The Curta mechanical calculator is an incredible feat of mechanical engineering. Developed in the 1940s, these devices are roughly the size and shape of a salt shaker. When a series of mechanical sliders and a carriage set correctly, it can makes calculations with the turn (or turns) of a handle. The results can then be reset using a lever, making for a very interesting piece of mechanical hardware.
Though it’s amazing what this device can do, its origin story is perhaps even more incredible. Its creator, Curt Herzstark, was taken to the Buchenwald concentration camp in 1943, where rumors spread that he was working on a new calculating machine. Strangely enough, development of this device was encouraged as something that could be presented to Adolf Hitler upon Germany’s predicted victory in World War II. As this prediction didn’t come to fruition, Herzstark was able to make working prototypes of this invention after the war, and then put them into production under his own name. This continued until they were phased out in the 1970s with the invention of the electronic calculator.
Though “obsolete” technology in practical terms, with its elegant design and mechanical brilliance, it still has a large fanbase. In an incredible homage to this device, Marcus Wu actually created a 3:1 scale Curta “cranker” from 3D printed parts. He first stumbled onto this design when he found the video below after researching for the design of mechanical counters.
He was fascinated with how the device worked, and decided to print some of its mechanisms for a simplified demo device. Of course, he didn’t stop there. After being shown the original engineering drawings at Museum Mura by Olaf, creator of the YACS (Yet Another Curta Simulator) website, he knew he had to make a complete device.
Even with these original drawings and a 3D printer, actually making this into something that physically worked in real life was a massive undertaking. Everything had to be modeled in 3D to send to the printer, and he used Onshape to generate 240 printed parts for 100 individual designs (many parts need more than one copy). They can be found on Thingiverse. To make the job easier on himself, he combined as many of the Cuta’s 600 components together that he could. In addition to the parts he printed, there are also roughly 100 non-printed parts, that include ball bearings, springs, screws, nuts, and the like.
It took months to print the whole thing, as he was assembling and checking tolerances along the way. He estimates print time itself was well over 9 days. However, fitting everything together on such a precise instrument took the bulk of the work, and in total it took Wu a full year to complete his working Curta model. This included enhancing the strength of components as needed, ordering springs, and figuring out where to lubricate everything. After going through this process, he can now disassemble and reassemble the entire device in a few hours.
As if that wasn’t impressive enough, what really sets this build apart visually is the paint job. Wu used a Cricut machine to cut out stencils for the numbers on the dials. The detail is exceptional, and as he puts it in his imgur writeup, “As I pull the stencil away slowly, the numbers reveal themselves. This part is totally like magic every time.”
To get a smooth finish on the external parts, he used Bondo and painted them before sanding them smooth. After quite a bit of painting and sanding, it’s very hard to differentiate his model from the real thing. In pictures, that is; in real life the relatively gigantic size would certainly give it away!
He says that learning how to paint was one of the hardest things about completing this job, however, he seems to have taken on the challenge with aplomb. He also said it was difficult manually fitting the parts together, as well as knowing when to take a step back from what he was doing and consider other solutions for a pesky problem.
Wu reports that he’s “very happy with the results,” and that he hasn’t seen any issues due to wear. He doesn’t, however, think that he’ll ever build a 1:1 scale Curta in this manner, as the plastic parts likely wouldn’t hold up at that size. He would eventually like to teach himself machining and build a normal-sized device.
If you’d like to see how it Wu’s build fits together and operates, check out the final video below: