I have a part that needs to be parallel within .0008" to datum 'A'
We cannot fixture datum A to the granite as it has a boss, we use three ground blocks to fixture the part on three points, then sweep with an indicator.
Old timey engineer says that is the wrong way to check it, and throws a pair of micrometers on it and gets .0005".
He's checking thickness correct? Not parallelism. At this point I don't know how else to explain it to engineering...
The datum should be a perfect plane, tangent to the material on the free side of it.
Sweeping with the indicator measure only the parallelism of the plane from a datum passing through the blocks, not the real datum (except if the blocks ha exactly the same height, and cover the whole surface)