I feel dumb, need help with flatness.

[[Mi...]]

"GD&T Flatness is very straight forward. It is a common symbol that references how flat a surface is regardless of any other datums or features."

This has always been my understanding of flatness, very straight forward, and only checked to itself, low to high points.

So can someone help me understand why the alignment of the part has an affect on flatness?

I wanted to just take a quick look at a surface, so I made a plane and a circle to locate the part. Rotation in space = plane, Z origin = plane, X/Y origin a simple circle in the bore, the part in rotationally symmetric so no Planar rotation. My flatness result is .0017"

So I changed the alignment, added an additional circle in the bore, created a 3d line using the 2 circles. Rotation in space = 3d line, Z origin = plane, X/Y origin = 3d line, no planar. Result is .0007", which is much closer to what I can see manually running and indicator over the surface on a granite table.

If flatness is only checked to itself, why does my alignment affect the result? Flatness is so simple that I feel really dumb that I can't wrap my head around where the extra .001" is coming from, because clearly I am introducing it.

 

Thanks.

[[Ze...]]

Are you measuring the exact same area of the plane each time with the same measurement strategy? Are you taking enough points to adequately define the plane? 

BTW... if you are putting the part on a granite plate and running an indicator across it, you're checking parallelism, not flatness.