An iconic image of the last century: the mechanical adding machine. In this dissection we'll look at the basics of how it works. Vin Marshall

Examining clever mechanisms often provides ideas and inspiration for our own designs. This week’s mystery tool was in fact a look at a partially disassembled adding machine. Here’s the rest of that dissection.

Congratulations to Midpipps for nailing it first, then later identifying the brand. Someday we’ll stump you guys.

Check out the adding machine’s dissection gallery here

Adding Machine

An iconic image of the last century: the mechanical adding machine. In this dissection we’ll look at the basics of how it works.

Adding Machine Buttons

This is a side view of the keypad on which numbers are entered. There is one row for each decimal digit (0 – 9) and one column for each significant digit of the number being entered. The mechanism releases all of the spring loaded buttons on that column when a new button is pressed, ensuring that only one number can be entered for each column. The selected number / button has a tab that protrudes from the bottom of the number pad (see number 3 in the most significant digit column selected in this picture). This tab limits the movement of another mechanism, which we’ll see in the next picture.

Guess This Tool #6

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Adding Machine Number Gears

As the number bars of the previous picture retract back to their normal position (remember that the distance they moved, and therefore the distance they will retract, corresponds to a numerical value), they turn these “gears.” These gears will turn from 0 to 9 positions, or teeth, depending on the number entered in the corresponding column of the keypad above. These teeth then engage the counter wheels – already removed in this picture – that we will see in the next few pictures.

Adding Machine Counter Wheels Installed

Pictured here are the counter wheels (beige plastic) and the “gears” of the previous picture (steel, barely visible, engaged with the top of the counter wheels). As those gear arms turn, they rotate the counter wheels. In using this machine, a user enters on the keypad a number to be added to the running total and pulls the lever. When the lever is pulled, the number bars are extended. The amount of their movement is limited by each digit of the number entered on the keypad. That movement is translated through a linkage into rotation of the “gear” arms pictured here, which is then translated into a rotation of the counter wheels. Because the counter wheels had an initial state (either 0, if the machine has just been zero-ed, or the previous running total), the additional rotations add the newly entered number to the running tally. The wheels have 10 positions, representing a value from 0 to 9 for each significant digit. For example, if the ones position of the current total was 2, meaning that the counter wheel is turned to the 2nd tooth already, and the number 3 is added to the total, the gear arm will be made to move 3 teeth, which will turn the ones counter wheel 3 more positions to a new total of 5.

Adding Machine Output

These number stamps are part of the “gear arms” seen in the previous pictures. Just as the gear arms move, so move these stamps, putting the right numbers into stamping position. When adding new numbers to the running total, each number is stamped onto the paper tape. When the total is printed, the counter wheels and gear arms work in reverse; the position of each counter wheel controls how far the corresponding gear arm moves, which in turn rotates the number stamps into position to stamp the total onto the paper tape.

Adding Machine Overflow

The parts of this machine shown thus far won’t handle overflow / carries from a column. That is where these arms come in. The counter wheels have 10 teeth on their counting side and one cam on the other side (Shown in more detail in the next picture). Every time a counter wheel goes past the tooth that represents 9, that cam actuates one of these arms, which will allow the next wheel over – the next significant digit – to rotate one extra stop. In this way, carrying in addition is accomplished.

Adding Machine Counter Wheels

There is one counter wheel for each significant digit in the machine’s “memory.” The counter wheels have 10 teeth on one side, representing the digits 0 – 9, and one cam on the other side. That cam actuates an arm that handles an overflow from the column each time the counter wheel rotates past the position that represents 9.

Adding Machine Paper Feed

In addition to everything supporting addition in this machine, there is a plethora of gadgets and geegaws that handle all of the ancillary tasks like advancing the paper and feeding the ink ribbon. Pictured here is some of the paper mechanism.