A mechanical calculator, which is capable to add and substrat, featured carry mechanism, keyboard, overflow mechanism and openable part for set up.


The Context

I wanted for a while to continue my first attempt to create mechanical calculator as shown in 2010 on youtube. I have retried it and finally succeed to create it.



The functioning is based on the Pascaline Machine created by Blaise Pascal in 1642. It uses a carry mechanism, featured 6 digits. I have added some functions like the single keyboard and overflow mechanism.

Regarding the carry mechanism, it is based on two ratchet gearing. When a digit makes full rotation, it grabs the next digit and raise it. When it is raised, it can’t moves because of a ratchet mechanism but the sautoir can. Thanks to a other ratchet mechanism, when  the moving digit is raised the ratchet travel for 2 tooth on a 20 tooth gear. When the sautoir is released after complete rotation of previous digit, it falls, and makes rotate the digit. The movement is equal to 2 tooth on a 20 tooth gears, result of a 1/10 rotation, which has made in fact a carry. See the video to understand more :

Regarding the system of keyboard, I have tried to make a full keyboard with 10 key but not figure out the problem of dead point because of lego. Therefore, I have build a version with a single key, when it is pressed, the key moves and a ratchet touches a 20 tooth gear. During the displacement, it moves the 20 tooth gear with 2 tooth of displacement, which result to add one to the digit. A free point of rotation with little weight enables to replace the system in correct position. Note that the spring used is from a dismantled lego shock absorbers. See it in action :

Concerning the system of substracting, It uses the principle of the complement, thanks to the moving panel. I let you see the video to understand. Regarding the overflow mechanism, it is a new feature which enables the user to know when the mechanical calculator is in overflow, that is to say that the needed number is more important than that it can display. For instance with 6 digit, 999 999 + 1 equal to  1 000 000 but the machine can not display 7 digit, therefore the overflow is detected. To create this, I use the rotation of the last digit, when it has made a full rotation, it pushes thanks to a lever an other lever which releases the overflow panel. This overflow panel is mounted with a rubber for its raising. See the video to understand more :


As a result, the mechanical calculator can make addition or substraction, the overflow is detected when to important number is in the box, and it is easy to use thanks to the keyboard mechanism. Also, I have made a openable panel at the rear which enables the user to dismantled each parts of keyboard or adder to set up finally. The only set up is to select the correct rubber band for tension, set up the end of upper ratchet (axle have to go over the parts for 1 or 2mm), ensure that all axle can rotate freely (including moving parts and digit). Also, the main roller which display the result can be raised to set up the position of the roller with number. See the final video to see it :

Building Instructions

I have created the free instructions for this creation, as a photo sequence. You can download it in pdf, or browsing it on the blog.

Here is the bill of material :

Or download as a pdf in html grid :rebrickableBI (1mo)

Want to check before if you can build the Mechanical Calculator  ? it has been indexed to Rebrickable.com, the website which matches your parts with mocs !

Don’t forget to download the sheet of sticker for the calculator :  Sticker Sheet (PDF 1mo)

Download the PDF of building instructions (24mo)

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