Summary

I have a rackmount server with a drive cage for one 3.5" hard drive and three 5.25" drives, but I wanted to mount four 3.5" drives.

This bracket adapts 3.5" drives to fit in the 5.25" cage and positions three 3.5" drives vertically in the space intended for two 5.25" drives.

Instructions

If desired, edit the OpenSCAD to select whether or not to make relief cuts for better screw hole perimeters ("relief" variable, default true) using Zach's incomplete polygon method from http://blog.makerbot.com/2010/12/13/design-tip-incomplete-polygons/ . The code is fully parametric but you're unlikely to want to change much else besides screw sizes and minimum plastic thickness.

Print four brackets. I had good luck with the heated build platform on my CupCake, and you will need most of your X-axis travel.

Remove any ooze and stringing from the small-drive mounting ridges. Drill through the supporting membrane under the six upper holes with a 9/64" or comparable bit. Clean out the four lower pilot holes if needed.

Set three 3.5" hard drives on edge, set two brackets onto them with the mounting holes aligned and the bulk of the bracket body toward the center of the drives (screw recesses toward the ends), and install the drive-mounting screws. Flip the assembly and repeat for the other two brackets.

Position the whole assembly inside the drive cage, aligning the pilot holes with the cage's mounting holes, and mount to the cage. Sheet-metal screws worked best to self-tap the plastic pilot holes.

Mounting Bracket for Three 3.5-Inch Drives in Two 5.25-Inch Drive Bays