Odhner

Specification

Donated in 2013.

Generally works, but:

if the first decimal of the register contains a 1, it can't be reset to 0. Not that much of a problem, you can subtract 0 while the carriage is on the first digit to set it to 0.
Mechanics needs some cleaning an maybe a little lubrification to make it go smoother.
a lot of the digits have lost their paint.

Input is given by setting the 9 levers.
The right hand counter is the accumulator. It stores the result of the addition or subtraction.
The left hand counter is the register. It counts the number of turns.
Input is transferred to the accumulator by turning the crank, clockwise turn for addition, counter-clockwise for subtraction.
The carriage can be shifted to align the input with a certain decimal of the accumulator and the register.
Both register and accumulator have a reset handle, give it a full turn to set all digits to zero.
The input can be easily reset by pulling the reset handle left while turning the crank until it pushes all levers to zero.
(other than the above) always turn the crank a full turn. You can always undo a turn by turning in the opposite direction.
A bell signals an overflow condition.

Reset accumulator and register.
Shift carriage all the way to the left.
Input the first factor.
Turn clockwise as many times until the last decimal in the register equals the last digit of the second factor.
Shift the carriage 1 position to the right.
Turn clockwise as many times until the fore-last decimal in the register equals the fore-last digit of the second factor.
Repeat the last two steps for all digits of the second factor.
Read the product from the accumulator.

Reset accumulator
Shift carriage to the right, allowing space for decimals.
Input the dividend.
Turn clockwise to transfer it to the accumulator.
Reset the register.
Input the divisor.
Shift carriage to align the leading digits of input and accumulator.
Keep turning counter-clockwise until the bell rings.
Turn clockwise to undo the last turn.
If accumulator contains all zeros, read exact quotient from register, else shift carriage one position left and repeat last two steps.
If not dividable, the process stops when the carriage is all the way to the left. Read approximate quotient from the register and remainder from the accumulator.

Reset accumulator
Shift carriage all the way to the right.
Input number
Turn clockwise to transfer it to the accumulator
Reset register
Shift carriage aligning left-most digit of input with left-most digit of the value in the accumulator
Set a 1 in the input that is aligned with the right-most digit of the value in the accumulator and turn counter-clockwise
Set the next odd number (3) and turn counter-clockwise
Repeat with next odd numbers (5, 7, 9, 11, 13, ...) until the bell rings.
Undo last turn by turning clockwise.
Subtract 1 from input
set a 1 in the next input digit (one decimal to the right)
shift carriage left.
repeat the process of subtracting odd numbers until the bell rings.
If accumulator contains all zeros, read exact result from register
If carriage reaches left-most position, read approximate result from register