Unstable processing of workpieces on CNC vertical lathes may be caused by many reasons. Here are some possible reasons:

1. Tool wear or damage: CNC vertical turning tools will be worn or damaged during use, resulting in uneven cutting force and increased vibration, thus affecting the processing quality and stability of the workpiece. Regular inspection and tool replacement of CNC vertical lathe are important steps to ensure processing stability.
2. Insufficient or uneven clamping force: If the workpiece is not firmly fixed or the clamping force is uneven during the clamping process of CNC vertical lathe, it will cause the workpiece to shift, vibrate or loosen, thus affecting the stability of the processing. Ensuring adequate workpiece holding force and using appropriate clamping methods and fixtures is key.
3. Insufficient rigidity of the machine tool: Insufficient rigidity of the CNC vertical lathe may cause vibration and deformation. This may be caused by the unstable structure of the CNC vertical lathe, insufficient bed rigidity, or aging seals. Choosing high-quality machine tools and maintaining machine maintenance and adjustments can improve processing stability.
4. Unreasonable cutting parameter settings: The settings of cutting parameters such as CNC vertical lathe cutting speed, feed speed and cutting depth directly affect the processing stability. If the CNC vertical lathe cutting parameters are set unreasonably, the cutting force may be uneven, too high or too low, causing vibration or a decrease in processing quality. According to the workpiece material and processing requirements, it is crucial to reasonably select and adjust the cutting parameters.
5. Lack of cooling and lubrication: During CNC vertical lathe cutting, proper cooling and lubrication can reduce cutting temperature, chip accumulation and friction, and improve the stability of the cutting process. It is important to ensure proper cooling and lubrication system operation and coolant selection.

The above are just some of the common reasons that may lead to unstable CNC vertical lathe workpiece processing. In actual operation, other factors need to be comprehensively considered, such as processing procedures, workpiece design, machine tool adjustment, material selection, etc. Regular inspection and maintenance of machine tools, reasonable setting and adjustment of cutting parameters, and optimization and improvement according to actual conditions will help improve processing stability and processing quality.