If irregular changes in the size of the workpiece occur during CNC lathe processing, it may be due to the following reasons:

1. Tool wear or damage: Tool wear or damage can cause the cutting edge to lose accuracy, thereby affecting the size of the workpiece. Check the wear of the cutting tools and replace or repair them in time.
2. Unstable tool installation: Unstable tool installation may lead to uneven cutting forces, resulting in irregular changes in the size of the workpiece. Make sure the tool is installed correctly and check the clamping force and rigidity of the clamping device.
3. Insufficient or uneven clamping force: Insufficient or uneven clamping force will cause the workpiece to move or deform during processing, thereby causing irregular dimensional changes. Check the stability and clamping force of the clamping device and make sure the workpiece is firmly clamped.
4. Thermal deformation of machine tools: Machine tools will generate thermal expansion during long-term operation, which may cause slight deformation of the machine tool structure or transmission system, resulting in irregular changes in the size of the workpiece. Control the temperature stability of machine tools, and regularly check and calibrate the geometric accuracy of machine tools.
5. Wrong processing parameter settings: Wrong processing parameter settings will have a direct impact on the size of the workpiece. For example, improper setting of parameters such as cutting speed, feed speed, and cutting depth will lead to changes in the size of the workpiece. Ensure machining parameters are set correctly and adjusted appropriately based on material and workpiece requirements.
6. Programming errors: Wrong processing procedures or operating steps may cause irregular changes in the size of the workpiece. Verify the accuracy of the machining program to ensure correct coordinate input and machining path definition.

If the problem of irregular changes in workpiece size persists, it is recommended to contact the CNC lathe manufacturer or professional maintenance personnel for further inspection and troubleshooting. They can perform more detailed machine tool inspections, system adjustments and troubleshooting to address dimensional changes.