The reasons for dimensional instability in CNC milling machines are complex and varied, mainly involving four aspects: mechanical, electrical, programming, and operation. The specific reasons and analysis are as follows:
Mechanical aspects
◆ Feed System Faults:
Loose connection between servo motor and leadscrew: This causes the leadscrew and motor of the CNC milling machine to be out of sync, resulting in dimensional errors. This usually manifests as the machining dimension changing only in one direction. Tightening the coupling screws can resolve this.
Large clearance or wear in the ball screw nut assembly: This increases the movement resistance of the milling machine table, making it impossible to execute movement commands accurately. This usually manifests as irregular variations in part dimensions within a few micrometers. Adjusting the clearance or replacing the leadscrew nut assembly can resolve this.
Weared or improperly adjusted ball screw fixed bearing: This causes excessive movement resistance, usually manifesting as irregular variations in dimensions within a few tens of micrometers. The worn bearing needs to be replaced and carefully adjusted.
Poor lubrication of the ball screw and nut, slider and guide rail: This increases the movement resistance of the CNC milling machine, affecting dimensional accuracy. Lubrication conditions need to be improved.
Excessive table movement resistance: This is generally caused by overly tight sprocket adjustment or poor lubrication of the machine tool guide rail surface. The sprocket needs to be readjusted and the guide rail lubrication improved.
◆ Spindle assembly malfunction:
Spindle bearing wear: High-speed rotation of the CNC milling machine spindle causes severe bearing wear, resulting in changes in machining dimensions. The axial and radial deviations of the spindle need to be checked, and the spindle bearings adjusted or replaced.
◆ Other mechanical component failures:
Wear of the slide guide rail: This causes mechanical crawling on the CNC milling machine, affecting dimensional accuracy. Machine tool maintenance is required, including timely cleaning of metal shavings and application of lubricating oil.
Decreased tool turret repeatability: Deviations occur during long-term use. The repeatability of the tool turret needs to be checked, and the machine adjusted or the tool turret replaced.
Electrical aspects
◆ Servo control system malfunctions:
Servo motor malfunction: For example, a faulty rotor bearing in a CNC milling machine requires replacement of the motor bearing or the entire motor.
Position detection device malfunction: For example, oil or dust inside the linear encoder position detection device requires cleaning or replacement of the linear encoder.
Improper motor control parameter settings: For example, unreasonable settings for parameters such as the gain parameter of the servo position loop or the mechanical load-inertia ratio parameter of the speed loop. Servo parameters need to be set and initialized according to the specific specifications of the motor, followed by servo optimization.
◆ Inappropriate system parameter settings:
Incorrect electronic gear ratio or step angle settings: This leads to unstable machining dimensions. The system parameter settings need to be checked for rationality, especially whether electronic gear ratios and step angles have been modified.
◆ External interference affecting the system:
Power fluctuations or interference: This causes the system to lose synchronization or receive excessive pulses, leading to the motor moving too much or too little. Voltage stabilization equipment needs to be installed, and anti-interference measures need to be taken, such as isolating high-voltage cables from low-voltage signal lines, adding anti-interference absorption capacitors, and using shielded cables for isolation.
In terms of programming
◆ Programming Errors:
The program ends without a corresponding beginning or end, or without canceling tool offset: This leads to unstable machining dimensions. The program needs to be modified, improved according to the workpiece drawing requirements, and a suitable machining process should be selected. The program should be written correctly according to the instructions in the manual.
◆ Inappropriate Parameter Settings:
Excessive feed rate: This causes the CNC milling machine to lose synchronization or produce unstable machining dimensions in circular arc machining. Machining parameters need to be set appropriately; for example, the machining rate F should not be set too high.
Operational aspects
◆ Tooling and Fixture Issues:
Unreliable Clamping: This causes the workpiece to move or deform during machining on the vertical milling machine, affecting dimensional accuracy. The tooling and fixtures need to be carefully inspected, and clamping methods improved to ensure reliability.
◆ Improper Operation Methods:
Incorrect operator methods, such as improper selection of cutting feed rates, lead to unstable machining dimensions. Operator training is necessary to improve operating skills.
