Machining a pen from A36 steel round
Metalworking tools change a lot of life equations about what's expensive. Having sunk a small fortune into building out a machine shop, and having lost a $30 metal pen on a plane flight, I figured it would be cheaper to use a few inches of 1/2" steel round than buy another one. At quantities of over 1000 units this logic makes perfect sense, so I'm also designing industrial robots to automate the manufacturing process. It's probably going to take a while.
The finished product
This was my fourth attempt. I managed to break the first three during manufacture in various ways. (TODO: upload some photos of those)
- It feels great to write with, once you get used to the weight.
- The knurl texture is awesome.
- Aside from the cap, there's no looseness anywhere; the refill feels like it was welded in.
- The pen is wide enough that you can stand it on end (including with the cap off). I consider this a feature, even though I can't imagine a practical use for it.
- Although they're common for pens, I like the fact that this one doesn't have a pen clip.
What doesn't work
- The cap has no retention mechanism; I ended up winding some teflon tape around the receiver to add enough thickness to keep it on. Every now and then I replace the tape because it wears out.
- There's a defect on the side of the pen: I milled the slot too deep, and it cut into the threads on the inside. I admit it does look cool though.
- Along those lines, the milled track slot is rough around the edges; it looks amateur.
- You have to use a vise and pliers to replace the refill.
- It's round and heavy, so it rolls easily.
- The heat-treated finish is starting to wear off a little; I probably need to take it apart and reapply it.
- I should probably have used stainless; the A36 has a metallic odor, which means it's corroding a little.
Planning the next one
I want to change a few things:
- The cap should screw onto the end of the pen.
- The barrel and cap should have a hex profile so it doesn't roll.
- I think the diameter could be a little bit smaller, but I'm running tight tolerances between the hex flats and the internal threads. (I'm just using regular M7/M8 taps; there's probably a finer thread profile that would work better.)
- It should be made from 304 stainless; I need to get some smaller round stock of that.
TODO: manufacturing process
I need to write this up, but I didn't document it while I was making it. I'll probably get some photo/video docs of the next one.