Saturday, July 5, 2014

3D Printed Mechanical Pencil

What better way is there to spend multiple consecutive weekends than sitting at your computer, redesigning a mechanism that has existed for decades, all to be able to 3D print something that can be bought at the store for less than $1? ... That's right, anything. However, when your co-worker throws down the gauntlet there is only one thing to do. Take it up.

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.

In this design, the lead is pushed out incrementally by rotating the back of the pencil, which rotates a screw. There are 12 detents per revolution that allow the lead to be locked in position. These detents give a nice "click" sound when the back end is rotated. Rotating it clockwise will push the lead out about 1.2mm per click. Rotating in the opposite direction will allow the lead to be pushed back into the pencil. The back end can also be popped apart from the pencil which allows the lead to be fully back driven. In this mode, the detent mechanism is declutched from the screw so that any force applied to the lead will cause it to back drive the screw and retract into the pencil.

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. 

One detail that stands out with the design, like all my printed assemblies, is that this has a lot of holes in it. It looks like Swiss cheese. This, again, is primary for cleaning out the support material. Although, I do try my best to not make all the holes look too stupid. Another side benefit of the holes, besides styling, is that they allow for viewing of the internal mechanisms.

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