I am trying to get a new Chinese 6040 machine set up. Not have any problems with the Y or X axis, but cannot solve the problem with the Z axis. A search of the forum revealed a number of threads on lost steps in the positive direction, but nothing about loss in the negative direction.
As I run test cuts using Mach 3 and the supplied roadrunner.tap Gcode, each progressive cut becomes shallower until after several movements, the end mill no longer contacts my material at all. I have ruled out most obvious problems such as a loose end mill.
The problem becomes worse at slower rates and lower accelerations. There is an occasional but noticeable catching or "clunking" only as the z-axis lowers. Repeated movements at high rates accelerations don't seem to cause the problem.
Given that it is only a problem during descending movements, an underpowered stepper or bad bearing on the ball screw seem unlikely too. Any suggestions on what to check next?