Lapping three surfaces together is the path to flats. Lapping just two together is how we make concave/convex hemispherical pairs for telescope mirrors. https

Lapping three surfaces together is the path to flats. Lapping just two together is how we make concave/convex hemispherical pairs for telescope mirrors. https://en.wikipedia.org/wiki/Flatness_(manufacturing) https://en.wikipedia.org/wiki/Flatness_(manufacturing)