1, check z motor, shaft coupling, ball nut, ball screw on the pin, ball nut seat and other connections of the clearance: place the percentage meter to measure the pallet, contact forward to 4MM, Z clear. Z backward to the tailstock, with a hand on the tool holder at the same time Z and then return to 0, see if there is any error.
2, check the clearance of the tool holder and the pallet: the percentage of the first hit the tool holder and then hit the pallet, use the lever to wrench the tool holder (this is a small amount of turning is unlikely).
3, wait for the yellow flowers! Tell us about the checking methods you've used and the results.