today I had my Z axis motor screws come loose which dropped the Z axis into my work piece, I don’t know how my 1/8 bit didn’t snap! the collet nut left a night burn mark.
The Z axis motor screws are probably the most susceptible to loosening due to vibration and the most difficult to see when they are coming loose.
Huh. That’s a new one.
Looks like I need to hurry and add that threadlocker to the shop. I have a big bag of tiny tubes. Z axis and pulley grubs…
This happened to me too. Three of the four screws holding the Z motor fell out during a job…found them on the bed when tidying up. Fortunately it didnt affect the milling. I have just made sure that the screws are back in and really tight.
Agree, its difficult to see them to check if they are loosening.
Mine also loosened, I only noticed after taking it apart. I did not use Loctite for those screws. Lesson learned. 
