Effective Training Inc. Message Board

Hi all

I am a new here and very happy when found this kind of discussion forum.

Does somebody has an answer to me to this question:

Is the flatnees a special case of surface profile or is there something more mystique behind? Of course You cannot set datums with flatness but if we don't use those datums with this case at all.

I don't see any difference if we has a flat surface and we measure those and comparing data against flatness or surface of any profile, the result should be same?

When "Profile of a Surface" (PoS) is applied to planar (flat) surface AND there are no datum references included in the feature control frame for the Profile tolerance, then the PoS control MIGHT BE the same as Flatness, but it may not necessarily be the same. What do I mean? Unlike the Flatness control, PoS can have its tolerance divided unequally or non-uniformly about the true profile (flat plane, in this case). Flatness is simply: "do all points on the surface lie between two parallel planes?" If PoS is applied to a planar (flat) surface and there are no datum references included then it is purely a FORM control only. And, if simply applied "as is" with no other requirements, PoS too is "do all points on the surface lie between two parallel planes?" BUT! With PoS I can add qualifying requirements that define how the tolerance is to be divided, or I can define non-uniform tolerance zones about the true flat surface. Such modifications of the tolerance zone dispersion and shape cannot be done with Flatness.