Curta Calculator Type I scaled at 3:1

by mwu, published

Curta Calculator Type I scaled at 3:1 by mwu Dec 6, 2016
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26017Views 14094Downloads Found in Engineering


The Amazing Curta Calculator is now 3D Printable!

New group for discussion, bragging rights, and help with Curta builds: https://www.thingiverse.com/groups/curta

Bill of Materials
Build Manual
Assembly Video (The build manual contains more details, but this is a good visual guide)
See video of the Curta in action.
See for my original build for details on the project.
The CAD files are shared on OnShape

Update 8/18/2017
Added missing bolt sleeves for zero positioning lever and anti reversal pawl.

Update 7/6/2017
After manually aligning all the 5mm number stencils on the lower and upper housing and then messing up the paint, I decided to create stencils for it (lower_housing.dxf and upper housing numbers.dxf). Those stencils are now uploaded.

Also, I have added a group for those building the Curta. I will be answering questions there and I look forward to seeing pictures of people's in-progress builds: https://www.thingiverse.com/groups/curta

Update 6/27/2017
Updated the main axle and step drum model. The old one works, but the hole for the crank handle pin was 90 degrees off. This change is basically cosmetic.

Update 5/3/2017

  • Updated a few files that were requiring too much manual fitting work.
  • The zero positioning disk is updated for easier printing / better teeth for the anti-reversal pawl.
  • Added missing crank pin file.
  • Adding dxf files representing paint masks for numbering and lettering. I cut these from vinyl with a Cricut machine.
  • Updating the BOM because the torsion springs for anti-reversal pawl and zero positioning lever recommended in the BOM were way too strong. I ended up making these springs by hand. The BOM is now available as PDF and XLS files.
  • Adding the initial draft of the build manual. This covers the entire build process -- including making the springs mentioned above. Feedback on this build manual is desired and welcome -- help me improve it for others please.

Update 3/29/2017 -- In the process of preparing the build manual, I have a list of several files which need correcting to get correct assembly and functionality out of the Curta. I recommend waiting until I can get these changes published before doing much printing if you haven't yet. Particularly, the mount hole for the crank handle on the main shaft was 90 degrees from where it should have been.

While the build manual is taking me some time, I am excited by the progress on it so far. In the process I have disassembled and reassembled the second Curta which helped me find and eliminate extra friction. I will have another video of the Curta going up soon which will cover a simple, but difficult calculation that has eluded a correct result on the 3D printed Curta until now. That calculation is simply starting at 0, adding 1, then subtracting 1. The result should be a completely zero result with zero showing on the turns counter as well. Since the Curta utilizes addition for subtraction, the calculation requires the Curta to rotate every result and turns counter dial to 9 and a carry operation to cascade all the way around bringing each digit back to 0.

Update 3/9/2017 -- I have assembled a second Curta and taken loads of pictures along the way. I can now either work on a build manual utilizing the photos I took or a series of build videos. There are a LOT of photos and a lot of steps involved. What would you prefer (google form poll)?

Update 1/18/2017 -- I have updated the transmission gear and lockout parts to be printable using a 0.4mm nozzle instead of a 0.25mm nozzle. Thanks to the feedback from MikeHenry. I also updated a repaired Lower Housing.stl due to non-manifold edges.

Update 01/06/2017 -- I found that the tens bell also had non-manifold edges causing problems when slicing. A repaired copy has been uploaded.

Update 12/30/2016 -- I found that the main axle and step drum had non-manifold edges that may cause problems when slicing (Simplify3D skips a few layers in the next to last row of teeth). I uploaded a repaired copy.

Update 12/17/2016 -- I found a part I had missed uploading a file for. It's the securing spring for the zero positioning plate. I have now added the file and added it to the BOM.

Update 12/12/2016 -- I just added layer height, infill, orientation, and support notes to the printed files in the BOM. First step in getting actual build instructions.

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I am soooo doing this one. I've been fascinated with those for years, and unfortunately they are quite expensive. Thank you for your work kind sir. As soon as I finish printing these accessories and prusa printer parts.

mwu - in reply to Mandor

Awesome, I look forward to another Curta make!

It's people like you that cause unrest. I am obsessed with the Curta calculators and now I have to print this and play with it. I can't stop thinking about it. BRAVO!

Here via the Make:magazine article, and from there the reddit post (which i sub to but apparently missed when it first went by). Just saying hi and congrats for a great model, it's truly a demonstration of how far the tech has come and what patience and determination can do to your hobby. I hope to get around to building it one day!

Welcome and thanks for the compliments!

Jul 30, 2017 - Modified Jul 30, 2017

Hey! This is probably my favorite thing on thingiverse, and my math professor dad (who I want to give this to as a gift) loves stuff like this, do you think that you'll ever make a version that is the original size?

mwu - in reply to Hexy

If I do, it almost certainly will be made from the original materials -- I ordered a 1:1 scale selector shaft and selector knob from Shapeways. The parts work, but the fingers on the selector knob are so small and fragile that it wouldn't hold up to use. The selector shaft is like a toothpick made from plastic with the helical groove and divots in it, so I'm afraid that would snap in time too.

I have a 200x 200 build plate with a 180 height, what items need the 220 height so I may find a way to print diagonally or outsource to get the full height?

The main shaft and step drum requires the height. The upper and lower housings require the full width and depth specified while the lower housing requires 170mm of height. The transmission shafts need 170mm of height as well. After the lower housing the next widest / deepest part (the upper sleeve) requires 143mm of width and depth.

How many individual parts make up this print?

Around 240

Dare I ask how long it takes to print all the parts ?

I calculated it out and it's around 9 days, 14 hours, and 44 minutes non-stop printing on my printer. That's assuming no failures and that someone is there to immediately remove one print and start the next. Filing and sanding the parts to fit properly and assembling it takes longer.

Very nicely done. Really beautiful work.
That's a lot of parts surely requiring a lot of work to get things to function together. I will be hoping for some printing time soon to have a go at this myself.

Awesome, thanks!

Note to everyone, I have a thingiverse group (https://www.thingiverse.com/groups/curta) where I would love to see progress photos and any questions people may have building theirs.

Hey Boss,
I'd really like to print one, but I'd like to print the most up-to date version. Any chance of an update soon? I'd be more than happy to work on some issues you're having. I'm proficient with Fusion360 and have 3 printers of my own. Currently work as an R&D engineer and just love the history behind the curta. Ever since I saw your post on Reddit, I've wanted to print one and the time is now.

Thanks again for your work, Luke

May 3, 2017 - Modified May 3, 2017
mwu - in reply to lukeduke1052

Just uploaded an update. The BOM is updated, paint masks are added, and an initial draft of the build manual is added. As far as I know, these files should now be correct and the main changes left are to the build manual as I find things that need better wording and as I am able to add more images to it. Feedback on the build manual is desired and welcome.

You are the man! I can't wait to get started. Thank you much!

I do have some updates coming soon as well as paint masks for numbering and lettering on some of the parts. I was out of town for a bit for a wedding so those have been on hold. I will try to get those uploaded today.

If Thingiverse counted multiple likes, I'd click this project every time I went on line. Amazing!

Thanks, I appreciate it!

Mar 9, 2017 - Modified Mar 9, 2017

I've updated the description of this thing to list a poll (https://docs.google.com/a/digitaltorque.com/forms/d/e/1FAIpQLSeJQcq5njLh1_Z-IPOROXWmYAR7QqbMc7tr924hxTVYikhf6Q/viewform?c=0&w=1) on how you would like to see build instructions for the Curta. Please fill this out -- it's just one question.

Wow! This is incredible.

I am waiting until the instruction set is complete, but I am definitely watching this. If you have a video I would love to see it in action.

mwu - in reply to lars0

I finished the second Curta and took tons of pictures on interval while working on it. I will be using those to document the build process soon. Here is a vid I took recently: https://youtu.be/ShFkJgck6Pw

Really great that you have made the effort.
I was trying to sort the files according to the POM and have few problems. I couldn't find few files (and obviously had few left that didn't fit into the POM table).
item # 2 --> I found 10061, but 10029 not really
item # 70 --> is it the "digits axle"
item # 71 --> ?
item # 72 --> is it the "main shaft pin", there is no number so I can't compare it with the original drawings

I'm thinking to build it or at least give a try. I have ordered almost everything except two springs.
How strong should "torsion spring for zero position lever" and "torsion spring for anti-reversal pawl" be? I have real difficulties to find the matching ones and was wondering if I could bend something similar by myself.

Do you have more pictures from the assembly process? Clearly it will be a huge effort to write instructions, thus I would appreciate already when I could see some pictures of the assembly process.

Thanks a lot!

10061 is the selector shaft and 10029 is the cylinder that goes around it with the number markings on it. I combined the two and then split the selector shaft in two (to reduce the support required). The selector shaft is named Selectorshaft-_selector_shaft_bottom.stl. The top of it is called Digit_Selector_Axle.stl.
Yes, #70 is the digits axle.
71 and 72 are mainshaft-_Main_shaft_pin.stl and Part_Studio3-_main_axle_dowel_rod.stl -- the longer one is to connect the main axle to the crank handle and the shorter one is to connect the zero positioning disc to the main axle.

The two torsion springs were the most difficult for me. I ended up making them myself the first time around, but not in a way that would be easily reproducible. I did some searching and linked to mrspring.com and McMaster Carr to items that should work, though I have not tried them yet. I'm working on building a second Curta -- if they don't work, I'll update the BOM with something that does.

I don't have a lot of pictures from the assembly process yet, but I will be taking a lot in this second build. I'm also working on assembly renderings from the 3D models.

Comments deleted.

What a terrific project. I'm starting now on a 3D print of the 3X version but wondered if you had given any thought to machining the 1X scale model from aluminum, steel, or brass bar stock? The idea intrigues me though it seems like an ambitious machining job.

I would love to, but I don't have the tools (Mill or lathe) and I don't have the experience working on them either.

Thanks - I have both lathe and mill, but as I saw it looks like a really ambitious machining job so I'm starting with a 3:1 scale 3D print first on my Zortrax M200 printer. So far it looks like larger pieces are best done in PLA+PHA because that doesn't warp, and smaller parts are better done in ABS because those print better with my printer. How well does your 3D print model work?

Jan 2, 2017 - Modified Jan 2, 2017
mwu - in reply to MikeHenry

Yes, it is a pretty ambitious machining job, but many of the parts are pretty simple with a few complicated ones in the mix (the selector shafts come to mind). I hope to one day learn to machine and build a 1:1 scale Curta.

Mine was working quite well until I made the mistake of lying it sideways on my desk. It rolled off and landed with a sickening crack. Multiple parts are broken, but I haven't had a good chance to reprint and repair because I am busy working on a build manual and a commissioned copy. I don't think I will do any more commissioned copies for those reading... I didn't think I would even do one, but that is a whole different story.

For me, some of the harder parts to print are those selector shaft parts and it seems like those would be really simple to machine, even at 3:1 scale so I may give that a shot if the printed parts have problems.

Really sorry to hear about your damaged model. I did the same thing with a large modular arm but only a couple parts broke on that so fixing it wasn't too bad. It's especially sad on something as complex and time-intensive to build as your Curta calculator so good luck on getting it repaired.

It must be tough to get a fair price on commissioned copies of the Curta - most folks have no idea how much time goes into the printing and post-processing, let alone the time to design it in the 1st place.

A fair price for a 3D printed Curta with all the time necessary to manually fit all the parts would run (significantly) more than a real Curta. In this case I'm doing it for someone I couldn't say, "No," to (in fact, I was honored to say, "Yes
) and plus I get a trade for a real Curta and some publicity too which will be fun.

I really didn't have much trouble with the selector shaft parts. The peg on the top of the selector shaft and the peg at the top of the bearing that the selector shaft sits on did break a few times. I printed those at 100% after having them break. I have considered (and still am) printing them with a hole instead of a peg and fitting a pin in its place. The pin could be a metal one, but I've had good experience with printing pins horizontally that are pretty strong and that would keep more of the Curta 3D printed.

Not really a big deal on breaking my Curta -- I'll reprint the parts when I get time and I've gotten pretty good at taking the thing apart and putting it back together. Really, the time consuming part has been printing and fitting the parts (more fitting than printing). I am preparing the external parts of my Curta for painting and that also takes significant time to get it right (especially with the knurling on it). I also need to find a good way to get custom made decals or paint masks to get the numbering and lettering on it before I can consider mine done.

Nice that you get an actual Curta and publicity out of the deal.

My Zortrax printer maxes out at something like 80% fill (they only have options for low, medium, high, and maximum) so solid is not an option. I also thought about using metal pins on the selector shaft parts if the plastic ones break off. It would be easy to drill holes in the applicable parts with a lathe and there might be an appropriately sized dowel pin available. By printing those parts horizontally, do you mean on their side? I don't think that would work at all well on the Zortrax but maybe you mean something else.

By the way, which brand and model of 3D printer are you using?

Yes, on their side. I can't print a larger diameter than about the size of the support columns on their side due to the angle of overhang at the bottom of the cylinder diameter and I have to use a raft or a big brim / skirt. I also slow down the print to reduce the likelihood of longer cylinders warping with the small contact area. For a small pin like the ones we are talking about or the carry pins for the results dials, I print them on their side on a raft then clean them up with a needle file afterwards because it isn't the best geometry to print that way, but it does make the pin much stronger.

I printed the first Curta on a modified gCreate gMax 1.5 (not the plus version). I am printing the second one with a triple c-bot (http://openbuilds.org/builds/triple-c-bot.1757/) running on a replicape (http://www.thing-printer.com/product/replicape/). Not having a large print bed moving back and forth quickly on the y axis for small features or infill in tight areas is a help. I am also enjoying the reduced moving mass of the core-xy. I am printing at twice the speed and getting significantly quieter operation and a better quality result.

Thanks for the info on your printers. Looks like they (or probably the slicers used) provide some nice capabilities that I don't get with the Zortrax. Oddly enough the 10207, 10208, 10209, and 10216 shafts print vertically pretty well for me so long as the raft sticks to the print bed. I'm having major problems with 10220-410003 and similar sleeves for those parts though. That little V feature that protrudes from the inner sleeve wall toward the center of those parts fails to render in my slicer, apparently because the slicer doesn't recognize a wall less than 0.5 mm thick and can't make a wall less than 0.9mm thick. I'm fiddling with slicer settings and changes to the Onshape part files to see what I can do to fix that but am not hopeful.

I just updated those parts (10218, 10219, 10220, 10221, 10222, and 10230) to be more easily printable with a larger nozzle. On my slicer I did have to reduce the extrusion width some, but it should print without a 0.25mm nozzle now.

Thanks - this comes at a good time for me as I have time to try those parts again and was about to see what I could do in Onshape. I'll report back in a day or two on how it goes for me with the new parts.

I've already printed the zero positioning disc and those little teeth seemed to come out pretty well for me. Time will tell if it works as good as it looks.

Thanks again for the revised parts - I really appreciate the effort.

I have been busy printing a second Curta and have been needing to print those for a while, but my 0.25mm nozzle is clogged and I don't have a tool that small to clear it out (tried some other methods already). So the updates weren't entirely altruistic :).

Currently I'm printing the large lower housing. On my old printer that was a 60 hour print. This one should finish in just over half of that time... assuming it succeeds.

There is no reason the curve needs to be a single wall (I just followed the engineering drawings directly) -- it can be capped and filled so that the outside of the is a cylinder and between the 'V' and the cylinder is filled with material.

In addition, there is also no reason that the spacers that make the part thicker can't reach all the way to the bottom of the part so that there are no thin portions of wall. I avoided that issue since I have a 0.25mm nozzle. If I get a moment, I can make those changes or if you have already made them, if you send them my way I'll update the files and add credit to the description.

In fact, there is only one other part I can think of I really needed the 0.25mm nozzle and that was on the zero positioning disc. The toothed portion of it that combined with the anti-reversal pawl prevents backwards rotation doesn't print with a great edge on the teeth when printed with a 0.4mm nozzle. It may have worked, but I didn't want to take a chance on it. My new printer got better definition with a 0.4mm nozzle, but I'll have to test it to see if it's good enough. I'll be able to check that out soon.

Comments deleted.

You just made my dream ! I have always wanted a curta !

For anyone watching, I just updated the printed files in the BOM to include details on how they should be printed -- layer height, infill percentages, etc. For parts that I said to print horizontally, I did that for strength even though they don't come out as round. Sometimes filing is necessary to smooth those out and fit them with other parts. There are two types of screws that need to be printed -- one for each of the selector knobs and one for the main crank handle. Those along with the tops and bottoms of the frame supports need to be threaded. I encountered a lot of splitting of parts when putting them through a die before I found that if I filed the layer lines down, it eliminated that problem.

I hope to begin writing up an actual build document soon.

Awesome job! I just decided to search for this to see if maybe someone had made this amazing piece of engineering printable and lo-and-behold you put it up just 5 days ago! I know what I'm printing next!

This is Amazing!

Thank you very much mwu for sharing! I've been watching your updates on your blog for months, hoping someday soon you'd release the files.

Mechanical Calculators are hobby of mine, and I've always dreamed of seeing and using a Curta in person, or, by some miracle, owning one myself. Looks like my dream just became that much closer to reality!

This is amazing work! I think people sometimes underestimate the mechanical marvels these devices were, the Curta being the apex of their design ingenuity. Thanks for shining a light back on a page of history many have forgotten!

Many Thanks,
Dan I.

I wish I could have gotten it released sooner -- real life intervenes more than I would like :) I also wish I had a build guide, but that will take longer.
Thank you for your interest and support!

Quick question. Is it possible for the chrome steel 5mm ball bearing that's available on Amazon to replace the 5.3mm steel shot or does it have to be exactly 5.3mm? Regardless, THANK YOU for posting this (I'm Mr. Pute on YouTube who's been bugging you about it)!!!

The 5.3mm ball is used with a spring to snap the selector knobs (used to input a digit) to a number so that it cannot sit between digits. Given that its main function is just to sit in the detents in the selector shaft, the 5mm ball will most likely work.

To be sure, I can update the selector knob's hole to be 0.3mm smaller to eliminate slop caused by the smaller ball. The springs listed for the selector knobs are small enough to account for the smaller hole so that won't be a concern.

Good suggestion -- I was thinking that steel shot would be a problematic part since the only way I could find it was an entire bag.

Awesome! I'll hold off on printing that part until you make a modification. I'll order the 5mm ball and confirm whether or not it works properly after I assemble. I'm looking to make two of these (one as a Christmas present and one for myself) within the next couple of weeks. Thanks again!

Dec 7, 2016 - Modified Dec 7, 2016
mwu - in reply to Emulsifide

It is updated now -- both the model and the BOM to reflect the new ball size.

It's very ambitious to make one of these before Christmas. Significant time is necessary to manually file and ensure proper fit of each part. I also had to do a lot of calibration to ensure carry operations work properly. Good luck -- Also I don't mind answering questions along the way.

Fantastic! Thanks! I'll keep you updated on how it goes.

cant wait to print this

Now I can own my own one. And I still know how to use it. Thanks ;-)

Now I can own my own one. And I still know how to use it. Thanks ;-)

So I'm not the only one who dreamed of owning an over-sized Type I. This is a work of art!

Thanks :). The over-sizing is simply a result of what my printer is capable of. If I had a mill and lathe and the skill, I would have made one at 1:1 as close to the original as possible.

No, it is perfect! I am a huge fan of up-scaled machines and cut-aways, so this is my catnip :)

Simply amazing work, congratulations!