Supposedly "the answer" is a discarded engine block: https://makezine.com/article/maker-news/patdelany/

"Open Source Muti-Machine": The heavy, ultra-stiff mass of the cast iron dampens vibrations and forces the cutting tool to move in a perfectly straight, micron-accurate line dictated by the factory-bored cylinders.

Ask an AI about it, but conceptually it's an amazing concept. There are probably billions of "nearly micron-accurate" machines, and if you're 1000% repeatable 90° then the world is (eventually) your oyster.