Basic Analog Computer using a differential. The input gears can be turned to compute the sum between two numbers. The result (without carry) will be shown on the output wheel.
 

It's a single plate, easy print that fits on the A1 mini plate. An optional stand with number indicators is on the 2nd plate.
I added a version that has much less prominent numbers (perhaps a bit harder to read), but removes the need for supports around the wheels entirely, limiting them only to the hub piece.

This model was inspired by the design of fire control system used on ships in the 1950s. Mechanical systems were used to compute real-time solution to complex problems involving dozens of variables, long before digital computers were available. Human ingenuity really is quite fascinating.

Assembly Instructions

No glue is required. All parts friction-fit together.
IMPORTANT: When you assemble the last gear-wheel make sure all 0 (zero) values on all three wheels are lined up.