This is how the 3D printed mechanical pencil came to be. Luckily though, it actually works pretty well and has enough style to spare.
This pencil has 4 separate parts and was printed fully assembled as shown in the image below. Its about 6" long and 1/2" in diameter at its maximum, not including the pocket clip. It takes standard 0.9mm lead and 7mm diameter erasers. Three extra pieces of lead can be stored behind the eraser. I would have liked to do a more common lead size like 0.7mm or 0.5mm but the feature sizes required to hold lead that small are very difficult to achieve even on high resolution printers. Its not impossible but I decided to focus more on getting something working than getting something that was perfect.
I went though a couple prototypes where I was just modifying the standard mechanical pencil design ("hysteresis collet") for printing. It didn't take long before I accepted that the standard design is ill suited for printing - there is too much reliance on spring force and tight fitting parts. The only way I had around that obstacle was to reinvent the design for 3D printing. I came up with a couple of concepts but settled on a lead-screw design.
At the front of the pencil, 3 small fingers grab the lead and apply some friction to prevent the lead from falling out of the pencil. New lead is inserted through the tip.
I tried to retain all the core functionality of a mechanical pencil with this design. I believe I accomplished that. Moreover, it looks cool and functions well. I am glad with how this project came out. For reference, this was printed on an Objet Eden printer.
I spent parts of the last 6 months modifying the design to work with less precise printers. The hope was to not only allow the pencil to be printed out of more common plastics, like Nylon, but also to allow it to be printed with more common and less expensive processes like SLS. I went through 5 iterations before I got a good working version printed in SLS Nylon, pictured above. After I opened up the design for purchase I started seeing, and hearing, about pencils not working for one reason or another. I found that, for some reason, there appeared to be much more variability in the production process than I originally saw during my design iterations. I saw large variations in dimensional tolerances as well as material properties. I have thoughts as to what is causing this but it is currently making the design non-producible. I'll continue to investigate different avenues to allow this design to be purchasable. Right now it is on Shapeways but not available for purchase: Mechanical Pencil.