Curta Calculator Scale Replica

[Robo_Pi]

I haven't seen any information on that. Would love to see it if anyone knows where it can be found. I doubt the original gears were punch pressed. The outside profile can easily be machined. But I'd like to know how they did the center hole with the wedge key. They could have had a special broach for that. The broach idea was my first idea, but for various reasons I chose to abandon the broach idea in favor of punch press.

We haven't made a gear yet, or even a punch and die. So we're certainly open to alternative options.

I'm looking into building a small punch press from a lawnmower crankshaft. But I've always wanted to have a small punch press anyway. So I'll probably build that in any case. This Curta Project was just the spark that ignited the flame of my old desire to build a punch press. I've had that dream for a very long time.

But yeah, if there is any original manufacturing information to be had, I would love to see it.

 

[Robo_Pi]

A Public Service Announcement​

Just for the record. I've been doing most of the posting in this thread. But it was never my intent to hijack this project. I'm simply offering my potential solutions. They may or may not even work. Obviously I feel strongly that they have potential or I wouldn't be suggesting them. But that's no guarantee that they will actually work.

If anyone else has another approach they would like to share, please feel more than welcome to chime in and share your ideas.

Obviously things like EDM and farming out machining to other sources kind of defeats the purpose of this project. We already know that we could do that at great expense. So that's not an idea we haven't already considered.

Buy yeah, if anyone has any alternative methods to approach this project, by all means chime in.

The first one to produce a set of Curta gears WINS a FREE "Congradulations!".

 

[Robo_Pi]

Alternative Curta Designs.

I just joined a Curta Forum. I can't post until I've been approved. I'll ask there for any information concerning how the original calculators were manufactured. That would be extremely valuable information to be sure.

In the meantime I searched through some of their posts and found a picture of this early Curta made in 1938, Notice that his one is quite different from the later model that Racer has the drawings for.

Notice that the numbers for the setting levers are just stamped onto the body. There are no number dials for the setting numbers. The only dials appear to be on the top for showing the results of calculations.

Note also in the slots of the setting levers you can see the setting dimples are all in a straight line. That too is quite different from the one Racer gave us the drawings for. The model Racer is trying to build has spiral setting arms and those dimples follow the spiral.

This 1938 model might be easier to build. But we don't have the drawings for it.

We could probably spend a full year just studying this antique calculator.

I think I'd almost rather build this 1938 model. But unfortunately we don't have the drawings for it. So we're stuck with having to build the more advanced version.

 

[Robo_Pi]

Another photo of the 1938 prototype Curta

Apparently this was the original prototype. There probably aren't any reliable drawings for it.

 

[Robo_Pi]

There's a guy on the Curta Forum from Switzerland who designed a Curta Watch. The following picture is a rendering from his CAD model. He's looking for a watchmaker who might be interested in making it for him. Not sure if he has the internal mechanism worked out or just wants this face put onto an existing mechanical watch. The numbers in the little windows don't move. It's just 1 thru 9 then 0 = 10:00, 1 = 11:00 and 2 = 12:00.

Kind of a cool idea for Curta Enthusiasts. Others on the forum say they would like to have one too. A person could probably buy an inexpensive quartz watch and try to put that mechanism in a case like this with a face like this.

Apparently there's quite a large Curta following.

If you stick with this project Racer and build a Curta Calculator to completion you could potentially set yourself up for a lifetime career just making replacement parts, doing repairs, and selling replicas. There appears to be a potentially large enough following out there. People who repair Curta Calculators are reporting as much as a year backlog. So apparently there's no shortage of interest in these calculators. And from what I hear repair parts bring a fortune if you can make them. So this is definitely a worthwhile project to follow up on.

 

[Robo_Pi]

Don't mind me I'm posting up a storm here:

I just looked up information on ordering replacement parts for a Curta Calculator. The Gears we are talking about making cost $40 EACH as replacement parts.

Let's see, there are 42 gears, that's 42 times $40,, . . . $1,680 worth of gears we're about to make. And that's if we only make 42 of them.

 

[Robo_Pi]

Something is Terribly Wrong!

The gears in the drawings we have don't look anything at all like the gears in this assembly video!

Why is that? Which gears are we making?

The gears we have drawings for are just flat gears. The gears on the transmission shafts in this assembly video have extended brass collars above and below the gears. You can clearly see this starting at 0:44 on the video.

I was going to just start making parts based on the Solidworks drawings we have. But there seems to be a difference between those drawings and this assembly video.

Which one is correct?

I'm working with the Solidworks drawings. But now I'm not convinced the drawings can be trusted. The gears shown in the video cannot be produced via a punch and die method.

 

[Robo_Pi]

First photo I was able to find of a Curta manufacturing facility.

Can't tell from this photo what anyone is working on, but it sure looks like they had some pretty crude tools to work with.

 

[Robo_Pi]

Solved the temporary panic over the gear design

The drawings we have are correct. The video just doesn't show enough detail. In the video this looked like a one-piece gear, but it's actually an assembly of 3 parts. A ferrule, gear, and sleeve. I'm not sure what holds them all together, but it's three pieces. So the drawings we have of the gears are correct after all.

 

[Robo_Pi]

This is becoming a nightmare Racer

After we manufacture all these gears, we need to make a whole lot of sleeves and ferrules. Far easier to make than the gears to be sure, but still require quite a bit of manufacturing processes.

Then when we get the sleeves, gears, and ferrules all made they need to be assembled as sub-units.

Here's the assembly drawing:

A complex sleeve, part number 410360 slips down through the center of the gear and through a ferrule, part number 410347, and then the whole assembly is crimped together as a unit at the bottom. Then you finally have an entire gear assembly that can be slipped onto a transmission shaft which you also have yet to make.

It's probably this 3-piece gear assembly that cost $40.

What do you think?

We need to build setting shafts and all their related parts.

Counter dials with all their gears, etc.

Let's not forget the pinion gear that goes on top of the transmission shafts too.

I don't know how long I'll be able to help with this project. I have a life to live.

I'd kind of like to see the gears made and maybe take a shot at figuring out how to make a setting shaft which I think is the most difficult piece on the whole calculator. If we actually did that much you should be able to take on the rest of the project by yourself. You will have already learned how to tackle the most difficult parts. Everything from there on out should be a piece of cake, at least in terms of how to make the rest of the parts. It won't necessarily be a piece of cake to actually make them though.

As you can see, even to make these gears requires quite a bit of external tooling and engineering skills. The same will be true for the setting shafts. And probably every other part on this calculator. Most of your time will be spent designing and building the necessary tools to make the parts. Making the actual parts will be a "secondary operation".

It's going to be this way the whole way through this project.