2n2r5 Based Threadless Ball Screw

I have altered the parameters of 2n2r5's part to allow four 3x10x4mm (ID x OD x W) bearings and a slimline size that fits the standard wooden bed and screws.

12mm M3 nuts and bolts are needed to mount and approximately 200mm of 8mm polished smooth rod( for SD2).

4mm pitch so 800 steps per mm in firmware.

Customized version of http://www.thingiverse.com/thing:124706

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