The Curta.org Bulletin Board

[Ravlen]

I saw an auction that was hard to resist here in Japan. In fact, it was impossible to resist, and now I have my first Curta. Type II, Serial 502002. The auction description explained that it was non-functional, but the seller was not sure how bad it was as they did not know what was wrong with it, only saying some sliders were sticking and the numbers on top weren't changing.

I was looking for a project to work on in the evenings, and with this being such a low serial number black body type II, I thought it would be fun to buy it (Under 700 USD) take it apart and clean it up. I figured it would be similar to https://hackaday.com/2023/08/12/opening-a-curta-with-great-care/, which is the same type/age.

Well, things are both better and much worse than expected. The good news is that externally it's in great condition, looks to have been used very little. It even had a custom leather case (not official).

Unfortunately, it is definitely not just "sticky". Here's my theory: At some point a long time ago, in the middle of a subtraction, it jammed somehow. The owner attempted to force it and likely broke something inside. Now the drum is midway through a turn, with some slider gears meshed with the step drum and non-movable. It's in subtraction mode (popped top), unable to go back to add mode, clear it, or lift the top, due to being mid-turn I assume.

But worse is that the crank still turns, despite the drum not moving, so the drum/shaft are somehow disconnected internally. The crank spins very smoothly, though there is some resistance as if there's some old lubricant or something.

Then I'm guessing they shelved it and it sat there for years. Someone eventually found it and either sold it to someone to try to fix, or tried to fix it themselves. When I took the bottom off, it was very obvious that they put a bunch of lubricant (I'm not sure what, but looks slightly golden) all over it to try to "work it loose" I guess. There's spots on the inside (bottom) where the lubricant dripped down.

So that's where I am right now. I've cleared the work bench and I'm waiting on a buddy to send me his ring clip pliers and 1mm pin punch, before I start to (slowly) take it apart and see how it's actually broken. When I know more, I'll decide whether to buy an ultrasonic cleaner, lubricant, etc.

I'm not sure how to post pics here yet, but I'll share when I can.

[Ravlen]

[murff]

[Knetball]

It’s nice to see where this Curta has ended up. I also contacted the seller. When he didn’t want to answer my questions about remaining functions, I suspected it might be completely broken. So, I offered $400. I would have then sent it to Romano.

Hopefully, it can still be repaired. I’ll keep my fingers crossed and look forward to you keeping us updated.

[Ravlen]

OK, first update! First identification of the actual problems.

The first hurdle was actually just getting the darn handle off. I made a little wooden holder for the Curta to support the handle while I tapped the pin with a 1mm punch, but no matter what I did, the pin just wouldn't come out, I couldn't figure it out.

I gave up for a while and looked at the drum and bottom of the shaft. The drum was in the middle of the slider gears, and you can see in the original pics above how the notch in the bottom of the shaft was not aligned with the wheel that helps it snap into the home position. With some gentle experimentation, I found I could turn the shaft from the bottom (after unsticking the ratchet that was severely jammed), and I carefully helped it finish the rotation.

But when it finally got into the home position, it still was not properly aligned. It would not exit subtraction mode, I could not lift the top carriage to shift registers or clear the results, so the drum was definitely out of position. With some more experimentation, I found I could lock the drum mechanism in position with one of my tools, and slowly rotate the shaft until everything was finally properly aligned. Then things finally started to move.

With everything aligned, I could now finally lift the carriage and change registers, smooth as could be. Clear ring worked perfectly too. I could go between add/sub modes, though this was sticky. This is how I found out the real problem with the handle. When pulling on it to change modes, it suddenly came off in my hand. But you can see from the pic that the pin still appeared to be in place. After things jammed internally, somehow had turned it hard enough to shear off the pin. With the handle now spinning freely on the shaft (invisibly), I could now see why I was unable to knock the pin out.

But with the shaft off, i could look at it under a loupe and just barely spot the location of a pin. After first tapping it the wrong way (whoops) I tapped it the other way and it came out.

I used the 1mm punch as a makeshift crank and could gently spin the mechanism and did some very simple additions. Like +1 repeatedly, and it actually works, including incrementing the turn counter, but you can see the gears are just barely aligned, and the carry mechanism isn't working yet. If you try a subtraction, or multiple digit calculations, it's too much and the drum stays in place while the shaft spins.

It's pretty clear to me that the pin holding the drum mechanism in place on the shaft has sheared off as well.

My current theory is that it got a little stuck, and someone that didn't know what they were doing turned it backwards as hard as they could to try to unstick it. The ratchet kept the shaft from moving, but also got jammed very tight as a result of the point digging deep into the ratchet gear, and the handle and drum pins sheared off.

I'm going to remain optimistic that this is repairable. I'll continue the disassembly when I have time, and give every part a gentle but thorough cleaning while looking for other broken parts.

Handle off, still showing the piece of pin in place, it's the same on the other side (have not been able to extract it yet):

Spinning the handle made the broken pin drag across the shaft, obscuring the edge of the pin. Can you spot it?

Tapped it with the 1mm punch, but had it on the wrong side (could not visually identify which side was correct, and I picked wrong).


One tap from the other side, and finally the remains of the pin was out.

[Ravlen]

I'm now browsing ultrasonic cleaners online, but there doesn't seem to be many options for ultrasonic cleaning solution/concentrate here in Japan. I won't put any anodized or plastic parts in it, or any super tiny or fragile parts either. Not yet sure how I'll clean the old lubricant off the sliders though.

Also pondering what lubricants to buy. I can get molybdenum paste and sewing machine oil easily enough.

[Kmcqueen]

Thanks for these updates and photos! I think you will be able to fix this. I completely cleaned and freed up my first Curta without an ultrasound by just carefully taking apart components and soaking in small containers of Isopropyl alcohol. Watch any painted parts but it won't harm the anodizing. It's clearly slower than using an ultrasonic - but it works. My sliders and the tens carry levers were completely frozen from what ever oil had been previously used.

Liberty synthetic oil is used by watchmakers and won't cause any corrosion over time.
https://www.amazon.com/Liberty-Synthetic-Lubricating-Grandfather-Clocks/dp/B00AZMGFI4

[Ravlen]

@kmcqueen Thanks for the tip! I see this is also the recommendation at https://www.vcalc.net/cu-lube.htm.

I actually have been meaning to order a few unrelated things from the US Amazon store this month, so I'll add this to the cart. Is there a specific type of molybdenum paste that is considered the best for Curtas? I could throw that in too, if it's not too expensive. That same site seems to recommend this one: https://www.amazon.com/dp/B0006N6ZUU/ ? If not, I was going to order this one from the local (Japan) Amazon site: https://www.amazon.co.jp/-/en/dp/B0C2BPQ537/

[Kmcqueen]

I have access to this product at work.......
http://www.microlubrol.com/MicroLubrol-Helimax-XP-Camera-Lens-Telescope-Optical-Instrument-Grease-1-oz.aspx

Mostly used in high end camera equipment. I just spoke to the techs and they also use this product (I think both would be fine for the relatively low speed use a Curta is put through)

https://www.amazon.com/Corning-Molykote-Performance-Synthetic-Lubricant/dp/B00CAYWNCW

I'm partial to the synthetic versions because I'm convinced it's not as prone to causing corrosion over time.

[Ravlen]

Here's what I've learned/discovered up to now:

Everything was almost completely seized up due to ancient lubricant, or potentially poor quality lubrication work at some point (see later). It took a very long time to disassemble everything as it was so hard to get things unstuck without breaking anything.

The carry gears were terrible, I knew they had to be able to move, but it took a lot of pressure to get them sliding again. Eventually things started to loosen up and I could slide them around on the shafts. The gears that are locked in place with c-clips were so stuck I feared someone had glued them in. Eventually, with a scary amount of force, I could loosen them and get them off.



I also took off the carry levers and discovered one is bent, and many springs are broken, though miraculously all have at least one leg still there, so it should still work if I'm gentle (I plan to fabricate new springs from 0.2mm spring wire). The surprise is how dirty these are, it seems they've been heavily lubricated, and the lubricant has deteriorated a lot. You can see in the pictures, though from the lubrication guide it looks like these are not supposed to be lubricated? I didn't see any in the old curta assembly video on youtube either.



Eventually got all the other parts off (reversing lever was another nightmare due to the stuck gears), and got it down to just the shaft with drum, and as I expected the pin was sheared off.



I managed to take the drum off as a result, and pop the broken bits out of the drum "collar". Unfortunately, the remaining bit in the shaft is proving extremely stubborn. You can see how it's really wedged in there. I'm going to pick up a watch band pin remover (screw type, not hammer type) and see if that works better than tapping on it with a pin punch. It's hard to see in the picture, but you can see the drum "collar" now clear (with 8mm tool pushed through to demonstrate), and two small bits of pin on the tray which is unfortunately acting a bit as a mirror, and the shaft with pin still in there.



So that's where I'm at. The lubricant has arrived, and if I can get that last pin out I'm ready to finish cleaning it and putting it back together (with replacement pins, yet to be sourced) as a first test, before attempting to make carry lever springs some point in the future.

[Ravlen]

Some notes on cleaning.

I started by doing it by hand with isopropyl alcohol. You can see one example here, with before on the right, and after on the left.



But it seemed to leave a bit of a very faint grey residue. I decided to pick up an ultrasonic cleaner (for only the all-metal parts) and do a comparison.

First, this was before/after cleaning of shaft and gears (were removed during cleaning) in the ultrasonic bath. (left = after, right = before).



Then I tested with ultrasonic cleaning with water with a drop of soap, vs isopropyl alcohol (in a ziplock). In all these pictures the left is water/soap, the right is IPA. Hopefully you can see how it's obvious the water/soap option worked much better, and the IPA left some light residue in a few spots. I'll be using just water/soap from now on. I could buy specialist solution, but a drop of soap seemed perfect.



Finally, one more before after to show that the ultrasonic cleaner, despite being the cheapest on amazon, is totally worth it. The first two pics are before a single 5 minute cycle, and the last is the after for all those parts.



The dirtiest parts will need a few cycles, but it's looking promising. I won't put in the tiniest parts though, I'm worried they'll be damaged (tiny c-clips and carry level springs).

[Ravlen]

Some updates for people following along.

I was busy with some business trips and other work, but now and then tinkered away at it. But I did do some interesting things.

The hardest was that I fabricated 14 new carry springs by hand. I got 0.2mm wire and tried a few techniques. The guy that created the 3D printed Curta shared a 1:1 scale "mold" that I could try to 3D print and wrap the wire around, but it didn't work.

In the end, I had some carefully drawn marker lines on some fine needle nose pliers, and when placed in those positions, it was easy to carefully bend the wire at the correct spots. It wasn't a problem if the angle was slightly off, that's easy to correct. The last pic shows plastic piece holding gear in place temporarily.

Here's a pic of an original spring and one of my handmade ones:



Then, and this one is painful, during reassembly the second last spring clip for the counter shafts snapped. I can't imagine how I'd fabricate a metal replacement, and I didn't see the specs for these specific clips in any of the engineering designs. For now, I've used a bit of plastic to hold it in place while I get a friend to 3D print some small "C-shaped" plastic replacements. Not ideal, but should work. (last pic shows the plastic bit holding the gear in place)

[Ravlen]

Now I have a problem related to the carriage lock pin. It's kind of hard to explain how it works, but there's a wheel deep in the curta (called the locking disc) that acts like a bearing and spins with the shaft. That disc has a small notch in it for the lock pin. When you lift the carriage, the pin slides out the notch, and you can spin the carriage.

The lock pin also has a groove in it for that locking disc. So when you turn the crank, the wheel turns and fits into the groove in the lock pin, preventing you from lifting the carriage mid-calculation.

Now, the problem is that after fully reassembling the Curta, with the new springs and attempting a test run, the pin wouldn't go in. Looking into the hole, I can see a very slight misalignment of the notch in the wheel (meaning it's slightly rotated from where it should be), which is blocking the pin:



After a full disassembly again, and some tinkering... I have NO idea why this is misaligned now. Nothing seems bent, there is no play or looseness anywhere, every screw is a perfect fit... Yes this wheel is slightly off, and I don't get it. I thought it was properly aligned when I started, because the carriage was the only thing that seemed to work (after some initial cleaning), but maybe not?

Here's views of the locking disc. 1. In ideal zero position. 2. In actual zero position when everything is screwed together. 3. Showing how the disc blocks the locking pin from sliding in. 4. With locking pin in place, and disc rotated to lock the pin during simulated cranking.




Does anyone have any idea how to fix this alignment? I've tried everything I can think of, but everything else seems perfectly aligned, with essentially zero "play" (looseness) for any adjustment. I even tried gently bending the prongs of the spring to coax the wheel into position, but the tens carry unit that screws into it is not affected by the position of the springs, and everything else follows that part. I'm mystified.

I'd rather not take a file to this disc to widen the groove, so if anyone knows what could cause this misalignment, I'd love to hear it!

[Ravlen]

Oh, I should have posted this first, but I was able to extract the last bit of broken pin from the shaft, which is why I was able to work on other things finally.



I found the correct size hollow pin to fit in there, but I was not able to get any to fit all the way in correctly (without bending). I tried a solid pin and that worked fine, so I'm using that for now.

[mwu]

I've been looking at some of the engineering drawings for the Type II.







It seems this disc is fastened to the tens bell via M2 screws. The tens bell is then kept in sync with the step drum by an offset pin at the top of the step drum that interfaces with the rings of the tens bell:



The red ring is where the pin from the step drum interacts with the tens bell ring. The blue ring is how the ring keeps position on the tens bell (prevents the ring from spinning on the tens bell)

As far as I can tell, this leaves a few different places for things to get messed up when a device was forced as yours was:

1. The step drum was off angle because the pin had sheared which put the tens bell off which then made the carriage lock when it shouldn't
   
2. The blue portion of the ring got damaged / sheared when the device was forced causing the ring of the tens bell to be rotated from where it should be. This would cause the tens bell to be at an incorrect rotation from the step drum and main axle and therefore cause the carriage to lock at the wrong time.
   
3. The M2 bolts binding the step drum to this disc sheared causing the step drum and the locking disc to be out of sync. I think this is unlikely and you would have noticed damage to those bolts when disassembling.
   

There are probably other options -- I haven't 3D modeled a Type II so all I'm going by is referencing the engineering drawings.