The solution to the unstable dimension of CNC cutting:

1. The size of the workpiece is accurate, and the surface finish is poor
cause of issue:
1) The tip of the tool is damaged and not sharp.
2) The machine tool resonates and the placement is unstable.
3) The machine has crawling phenomenon.
4) The processing technology is not good.

Solution (contrast with the above):
1) If the tool is not sharp after being worn or damaged, re-sharpening the tool or selecting a better tool to re-align the tool.
2) The machine tool resonates or is not placed smoothly, adjust the level, lay the foundation, and fix it smoothly.
3) The cause of mechanical crawling is that the carriage guide rail is badly worn, and the screw ball is worn or loose. The machine tool should be maintained, and the wire should be cleaned after get off work, and lubrication should be added in time to reduce friction.
4) Choose a coolant suitable for workpiece processing; if it can meet the processing requirements of other processes, try to choose a higher spindle speed.

2. The phenomenon of taper and small head on the workpiece

cause of issue:
1) The level of the machine is not adjusted properly, one high and one low, resulting in uneven placement.
2) When turning the long shaft, the workpiece material is relatively hard, and the tool eats deeper, causing the phenomenon of tool letting.
3) The tailstock thimble is not concentric with the spindle.

solution
1) Use a spirit level to adjust the level of the machine tool, lay a solid foundation, and fix the machine tool to improve its toughness.
2) Choose a reasonable process and appropriate cutting feed to prevent the tool from being forced to yield.
3) Adjust the tailstock.

3. The drive phase light is normal, but the size of the workpiece is different

cause of issue
1) Long-term high-speed operation of the carriage of the machine tool leads to wear of the screw rod and bearing.
2) The repeated positioning accuracy of the tool post produces deviations during long-term use.
3) The carriage can accurately return to the starting point of processing every time, but the size of the processed workpiece is still changing. This phenomenon is generally caused by the main shaft. The high-speed rotation of the main shaft causes serious wear of the bearing, leading to changes in machining dimensions.

Solution (compare with above)
1) Lean on the bottom of the tool post with a dial indicator, and edit a canned cycle program through the system to check the repeat positioning accuracy of the carriage, adjust the screw gap, and replace the bearing.
2) Check the repeat positioning accuracy of the tool holder with a dial indicator, adjust the machine or replace the tool holder.
3) Use a dial indicator to check whether the workpiece can be accurately returned to the starting point of the program; if possible, check the spindle and replace the bearing.

4. Workpiece size changes, or axial changes

cause of issue
1) The rapid positioning speed is too fast, and the drive and motor can’t react.
2) After long-term friction and wear, the mechanical carriage screw and bearing are too tight and jammed.
3) The tool post is too loose and not tight after changing the tool.
4) The edited program is wrong, the head and tail do not respond or the tool compensation is not canceled, it ends.
5) The electronic gear ratio or step angle of the system is set incorrectly.

Solution (compare with above)
1) If the rapid positioning speed is too fast, adjust the G0 speed, cutting acceleration and deceleration and time appropriately to make the drive and motor operate normally at the rated operating frequency.
2) After the machine tool wears out, the carriage, the screw rod and the bearing are too tight and jammed, and they must be re-adjusted and repaired.
3) If the tool post is too loose after changing the tool, check whether the reversal time of the tool post is satisfied, check whether the turbine wheel inside the tool post is worn, whether the gap is too large, whether the installation is too loose, etc.
4) If it is caused by the program, you must modify the program, improve 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.
5) If the size deviation is found to be too large, check whether the system parameters are set properly, especially whether the parameters such as the electronic gear ratio and step angle are damaged. This phenomenon can be measured by hitting a hundred percent meter.

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

cause of issue
1) The overlap of vibration frequency causes resonance.
2) Processing technology.
3) The parameter setting is unreasonable, and the feed rate is too high, which makes the arc processing step out of step.
4) Loosening caused by large screw gap or out-of-step caused by over-tightening of the screw.
5) The timing belt is worn out.

solution
1) Find out the resonant parts and change their frequency to avoid resonance.
2) Consider the processing technology of the workpiece material, and compile the program reasonably.
3) For stepper motors, the processing rate F cannot be set too high.
4) Whether the machine tool is installed firmly and placed steadily, whether the carriage is too tight after being worn, the gap is increased or the tool holder is loose, etc.
5) Replace the timing belt.

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

1) Occasionally a piece of size has changed in mass production, and then it is processed without modifying any parameters, but it returns to normal.
2) Occasionally an inaccurate size occurred in mass production, and then the size was still unqualified after continuing to process, and it was accurate after re-setting the tool.

solution
1) The tooling and fixture must be carefully checked, and the operator’s operation method and the reliability of the clamping must be taken into account; due to the size change caused by the clamping, the tooling must be improved to avoid the misjudgment by the workers due to human negligence.
2) The numerical control system may be affected by the fluctuation of the external power supply or automatically generate interference pulses after being disturbed, which will be transmitted to the drive and cause the drive to receive excess pulses to drive the motor to go more or less; understand the law and try to adopt some anti-interference measures, For example, the strong electric cable with strong electric field interference is isolated from the weak electric signal signal line, and the anti-interference absorption capacitor is added and shielded wire is used for isolation. In addition, check whether the ground wire is firmly connected, the grounding contact is the nearest, and all anti-interference measures should be taken to avoid interference to the system.


Post time: Mar-10-2021