Hello everyone,

I'm working with a group of hobby machinists who would like to try to build a replica of a Curta Calculator. We're going for an original size metal replica. We know there already exists a much larger 3D printable version. But that's not what we want. We also have all the original drawings, so we're all set there. We have ideas on how to proceed with the build, but we would like to know how they were originally manufactured if possible.

Can anyone point me to any information on how the original calculators were manufactured? Not the parts drawings, we already have those. I'm talking about the actual manufacturing methods.

I've been searching around and haven't been able to find any actual manufacturing information. Even pictures of the inside of a Curta factory could be useful. We might be able to glean some information from those photos if they exist.

Thanks,
James

The Oughtred book has a few factory pics - http://www.oughtred.org/curta-book.shtml. But you'd be best advised to talk to Murff or Romano or someone who's familiar with assembly/disassembly. This will be a labour of love for you, I think. The machine tools that stamped out all those little parts are long gone now. Best of luck to you!

Not sure that I know more than others to the topic of producing a Curta...
There are no specific fabrication processes known - at Contina they used a lot of smaller supporting tools to assembly the Curtas, some of them are known. In the SERVICEHANDBUCH (dis)assembling and oiling is described.
But your project is more focused to prototyping - so, you don't need to optimize the fabrication processes. I think the parts are the challenge: along with the technical drawings you have to be chosen the right materials for the parts, all the different kinds of bronze brass aluminum and steel. Another difficulty could be the small parts (springs, screws, balls...).

Marc from curta.fr has some deeper knowledge - maybe he can support

What is your plan (time, financially) for this project?

murff wrote:

What is your plan (time, financially) for this project?

The group is planning on a year long project. I'm not concerned with necessarily meeting that deadline, so if it takes longer that will be alright by me.

For me the finances must be absolutely minimal. Our goal as a group is to build this thing using "old-fashioned" or traditional machining methods. So we won't be using any CNC, Laser, or EDM. Everything we do will be old-school.

Our current plan is to begin by making the transmission gears. We have decided to make them from 260 half-hard brass. We aren't concerned with using the same exact materials used in the original Curta.

We also aren't concerned with our parts fitting an original Curta to be used as replacement parts. For this reason we can deviate somewhat on many of the non-critical dimensions. As long as it works in our replica Curta that's all that's required.

My personal plan is to make 10 Curtas while I'm building them. In the case of these transmission gears this means that I will need to make 420 of them instead of just the 42 used in a single Curta.

Our current plan for making the transmission gears is to punch them out on a punch press. We are currently working on designing a die to make the gears.

Our hope is to be able to punch out an entire gear in one operation using using a compound die. Because of the extremely small size of these gears this may not be possible. If that turns out to be the case, are second approach is to just punch out the center holes with the wedged key in the center of a circular blank. Then take those blanks and mount them on a transmission shaft arbor to mill the outside profile of the gears. This will most likely result in higher quality gears as well. It appears that we will most likely go in this latter direction. Although we haven't yet given up on the idea of punching out entire gears. We'll only move onto the second method if the first idea fails.

It would be nice to know how these gears were made in the original factory. I'm curious whether they punched out their gears, or if they had an alternative method to manufacture them.

We're starting with the most difficult parts to manufacture first. The next part we plan on making, after the transmission gears, are the setting shafts.

If we can produce usable transmission gears and setting shafts we feel confident that we can manufacture all the rest of the parts.

We chose these most difficult parts to tackle first in the event that we cannot make them efficiently. If that turns out to be the case, we'll resign the project.

I'm hoping we can be successful making these gears and setting shafts. If we can get that far I think we'll be able to make the rest of the parts without any serious issues.

Again, I'd like to add, that we aren't concerned with making everything identical to the way it was made in the original Curta. So we have some freedom in terms of redesigning parts of it if required to meet our needs. This gives us some leeway when it comes to manufacturing the parts.

I would personally like to keep as many parts as close to the original drawings as possible so that parts I make could potentially be used as replacement/repair parts for an actual Curta. But that goal is secondary.

The main goal is to produce a replica Curta that actually works. If that requires some redesigning so be it.

But currently, right now, the transmission gears are what we are working on. And so far we are still at the design stage of making the punch and die to produce them. We'll most likely need to make several prototypes of the punch and die system before we get one that produces gears that we are happy with.

This is all done by hobbyists. No one is working on this project full time. This is all spare-time hobby work. So we do what we can when we have free time. No one is in it for the money. We're only in it for the Curtas.

Although, as I say, I'm hoping to build 10 of these while I'm at it and if they turn out well I hope to try to obtain some cash return for all my work. If successful I'll probably put 9 of them up for sale on eBay and see what they go for. If they sell, I might be tempted to make some more since by then I will have both the experience and the tooling on hand to make more.

I'd advertise them as "Handcrafted Curta Replicas", I wouldn't try to pass them off as the real thing. In fact, I plan on engraving on the bottom, "Handcrafted Curta Replica". I see no reason to pretend otherwise.

And like I say, for me, if I got these made inside of a year I'd be both thrilled and amazed. I personally expect it to take longer than that. That's not a problem for. As long as I see progress with every part made I'll just keep going until I have them built. However long it takes.

And if I give up I want to give up early.

That's why I'm starting with parts I consider to be the most difficult to make. The deeper I get into it, the easier the parts will be to make. So if I can't make these most difficult parts, then I'll just abandon the project early on.

Or to state that another way. If I can succeed in making the gears and setting shafts, I'll be confident that I can make the rest of it.

Thank you for the detailed feedback...




Not knowing if you already read all the past threads in this forum, I like to ask this:
As a group of skilled mechanics you maybe could also produce all the parts of my own project idea: The Curta Watch (except the watch movement itself)?


... maybe as a follow-up project

I haven't read all the other threads on this forum by far. I did read your thread on your Curta Watch project. It's very nice.

I personally have no interest in trying to build a watch. The parts are extremely tiny and fragile. The Curta Calculator parts are already a huge challenge precisely because of their small size. If I could build a Curta Calculator about 5 times large it would be a piece of cake.

I would imagine that the case for your Curta Watch would be fairly simple to machine as the case itself is large enough to deal with. The face and hands would be much more difficult simply because of how delicate they are.

If you insist on a mechanical watch, your best bet would be to buy an existing mechanical watch. Remove it from its case. Replace the face and hands with your design, and then put the whole thing back into the case you have designed.

If you want to experiment first you might try doing this with a less expensive electronic quartz watch. After getting that down pat, then you could move on to trying it with a more expensive mechanical watch.

The face itself doesn't look too difficult. I would suggest making two thin disks. One for the outer face, and one for the inner face. The outer face would need to have the square windows punched out. And then both faces would need to be delicately silk-screened with the artwork. Either that, or you could use some type of printed decals.

Making the thin delicate hands representing the Curta zeroing ring and decimal point arm would be beyond my ability simply because they are too delicate. I would imagine that a punch and die would be in order for those as well, unless you are willing to go with laser cutting or EDM.

Also if you want the little red arrow to count the seconds you'd need to find a watch that has that shaft available to start with. The shaft would need to be in the position you want unless you are willing to reposition the red arrow to match and existing watch mechanism.

I would say your watch is a far simpler project. Assuming you aren't building the actual watch mechanism. And it would also be much easier if you are willing to pay for laser cutting or EDM manufacturing of the parts.

If you used a punch press, you would either need to make the punch and die set yourself, of have it made. I don't imagine it would be cheap to have it made.

We plan on making our own punch and dies. So our project will be entirely made by us in every detail including all the tooling.

I don't have a lot of money so I can't afford to have anything made by someone else. If I can't make the part, I'm done. I quit. I'm just not interested in paying someone else to make a Curta Calculator for me. That would end up costing more than what I can buy an original Curta Calculator for.

murff wrote:

Not sure that I know more than others to the topic of producing a Curta... There are sausage stuffers that are built to last and no specific fabrication processes known - at Contina they used a lot of smaller supporting tools to assembly the Curtas, some of them are known. In the SERVICEHANDBUCH (dis)assembling and oiling is described. But your project is more focused to prototyping - so, you don't need to optimize the fabrication processes. I think the parts are the challenge: along with the technical drawings you have to be chosen the right materials for the parts, all the different kinds of bronze brass aluminum and steel. Another difficulty could be the small parts (springs, screws, balls...). Marc from curta.fr has some deeper knowledge - maybe he can support What is your plan (time, financially) for this project?

Why would anyone build a Curta Calculator is it costs more than just buying one? What am I missing?

JohnsonQ wrote:

Why would anyone build a Curta Calculator is it costs more than just buying one? What am I missing?

And Lego makes a LOT of money. People like building things, it's part of the human condition