Pen project

I'm at the brainstorming phase of a pen project myself, actually. My goal is to design and machine a complex multipart pen that functions normally and would be entertaining to field-strip during long meetings.

My first suggestion is to identify a cheapish pen that has a clicking action that you like (this one is my choice) and plan to use the internals of it (spring and refill, mostly). Working these dimensions into your design shouldn't be too difficult, but the trick is to make sure you can insert and remove it easily! I was halfway through designing a mechanism for my project before realizing that it would make it almost impossible to retract the pen without partially disassembling it. If you don't feel too bad about it, you can borrow the mechanical clicking features from the stock pen in question too.

Also, since you're 3D printing this, you need to think carefully about orientation. If you print it upright, you have to worry about sticking to the bed firmly enough (since this is a tall, narrow object), and any text on the sides needs to be designed properly or it won't show up very well. If you print it horizontally, you will either need to design one side to be flat to adhere to the bed or rely on support, which will reduce the quality of your finish. You also have to worry that the internal barrel for the refill will be out of round (won't be 100% circular), which might require post-processing or a looser fit.

If these pens can be one-time use for refills, consider designing them to insert the refill tube during the print and have it firmly held in place (no clicking retraction). This will have a 'cool' factor to it, and also demonstrate that 3D printing is so cheap you don't have to think twice about tossing something out when you're done with it.

Good luck with your project!