A program that I made during an internship in a couple weeks to determine the magazine type to be used in a toroidal winding machine based on inputs, a table with pre-set magazines and their data like the diameter of the wire, and so on.
The original program that they had made 20 or so years ago wasn't able to print the results using the company printer since the IP of the printer was hard-coded and the source code was lost.
This program was therefore made completely from scratch in cooperation with a co-worker with whom we figured out what the necessary formulas are to calculate which magazines the company can use.
This was the first program that I made using WPF.