The decline in CNC machine tool machining accuracy may be caused by a variety of factors. The following are some common factors that cause the processing accuracy of CNC machine tools to decrease:

1. Wear and aging: Various components of CNC machine tools, such as guide rails, screws, bearings, etc., may wear and age after long-term use. This will cause the machine tool's motion system to become unstable and affect the machining accuracy.

2. Impurities and contamination: Impurities, dust and contaminants present in the working environment of CNC machine tools may be deposited on key components such as guide rails and lead screws, causing sluggish movement and thus affecting machining accuracy.

3. Material deformation: CNC machine tool parts may be affected by thermal deformation or stress deformation during processing, especially under long-term or high-load conditions, which may lead to a decrease in processing accuracy.

4. Temperature fluctuations: Changes in the temperature of CNC machine tools will cause thermal expansion and contraction of machine tool parts, thereby affecting the dimensional stability of the machine tool. Unstable temperature environments may cause fluctuations in machine tool processing accuracy.

5. Tool wear: CNC machine tool tool wear will affect cutting performance and lead to processing dimensional deviations. Timely replacement of severely worn tools is the key to maintaining machining accuracy.

6. CNC system problems: Faults or errors in the CNC system of CNC machine tools may cause inaccurate machine tool motion, thereby affecting machining accuracy. This includes problems such as CNC program errors and control system failures.

7. Workpiece clamping problems: Unstable workpiece clamping on CNC machine tools or problems with the fixture itself may cause the position of the workpiece to change during processing, thus affecting processing accuracy.

8. Operational errors: The incorrect use of CNC machine tool operators, incorrect setting parameters or unreasonable cutting conditions may lead to a decrease in the machining accuracy of the machine tool.

9. Power supply problem: The unstable power supply voltage and current of CNC machine tools may affect the operation of the machine tool, especially when it involves the servo system, which may lead to motion errors.

To ensure the machining accuracy of CNC machine tools, regular maintenance, cleaning and calibration work is required. In addition, operators need to be professionally trained to use CNC systems and equipment correctly to reduce operating errors. Regular inspection and maintenance of CNC systems, machine tool parts and working environment can effectively reduce the impact of these factors on machining accuracy.