The ball screw nut pair wears unevenly, and the positioning accuracy of the tool holder is poor under the action of the cutting force. Preloading the lead screw nut or using the clearance compensation cannot solve the overrun fault caused by it. The screw nut pair must be replaced. Strengthen the daily maintenance of the equipment, regularly check and eliminate the screw nut clearance.
The nut at the connection of the X axis to the motor is loose. When the X-axis and the motor are connected, the nut is loose, causing the screw to move axially. The looseness of the lead screw is constantly changing due to looseness, resulting in unstable machining dimensions. Tighten the nut after tightening the nut or adding a loosening device. Regular inspections and adjustments.
(3) The X-axis carriage rail surface or the strip is unevenly worn. The X-axis carriage rail surface and the strip are unevenly worn, the machine tool's motion characteristics are deteriorated, and the frictional resistance varies from place to place. Under the action of the cutting force, the tool holder moves loosely and the positioning accuracy is deteriorated. The size tolerance caused by this type can not be eliminated by adjustment, and only the accuracy of the recovery rail can be completely solved. The elimination method is the repair, scraping the X-axis carriage rail surface and the strip. Strengthen the daily maintenance of the equipment, regularly check the scraping board, and replace the failed parts in time.
(4) The tool holder locating pin or hole is severely worn. Since the tool holder positioning pin or the hole is worn, the position of the rotary tool holder is not unique each time, and the clamping tool holder is maintained at the positioning position, thereby causing the processing size fluctuation. Replace worn parts. Usually pay attention to the lubrication of the tool holder to prevent impact events and overload cutting.
(5) The machine tool guide rail is poorly lubricated or the iron filings enter between the dynamic and static guide rails during the movement. If the machine guide rail is poorly lubricated or the scraping plate fails, the iron filings enter between the moving and static guide rails, resulting in large and uneven frictional resistance and poor machine motion characteristics. Adhere to the rails and replace the broken scraper. Strengthen the daily maintenance of equipment.
2. On a H1-K001 three-axis unit, the 11A main reduction shell has a diameter of l00mm, 90mm, etc. The machine consists of two machining center units, wherein the NC1 unit collides with a diameter of l00mm holes, and the NC2 unit collides with a diameter of 90mm holes. When machining, the NC1 unit has a diameter of 100mm, and the hole has never been out of tolerance. The NC2 unit has a diameter of 90mm, and the size is out of tolerance. When the probability of waste is small, it is about 1‰, and when it is high, it is about 2%. Waste products caused by abnormalities such as tool wear and tool setting errors).
As we all know, once the knives are adjusted, they become fixed-value tools. For example, if the knives are wrong, the resulting accuracy is also systematic error. In the current processing process, the pupil size is often out of tolerance, and the randomness accuracy failure is restored without any adjustment and returning to the normal size. To this end, a special research team was set up to list the workpiece material, tool clamping force, coolant, blowing system, spindle and tool 7:24 cone, rivet and gripper, sleeve, butterfly spring, etc. 7 items, such as the situation, may cause the cause of the accuracy failure and check the analysis one by one. When the machine is tracked and monitored abnormally, the following four cases are found:
(1) The probability of scrap appearing in different manufacturers' blanks is different;
(2) It is found that the contact area of ​​the 7:24 taper surface of the spindle and the shank does not meet the requirements;
(3) Two disc springs are broken in the spindle cutter clamping device, which makes the scrap writing increase;
(4) When the NC2 unit exchanges the tool, the NC1 unit collides with a diameter of 100 mm and the hole is still in progress, and the iron filings fall in the NC2 unit main shaft.
From the perspective of the fault treatment effect, the main cause of the sporadic size error is that the clamping force of the spindle shank is insufficient and the contact stiffness between the main shaft cone and the shank is poor. When the material is hard, the contact rigidity between the main shaft and the shank is poor due to the long arbor. When the cutting force is large, the position of the blade changes, so the boring hole is too large.
3. In 2004, Jiangling Chassis purchased a batch of CKA6150 CNC lathes. One of them started to produce Occasional X-axis dimension tolerance (0.035mm in processing parts) after three months of production. The scrap basically appeared after returning to the reference point. One, but not every time the reference point is processed, the first one is scrap, only occasionally. After checking that the machining parts are out of tolerance by a few millimeters, as long as the reference point is returned again, it is normal.
After eliminating the operator's possibility of misoperation, it is judged that the fault is related to the reference point return, and the speed stop block and the electric switch are checked, and there is no looseness, so the machine manufacturer's "three guarantees" personnel are notified, and the waste inspection is performed together with the factory personnel. In the middle, it was found that the tolerance was about one screw pitch, but it was normal to check the distance between the reduction gear and the reference point. After an occasional inspection, it was found that due to a lot of dust accumulation at the deceleration switch, the switch sensitivity was deteriorated, the deceleration switch was replaced, and the fault was eliminated.
Sixth, fast processing random accuracy fault method
Due to the handling of random precision faults, the search and judgment takes a period of observation and accumulates the original data. If the data has been accumulated in the early stage, it is easy to analyze the trend and make correct processing in the process of searching.
In order to quickly deal with random accuracy faults, in the aspect of equipment management, the habit of establishing equipment maintenance files should be developed. Each file has a file book that records every time the equipment malfunctions, cause analysis, processing, spare parts replacement, post-repair effect tracking, repairer, etc., and also records the equipment maintenance to ensure the authenticity of the records. Finding faults is beneficial.
In terms of quality management, quality control charts should be adhered to according to the requirements of quality management standards. X-R charts are common, which is convenient for finding random errors.
Seven, random precision fault prevention
1. Good equipment procurement and installation quality. Procurement equipment should be selected as brand-name products. The machine tool itself has congenital defects such as poor structural rigidity and poor manufacturing quality. It is difficult to eliminate the accuracy. Equipment installation should be carried out in strict accordance with the requirements of the instructions for use. The foundation must be strong and vibration-proof. The bearing capacity should meet the requirements. The position of the block should be adjusted reasonably. The horizontal adjustment should meet the requirements. The grounding of the machine should be standardized. The power line and signal line in the electric cabinet should be separated. Trace, the spacing meets the requirements to improve the anti-interference ability of the machine.
2. Careful implementation of the equipment maintenance system caused by the accuracy of the main cause is: machine maintenance is not in place, machine tool guides, ball screw nut pair, indexing table, rotary tool holder, spindle drive and other key components are poorly lubricated, worn, Looseness and the like cause a decrease in motion accuracy. In addition, improper use of the machine tool, frequent overload cutting, and moving parts beyond the specified speed are also one of the causes of accuracy failure.
3. Improve the operator's proficiency The operator is not skilled. If the tool hits the machine and damages the spindle or the tool holder, destroys the work table or the guide rail, this will increase the randomness accuracy failure rate. The reason is difficult to rule out even if you know it.
4. Correct and rational use of CNC machine tools Improper use of CNC machine tools is one of the reasons for the randomness of randomness. The use of stepper motor driven open-loop CNC lathes such as overload cutting or excessively fast moving speed can cause the motor to lose steps.
5. Improve the repairman's repair skills In the actual work process, it is often found that due to improper adjustment and repair of the repairman, the accuracy of the equipment is expanded or the quality of the repair is not high. Therefore, it is important to improve the repair skills of the repairman.
With Push-in connection technology, you can connect conductors easily – both directly and without tools, insert forces reduced by 50%. it is suitable for various applications. Insert rigid conductors or conductors with ferrules from 0.25mm² into the conductor shaft.The contact spring opens automatically and provides the required pressure force against the current bar.
When installing smaller conductors from 0.14mm², use a standard screwdriver to push the orange button and actuate the contact spring.
Push-in Connection have test and passed various certificates. For example, Vibration resistance in accordance with railway standard DINEN50155,shock and corrosion resistance in accordance with current shipbuilding registers.
Spring Push Terminal Block,Spring Loaded Terminal Block,Twin Terminal Block,Twin Spring Terminal Block
Wonke Electric CO.,Ltd. , https://www.wkdq-electric.com