I paid € 89 for a 12 inch surface plate from China that is reported to be flat within 7.1 microns. Is that true? And how do you measure that without very expensive professional tools?

Maybe you wil get some inspiration from my approach. Anyway, have fun watching.

I see that in this video I forgot to mention an important setting of the laserpointer. I went to great length to ensure that the beam was exactly parallel to the surface of the plate. That is very important to avoid misinterpreting the results of the measurement.

Excellent information about the repeat-o-meter and the auto-collimator in these video's:

Tom from Oxtoolco:    • Repeat O Meter  
Robin from Robrenz:    • ULTRA PRECISION REPEAT O METER  
NYC CNC:    • DIY Rahn Repeat-O-Meter Part 1: CAD D...  
Machine Tech Video Blog:    • Autocollimators 1: Introduction  
ADTW learn:    • Autocollimator Working animation