# Curta Calculator Scale Replica



## racer8412 (Apr 6, 2019)

As a beginner to metalworking and machining I've taken on the challenge of creating a scale replice of an original type 1 Curta Calculator, I will be providing documentation, pictures, resources and tips along the way for anyone interested in the project or wanting to make their own. Solidworks part drawings https://drive.google.com/drive/folders/12UV_sGF0ronaqxmvK0PDOdtrl5kVPG28?usp=sharing not all drawings have been remade still working on the last few parts.


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## Robo_Pi (Apr 6, 2019)

Fantastic.  I'll definitely be following along.  I'll try to build one along-side you if possible.   I use Fusion 360 now so I'll see if I can open those solidworks drawings in Fusion 360.   Thanks.


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## racer8412 (Apr 6, 2019)

You can also use eDrawings in order to open them if necessary.


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## racer8412 (Apr 6, 2019)

Most parts are made of aluminum, phosphor bronze, stainless steel, and nickel silver, the exact material is listed in german on most of the drawings. IMPORTANT: ALL MEASUREMENTS ARE IN METRIC


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## racer8412 (Apr 6, 2019)

Im also uploading the original part drawings in pdf form, they should appear soon in the drive repository.


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## Robo_Pi (Apr 7, 2019)

Just for general information here's the ReCurta GitHup page and the CurtaWiki page.

ReCurta on GitHub

Curta Wiki

The Curta Calculator Page

The Curta Image File (shown below)


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## Winegrower (Apr 7, 2019)

This is an unbelievably heroic undertaking.   I have many times wished for one of these.   This will be a fascinating thread...


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## Robo_Pi (Apr 7, 2019)

The following video is golden for anyone who is interested in how a Curta Calculator works.


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## Robo_Pi (Apr 7, 2019)

Done machining all the parts?

Good.  Now assemble it,...


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## ch2co (Apr 8, 2019)

Best of luck to anyone who attempts this endeavor. 
I stumbled upon a Curta type II at an estate sale and felt very guilty paying only $20 for it. Nobody knew what it was except for me who had been drooling over one ever since I worked for a surveyor back when I was 17. (Around 60 years ago). The condition of mine is perfect except for a tiny ding in the storage case. I still can’t see how someone in a German concentration camp could dream this thing up in his cell and then start manufacturing them soon after the war was over. They are AMAZING! And are now worth in the thousand plus plus dollar range. I will be watching your efforts as you proceed on this monumental task. Best of luck. 

Chuck the grumpy old guy


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## ch2co (Apr 8, 2019)

Whoops, it’s a Type 1. Serial number shows that it was made in Oct. 1952. 4000 of them were manufactured in 1952. 
Chuck the Grumpy Old Guy


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## Robo_Pi (Apr 8, 2019)

I'm emerald with envy Chuck.  Can you post a photo of it and make me drool even more?


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## rgray (Apr 8, 2019)

ch2co said:


> I stumbled upon a Curta type II at an estate sale and felt very guilty paying only $20 for it.



I would give you a $100.00 for it just to make you feel better.LOL


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## ch2co (Apr 8, 2019)

Happy to oblige! 



	

		
			
		

		
	
 .


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## ch2co (Apr 8, 2019)

rgray said:


> I would give you a $100.00 for it just to make you feel better.LOL



I don’t feel quite that bad about the transaction. It would take about 15 times your offer before I would start thinking about it. But then maybe a little more.
Postage would be included though!


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## Robo_Pi (Apr 8, 2019)

I'd like to have an old busted up junk one.  Something I could take apart so I can look at the parts first hand.   I wouldn't want to have pay much for it though.


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## ch2co (Apr 8, 2019)

This is ridiculous. I just finished reading Robo Pi’s
last message and turned back to what I was doing before this whole conversation began. And that was looking up Mahlers 7th Symphony because I never cared for it even though I love many of his other symphonies and a friend suggested reading an internet discussion that he thought was interesting and.......Anyhoo, I was presented with a YouTube page the top item of which was assembling a printed Curta Calculator! Weird! Here’s the address. 






The thing is rather large and couldn’t really be called a handheld, but possibly interesting to some of you that have printers.  

By the way, the first movement of Mahlers #7 just doesn’t please me. His Sym. 2 on the other hand is my favorite work in all of classical music. I’m now listening to another favorite musician, Tom Waits.


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## ttabbal (Apr 8, 2019)

I've thought about building a printed one. Then I got into metalworking... 

That 2mm shaft sounds like a bear though. 4x upscale?


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## Robo_Pi (Apr 8, 2019)

Machining the parts for one that big would be a lot easier than trying to replicate the original size.  The one we're hoping to build is next to impossible to build because of the microscopic size of the parts.   But hopefully we'll give it a shot.  We're going to need to be clever.  No need for a Briidgeport mill on this project.


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## rwm (Apr 8, 2019)

I double-dog dare you....
Robert


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## Robo_Pi (Apr 8, 2019)

rwm said:


> I double-dog dare you....
> Robert



You'll have to double-dog dare Racer.   I'm just a pain in the tailstock going along for the ride.


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## ttabbal (Apr 8, 2019)

I might have to cheat and use the CNC router for some bits. 

Bridgeport and PM1127 here, neither lends itself to micro parts. Maybe I should use them to build a smaller lathe and mill...


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## Robo_Pi (Apr 8, 2019)

ttabbal said:


> Maybe I should use them to build a smaller lathe and mill...



I've been thinking about this myself. 



ttabbal said:


> I might have to cheat and use the CNC router for some bits.



I confess that I've been thinking the same thing.  Especially for the setting shafts part# 10061 it's begging for a CNC machine. 

Like you, I thought of making my own micro mill.  Then when I saw the setting shaft I instantly thought that I could automate this using an Arduino and stepper motors to control the micro mill.

Then I just realized that I will have built a CNC machine. 

Can't blame us. Who wants to sit there a mill a thousand dimples when an Arduino would do it with no complaints.  Then I could go work on my '47 Chevy while the Arduino drills the perfectly spaced dimples.


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## ch2co (Apr 8, 2019)

Yeah, but that takes all the fun out of it. 

*I’m just too old to even think about taking on this complex of a project. I can see a LOT of time needed for this job. Kudos to any and every body who proceeds into this endeavor.  Go get’em!*


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## Robo_Pi (Apr 8, 2019)

ch2co said:


> Yeah, but that takes all the fun out of it.



In that case when I get the setting shaft blanks cut to size I'll send them off to you for dimple drilling.

My donation to helping a grumpy old guy have some fun.


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## ttabbal (Apr 8, 2019)

Be careful, I tried to volunteer @mikey to make everyone's HSS tools and we ended up with a huge thread. And I ended up getting decent at grinding.  

Maybe I should volunteer him for this too... I know he has a Sherline...


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## mikey (Apr 8, 2019)

Nope, don't even think about it. A Sherline is the right size for this and it is precise enough but you have to WANT to take this on and I DON"T! It would be an awesome project, though.


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## Robo_Pi (Apr 8, 2019)

We better quit.  We're filling up Racer's project thread with silly chit-chat.  

Just want to extend a gracious thanks to Chuck once again for sharing the beautiful photo of his awesome Curta.  

Now everybody get back to work and find some dimples to drill or something.


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## rgray (Apr 9, 2019)

Swiss lathe parts for sure.
I've been following a few on auctions.
Every once and a while one sells in my price range.
I'm probably more worried about the learning curve to run one than anything.


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## Robo_Pi (Apr 9, 2019)

Racer has agreed to accept my proposition to be one of his mentors.   I live in Pennsylvania, USA and he lives in Flordia, USA.  So I won't be able to mentor him live on how to correctly operate any machines he might use.   Hopefully he can find someone in person to help with that end of things. 

What I am going to offer are lessons on how he might take on a project like this.   My plan is to just start making some parts.  He'll need to make all the parts eventually so it really doesn't matter which parts we start with.  I've chosen some of the more difficult parts to start with.

Racer is under no obligation to actually follow my mentoring suggestions.  He can also continue to seek other mentors and input from anyone else at any time.   In fact, I encourage that.   All of my mentoring will be suggestions only.  So Racer is free to decline my suggestions at any time.  

My first suggestion is to hold off on buying any machines or tooling for now.  There is much that will need to be done before any actual parts will be made anyway.   As I see this project there will also be two main categories of parts.

1. Larger parts that can actually be made on standard size machine shop equipment. 
2. Micro parts that are going to need special techniques to make. 

My proposal to begin is to consider a single part.   We going to consider making the transmission gears.   These are "_micro parts_" that will be extremely difficult to make.   In fact, I think that making these transmission gears can be a real test of whether or not it's worth moving forward on this project. 


Here are the parts.

*Part #10038 Transmission Gear I   (20 required)*




*Part #10053 Transmission Gear II   (22 required)


*

This is a total of 42 gears.   I don't know about anyone else, but I'm not going to sit around trying to machine out 42 micro gears by hand.  

Not only this, but if I build one of these things I want to tool up to be able to make more than one of them. 

So my proposal (_and that's all it is at this time_) is to make punch and dies to punch these gears out from sheet brass.   Making the punch and dies will be the machine shop work.    We are still going to need to machine these gears.  Potentially four times.  Once for the punch profile, once for the die profile, and we'll probably need to do this twice to make separate punch and dies for the above two gears.

What's the difference between part 10038 and 10053?   Thickness.  Period.   I double checked this by laying the two drawings over each other.  The drawings are identical in terms of gear profile.  The only difference is that Gear I is 0.5 mm thick, and Gear II is 0.6 mm thick. 

Because of these different thicknesses we may need to make special punch and dies for each gear.

This is where Racer comes in.  

His mission, if he should decide to accept, is to design the punch and dies to punch out these gears.   We need to have them designed before we even start machining anything.

So here's my proposed homework for Racer:


Assignment:

Download and read this 16 page booklet
Stamping Basics

If you have a printer print out the 16 pages and put them in a binder.  You'll find this information useful for future operations.   This will be a good place to keep all your notes and calculations as well.

*Detailed Study:*

Read on pages 2 - 3 the sections marked:
*Punch Press
Simple Die
Compound Die*
(You will need to design a compound die for this)

Read and take note of all the different types of Punch Operations
Page 7 - *Stamping Terminology*.
(for now we will only be using a "perforating" operation)

Read also on page 7  - *Perforating

On page 8 copy the Perforating Pressure Formula information*.
(You'll need this to make your calculations)

On page 14 read
*Shear Angles*
(this will just give you some understanding of the different types of cutting edges a punch can have. You don't need to understand shear angles at this time.  Just be aware that they are important)

When you're done with this we can move forward to the next stage which will be a second homework lesson concerning the design of punch and dies.   You'll need to calculate punching pressures, and required clearances, etc.

Let me know when you have completed the above homework and I'll give you the next homework lesson. 

_Note to Racer: This all assumes, of course, that you agree with my proposal to punch out these gears.  If you have another manufacturing process in mind, then you'll need another mentor.  

Note to other Machinists: If you have a better idea of how to make these gears by all means chime in.  I am in no way suggesting that my ideas are the only way to go, nor the best way forward.   All I know is that I'm not about to sit down and try to machine out 43 micro gears individually.  So this is my proposed solution.  And just for the record I did think about cutting all the gears at once on a shaft, and then parting them off individually.   The problem with that is the key in the center hole.   You can't just drill a round hole through the stock.  You need that triangular key.   So it has already been suggested to me that after machining the rod and parting off the individual gears, I would still need to punch out the center hole.   So why bother with double work?  Why not just punch out the whole gear in one fell swoop and be done with it?   The gears can then be deburred in a tumbler with the correct grit medium.  That's my proposal.  If you have a better idea that isn't CNC or Laser Cutting, by all means offer it up. _

_Second note to Racer__: Don't worry about machining.  You'll need to machine the punch and dies.  So you'll be machining these micro parts.  You'll have ample opportunity to be machining all sorts of things as a tool and die maker. _


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## ttabbal (Apr 9, 2019)

I'm not sure those can be CNC milled. Inside corners and the only radius spec is 0.1mm max on the key. That's a pretty small endmill.. Punching is probably the best option outside of laser, or EDM. Maybe broaching the corners, but that could be tricky too. And tedious.


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## Robo_Pi (Apr 9, 2019)

I agree.  In fact, I don't even know if we can punch these out to within acceptable tolerances.   This whole adventure is new territory.   I'm going to challenge Racer to study up on punch and die and let us know if he can build tooling that will meet the required tolerances.  

If nothing else he will have learned quite a bit about about punch and die stamping, perforating pressures, die clearances, and other important information conerning this manufacturing technique.   He may even discover that he can't meet the tolerances he requires, and have to reject the punch and die method.   I'm just proposing lessons.  

It's up to Racer to do the "_homework_". 

*A Note on Tolerances*:

I'm thinking that, although we should strive to meet the tolerances called out on these drawings, in reality, we can probably loosen up on the tolerances a bit.   The reason I say this is because we are building this as a handcrafted item.    Therefore we can work with looser tolerances via the assembly of a one-off handcrafted item.

Keep in mind that the tolerances called out on these drawings are for manufacturing to assure that all these parts will fit on any Curta Calculator even as replacement parts.   If our parts are slightly off on some dimensions, we can probably take care of that during the handcrafted assembly.   For example, my handcrafted gears may not fit your replica of the Curta Calulator, and your gears may not fit directly into mine.    But as long as they work in the calculator they were handcrafted into that's all that matters for our purposes. 

So we have a little more room for error in tolerances than are called out on these drawings simply because we are building a handcrafted replica,. 

~~~~~

None the less, Racer is going to need to design a punch and die that is going to punch out gears as close to the drawing tolerances as possible.  This is going to require that he gains a really good understanding of the punch and die processes, especially concerning clearance issues.  He may need to consider shearing angles of the punch and die edges as well.

He'll no doubt need to do some preliminary trial and error work.  I would (_and will_) suggest in future homework lessons to begin by making a simple circular punch and die.   Get the brass sheeting that will be used.  And punch out some circles.   And see what you get.   Try different clearance tolerances, etc. 

Hey, I never said this was going to be EASY!  

I'm not claiming to know how to build these things.  All I plan on doing is teaching Racer how to go about figuring out how to build it.   He'll need to overcome all the obstacles.  I'm just an innocent by-stander. Honest.


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## racer8412 (Apr 9, 2019)

So, are you looking for something like this?


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## Robo_Pi (Apr 9, 2019)

I was actually planning on having you build something more like this one.  Only one that runs at a higher speed and has a guard on it so you can't stick your fingers into the punch press. 






The punch and die will be a lot more complicated too.  Although when you first build the punch press you will want to try it out with just punching circles out first.

But that wasn't the homework assignment I gave you.   You're supposed to be learning about the process of stamping metals.   And learning the formula for calculating perforating pressure. 

You'll need to be able to calculate how much pressure you'll need to punch out the gears you want to make.   And that will change as you design the dies and choose the material you'll make the gears from.

The final punch and die you'll need to make will look a lot like the compound die on page 3 of the booklet I asked you to read. 

And the final punch press I'll have you build (_assuming you ever get access to a lathe_), will be more like the one in the video I posted above.  Except it will have a much faster punch speed and more safety guards around the actual punch. 

I might be expecting too much.  

If you tried to punch the gears out using a hydraulic jack like in the video you posted it would take you a week to punch out 20 gears.  

You want to be able to punch out 100 gears in a few minutes.    So you'll need a fast press like the one in the video I posted only even faster.

Besides a faster punching speed actually produces a far better finished product.   So you don't want to be pushing the punch through slow with a hydraulic jack.   You want it to just punch the part out in a split second.   That will make a much cleaner punch.


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## Robo_Pi (Apr 9, 2019)

*A Sneak Preview*

I ran across this video animation.  This is a bit advanced as I haven't addressed all the design considerations for the punch and die.   But this animation is almost precisely what we need to build.    It's a simplified version of our compound punch and die. 

The animation is great.  The narration not so hot.  Kind of robotic sounding. 






There are two top punches.  One (_in blue in the video_).  This will punch the outer profile of the gear.  The second punch is the center punch wrapped in a spring.  That punches out the center hole of the gear.    The pink block (_held down by the spring around the top center punch_) holds the gear blank in place during the punching operation. 

The bottom die is the reddish part in the center of the bottom.  It has both a center profile to accept the top center punch, and the outer profile of the gear teeth to match up with the outer blue punch on the top.

The spring-loaded yellow stage is used to hold the feed material flush with the top of the bottom die.

In this animation they are just punching out round washers with a hole in the center. 

In our case the finished product will be a profiled gear with a center hole that has the wedge key we need. 

This is a very complex punch and die set up.   Making this alone will be a masterpiece of machining.    So this punch and die setup is not something we want to rush though.  How well this is made will determine how nice the gears are that it punches out. 

But this is the idea.   I'm really glad I was able to find this animation.   This animation saves a lot of explaining.  

Key issues of importance will be the clearance between the punches and dies.  I've done punch press work before and I can tell you that another factor of how well things go will be determined by the speed of the operation.   This tends to be something we find by trial and error once it's all up and running.

There will be two perforating forces.  Each one being applied at different points in the punch stroke.   In this animation the Blue top punch cuts the outer profile first.  Then the center punch punches through the center hole.  Because of this staggered operation the punch press only needs to be rated for one of these two operations.  Obviously it will need to be rated for the higher pressure operation which will be the outside profile. 

In any case, there you have it.  It's almost designed for us.   All we need to do is calculate the clearances we need for our material and the exact dimensions of the two top punches and the two cutting edges on the bottom die. 

As soon as we get this made we can start punching out gears.   

Of course, we need to build the punch press itself too.  But let's not get too bogged down in the details.  

Let's take it one step at a time.


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## racer8412 (Apr 11, 2019)

These videos are the best I can find on the subject


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## Robo_Pi (Apr 11, 2019)

Good work.

I've been off in this thread,  A Lawnmower Punch Press, preparing to design a punch press for us to build. 

I even got a lawnmower to use.  

I was going to take you though building the compound die one step at a time, but perhaps you can design the compound die on your own now?

From what I've learned in the Lawnmower Punch Press thread from Jim Dawson, using half-hard 260 brass should work well for this purpose. 

The shear stress for half-hard 260 brass is about 52400 psi.

Have you learned enough from those videos to take a stab and calculating the clearances we'll need?

You could probably start simple with just the center hole punch and die.

See if you can figure out what size the punch and die need to be.  We can deal with the more complex outer gear profile later. 

The center punch will be extremely simple to make.  It's just a rod with a v-groove in it. 

The die will be a round hole with the triangular key sticking out of it. 

Personally I would make the die in two pieces by just drilling and reaming a round hole.  And then cutting a keyway slot to accept an insert we can slide into it from the bottom to stick out as the triangular groove cutter. 

Don't worry about actually making the punch and die.  Just calculate the clearance.   And then see if you can figure out what size the punch and die should be to produce the correct size hole.

Tip:  In real life engineers often cheat by doing things trial and error to some degree.   You can make a round punch, a round hole die, set up a way to keep them lined up.  Then put some brass material between them and whack the punch with a nice heavy hammer.   This should produce the hole and the a piece that comes out of the hole. 

You can then measure the hole and the piece that came out of the hole and see if you get the results you are expecting.  If not, back to the drawing board. Adjust the sizes of the punch and die and try again.   

You can use this same method for the larger circle on the outside diameter of the gear.  You  want to make sure you can punch out a nice blank that has the precise diameter you are hoping to punch out. 

*Note:*  If this proves to be too difficult to produce a perfect gear we can move forward to option #2 which I haven't yet mentioned.  Option #2 still requires that we punch out round washers.   So we'll still need the punch press and compound die to punch out are washer blanks.   We can then machine those blanks to finial precise profile dimensions by stacking them all on a transmission shaft arbor and machining them all at once in a batch milling operation.  

So we'll need to make these round brass washers either way.   And we'll still need to punch out the center hole with the wedge key.   So this is the path forward.  Unless you have better ideas. 

Others have been suggesting using EDM, but that requires sending the brass out and having someone else make the gears.  That would kind of defeat our whole purpose right?    Another suggestion was to have someone else make the compound punch and die tools.   And then we just punch them out.  But again, doesn't that defeat our purpose?  That would also be extremely EXPENSIVE. 

My suggestions have been within the scope of what we might actually be able to do. 

If these parts were 10 times bigger this would be a lot easier to do.   But they aren't.  They're tiny.   So that's what we're stuck with.


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## Robo_Pi (Apr 11, 2019)

Racer,

What I'm proposing may even be physically impossible.   It's up to you do find out. 

The hole we want to punch in the center is only 2.3 mm in diameter.

The material we hope to punch the hole into is 0.6 mm thick.

That means our hole diameter is only about 3.8 times larger than the thickness of the material.

We must be dangerously close (_if not already exceeding_) what a punch and die can even do.

See if you can find out what the limits are for the smallest hole you can punch in a given thickness of material.

We may need to give up on the punch press idea entirely.

I honestly don't know the answer.  I've never worked with such small parts before.  This is new territory for me.


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## Robo_Pi (Apr 11, 2019)

I found this on this web page: Design Tips

*Minimum Hole Diameters*

Holes can be punched most economically when the hole diameter is 1 1/2 times greater than the stock thickness. Minimum diameter is related to the shear strength of the material. In softer materials this can be equal to or less than stock thickness. Stainless steel, on the other hand, would normally require a hole diameter equal to 2 times stock thickness. If the hole diameter is less than material thickness (or less than .032 dia.), it must normally be drilled and the burr removed.

Based on this "Design Tip" it looks like we might be ok.   We're 3.8 times larger than the stock size.  That should be ok according to this shop tip. 

See, I'm even doing your homework for you.


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## rwm (Apr 11, 2019)

Maybe I missed it but does anybody know how these parts were originally made?
Robert


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## Robo_Pi (Apr 11, 2019)

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.


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## Robo_Pi (Apr 11, 2019)

*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!".


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## Robo_Pi (Apr 11, 2019)

*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.


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## Robo_Pi (Apr 11, 2019)

Another photo of the 1938 prototype Curta






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


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## Robo_Pi (Apr 12, 2019)

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.


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## Robo_Pi (Apr 12, 2019)

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.


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## Robo_Pi (Apr 12, 2019)

*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.


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## Robo_Pi (Apr 12, 2019)

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.


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## Robo_Pi (Apr 12, 2019)

*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.


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## Robo_Pi (Apr 12, 2019)

*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.


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## rwm (Apr 12, 2019)

I feel like the 1938 prototype would be a lot easier to make. The spiral shafts add a lot of complexity just to add numerals at the top. I would start there. For those small gears it would be easy to machine a log bar to make the outside contour and then you could part off individual gears. I am just not sure how to make the center holes. I would consider soldering the gears to the ferule. 
Robert


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## Robo_Pi (Apr 12, 2019)

I am in total agreement with EVERYTHING you said Robert.   



rwm said:


> I feel like the 1938 prototype would be a lot easier to make



I would rather build the 1938 prototype too.   The problem there is that we don't have the drawings for that.   We could probably figure it out.   But we'd surely need to spend a lot of time doing that before we even begin. 

If we knew where we could get the drawings for the prototype that would be cool.



rwm said:


> The spiral shafts add a lot of complexity just to add numerals at the top.



Agreed.  The one we have the drawings for is a monster to be sure.  



rwm said:


> For those small gears it would be easy to machine a log bar to make the outside contour and then you could part off individual gears. I am just not sure how to make the center holes.



Well, I do have an idea to go in reverse order for perfect gears.   In other words, punch out a bunch of round washers with the correct shaped center hole.   Then stack them all together on a transmission shaft arbor, and machine the outside profile that way using a forming tool bit maybe even on a flycutter?   That would be hobby-machinist friendly.  We'd only need to make 5 milling passes to cut the five complex teeth.   This would ensure that the outer profile of the gears was well machined and in perfect tolerances too.



rwm said:


> I would consider soldering the gears to the ferule.



Yeah I like that idea too, but we'd still need to make the sleeves and ferrules anyway.   So soldering rather than crimping wouldn't save much, if anything.    And then there's always the possibility that solder could interfere with the fit on the transmission shaft.   These gears need to slide up and down the transmission shafts effortlessly. 

The whole thing is going to take quite a bit of handcrafted care during assembly no matter what. 

I mean, if we currently had a whole box full of parts that we just made, putting them all together would not be easy.  Each and every part would need to be checked to be sure it slid over every place it needs to go without binding.   It also couldn't be too loose either lest it might get jammed up from being crooked. 

This is almost as bad as building a watch.  Maybe not quite that bad, but pretty darn close to it.


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## rwm (Apr 12, 2019)

"Well, I do have an idea to go in reverse order for perfect gears. In other words, punch out a bunch of round washers with the correct shaped center hole. Then stack them all together on a transmission shaft arbor, and machine the outside profile that way using a forming tool bit maybe even on a flycutter? That would be hobby-machinist friendly. We'd only need to make 5 milling passes to cut the five complex teeth. This would ensure that the outer profile of the gears was well machined and in perfect tolerances too. "

That is a great idea. 

I think you could use the diagram you have and modify the shafts and eliminate the numeral dials without changing much else, no?

Robert


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## RJSakowski (Apr 12, 2019)

How  determined are you to make exact replicas of parts?  Parts are designed with ease of manufacturing in mind.  The purpose of the vee shaped protrusion in the bore of the gear is to follow the keyway on the shaft.  A conically pointed pin inserted through the bushing and brazed in place could accomplish the same goal.


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## Robo_Pi (Apr 12, 2019)

rwm said:


> I think you could use the diagram you have and modify the shafts and eliminate the numeral dials without changing much else, no?
> 
> Robert



I imagine this could be done fairly easily.  That would not only eliminate the numerical dials, but it would eliminate the need for the pinion gears on the top end of the transmission shafts as well.   It would also eliminate the need for the "Spider Spring" and all the steel balls that hold the numerical dials in place. 

But if you look on the first page of this thread at the video on how this thing works, you might take note that it's important to have the balls and spider spring holding the numerical dials in place, which in turn, hold the transmission shafts in place, when they aren't being locked in by the locking gear.   

In other words, changing anything on this design could potentially have cascading effects that could result in other problems.  For all we know the original prototype model had these problems and was prone to jamming up?  Maybe that's part of the reason they redesigned it?

Changing things without being clear on what the consequences are could result in trouble down the road. 



RJSakowski said:


> How determined are you to make exact replicas of parts? Parts are designed with ease of manufacturing in mind. The purpose of the vee shaped protrusion in the bore of the gear is to follow the keyway on the shaft. A conically pointed pin inserted through the bushing and brazed in place could accomplish the same goal.



I agree, there is a lot of opportunity here for some design changes that would make things much easier for a hobby machinist to be sure. 

I don't know what Racer's full purpose is.  Nor am convinced of his conviction to the project at this point.  

I'm thinking that anyone who takes this project on would probably do well to stick to the original design.  Unless they are only interested in building a single unique hand-made calculator for their own personal accomplishment and satisfaction. 

Seems to me the latter would be a terrible waste of effort.   For just a little more effort making it correctly could result in a potential for some real profit down the road.    Apparently there are people willing to pay good money to have their repaired.   If a machinist put in the effort to do this project by the book it could result in a potential pay off in the long haul.

Let's face it, no matter how we approach this project it's going to be at least a year long (if we're LUCKY!)  Maybe longer than that. 

This isn't a project you're going to want to look back on and say, "_Darn I wish I would have taken the time to do it right_".

Probably better off just doing it right from the get go. 

Forty dollars a gear?  Once you are set up to make them, it could potentially pan out.  

Just say'in.

Especially for someone just starting out in life like Racer.   This is an opportunity he really shouldn't pass up.   He won't have time to do this after he graduates and starts working full-time.   And if he does it now, he could get himself set up in a Curta Calculator repair business and even sell the "Replicas" that he makes.  This could turn out to be a full time career if someone takes it seriously.

Look around.  I just came from a web site where people are selling Curta Calculators for as much as $5,000.   A working replica will easily bring in a grand.   In fact, it's hard to find even used beat up one for much under $1000.

Although some people do get lucky.   I just read a post on the Curta forum where a guy just picked one up at an antique store for $10.  No kidding!  Apparently the owner of the shop had no clue what it was worth.   And the photo of the one he got appears to be immaculate.


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## rgray (Apr 12, 2019)

Have you seen the Reddit post and the github files on the Curta?
Someone started an effort 4 years ago.


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## Robo_Pi (Apr 12, 2019)

rgray said:


> Have you seen the Reddit post and the github files on the Curta?
> Someone started an effort 4 years ago.



Are you talking about this one  ReCurta

Or is there another one?

Edited to add:

It looks like the ReCurta project was started (or possibly last updated) in 2015.
It also appears that their goal was to post all the drawings and parts lists, etc.  Which they appear to have completed. 
I don't think it as ever their intention to actually build one.   Just make the plans available for someone else to build one. 

Ah,... I think that's supposed to be us?   

It's fate.  We're supposed to be building this thing.


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## tcarrington (Apr 12, 2019)

I have worried less about the gears and assembly thereof than how you are going to make the shaft with the spiral groove.


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## rgray (Apr 12, 2019)

Robo_Pi said:


> Are you talking about this one ReCurta



Yep that's it.
I originally found the reddit post. The wording there had me thinking their intent was to build one.

__
		https://www.reddit.com/r/Machinists/comments/3qc6gh
Says he was gonna build a few for himself and friends.
Wonder if that ever happened.


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## Robo_Pi (Apr 12, 2019)

tcarrington said:


> I have worried less about the gears and assembly thereof than how you are going to make the shaft with the spiral groove.



That's next in line.  

I have a dividing head so I'm prepared for that one. 







I didn't want to scare Racer off with more expensive tools.  

Although I think there are tricks that can be used to cut a spiral without using expensive equipment.


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## Robo_Pi (Apr 12, 2019)

rgray said:


> I originally found the reddit post. The wording there had me thinking their intent was to build one.



Sounds like the guy has some cash.   He says he paid someone to make all those Solidworks drawings.   I can't imagine that being too cheap. 

He also mentions that he might have some parts laser cut.   If he's willing to farm out a lot of the machining work he might do ok.  That post on Reddit has been archived.   I wouldn't know how to contact this fellow.  Maybe through his GitHub page?


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## ttabbal (Apr 12, 2019)

Those spiral parts make me want to put a stepper on my rotary table and use it as a fourth axis on my CNC router. It would need a jig to support the shaft, but I think it's at least possible. 

For the gears I think if I were going to try for myself, I would make a stack of off center washers. Then file the keyway, put them on a mandrel and file or maybe flycut the gear teeth. 

Or maybe I'll wait for @Robo_Pi to make a punch press and buy from him.


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## Robo_Pi (Apr 12, 2019)

ttabbal said:


> Or maybe I'll wait for @Robo_Pi to make a punch press and buy from him.



I walked by the lawnmower today.  

I was really tempted to start tearing it apart but I had to weld up my tractor hitch today.   I know I know,....  excuses, excuses!

It's raining out now and I'm also hungry.   So not it's likely I'll tear the mower down today. 

Maybe tomorrow.  Who knows?


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## Robo_Pi (Apr 14, 2019)

*How to Calculate the Square Root of 2 on a Curta Calculator*


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## racer8412 (Apr 15, 2019)

Jeez a lots gone on since I last checked the thread, school was a pain but I did more research over the weekend and did START working on a punch die design, it's nowhere near being finished but i'm working on it.


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## Robo_Pi (Apr 15, 2019)

racer8412 said:


> Jeez a lots gone on since I last checked the thread, school was a pain but I did more research over the weekend and did START working on a punch die design, it's nowhere near being finished but i'm working on it.



That's cool. 

There's no rush on my part.   I have no deadline.   Obviously you would like to have a home-built Curta in hand inside of a year.   I make absolutely no promises that you can meet that deadline.     Personally I feel that if we meet that deadline I'll be tickled pink. 

Is it possible?  I have no clue.   I don't even know if the punch press idea is going to pan out.   I have been drawing up a punch and die in Fusion 360, myself, and I confess this doesn't look easy.   We will either become the best punch and die makers around, or we'll fail.  That's how life works. 

I'm slightly optimistic in that if we can manage to punch out the center hole that alone will be progress.   I don't know if you've read the details in this thread, but as a second alternative to punching out the entire gears we could just punch out disks with the proper center hole, and then machine the outside profile using a milling technique.   So I'm confident that we'll eventually have some gears in hand.  The only real question will be how much work it will take to produce them. 

If we could succeed in punching out entire finished gears in one fell swoop, that would be much preferred.   That production method would loan itself to ultimately building multiple Curtas with much more ease.   On the other hand, if we end up having to machine the gears, even on a batch arbor that's going to slow down the whole process (_although it might ultimately produce better gears that way_).   Either way we can still move forward.   I'm thinking we're going to need the punch press either way.   You may not need one as elaborate as I'm hoping to build with a lawnmower crankshaft.  It may be possible to press these tiny gears out by hand on a manual press.   I've always wanted a crankshaft punch press so I'm going in that direction for personal reasons. 

But yeah, there's no rush here. I'm working on other projects myself.  I won''t bother to list them all here.    Too many to list. 

It probably won't hurt to start thinking about how to make the sleeve that goes inside the gear:  There will be over 30 of these needed.  Three different flavors. 

These are part numbers 10055 (aka 410354), 10056, and 10140 (aka 401359).  They will all require similar machining and tools but there are subtle differences in their lengths and shoulders. 











The manufacturing process for these will be fairly easy.   Just start with tubing of the right size, then turn down the smaller diameter on the lathe, and finally using a compression die to fold in the wedge shape.   This can easily be done with a small hand press.  But more dies will need to be made.

Time consuming to make them all, but fairly easy to make. 

There will also be ferrules to go with these,  But the ferrules are just short pieces of tubing, so those will just need to be cut to proper length. 

Then the transmission gears will be finished and we can move on to making a transmission shaft.   Although, in reality we might want to make some transmission shafts early on so we can have something to test-fit these sleeves on.    The transmission shafts will also eventually need to have pinion gears that fit on the top of the shafts.  But let's take one nightmare at a time.  

The quickest way to turn these sleeves would be to feed in the tubing through the lathe spindle till it hits a preset stop.  Then turn down the smaller diameter and part it off.  Do this again and again until you're blue in the face or have all 30 some sleeves done.   A collet would be preferred over a chuck for this operation, but not absolutely necessary.   Then when these are all machined they would all need to be run through a compression die to form the wedge shape.  

Just looking ahead.   The ferrules would be parted off in a similar fashion.   No other machining needs to be done on those. 

I'm played out just having typed all this information into this post.


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## Robo_Pi (Apr 15, 2019)




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## hman (Jul 7, 2019)

OK, thread is a bit old ... but I've bee binge-watching Adam Savage videos, and ran across this one.  Enjoy!


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