What is a millwright?
A millwright is a craftsman who builds or maintances mills and/or machines.
To your Great-Grandpa, a millwright would build and repair gears, mechanical belts and driveshafts at sawmills, flourmills, and papermills.
Today millwrights work at such places as nuclear power plants, food processing plants, steel mills and in the aerospace industry, maintaining and building machines to help in production.