Why Sigma? In mathematics, Sigma represents the summation of parts to form a whole. Our customers have been asking for a clicky pen for years, but I wanted to deliver something exceptional. Turns out that took a while, but we've arrived. Over two years in the making, the Sigma Pen embodies this principle—it's the sum of precision-engineered components, innovative manufacturing, and thoughtful design, all coming together to create an exceptional writing experience. It's also a Gen-Alpha nod to the pen's subtle awesomeness.

The Story I launched the Alpha 3-Piece Executive Pen on Kickstarter back in 2013. That project funded the purchase of my very first CNC lathe and helped transform Prometheus from a hobbyist workshop into a real business. Since then, I’ve watched design trends come and go. Personally, I’d rather set trends than chase them. Over the following few years, bolt-action pens became all the rage, and it felt like literally everyone was making one. They’re cool, but they just weren't for me. So, I waited.

In 2020, our friends at Tactile Turn launched their Side Click pen. I was intrigued and became an early adopter. As far as I know, it was the first machined pen to feature a side click, utilizing a mechanism based on the iconic Mitsubishi Boxy 100. I fell in love with the stupidly simple engineering behind it—the exact kind of design philosophy that drives everything I do. It looks like nothing, yet it does everything.

But calling it "simple" is a bit of a misrepresentation. Achieving true simplicity is actually the hardest thing in the world to do. The Sigma has been a grueling three-year project, with a final push over the last 12 months. The final mechanism consists of just two parts: a plunger and a spring. To get the form and function exactly right, it took 9 revisions of the plunger and 11 iterations of the spring just to cross the finish line.

The breakthroughs came from collaboration and a bit of creative sourcing. Joe had the brilliant idea to repurpose the ultra-smooth ruby ball from our Lambda tops to serve as the side-click push button. Initially, we actually started this project by buying the raw plungers and springs directly from Tactile Turn, but both required heavy modifications to suit our unique form factor. We still proudly buy our specialized screws from Tactile Turn, which they manufacture in-house on their Swiss CNC lathes.

Eventually, a thought hit me: Could I just 3D print these internal parts instead of buying them and modifying them? It took a long time and endless testing to say for certain, but my final answer is a resounding yes. Shifting to advanced 3D printing allowed me to fully customize the mechanism for our exact use case, manufacture cleanly on-demand, and push the absolute limits of what manufacturing technology can achieve in 2026.

In-House 3D printed mechanism: A masterclass in modern manufacturing. Inspired by the designs of our friend Will Hodges at Tactile Turn and the Mitsubishi Boxy pen, we've refined every aspect to deliver an exceptionally smooth and consistent action. Each click is a confirmation of quality, a testament to the seamless integration of its many parts. The mechanism is entirely 3D printed in-house on our Formlabs Form3 resin printer from engineering-grade Tough 1500 material.

Just like our acclaimed Lambda tops, the Sigma Pen features our signature instrument-grade ruby ball as its retract button. This novel element provides a satisfying tactile experience and connects your new pen to the heritage of our product line, and the quality you've come to expect.

We prefer (and ship with your pen) the Schmidt Easyflow 9000 refill. This is the same refill we ship with our Alpha pen adapter kit. There are a lot of parker compatible refills out there, which you can use, but the Schmidt is our favorite.

Our USA-made spring is specifically designed for Parker refills and has just the right amount of force. No generic coil springs here.