I calibrated the machine the first time with no problems. Then I had a problem with the chain jumping on the sprocket, so I took the machine apart, and tried to re-calibrate.
The process went fine until I got to the z-axis part. When I tried to lower the bit to zero, the entire machine would lower, and the z-axis did not move. Then, any action I would try to take would result in the sled lowering toward the ground.