The Curta.org Bulletin Board

[mwu]

I got into 3D printing a while back and while working with my brother on designing a mechanical device, he introduced me to the Curta. From there I found the engineering drawings and over the last month I have modeled every part.

All parts are modeled to their nominal sizes, so it is not yet produceable. I need to calculate tolerances for 3D printing and alter the design to make it more printable.

Also, it's definitely not printable at 1:1 on an FDM printer. I may try for a 4:1 scale on my FDM printer, but I am also planning on either building an SLS printer or ordering through Shapeways for something closer to 1:1.

I'm doing this project for myself, but I also wanted to gauge interest in it. I don't know for sure how much printing through Shapeways would cost, but other costs to cover are the tooling I would need to finish the parts (many will need threads cut into them) and the tools / time it would take to do proper assembly.

I know I'm crazy for attempting it, but the thing fascinates me.

[murff]

Hi mwu
Interesting try. But I don't think that you will be able to build a running Curta based on 3D printed parts. Other importance beside the form of the parts is the choice of the material - in earlier Curtas all kind of metal (carefully choosen for each element) - later some unimportant parts were made in plastic.
So I think it would be interesting to have some 4:1 (or larger) parts for displaying purpose (kind of functional models).

But however - good luck

And - would you share your transposed drawings? (as we done with the originals!) - If so I would offer a hosting on my site curta.li
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[stenella]

It does sound interesting enough - a Curta in 4:1 size especially, that would be quite the conversation piece. Do keep in mind that it is a quite well-engineered thing, also as far as materials and their strengths go. There are plenty of springs inside, some of them rather weird, such as the spider spring in the carriage, which you will be unable to make from plastic. But I would certainly love to see the end result!

[stenella]

haha murff - noone post anything for days, and then there's the two of us within a minute of each other

[murff]

stenella you're right - it's a very lively forum
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[mwu]

[stenella]

Well, do keep us posted (with pictures!). As far as assembly goes, it's hard to beat this video:

https://www.youtube.com/watch?v=AnTb26WHx2Q

Good luck!

[mwu]

Stenella, thanks for the link. Yes, I watched that video a few times now and have written step by step notes (though they will require some refinement and organization) for when it comes time to attempt an assembly.

[Pete]

I'm sure I'm not the only collector who will be watching this with interest. While I've dismissed the possibility of 3DP versions of the Curta as-is, scale models should indeed be possible with added parts. Good luck!
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Cheers, Pete

[mwu]

[murff]

cool

Please PM me some sample files (in different formats) - I own Cinema 4d so I can try to import them. The 1:1 size is ok - and for a 3D picture it's not important.


PS:
here is a sample of my Cinema 4 usage...
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[mwu]

I ordered a couple of parts from Shapeways. They use laser sintering (SLS) for very accurate / highly detailed prints. I chose two parts that have some of the smallest detail and interact together -- the selector shaft and knob. I ordered these at 1:1 scale so they are quite small.

The screw which follows the groove in the selector shaft is modeled directly into the knob. I could have simplified it by just modeling a nub onto the inside of the knob, but it was easier to just mate the screw I already modeled to the knob and export the two as a single model.

I used what I learned from my own printing to estimate how to tweak tolerances to allow these parts to fit. They did fit, but the fit was too loose. My calculations were for a different type of printing, so I figured they wouldn't be perfect.

While the parts came out right, the little axle at the top of the selector knob and the thin fingers that slide the gear up and down the transmission shaft are too weak to be useful. I can thicken them both up for strength, but I am skeptical that it would be enough. I will also need to increase scale which I knew I would need to.

This was more a test of SLS printing and tolerances than it was an attempt at creating usable parts. However with what I have learned, a smaller scale is possible with SLS than with FDM printing. With FDM, nozzle size is a big limiting factor. SLS printing is not nearly as cheap as FDM and takes much longer due to shipping to get the parts, so it's a longer iteration cycle. I plan on getting a smaller nozzle for my FDM printer and printing at scale for now. There are a few ways I can adjust the design to be better suited for 3D printing, but I will need to iterate more quickly than Shapeways would allow for.

At some point I may have to try printing out a clearing ring to see how strong it comes out at 1:1 through Shapeways. I know that is one of the more frequently broken parts.

Here are some animated gifs made from video of the parts:

[murff]

Cool, congratulation to this first success!
I don't exactly understand how the spring/ball is snapping on the shaft...


(and... I'm looking forward to some more 3d files)
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[mwu]

The spring and ball are intentionally missing -- those would have been added during assembly (they are not part of the models), but I didn't want to order those until I knew the scale I would be able to get things to work.

In other words, it does not snap to the number offsets. It does have the pin matched to the groove so the axle spins as the knob position is changed. It functions as far as I've given it the ability to.

[mwu]

I've made some progress printing usable parts at 3:1. I wrote a blog post about it.

murff -- I am still working on getting those models to you, but I found some flaws in them. I may need to complete the Curta before I know the models are correct.