3D Printed Curta Part II

Initial Modeling Complete

I completed modeling all parts for the Curta Type I from the engineering drawings. All parts are modeled to the nominal sizes (no tolerances are applied yet). It took me about a month to get through them all (working on it during evenings and on weekends). I used OnShape to model the parts and the excellent disassembly at vcalc.net as an additional reference to the engineering drawings.

There were a few parts in the disassembly at vcalc that differ from the engineering drawings (namely the results and turns counter carriage casting). It may be that those parts were easier to manufacture and assemble in a slightly different way that they were drawn up. As long as it was functionally the same, I opted for the design that would be easiest to print.

Print Process Change

After modeling the parts, looking at the tolerance specifications, and watching a video on assembling the Curta; I decided that even at 4:1 scale FDM printing may not be up to printing a Curta. It may produce something that could be assembled, but function may be inhibited by parts that have too much friction or too much wiggle room depending on how the layers match up between parts.

Update: I did end up going with FDM and decided on 3:1 scale. Friction was eliminated by a combination of manually fitting the parts and using a PTFE dry lubricant.

Instead of FDM, I have decided that I will use SLS. I toyed with the idea of using SLA, but decided I might as well go with the process that requires no support material to make things easier. I have not decided yet whether I will use Shapeways or build an SLS printer via the You-SLS project (I contributed to the project). While I have a ways to go before I have anything printable, You-SLS is very experimental. We'll have to see where it is and what quality it can produce when I get to that stage. I have not fully decided what scale to print at. I will need to print some tests before I know for sure, but I want to approach a 1:1 scale.

Design Considerations

When producing the 3D models for the Curta parts, I took into consideration 3D printing constraints (initially for FDM printing) as well as affordances gained by using 3D printing. Some parts manufactured separately could be combined and manufactured together as one piece. For instance, the gears and sleeves that the step drum turns in order to add or subtract each digit are combined. Unfortunately they cannot also be combined with the shafts or I would have no way to assemble it. The tens bell and step drum are normally made up of segments that are stacked together and fastened. Those will be printed as one piece. From 145 engineering drawings of parts, I have 116 models. This is partially due to the combination of parts and partially to somth the parts being fasteners and springs which will not be printed.


There are parts that must receive machine screws and parts that are designed to thread onto one another. I will need to use a micro tap and die set for this. However, two of the parts that screw together require M50 thread (larger if I scale up the Curta). M50 or larger taps and dies are expensive. Instead, I will either design these two parts to snap together in a keyed way or drill and pin them together in the correct orientation.


The Curta uses many very small metric screws. Thank goodness for the Internet because I don't know how else I would have found them without it. A friend, Ray Kholodovsky, maker of the Cohesion 3D printer suggested a site where I found all of the screw sizes I need. There are some screws which have a nub tip that are intended as a pin on the digit selector knobs and a guide for the rotation of the crank handle which are custom that I won't be able to find. Instead I will integrate those into the design of those parts or print the body of the screws and thread them myself. Some of the screws are shoulder bolts at very specific sizes. I will likely use standard screws combined with printed sleeves / spacers to add the shoulders.


There are also a few very small springs that are needed. I found equivalents to many of them online, but there were a couple of torsion springs (for the zero positioning lever and anti-reversal cam) that I could not find equivalents for. In addition, there are non-standard springs in all of the tens carry levers that are needed. Due to all of that, I looked into making springs.

I found a few articles on the subject (Making Springs, Homemade Torsion Springs, Make Your Own Springs in Seconds). At the small size I need, I think I should be able to fabricate the springs I need. I will need to create a couple of jigs for it, but making them and the springs should not be terribly difficult.


Tolerances between parts will be the defining factor on whether I can pull this all off. The engineering drawings specify tolerances which are smaller than even an SLS printer's accuracy. I will need to experiment and find values which create parts that have the proper fits.


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