I wanted to create a new thermal barrier tube for my Stepstruder MK7, but I don't have access to a drill press. Instead I printed out these guys.
I've been tapping the larger end with a m6 tap and threading them on to bolts. I clamp the head in place using a bench vise, and drill from the top down. The bits are a tight fit in the holes, but they seem straight so far.
This is still a work in progress and I've only drilled out "most" of two 30mm bolts so far with the 5/64 bits.
This upgrade has been inspired by: http://www.mastersketchup.com/qubd-extruder-jam-fix/