I have had several problems working with stepper motor based techno davinci's. It all comes down to their horrible control software. I use software called KAY to control the mill and it works perfect every time. If you have a servo davinci, that is a different animal, the z axis encoder might be broken! There is usually no way to fix the encoder itself if the glass wheel inside it is broken. A replacement can be ordered from techno for $180! I went though the same problems with my LC4896 when I purchased it from e-bay!