Robo3D Z Rod Constraint

#### Description
This constrains the threaded Z rod so it can't wobble and thereby decreasing Z ribbing, especially when coupled with a Z isolator such as the following.
http://www.thingiverse.com/thing:1122480

#### Instructions
Requires
- 2 608zz bearings.
- 2 M3 socket head bolts (12mm long I think).
- 2 M3 nyloc nuts.

Assembly
- Print 2 of these.
- Press fit the bearings (you can use a block of wood and hammer to make sure it's fully inserted).
- Press fit the nyloc nuts.
- In