Controlling the spindle speed of the CNC vertical lathe is achieved through the CNC system. The following are the general steps and methods:

1. CNC programming: In CNC programming, the spindle speed needs to be defined. This is usually achieved by specifying relevant instructions in the CNC program.

2. Instruction selection: Use appropriate CNC instructions to set the spindle speed. Common instructions used by CNC systems include S instructions. For example, S1000 means the spindle speed is 1000 rpm.

3. Speed unit: Pay attention to the unit of speed specified in the CNC program. The unit of speed is usually revolutions per minute (RPM), but other units may sometimes be used. Make sure you use the correct units in your program.

4. Spindle range setting: CNC systems usually allow setting the spindle speed range. Before setting the speed in the program, make sure that the set speed is within the acceptable range of the spindle.

5. CNC system parameter settings: In the CNC system, there may be some parameters related to adjusting the spindle speed. System parameters are adjusted to meet specific processing needs.

6. Interpolation motion control: Some CNC systems support interpolation functions, allowing the spindle speed to be dynamically adjusted during the machining process. This can be achieved by specifying the relevant instructions of interpolation motion in the CNC program.

7. Manual control: In manual mode, the operator may be able to manually adjust the spindle speed directly through the interface or panel of the CNC system. This is typically used for debugging or special case operations.

8. Safety considerations: When setting the spindle speed, ensure that the speed does not exceed the safety range of the machine tool and tool.

Please note that specific operating steps and CNC instructions may vary depending on the model and manufacturer of the CNC system. When using a CNC vertical lathe, it is recommended to refer to the relevant CNC system manual and operating manual to ensure that the spindle speed is correctly set and controlled.