Start with researching the "three plate method" to generate a flat surface. A flat reference plane is the first thing you need to make precision machines. (The

Start with researching the "three plate method" to generate a flat surface. A flat reference plane is the first thing you need to make precision machines. (The second is an accurate way to divide a circle.) But the three plate method is one of those foundational, from-first-principles realizations that changes everything. It basically means you can start with three rocks and manufacture your own reference plates that are accurate in flatness to any arbitrary tolerance you have the patience to achieve. With a truly flat plate in hand, you can generate a reference square - again, to any tolerance you like. If reading about that stuff (it's everywhere on the web if you know the right codewords "three plate method"), your next step is to find a copy (electronic, probably, it's long out of print) of Moore's "Foundations of Mechanical Accuracy". I keep a copy beside my bed so on nights when I can't sleep, I can caress the covers and ease my soul.