Mechanical calculators: Olivetti Summa 15

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I purchased this machine in January 2016 at an online auction site. Even though the vendor warned in his ad that the machine was not working at all, its exterior appearance was good, the case was intact and all the keys were present.

Upon arrival, all number keys were firmly stuck, the main lever was completely loose and the joystick was locked in the total (*) position. The ribbon was completely dry, moldy and immobile. The only working part was the manual paper advance mechanism.

I bought this machine in hopes it would give me some clues to repair an Olivetti Summa Prima 20, manufactured about 16 years later. However, the machine is very different internally, although the working principles are the same and the same engineer designed both. I find both the Olivetti Summa 15 and the Summa Prima 20 to be more dependent on good lubrication and maintenance than the FACITs. Olivetti resorts many times to mechanisms that are more fail-prone, if lubrication isn't very good. There are also more clips, slots, rivets, and less regular standardized screws. Maybe FACIT's tolerances are lower, or the machining is superior, but I found Olivettis much more dependent on fine tuning than the FACITs.

Anyway, the Summa 15 was stuck exclusively due to old lubrication. It seemed the machine was lubricated with molasses or glue. Many parts did not move, some only in slow motion. Using lots of patience and kerosene, I managed to get the main mechanisms unstuck, only to be able to identify functions and prepare for disassembly. It turned out that the Summa 15 has a rather modular design, with sections such as the keyboard and the paper handling mechanisms attached by just a few screws and springs. The keyboard carriage, however, had to be forcibly unstuck – after cleaning the old grease/glue, it worked smoothly.

Later on I disassembled the main lever locks, the ribbon carriages and some assorted locks and levers. Many small parts went through the ultrasonic washer. The counting gears and the printer were OK, except for locks and articulations that were stuck at first, but the kerosene wash and lubrication were enough to get them working properly. Thus, the disassembly of this machine was not total, but I intend to do it in the future. Some small articulations have a decisive role in the result of the calculations, including mechanisms to avoid printing left zeroes, negative balance adjustments, and the hammers that press the moving types in the printer.

As opposed to the FACITs, this and other Olivetti machines include mechanisms to perform tens complement automatically and print correct negative sums. In the FACITs, the operator is required to do a back transfer and a subtraction to get the negative result.

There's no reference as to the year of manufacture, but the Summa 15 was introduced in 1949, and was popular throughout the 1950s. Since the more modern Summa Prima 20 was manufactured in Argentina from the early 1960s, I think it's safe to assume that this machine dates back to the early 1950s. The Summa 15 was also manufactured in Brazil for a period, but I have no precise information on that so far. This unit was made in Italy, and remained in operation in Brazil until 1984 – at least, that is the date marked on an inspection stamp posted inside the ribbon cover.

Have a mechanical calculator stored somewhere, and want to get rid of it? Send it to me!

c1950 Olivetti Summa 15 s/n 470934

As purchased, in January 2016. Not bad on the outside, but completely locked up

 As purchased

Detail of the degraded lubrication

 Detail of the degraded lubrication

Insides, pre-cleaning

 Insides, pre-cleaning

Partial disassembly, pin board visible. The pins are set by the keys on the top, shifting them to the bottom. There the pins are captured by longitudinal bars when the main lever is operated, transmitting the digit to the printer, to counting gears and to the accumulator at the back.

 Pin board

Curiously, the serial number of this machine is on the inside.

 Serial number

Label posted in the back of the ribbon cover, indicating its last maintenance: April 1984! It's surprising that a manual machine has been used for that long. In 1984 programmable calculators were common, and small electronic printing calculators were very cheap already.

 Maintenance label

Curious and artsy ad for the Summa 15. Reminds me of “Mad Men”, the TV series.

 Olivetti Summa 15 ad

A look at the double set of counting gears, one for the individual entries, the other serves as an accumulator. Notice the carry teeth between the gears.

 Counting gears

A view of the bottom, the long springs hide the longitudinal bars that connect keyboard and accumulator

 Bottom view

Another unique feature is this counter, used in multiplication

 Multiplication cycle counter

Almost ready. The most important feature of this machine is the innovative “joystick”, used to set functions for total, subtotal, subtraction and label printing. The joystick was introduced first in this model, and is patented by its designer, Natale Capellaro.


A sample printout

 Sample printout

Final assembly

 Final assembly

More about this machine

Operations manual (in Spanish)

Operations manual (in Italian)

John Wolfe's Web Museum, description of the entire Olivetti line along time

Description of the Summa 15 (in German)

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summa15.txt · Last modified: 2017/05/14 18:26 by clodoveu
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