Re: [CAD_CAM_EDM_DRO] Horizontal Planes

One method (that I plan to use) is to make a fixture with ball-bearings
on a vertical rod.

This is like a big blanchard-type grinder, but lightweight.
Make or buy a contractors metal 90-degree triangle, about 2-3 ft high (1m).
fix top and bottom end on ball-bearings.
You should have a vertical line and a triangle come to the side maybe 50
cm (you can extend this as long
as whatever extension you use is rigid, and does not bend appreciable
under its own weight.

Put a needle on the end of the outward part, pointing downwards. Measure
from the needle to the surface,
you can use a known-thickness calibrated piece, like for measuring
clearances in car valves.
Rotate the toe, measuring from the needle downwards. You and also scribe
an arc with the needle,
or put a fine-point pen in it. Yur favriation from a perfect arc or
circle tells you how far out you are.

You should easily get to 0.01 mm or 0.0005 inch over 1 m. It will give
you a true plane, not necessary vertical.
I think only expensive optical measuring equipment is more accurate.

Your tolreance is the woblle in the ball-bearings, but as they are
lightly loaded even any slop may not show up.

Good luck,
c.