A lot of people have been requesting this although it is difficult to print. Attempt on your own discretion.
This is a fully in-situ printed mechanical pencil that I designed several years ago. More information can be found here: http://thecornercase.blogspot.com/2014/07/3d-printed-mechanical-pencil.html
All STL file units are in inches. Scale accordingly to convert to mm.
These parts can only be printed on high-resolution printers that can do SLS Nylon, SLA, or Polyjet The 'Normal' version is difficult to get right in most cases while the '30 Percent Larger' version works most of the time. Both require a lot of delicate cleaning to get functional.
The 'Normal' version takes 0.9mm lead and 7mm diameter erasers.
The '30 Percent Larger' version takes 1.3mm lead and 7mm diameter erasers.
The parts need to be freed very carefully, after printing, for the pencil to work properly. The screw mechanism needs to be lubricated and broken-in for the pencil to work correctly. I've used crisco, vaseline, and silicone lube for this. The break-in process is slow - it takes about 5-15 min of running the screw back and forth until the nut can finally travel the full length of the screw. I usually do this w/o lead in the pencil first, then add a piece of lead and do it again. Sometimes, initially, the lead breaks when I try to tap it back in but once the screw is broken-in the lead slides back in easily. Once this is done the pencil should run smoothly.