Rostock is a delta robot 3D printer.
This part is the top bracket that holds a pair of 8mm smooth rods, two ball bearings, a micro switch (endstop) and a plywood board.
Newer versions of the OpenSCAD source files will be published here: https://github.com/jcrocholl/rostock
Print three of these and install them at 120 degree angles on a plywood board (see RostockBaseDims.pdf by Modpod http://www.thingiverse.com/thing:24973 or print the drill jig http://www.thingiverse.com/thing:25000). The plywood mounting holes are sized to work with #11 wood screws or M4 screws, no nuts needed.
Clamp the smooth rod with 16mm M3 screws, washer on the outside, hex nut on the inside. Stainless steel fasteners are recommended.
30mm M8 socket screw holds two 608 bearings (skateboard bearings) followed by two M8 washers, then the printed part, another M8 washer, and a M8 hex nut.
The printed part forces a 2 degree upward tilt for the M8 screw, which will prevent the timing belt from falling off. I like to add a printed plastic flange ring on the outside of the rear bearing.
UPDATE: It's better if you can use a flanged 608 bearing, http://www.vxb.com/page/bearings/PROD/F608ZZ works really well, see pictures.
The micro switch is the common ZM series with 9.5mm center distance between the mounting screws. If you use M2.5x12 or similar, they will screw directly into the plastic, no nuts needed.