There may be the following reasons for the lock of the tool holder of the CNC vertical lathe for wheel processing:

1. Improper use: The operator does not use the knife holder locking device correctly or does not lock the knife holder properly. When processing the wheel hub, it is necessary to ensure that the tool holder is installed correctly and locked firmly, otherwise the tool holder may become loose.
2. Tool holder wear: After long-term use, the fixed parts of the knife holder may wear out, resulting in a decrease in locking force. If the locking parts of the knife holder are severely worn, they may not provide sufficient tightening force.
3. Loosening bolts or nuts: The fixing bolts or nuts of the knife holder may be loosened due to vibration or vibration, resulting in the lock of the knife holder not being tight. It is important to check and tighten the bolts and nuts of the tool holder regularly.
4. Poor lubrication: If the sliding surfaces of the tool holder or the locking mechanism lack proper lubrication, they may rub, seize or have poor locking effects. Make sure that the lubricating system of the knife holder works normally, and add an appropriate amount of lubricant in time.
5. Equipment aging or damage: During long-term use and operation, the tool holder components of CNC vertical lathe may affect the locking force due to aging, wear or damage. This may require repair or replacement of damaged parts.

If it is found that the wheel hub processing CNC vertical lathe tool holder is not tightly locked, the operation should be stopped immediately and the following measures should be taken:

1. Confirm that the operator uses the knife holder locking device correctly, and re-lock the knife holder.
2. Check the fixed parts of the knife holder, such as bolts and nuts, to ensure that they are not loose, and perform necessary tightening.
3. Check the wear condition of the knife holder, if serious wear is found, the damaged parts should be replaced in time.
4. Ensure the normal operation of the lubrication system of the knife holder and maintain a good lubrication state.
5. If the above measures are ineffective, it may be necessary to repair or replace the damaged tool holder parts.

Please note that it is recommended to consult a professional service technician or the equipment manufacturer for advice and guidance before undertaking any repair or replacement work.