Summary

To help you do long addition, subtraction, multiplication (and even division) when you don't want to or can't use a pen & paper e.g. in the rain.
Can be used to do latitude/longitude/time calculations with the Hangdial, StarSquare, and StarRings.

Better than an abacus- nothing to accidentally slide on you, and I keep forgetting where I'm up to with an abacus when doing carries and end up making mistakes: but not with this! Could also be used as a teacher aid.

There are TWO designs- one for long addition and subtraction only (it has a '10+' carry/borrow plate); and one which also supports multiplication (it has +10 up to +90 carry plates, and all plates are a little thicker too and all text more raised for no particular reason).

Both designs include a '10' digit plate, which is sometimes a convenience- but if you don't like it you don't have to use it of course.

For decimal places, just use remember that the last digit or last two digits or whatever are after a decimal point, in each print. Or use a stick or something.

Usage (Addition or Subtraction):

Consult the two rules on the bottom of the print to help you resolve any negative signs, and proceed as per the long addition/subtraction process.
For example, let's say we want to do '15 minus 23'. Then (via the second rule) we have two input numbers, '+15' and '-23'. Here the number with largest absolute magnitude (i.e. ignoring any negative sign) is '23', and it's negative, so (via the first rule) we will flip the signs of both numbers then at the end flip the sign of the result. So we enter on two prints '[+]000023' and '[-]000015', one below the other in that order: the positive number is always the first one, which ensures when we do our computation we never cross '0'. We use the '[..]' versions of plus/minus signs as a reminder that we've temporarily flipped the signs and need to flip the sign of the result.
We then proceed digit by digit from the right, updating the first number and zeroing out digits of the second number as we go to keep track of where we are up to:

[+] 0 0 0 0 2 3 [-] 0 0 0 0 1 5

-> (we see that the second number has a negative sign, so we subtract rather than add it)

[+] 0 0 0 0 1 10+ 3 [-] 0 0 0 0 1 5

->

[+] 0 0 0 0 1 8[-] 0 0 0 0 1 0

->

[+] 0 0 0 0 0 8[-] 0 0 0 0 0 0

then as the second number is all 0s we are (nearly) done, with the first number holding our result: we notice we have a '[..]' sign for that first number so we need to flip it's sign, giving:

- 0 0 0 0 0 8

i.e. -8 as our result.

Similarly, if we had '-23 plus -15' we'd have the prints display '[+]23' and '[+]15', do the addition, end up with '[+]38', then notice the '[..]' and flip the sign to give the result of '-39'. But if we had '23 plus -15' or '23 minus 15' we'd have the prints display '+23' and '-15' and not have to flip any signs.

When adding numbers, I like to add each digit and set the carries, then resolve all the carries at the end; rather than resolve the carries as I go. Likewise for subtractions you could do all the borrows as the first step.

Usage (Multiplication):

Enter on individual prints the two numbers to multiply, then below them sufficient 'working' prints, then finally at the very bottom the print to hold the 'result'. Ignore signs for the 'working' numbers and just put in the correct sign against the result (a positive and a negative being multiplied gives a negative result, otherwise the result is positive). Proceed as per the long multiplication process.

Usage (Division):

Enter the number to divide, and the number to divide by, on separate prints. Have a 'result' print', and some 'working' prints. Set up a separate set of prints to do the 'temporary' multiplications with. Proceed as per the long division process. Ignore signs when working, just put in the correct sign against the result (a positive and a negative being divided gives a negative result, otherwise the result is positive).

Printing and Assembly:

You should be able to scale it a fair bit if you need to.

Print at 0.2mm layer height, you can turn on ironing of top surfaces if you like. Infill of say 20%, Zseam position doesn't really matter. No supports required.

Then install the plates: where there is a '0' on the main part plates should be installed above it with '1' at the bottom, going up to '10' at the top. Where there is a blank on the main part the plates should be installed above it with '10+' at the bottom (going up to '90+' on top for the second design). Where there is a '+' on the main part the '-', '[+]' and '[-]' plates can be installed above it in any order really.
Finally, glue the two main elements together (easiest to print in PLA then use superglue and clamp until the glue sets).

Repeat for as many as you want. You could get away with just one for addition/subtraction, but it's good to have two: one for each of the two input numbers. Then when doing the addition/subtraction you can flick the digits of the second number to '0' as you go, to help you keep track of where you are up to.

For multiplication, print out 9 or so: two for the two numbers to multiply, one for the result, and the rest for the intermediate 'working' lines. Note that you only need the one print of the second design with its carry plates up to '90+', the others can be the first design (and you don't even need the carry plates for those).

For division, you'd need quite a few.