This simple tool attaches to your Dremel and allows you to do centered holes on the edges of 18mm-thick wood. The thread to attach it to the Dremel was taken from the Dremel Holder for The little bolt hobbit by elk (http://www.thingiverse.com/thing:25062), which was in turn derived from the Screw Library by syvwlch (http://www.thingiverse.com/thing:8793) I'm providing the SketchUp file so you can adapt it to other wood thicknesses.