Requires the knurled surface library at http://www.thingiverse.com/thing:9095
With the parameters set as they are, for me the screwdriver bits fit snugly and securely. If tight or loose for you, just scale appropriately, or use the SCAD to modify.
I'm a computer person who grew up on a farm. I enjoy all kinds of making. Programming, electronics design, rough carpentry, whatever.I am a mentor for FIRST Robotics FRC team 1502, "Technical Difficulties" at Chelsea High School.