I need a PRECISION straight-edge, so I made three

Have you seen the price of precision metrology equipment lately? Neither have I, I've been too busy rubbing these sticks together in an attempt to find flatness.

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Here's Alex's channel: https://www.youtube.com/@anengineersfindings

And some better reference material on the three plate method, if you're interested:

Accidental Science:

Flat From Scratch, revised - The 3 plates method

Robrenz:

ROBRENZ ATD #1 INTRO, 3 PLATE METHOD

Oxtoolco:

Making flat lapping plates 1