There may be many reasons why the CNC vertical lathe tool holder is not locked tightly. Here are some common possibilities:

1. The tool holder locking bolt is loose:

CNC vertical lathe tool holders are usually locked by bolts or nuts. If these bolts or nuts are loose, the tool holder will not lock securely in place.

2. The tool holder locking device is worn:

CNC vertical lathe tool holder locking devices, such as locking nuts, locking blocks, etc., may be worn due to long-term use. Wear will reduce the locking effect and prevent the tool holder from locking securely.

3. Hydraulic system problems:

If the CNC vertical lathe uses a hydraulic system to lock the tool holder, a failure of the hydraulic system may cause the lock to be unstable. This could include hydraulic pump failure, hydraulic line leaks, hydraulic cylinder problems, and more.

4. Machine tool structure issues:

Problems with the CNC vertical lathe structure, such as guide rail wear, loose sliders, etc., may cause the tool holder to be unable to be firmly locked in the required position.

5. Operation error:

The operator may make operational errors during the operation of the CNC vertical lathe, such as not using the tool holder locking handle or button correctly, resulting in loose locking.

6. Parts damaged:

The components of the CNC vertical lathe tool holder locking system, such as locking nuts, bolts, hydraulic cylinders, etc., may not work properly due to damage.

7. Lack of lubrication:

If the CNC vertical lathe parts lack sufficient lubrication, friction may increase, thus affecting the locking effect.

In order to solve this problem, you first need to carefully check all parts of the tool holder locking system, including bolts, hydraulic system, machine tool structure, etc. If any loose or damaged parts are found, they need to be repaired or replaced in time. In addition, regular maintenance and upkeep of machine tools is also an effective means to prevent such problems. If you are unsure of the cause, it is best to ask a professional technician to inspect and repair it.