modified two-bearing Z mount for three-leadscrew Z axis system for D-bot

This refers to the 3-leadscrew Z axis solution originally posted by karltinsly.

I was unsatisifed with the use of a single, unflanged 608 (skateboard) bearing held by a plastic box to the underside of a modified Z motor mount. I felt that the leadscrew would not be supported enough against forces acting to tilt it from side to side. I also wanted a solution which left the face of the bearing flush with the other surfaces of the mount, in order to make it easier to find locking or clamping 8mm