Curta Calculator Scale Replica

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
 
I am in total agreement with EVERYTHING you said Robert. :grin:

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.

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.

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.

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.
 
"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
 
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 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. :grin:

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.

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. :grin:

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. :grin:

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

It's fate. We're supposed to be building this thing.
 
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I have worried less about the gears and assembly thereof than how you are going to make the shaft with the spiral groove.
 
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. :grin:

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

71w052ThtcL._SX425_.jpg


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

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