If you constantly tweak the z stop on your printrbot, this dial will keep you from needing a screwdriver and help you keep track of your changes.
Use one of the premade STLs, or use the customizer function to create your desired knob style. Note, a 65mm length is good for a stock Printrbot. Use 80mm if you have added Brook's three point bed leveler.
I printed the stl using normal 40% infill. You will probably need cooling as the long shaft prints fast. While the print is printing, cut the head off of a 2 inch 6-32 screw. Once printed, insert the cut end of the screw into the hollow end of the knob. You may be able to just screw it in with pliers, or you may need to drill it out a bit, or glue in the screw depending on your print quality. You can remove and replace the existing 6-32 z stop screw with your new one. I find it helpful to mark the top with a Sharpie so I know how far I turn it. From my calculations, each revolution of the knob should move the z axis stop about 0.79 mm.
Update: added a second version with a thicker knurled knob if that fit's your design better.
Update: Per Michele31415's concept a dial with a pointer has been added.