There are many reasons for the instability of CNC vertical lathe workpiece processing. The following are some possible reasons:

1. Improper setting of cutting parameters: cutting parameters include cutting speed, feed speed, cutting depth, etc. If the cutting parameters are set unreasonably, it may lead to unbalanced or excessive cutting force, causing vibration and unstable processing.
2. Improper selection of tools: The selection of tools is closely related to materials and geometric features. If the selected tool is not suitable, such as the tool material does not match, the tool geometry is not suitable for the processing requirements of the workpiece, it may lead to reduced cutting quality, increased vibration or unstable processing of the workpiece.
3. Tool wear or damage: Tool wear or damage will lead to uneven distribution of cutting force and reduced cutting quality, thereby affecting the processing stability of the workpiece. Regular inspection and replacement of tools is an important step in maintaining process stability.
4. Insufficient rigidity of the machine tool: Insufficient rigidity of the machine tool may cause vibration and deformation during processing. The rigidity of the machine tool mainly depends on the structure and material of the bed and each component. If the rigidity is insufficient, it is necessary to consider improving the rigidity of the machine tool or re-select a suitable machine tool.
5. Uneven clamping force: The uneven distribution of the clamping force of the workpiece in the fixture will lead to unstable processing of the workpiece. Good clamping force distribution balances cutting forces and reduces vibration. Therefore, it should be ensured that the clamp is designed and adjusted to provide uniform clamping force.
6. Environmental vibration and interference: If there is vibration or interference in the working environment of the CNC vertical lathe, such as the operation of surrounding equipment, uneven ground or other external factors, it may lead to instability in workpiece processing. Where possible, ambient vibration and disturbances should be minimized or eliminated.

In addition to the above factors, there are other factors such as the nature of the material, the cooperation between the tool and the workpiece, the lubrication and cooling conditions, etc., which will also affect the stability of the workpiece processing. Therefore, when dealing with the problem of workpiece processing instability, it is necessary to consider these factors comprehensively and make appropriate adjustments and optimizations.