Wheelbuilding / Wheel Building / Wheel build help

How can you measure erd to 0.2mm

Clamp the rim to the 28" face plate on your big lathe and check the runout with a dial indicator. You could easily measure polar coordinates to within 0.05mm and plot the circularity without even having to get it centred properly. Obviously a CMM would be better, but not everybody has one of those, at least not in the size required.