Industry Information

Reasons for the CNC machine tools “dimensional instability”

Reasons for the CNC machine tools “dimensional instability”

1. Accurate workpiece size and poor surface finish

The cause of the failure: the tool tip is damaged and not sharp; the machine tool resonates and is not placed smoothly; the machine tool has a crawling phenomenon; the processing technology is not good.

Solution: If the tool is not sharp after being worn or damaged, re-grind the knife or choose a better tool to re-set the tool; the machine tool resonates or is unstable, adjust the level, lay the foundation, and fix it smoothly; the reason for the machine to crawl is dragging The plate guide rail wears badly, the screw ball wears or looses, the machine tool should pay attention to maintenance, clean the iron wire after work, and add lubricating oil in time to reduce friction; select a coolant suitable for workpiece processing, which can meet the processing requirements of other processes Under the circumstances, try to choose a higher spindle speed.

2. The phenomenon of taper size of the workpiece

Causes of failure: The level of the machine tool is not adjusted well, one high and one low, resulting in unstable placement; when turning the long axis, the contribution material is harder, the tool eats the knife deeper, resulting in the phenomenon of tool giving; the tailstock thimble is not concentric with the spindle.

Solution: Use a spirit level to adjust the level of the machine tool, lay a solid foundation, fix the machine tool to improve its toughness; choose a reasonable process and appropriate cutting feed to avoid the tool being forced to make the tool; adjust the tailstock.

3. The phase lamp of the driver is normal, and the size of the processed workpiece is large and small

The cause of the failure: the machine tool pallet runs at a high speed for a long time, resulting in wear of the screw and the bearing; the repeated positioning accuracy of the tool holder produces a deviation in long-term use; the pallet can accurately return to the processing starting point every time, but the size of the processed workpiece still changes. This phenomenon is generally caused by the main shaft. The high-speed rotation of the main shaft makes the bearing wear seriously, resulting in changes in the cnc machining dimensions. The metal processing WeChat has good content and is worthy of attention.

Solution: lean on the bottom of the tool holder with a dial indicator, edit a fixed cycle program through the system at the same time, check the repeat positioning accuracy of the carriage, adjust the screw clearance, replace the bearing; check the repeat positioning accuracy of the tool holder with a dial indicator, Adjust the machinery 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, overhaul the spindle and replace the bearing.

4. The size of the workpiece differs from the actual size by a few millimeters, or there is a big change in a certain axis

The cause of the fault: the fast positioning speed is too fast, and the drive and the motor cannot react; after a long-term friction loss, the mechanical carriage screw and bearing are too tight and stuck; the tool holder is too loose after the tool is changed, and the lock is not tight; the edited program Error, the head and tail end without echoing or canceling the tool compensation; the system’s electronic gear ratio or step angle setting is wrong.

Solution: If the fast positioning speed is too fast, adjust the speed of GO appropriately, and the cutting acceleration and deceleration speed and time will make the drive and motor work normally at the rated operating frequency; after the machine tool wears out, the carriage and screw crane bearings are too tight If it is stuck, it must be readjusted and repaired; after the tool holder is too loose, check whether the reverse time of the tool holder is satisfied, check whether the turbine worm inside the tool holder is worn, whether the clearance is too large, whether the installation is too loose, etc.; if it is a program For reasons, you must modify the program, improve it according to the requirements of the workpiece drawing, select a reasonable processing technology, and write the correct program according to the instructions of the manual; if the size deviation is found to be too large, check whether the system parameters are set reasonably, especially the electronic gear and Whether the parameters such as step angle are destroyed, this phenomenon can be measured by playing a dial indicator.

5. The effect of processing arc is not ideal, the size is not in place

Causes of failure: Resonance caused by overlapping vibration frequencies; processing technology; unreasonable parameter setting, excessive feed speed, which makes the arc processing out of step; loosening caused by large screw clearance or out of step caused by excessively tight screw; timing belt Wear.

Solution: Identify the parts that generate resonance, change its frequency, and avoid resonance; consider the processing technology of the workpiece material and program reasonably; for stepper motors, the processing rate F cannot be set too large; whether the machine tool is firmly installed, placed smoothly, dragged Whether the plate is too tight after being worn, the gap is increased or the knife holder is loose, etc.; replace the timing belt. The metal processing WeChat has good content and is worthy of attention.

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

Cause of failure: The tooling fixture must be carefully checked, and the operator’s operation method and the reliability of the clamping must be considered. Due to the dimensional changes caused by the clamping, the tooling must be improved so that the worker can avoid misjudgment by human negligence; the CNC system may Interference pulses are automatically generated after being subjected to fluctuations in external power sources or interference, and transmitted to the drive, causing the drive to accept excessive pulses and drive the motor to take away or reduce the phenomenon.

Solution: Understand the rules, try to adopt some anti-interference measures, such as: strong electric field interference strong electric cable and weak current signal signal line isolation, add anti-interference absorption capacitor and use shielded wire isolation, in addition, check the ground wire Whether the connection is firm and the grounding contact is nearest, take all anti-interference measures to avoid interference from the system.

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

Cause of failure: whether the parameters of the program in this block are reasonable, whether it is within the predetermined track, and whether the programming format meets the requirements of the manual

Solution: When the thread block has disordered teeth and the pitch is not correct, the peripheral configuration (encoder) of the processing thread and the objective factors of this function are immediately associated.

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

Causes of failure: incorrect programming; unreasonable system parameter settings; improper configuration settings; regular and periodic failures of mechanical transmission components

Solution: Check whether the instructions used by the program are executed according to the requirements of the manual. You can judge by dialing the dial indicator, positioning the dial indicator at the starting point of the program, and whether the carriage returns to the starting point after the program ends, and then repeat the execution Even if you observe the results and master the rules; check whether the system parameters are set reasonably or considered to be changed; whether the relevant machine tool configuration meets the requirements on the connection calculation coupling parameters and whether the pulse equivalent is accurate; check whether the transmission part of the machine tool is damaged or not. Whether the coupling is uniform, check whether there are periodic and regular faults, if so, check the key parts and eliminate them.

9. Unstable dimensional changes caused by the system

Reasons for failure: Unreasonable system parameter setting; unstable operating voltage; system interference due to external interference, resulting in system out-of-step; added capacitance, but impedance mismatch between system and driver, resulting in loss of useful signal; signal between system and driver Abnormal transmission; system damage or internal failure.

Solution: Whether the speed, acceleration time is too large, the spindle speed, the cutting speed is reasonable, whether the operator’s parameter modification causes the system performance to change; install voltage stabilizing equipment; ground wire and make sure that it is reliably connected, and the pulse output contact of the driver Add anti-interference absorption capacitors; select the appropriate capacitor model; check whether the signal connection line between the system and the driver is shielded, whether the connection is reliable, check whether the signal generated by the system pulse is lost or increased; send it to the factory for repair or replace the motherboard.