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Analysis of nine reasons for workpiece instability

Dec 25, 2019

cnc machine

1. Accurate workpiece size and poor surface finish

Causes of the problem: The tool tip is damaged and not sharp; the machine tool resonates and is not stable; the machine tool has a sting phenomenon; the processing technology is not good.

Solution: If the tool is not sharp after being worn or damaged, re-sharpen or select a better tool to re-set the tool; resonance occurs in the machine or the placement is not stable, adjust the level, lay the foundation, and stabilize it; The plate guide rail wears fiercely, the screw ball wears or looses, the machine tool should pay attention to maintenance, should clean the iron wire after work, and add lubricating oil in time to reduce the conflict; choose the proper coolant for the workpiece processing, and reach the processing requirements of other processes. In the case, try to choose a higher spindle speed.


2. The taper size of the workpiece occurs

Causes: The machine's placement level has not been adjusted properly, one high and one low, and the placement is not stable; when turning the long axis, the dedication data is relatively hard, the tool eats the knife deeper, forming the phenomenon of giving up the knife; the tailstock thimble is not concentric with the spindle.

Solution: Use a spirit level to adjust the level of the machine tool, lay a thick foundation, fix the machine tool to improve its resistance; choose a reasonable process and appropriate cutting feed to prevent the tool from being forced to give up the tool; adjust the tailstock.


3. The driver's phase light is normal, and the workpiece size is large and small when processing

Causes of the problem: The carriage of the machine tool runs at a high speed for a long time, causing wear of the screw rod and bearing; the repeated positioning accuracy of the tool post is wrong in the long-term use; the carriage can accurately return to the starting point of the machining every time, but the dimensions of the workpiece are still changed . This phenomenon is generally caused by the main shaft, and the high-speed rolling of the main shaft causes severe bearing wear and changes in processing dimensions. Metal processing WeChat, the content is good and worthy of attention.

Solution: Use a dial indicator to lean against the bottom of the tool holder, and then modify a fixed cycle program through the system, check the repeat positioning accuracy of the carriage, adjust the screw clearance, and replace the bearing; use the dial indicator to check the repeat positioning accuracy of the tool holder Adjust the machine or replace the tool holder; use a dial indicator to check whether the workpiece is accurately returned to the starting point of the program. If possible, repair the spindle and replace the bearing.


4. The difference between the workpiece size and the actual size is a few millimeters, or there is a large change in an axis

Cause: The speed of fast positioning is too fast, and the drive and motor cannot respond; after a long period of time, the mechanical carriage screw and bearing are too tight and stuck; after the tool holder is too loose, the lock is not tight; The program is wrong, and the head and tail are not taken care of or the knife compensation is completed. The system's electronic gear ratio or step angle is set wrong.

Solution: The fast positioning speed is too fast, then adjust the GO speed appropriately, cutting acceleration and deceleration, and the time to make the driver and motor work normally under additional operating frequency; after the machine tool wears out, the carriage and screw crane bearings are too tight If it is stuck, it is necessary to readjust and correct it; if the tool holder is too loose after changing the tool, check if the tool holder is reversed at the right time, check whether the turbine worm inside the tool holder is worn, whether the gap is too large, whether the device is too loose, etc .; if it is If the program is caused by reasons, it is necessary to modify the program, improve it according to the requirements of the workpiece drawing, choose a reasonable processing technology, and write the correct program according to the instructions of the instruction; if the scale error is found to be too large, check whether the system parameters are set properly, especially electronic Whether the parameters such as gear and step angle are damaged, this phenomenon can be measured by dial indicator.


5. Processing arc is not ambitious, the scale is not in place

Causes: resonance caused by overlapping vibration frequencies; processing technology; parameter settings are unreasonable, feed speed is too high, making arc processing out of step; loosening caused by large screw clearance or out of step caused by too tight screw; timing belt abrasion.

Solution: find out the parts that have resonance, change their frequency to prevent resonance; consider the processing technology of the workpiece data and reasonably program; regarding the stepper motor, the processing rate F cannot be set too large; whether the machine tool is sturdy, placed smoothly, and dragged Whether the board is too tight after abrasion, the gap is enlarged or the knife holder is loose, etc .; replace the timing belt. Metal processing WeChat, the content is good and worthy of attention.


6. In mass production, occasionally the workpiece is out of tolerance

Cause: It is necessary to carefully check the tooling fixtures, and consider the operator's operation method and the reliability of the clamping. Because of the scale changes caused by the clamping, it is necessary to improve the tooling to prevent workers from making mistakes due to human negligence; CNC The system may be subject to fluctuations in the external power supply or to disturb pulses actively after being disturbed, which will be transmitted to the driver to cause the driver to accept the remaining pulses to drive the motor to take away or less.

Solution: Understand the rules and try to choose some anti-interference methods, such as: blocking of strong electric cables with strong electric fields and signal lines of weak electric signals; participation of anti-interference absorption capacitors and selection of shielded wire barriers; in addition, check the ground wire Whether the connection is strong and the ground contact is nearest. All anti-interference methods are adopted to prevent the system from being disturbed.


7. There is a change in the processing of one part of the workpiece, and the other processes are accurate

Cause: Whether the parameters of the program in this block are reasonable, whether it is within the reserved track, and whether the programming pattern meets the requirements of the manual

Solution: When the thread block is displayed with chaotic teeth and the pitch is not correct, the peripheral equipment (encoder) for processing the thread and the objective factors of the function are immediately associated.


8. Each process of the workpiece has a phenomenon of increasing or decreasing

Causes: faulty programming; system parameter settings are not reasonable; equipment settings are not correct; mechanical transmission components are regularly and regularly changed

Solution: Check whether the instructions used by the program are implemented in accordance with the requirements specified in the instruction manual, and can be judged by dialing the dial indicator, positioning the dial indicator at the starting point of the program, and letting the tray return to the starting point after the program is completed, and then repeat the implementation Even after investigating the results, grasp the rules; check whether the system parameters are set reasonably or are considered to be changed; whether the single calculation of the relevant machine tool equipment meets the requirements and the pulse equivalent is accurate; check whether the transmission part of the machine tool is damaged and the gears Whether the coupling is uniform, check whether there are periodic and regular faults, and if so, check the key parts and clean them up.


9. The scale change caused by the system is unstable

Causes: System parameters are not set properly; Operating voltage is unstable; The system is out of step due to external disturbance; Capacitors have been added, but the impedance between the system and the driver does not match, resulting in the loss of useful signals; The signal between the system and the driver Transmission is abnormal; system is damaged or internal malfunction.

Solution: speed, whether the acceleration time is too large, whether the spindle speed, cutting speed is reasonable, whether the operator's parameter correction has caused the system function to change; install voltage stabilization equipment; ground the wire and determine that it is firmly connected, and the pulse output contacts of the driver Add anti-disturbance absorption capacitors; select the appropriate capacitor type; check whether the signal connection between the system and the driver is shielded and whether the connection is secure; check whether the system pulse signal is lost or added; send it to the factory for repair or replace the motherboard.

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