Articulating arms, bearing sources, FREE gauges! A three-fer!

[DIYSwede]

I can see how the races make parallels, but how do you use 12mm bearings, to test flatness of a surface gauge? How does the convex profile make a surface gauge? Kind regards.

By pressing the bearing balls into holes in the meter base, then grind away most of the sphere so you'll get a flat, small <1 mm protrusion,
becoming the hardened (@ 60-ish HRC)3 feet as in the post: Repeat Reading Gage - DIYSwede's "Cheap-O-Meter"
For more on the grind elsewhere: Grinding the feets (yuk, yuk) - Meccano Gallery &
on honing them: Diamond honing the feet - Meccano Gallery

You don't need to use bearings for feet on a surface plate tool, you could for instance epoxy old carbide inserts and grind/ hone theose too.
I wanted to get flat, dead sharp feet that plows away any small particles as the meter gets shoved across the surface plate.

Hope this explains my cheap "design considerations".
Cheers
Johan