I. Differences between CNC milling machines and conventional milling machines
1. CNC milling machines generally have a CRT screen display function, showing the machining program, various process parameters, machining time, tool movement trajectory, and workpiece graphics. CNC milling machines also generally have an automatic alarm display function, which can quickly locate machine faults based on alarm signals or alarm prompts. Ordinary milling machines do not have the above functions.
2. CNC milling machines use DC or AC continuously variable servo motors for the main drive and feed drive. They generally lack a spindle gearbox and feed gearbox, resulting in a short transmission chain. In contrast, conventional milling machines typically use three-phase AC asynchronous motors for the main drive and feed drive, with a gearbox providing multi-stage speed changes to meet process requirements, resulting in a longer machine tool transmission chain.
3. CNC milling machines generally have a workpiece measurement system, so manual measurement of workpiece dimensions is usually not required during the machining process. In contrast, conventional milling machines require continuous manual measurement during the machining process to ensure the machining accuracy of the workpiece.
4. The most significant difference between CNC milling machines and ordinary milling machines is that when the object (workpiece) changes, CNC milling machines only need to change the machining program (application software) and do not require major adjustments to the machine tool to process various different workpieces.
II. Common Faults in CNC Milling Machine Servo Systems
(1) Overtravel When the feed motion exceeds the soft limit set by the software, an overtravel alarm will occur. The fault can be eliminated and the alarm cleared according to the CNC system manual.
(2) Overload: When the load on the feed motion is too large, there are frequent forward and reverse movements, or the feed transmission chain is not properly lubricated, an overload fault will occur. Generally, alarm information such as servo motor overload, overheating, or overcurrent will be displayed on the CRT. At the same time, overload and overcurrent information of the feed drive unit in the power cabinet will be displayed.
(3) The reason for the axial movement during feeding is:
① The speed measurement signal is unstable, such as due to a malfunction in the speed measurement device or interference with the speed measurement feedback signal.
② The position control signal is poor, unstable, or subject to interference.
③ Poor contact at the wiring terminals, such as loose screws.
④ When surging occurs at the instant of transition from forward to reverse motion, it is generally caused by the backlash of the feed transmission chain or excessive gain of the servo system.
(4) Creep is generally caused by factors such as poor lubrication of the feed transmission chain, low gain of the servo system, and excessive external load. In particular, it should be noted that defects in the servo motor and ball screw coupling itself (such as cracks) can cause the rotation of the ball screw and the rotation of the servo motor to be out of sync, thereby causing the feed motion to be sometimes slow and sometimes fast, resulting in the creep phenomenon.
(5) Machine tool vibration analysis: Is the vibration period of the machine tool related to the feed rate?
① If it is related to the feed rate, the vibration is generally related to the speed loop gain of the shaft being too high or the speed feedback failure.
② If it is unrelated to the feed rate, the vibration is generally related to the position loop gain being too high or the speed feedback failure.
③ If vibration occurs during acceleration or deceleration, it is often caused by the acceleration/deceleration time being set too short.
(6) When the servo motor does not rotate, the CNC system should have an enable control signal in addition to the speed control signal, which is usually the +24VDC relay coil voltage.
① Check if the CNC system outputs a speed control signal.
② Check if the enable signal is on. Observe the I/O status through the CRT and analyze the PLC ladder diagram (or flowchart) to determine whether the start-up conditions of the feed axis, such as lubrication and cooling, are met.
③ For servo motors with point-to-point electromagnetic braking, check whether the electromagnetic brake has been released.
④ Feed drive unit failure.
⑤ Servo motor failure.
(7) Position following error over-tolerance alarm When the servo axis movement exceeds the position tolerance range, the CNC system will generate an alarm for excessive position error, including following error, contour error and positioning error. The main reasons are: mechanical transmission system failure, speed control power supply failure, improper servo system gain setting or incorrect position deviation value setting, excessive cumulative error of feed transmission chain, servo overload or failure.
(8) Drift: When the command value is zero, the coordinate axis still moves, resulting in position error. This is eliminated by drift compensation and zero velocity on the drive unit.
(9) Reference point failure Reference point failure is generally divided into two categories: not being able to find the reference point and not being able to find the reference point accurately. The former type of failure is generally due to a fault in the signal of the reference point deceleration switch, the zero mark of the zero position pulse encoder, or the zero position of the light grid ruler. The latter type of failure is caused by an improper setting of the reference point switch block position, which requires readjustment of the block position.
(10) The servo motor rotates automatically after being powered on. The main reasons are: incorrect polarity of position feedback, position offset of the coordinate axis due to external force, malfunction of the driver, tachogenerator servo motor or system position measurement circuit, or motor or driver failure.
