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Calculating Machine Trivia, Tips, & Musings

The following are messages of interest from the Calculating Machines Mailing List run by Erez Kaplan. Some are requests for information, some are tips and trivia, some are just interesting musings.
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Subj: (Long:) Curtas: Curiosity and maintenance; some musings
Date: 97-04-02 02:19:50 EST
From: (Nicholas Bodley)

It was a few months before I gingerly opened up my Curta to see what was inside. I have been almost a mechanical technician, in all but name, since I was old enough to hold a screwdriver, and the son of a Russian mech. engineer, so the bottom screws had no especial fear for me. I guess one develops a sense for which screws one can reasonably remove, and which screws one shouldn't. Sony's electronic devices have a special symbol next to the screws you can remove without creating trouble.

(For those people whose first language is not English, "gingerly" means approximately: "carefully".)

As to opening up anything to see what's inside, that is part of one's self-education. I was distressed to see a professional educator discouraging curiosity, but that is a topic for another realm of the Internet. The proverbial cat had nine lives, after all.. Nevertheless, one should use reasonable judgment about what to dig into.

Removing the bottom (and "side") cover of a Curta is sensible provided that you pay attention to a few matters. First, use a good screwdriver. Perhaps the best that are easily available are the inexpensive sets of flat-blade screwdrivers packaged in sturdy plastic cases; try Radio Shack in the USA. A screwdriver should have a flat end that is square to the sides; the sides at the tip should be parallel, or very nearly so. DO NOT use a cheap, worn screwdriver; if the edges of the tip are rounded off where they meet the flat sides, do not use it, or carefully reshape the end with a fine sharpening stone.

The blade should fill most of the slot; it should be just a wee bit thinner and narrower. (This is basic stuff, but where else do you find two screws that are as important as these, and as hard to find replacements for?)

If you have a pet cat, be totally sure that Kitty can not get access to your work area.

Take extraordinary precautions to avoid losing those screws. I don't know about parts sources for Curtas, but finding a replacement will not be easy, believe me. A really-competent watchmaker could duplicate them (if not electroplate them) for perhaps $25 US.

Removing the cover is easy enough. (You might need to tap it with the handle of your screwdriver...) Once it's off, keep in mind that dropping the Curta in this state is a sin even greater than dropping it with the cover on! Dropping the cover is also Very Bad Manners. :)

If you are normal, you will want to learn something about how the innards work. Go to and look for Pre-HP items; look for Curta. Note that the Curta's "stepped reckoner" (sorry; that's the peculiar term I learned for it) has complement teeth; the rightmost column uses tens complement, while all other use nines complement. (Subtract zero from the accumulator to see what happens.)

Everyone should know that you NEVER turn the handcrank backwards; my Curta had a delicate ratchet and pawl to give you a hint, but careless persons could force the crank. (Probably quite a few handcrank-operated calcs. were built so that you turned the crank backwards to subtract, but not the Curta.)

When you look up inside as you subtract zero, look for the carry slides that set and reset. (As I remember, they aren't reset (the mechanical folk might say, "restored") until you turn the crank for the next operation. Mechanical one-bit memories...).

I felt confident enough to disassemble the entry-slide shafts to get access to the innards to "clean house". Thinking that Curtas were mass-produced, I very carefully saved all parts, but didn't realize that (in mine) the setting shafts were individually hand-fitted! The tiny porous-bronze bearings at the ends of the shafts were individually sized. It took me a few hours of repeated trial assembly and semi- experienced judgment of what watchmakers (I think!) call "end shake" to get things back to something acceptable. Of course, I also had to ensure that the selector gears that were positioned by the setting knobs were centered on the teeth of the central "stepped reckoner". Fools march in....

Note that the gears on the Curta have uniquely-shaped teeth; they are designed to drive in one direction only, and the driving face looks like the involute curve that properly-designed gears have.

As much as I wanted to, I have never removed my (metal) handcrank. Mine is pinned in place. It is quite likely to be a taper pin, and for the life of me, I cannot figure out which end is the smaller end! I know that one could drive the pin tighter, which is a bad idea.

With the proper information, and a small-diameter pin punch that fits (NO MAKESHIFTS!), I could drive out the pin, but only with a backing block of sufficient mass, so that the impact of the hammer would not make the whole calculator jump. The backing block (lead is probably ideal, probably at least 2 lbs/1kg. approximately.) (Yes, I know "2.2".) The crank should rest on the backing block.

There is a video tape available (or was, recently) showing a Curta being assembled at the factory; it's probably PAL, perhaps SECAM, and would need to be converted to be viewed on an NTSC (US) VCR. See Erez Kaplan's site,

For those who have some knowledge of fine mechanisms, suitable oil and grease might well be those used for clocks and cameras.

Sorry, but the hiatus has made me forget some of the other things I had planned to include in this message...

Certainly the Curtas are among the true treasures of our civilization. I'm really glad that it was possible to manufacture such elegant mechanisms at a a profit!

Could someone tell us about what's inside a Bohn Contex? Are there any images of it on the Web? (The Contex was the "other" handheld mechanical calc. that had a significant amount of mechanism; it was not rotary, and had a 10-key serial-entry keyboard, I'm fairly sure.


One of my dreams is of a mechanical calculator that could calculate transcendental functions, and would use Curta-size parts; it would be a mechanical counterpart to the commonplace electronic handheld scientific calculators. It would probably cost $tens of millions to design and build, and would have no commercial justification (other than for those who wish to avoid use of electricity on the Sabbath!).

The innards of the Monroe PC-1421 (which I would passionately LOVE to see!!!) would be a start; that machine had a modular construction. Such a machine might best be designed upon the principles of a single-chip microprocessor, ROM, RAM, and all. It would be horribly impractical and delightful beyond measure.

When I can offer a decent sum, at some point in the future I'd love to buy a Monroe PC-1421, or at least read its service manual.

Not sure whether I still have it, but I used to own a detailed service manual for one of the STW-like Fridens, perhaps for the Model STW, which was the most popular. I'd love to get copyright permission to scan and maybe post copies of it on the 'Net. However, the whole manual would soak up the megabytes horribly; I can imagine 40 MB if in Adobe Acrobat format.

It seemed to be that these manuals had deliberate errors included that were carefully considered to be very obvious to any technician, but intended to deter copyright violators. (People unfamiliar with the machine would copy the errors faithfully.) Reminds me of an early (Intel?) semiconductor memory chip that had some errors in its layout that were bypassed instead of being redesigned out; when the Russians (no offense meant!) copied the chip, they copied the design errors as well.

Arabic calculators:

On an entirely different topic: Can someone tell me about calculators (either mechanical or electronic) used in Arabic-speaking countries? In the early days of the affordable handhelds, I saw (in a NY City store) an electronic handheld with an Arabic keyboard; I greatly regret that I didn't at least ask to see its display.

I trust that most List readers are aware that Arabic-speaking countries use numerals that are quite differently-shaped from our own. If memory serves me right, their one, two, and three are easy to guess at, but a zero looks like a decimal point, and a five like a zero.

There are a couple of books in existence about the history of numerals; I found one of them (by Menninger?) to be utterly fascinating; it discussed all sorts of context.


My best regards to all,

Nicholas Bodley



Subj: Dutch Burroughs
Date: 97-03-25 08:16:35 EST
From: (N. Baaijens)

My very first mechanical adding machine was a Burroughs from 1920 with an integrated printer, weighing 20 kilograms at least. This calculator is still in perfect condition. The machine does not reveal any type or model but I retrieved it in a German handbook: Die Rechnenmachinen und ihre Entwicklungsgeschichte Band 1 (The Calculating Machines and their History of Development (Volume 1).

It was in 1977 when the first Japanese electronic calculators with LEDs became affordable. In those days I visited the laboratory of the Dutch Governmental Geological Dept. and noticed one scientist making a hell of a noise. This drew my attention. The man was producing a meters-long papertape with numbers, using a weird looking contraption.

"What are you doing, my good man?", I asked. A bit annoyed he looked up to me and with a little sigh he muttered: "Calculating... And I do hate this machine, you know", he added. "Does it make mistakes?", I informed. "Oh no. Never. The only one who makes mistakes is I. But the noises give me a headache." "Are you aware of the state of modern technology and that there are small, silent, electronic calculators around which are much faster than this... eh... this thing? "Yes", he whispered, "but this is the government, you know. The Dutch government. We only replace machines when they are totally worn out and absolutely beyond repair."

I inspected the machine carefully and saw that it was built for eternity. And I liked it. I liked it very much.

"If I give you an brand new electronic calculator, will you let me have this one in exchange", I asked with a little tremor in my voice. "You're kidding", he answered and resumed his noises. "No. It's a deal", I replyed but he could'nt hear me.

I went out and hurried to a department store nearby where I bought an Omron calculator, capable of adding, substracting, multiplying, dividing and even power raising with the speed of light. Batteries not included. Price about 100 Dutch florins ($ 60).

Back in the geological laboratory I put this plastic miracle with batteries included on the desk of the sufferer. I never met a man more grateful than he was.

I took the Burroughs with me and after hours I reached my home totally exhausted.

So happy I was with this mechanical calculator that I realized that I had become a collector. A collector with already one beautiful machine.

Nico Baaijens



Date: 97-03-25 21:07:30 EST
From: (Wm. F. JOHNSON)

I have been interested for some time in early Burroughs (American Arithmometer) calculating machines. I believe that these machines are among the most beautiful ever made for their purpose. It is possible to view the interior of these machines through beveled-glass panels on the front and the sides of the machines.

My interest in the Burroughs machines began when I was a High School student in the 1970's. The family of one of my school friends had a machine of this type which they were going to sell in a garage sale for a very low price. I was amazed by the mechanical beauty of this machine, and suggested to my friend that I was interested in it. My friend suggested that I was crazy for being interested in such an old machine which was functionally inferior to an electric calculator. I was persuaded not to buy (unfortunately).

About three years ago, I happened upon a 1908 Burroughs "Style 4" glass-sided calculating machine. I found that I was still fascinated by the beauty of these machines, so I bought this machine (price $65.00 US). I have been very pleased with this unit, which still works perfectly. This machine is interesting, as it appears to be a 7 figure Burroughs factory reconditioned unit which originally carried an American Arithmometer label. I also own another glass-sided unit, a 1909 Style 3.

I am constantly on the lookout in my travels for machines of this type. I do not see very many for sale. Is there anyone else out there who likes the older Burroughs machines?

W.F. Johnson

Vintage Calculators

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