DIY measuring and lapping a worn surface plate

[DIYSwede]

Thanks for stating what I perhaps silently assumed, Paul.
My quest started with three pieces of marble plate and different grits eventually leading to making
a few tools: first the spherical comparator, which then with an indicator became the "Spherometer".

These could only prove down to app 1-2 micrometres deviation, but not where the highs and lows were.
But that was good enuff for my workshop use - I didn't mind if it was a sphere with a 600 m radius.

Then I suddenly found the free, uncalibrated diabase plate, which were to become my "master plate"
Hence I winged the Cheap-o-Meter together, finding the deviations less than 1,5 mumeter in any axis.

If it is "flat"? - I still dunno - it can be either somewhat concave or convex within those 1,5 my of deviation, but it's NOT warped,
and absolutely good enuff for my walk-in closet workshop.
And it didn't cost me 250 bucks for some other dude's calibration of it either - but then I'm real cheap.

My initial fundamentalist quest for flatness took a real hit when I read a few old books on metrology,
realizing that I've been standing on a relativistic quagmire all the time...

Tip (p 24ff): https://pearl-hifi.com/06_Lit_Archiv...l_Accuracy.pdf

Just my 2 cents