The reasons why the tool does not return to zero after the CNC lathe program is completed may be due to the following reasons:

1. The zero point return switch of the CNC lathe may be damaged or have poor contact, resulting in the tool being unable to return to the zero point. 2. There may be a problem with the servo motor of the CNC lathe, causing the tool to fail to return to the zero position correctly.

3. There may be a fault in the control system of the CNC lathe, causing the tool to be unable to correctly execute the zero return command.

4. There may be a fault in the mechanical transmission system of the CNC lathe, such as a loose or broken transmission belt, causing the tool to be unable to return to the zero point.

To solve this problem, the following measures can be taken:

1. Check whether the zero return switch of the CNC lathe is working normally. If it is damaged or has poor contact, it needs to be replaced or repaired.

2. Check whether the servo motor of the CNC lathe is working normally. If there is a problem, it needs to be replaced or repaired.

3. Check whether there is a fault in the control system of the CNC lathe. If there is a fault, it needs to be repaired or replaced.

4. Check whether the mechanical transmission system of the CNC lathe is working normally. If there is a problem, the corresponding parts need to be adjusted or replaced.

During operation, be sure to pay attention to safety to avoid harm to equipment and people. If you are not familiar with the steps, you can seek professional help.