This is a working mechanical pencil that is designed to be printed in situ. It takes 0.9mm lead and 7mm diameter erasers. The top turns clockwise, and clicks to position, to push the lead out by small increments (~1.2mm). Turning it counter-clockwise will allow the lead to be pushed back in small amounts by a screw & nut system. Alternatively, pulling the top away from the body allows the lead to be pushed back in until it is completely stowed. Pushing the top back down, apart from giving a satisfying *click*, will lock the lead in position. A cavity below the eraser can hold an extra 3 pieces of lead. All in, there are 4 captured moving parts in this pencil.
I'll be posting a video soon detailing how to clean and grease up the pencil in order to make it functional. Due to the roughness of the SLS Nylon, some lubricant is needed on the screw and nut to make it rotate freely. In the mean time, here is a quick set of instructions to get this pencil working (once you read all of it you'll see why a video would be helpful):
1) Cycle the top section in and out in order to get it to move freely and clear out any nylon powder
2) Make sure the hole below the eraser and the hole that the lead goes in are free of nylon powder
3) Use a toothpick to liberally deposit Vaseline on the screw and nut. This will take some time. Do not try to rotate the top section too hard to get the nut to move. Quick, impulse, rotations are better than brute force. Cycle the nut up and down the screw to wear in the system. If it doesn't move well in some spots then add more Vaseline. Clean up excess Vaseline with a Q-tip or napkin.
4) Once the nut can be moved by rotating the top section, its time to add a piece of lead. The pencil is designed so that when the top section is pulled out, the lead can be pushed in freely from the front. To allow this, put the nut all the way down at the front of the pencil and then pull the top section out (disengage it). Now insert a piece of lead in the front of the pencil (the lead should not go in far since the nut is all the way down), rotating it as its inserted to help it go in earier. Hold the pencil and tap the lead on a hard surface (quick, impulse movements) to force the lead into the pencil. At first nothing will happen but after a bunch of taps the lead will start to move, back driving the nut. Once the nut is all the way back in, push the top section back down (engaging it), drive the lead all the way back out, pull the top section back out, and repeat the tapping process. The goal is to repeat this until the lead can be pushed back in with one steady, light, push. This usually takes about 10 tap cycles (a couple minutes). If the lead breaks, remove the broken pieces and get a new one.
5) The hard part is over! Put some extra lead in the hole under the eraser hole, add an eraser, and draw stuff your your 3D printed mechanical pencil!
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Update 11/20/14: The last two pencils I have received have have wildly different quality than the previous 10 I had iterated on in order to get this design to work in SLS Nylon. I believe something has recently changed in the manufacturing process that has caused this. Until I decide if its worth addressing this I am not making this model available for sale. I hoped that this design could work with plastic SLS processes but it seems like its still pushing the limits a bit too much.
Update 1/10/15: I've ordered 4 pencils so far, of the new design, and of those I've had a 50% success. I've seen large variations in both dimensional tolerances and material strength. It looks like this process is too variable to make good prints of this design.