Boost Your CNC Machine Tool Accuracy: 9 Common Issues Solved

Reasons Affecting The Accuracy Of CNC Machine Tools

1. The workpiece size is accurate and the surface finish is poor.

Causes of Failure:

  1. Damaged or Dull Tool Tip;
  2. Resonance and Unstable Placement of Machine Tool;
  3. Crawling of Machine Tool and Poor Processing Technique.

Solutions:

  • To fix a dull tool, regrind it or replace it with a sharper one.
  • If the machine tool is resonating or not placed firmly, adjust its level, secure its foundation, and stabilize it.
  • The creeping of the machine is often caused by severe wear of the carriage guide rail or wear or looseness of the lead screw ball. Regular maintenance of the machine tool should be performed.
  • After each use, clean the iron wire and add lubricating oil to reduce friction.
  • Select a coolant that is suitable for the workpiece being processed and, if possible, choose a higher spindle speed while still meeting the requirements of other processing steps.
Reasons Affecting The Accuracy Of CNC Machine Tools

2. Large and small taper head of workpiece

Causes of Failure:

  • Poor Adjustment of Machine Tool Level, leading to Unstable Placement;
  • Cutting Phenomenon during Turning of Long Shaft with Hard Material and Deep Cutting Tool;
  • Misalignment of Tailstock Ejector Pin with Main Shaft.

Solutions:

  • Use a level gauge to properly adjust the level of the machine tool, secure a solid foundation, fix the tool firmly, and enhance its stability.
  • Choose an appropriate process and cutting feed rate to prevent excessive stress on the tool.
  • Align the tailstock.

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

Cause of Failure:

  • The high-speed operation of the machine tool carriage for an extended period leads to wear and tear of the screw rod and bearings.
  • Over time, the repeated positioning accuracy of the tool holder deviates.
  • Although the carriage returns to the starting point of processing accurately each time, the size of the processed workpiece still varies.
  • This is often due to the high-speed rotation of the spindle, which results in serious wear of the bearings and a change in machining size.

Solution:

  • Use a dial indicator to check the repeated positioning accuracy of the carriage. Edit a fixed cycle program through the system, adjust the screw clearance, and replace the bearing if necessary.
  • Check the repeated positioning accuracy of the tool holder with a dial indicator and adjust the machinery or replace the tool holder if necessary.
  • Use a dial indicator to verify that the processed workpiece returns to the starting point of the program accurately. If feasible, overhaul the spindle and replace the bearings.
Reasons Affecting The Accuracy Of CNC Machine Tools

4. The size of the workpiece differs from the actual size by a few millimeters, or there is a great change in an axial direction.

Cause of Failure:

  • Fast positioning speed is excessive, causing the drive and motor to be unable to respond.
  • After prolonged friction, the mechanical carriage screw rod and bearing become overly tight and seize.
  • The tool holder is not secure after tool changes and cannot be locked in place.
  • The program has errors, causing the head and tail ends to not match or the cutter compensation to not be cancelled.

5. The electronic gear ratio or step angle of the system is set incorrectly.

Solution:

  • If the fast positioning speed is excessive, adjust the speed, cutting acceleration and deceleration speed, and time to ensure the driver and motor perform normally at the rated operating frequency.
  • If the carriage and screw crane bearings are tight and stuck after machine tool wear, readjust and repair them.
  • If the tool holder is loose after a tool change, check if the reversing time of the tool holder is appropriate, if the worm and worm inside the tool holder are worn, if the clearance is excessive, or if the installation is too loose.
  • If the issue is caused by the program, modify it to meet the requirements of the workpiece drawing, choose an appropriate processing technology, and write the correct program according to the manual instructions.
  • If the size deviation is excessive, verify if the system parameters are set appropriately, paying special attention to parameters such as the electronic gear and step angle to ensure they are not damaged.
  • This phenomenon can be measured using a dial indicator.

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

Cause of Failure:

  • Overlapping of vibration frequencies results in resonance.
  • Inadequate processing technology.
  • Unreasonable parameter setting and excessive feed speed result in out-of-step arc machining.
  • Looseness due to large screw clearance or out-of-step caused by overly tight screw.
  • Worn synchronous belt.
Reasons Affecting The Accuracy Of CNC Machine Tools

Solutions:

  • Identify the components causing resonance and adjust their frequencies to eliminate resonance.
  • Evaluate the processing technology for the workpiece material and plan procedures accordingly.
  • For a stepper motor, avoid setting the processing rate (f) too high.
  • Ensure proper installation and stability of the machine tool, and check for looseness of the carriage due to wear, increasing gap, or loose tool holder.
  • Replace the worn timing belt.

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

Cause of Failure:

  • The tooling and fixtures must be thoroughly inspected and evaluated based on the operator’s methods of operation and the reliability of the fixtures. Any dimensional changes caused by the fixtures must be addressed in order to minimize the risk of misjudgment due to human error.
  • The numerical control system can be affected by fluctuations in the external power supply or interference, leading to the generation of interference pulses. These pulses can be transmitted to the drive, causing it to accept redundant pulses and misdirect the motor.

Solution:

  • To prevent interference, it is important to understand and follow the rules and take appropriate measures, such as isolating the strong current cable from the weak current signal line, adding anti-interference capacitors, and using shielded wires for isolation.
  • Check that the ground wire is securely connected and that the grounding contact is as close as possible.
  • Take all necessary steps to prevent interference with the system.

7. The processing of one process of the workpiece changes, and the dimensions of other processes are accurate.

Cause of Failure:

The reason for the failure could be due to several factors, such as the reasonability of the program parameters, whether they are within the predetermined range, and if the programming format meets the requirements specified in the manual.

Solution:

In case of disordered teeth and incorrect pitch in the thread program section, it is likely to be related to the peripheral configuration (encoder) of the processing thread and the objective factors affecting its performance.

8. Each working procedure of the workpiece has the phenomenon of increasing or decreasing.

Cause of failure:

  • Programming error and unreasonable setting of system parameters.
  • Improper configuration and settings.
  • Regular and periodic faults in the mechanical transmission parts.

Solution:

  • Verify that the program instructions are being executed according to the specifications in the manual.
  • Evaluate by using a dial indicator.
  • Place the dial indicator at the start of the program, run the program, and return the carriage to the starting point after completion. Repeat this process, observing the results and understanding the patterns.
  • Ensure that the system parameters are set reasonably or consider making changes.
  • Check that the single calculation on the coupling parameters for machine tool configuration connection meets requirements and that the pulse equivalent is accurate.
  • Check for damage to the machine tool’s transmission parts, examine the uniformity of the gear coupling, and look for regular and periodic faults. If any are found, inspect the key parts and rectify them.
Reasons Affecting The Accuracy Of CNC Machine Tools

9. The dimensional change caused by the system is unstable

Cause of Failure:

  • Improper system parameter settings
  • Unstable operating voltage, leading to external interference and system malfunctions
  • Incompatible impedance between the system and driver causing loss of signals
  • Abnormal signal transmission between the system and driver
  • System damage or internal malfunction

Solution:

  • Evaluate the reasonableness of speed and acceleration time, spindle speed and cutting speed, and any impact of operator’s parameter modifications on system performance
  • Install voltage stabilizationizing equipment to mitigate the issue
  • Ensure proper grounding and add anti-interference capacitance at the pulse output of the driver
  • Choose an appropriate capacitor model that matches the system and driver
  • Inspect the signal connecting line between the system and driver for proper shielding, reliability and to ensure that pulse generation signals are not lost or increased
  • If the issue persists, send the system to the factory for repair or replacement of the main board.
Reasons Affecting The Accuracy Of CNC Machine Tools
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Shane
Author

Shane

Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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