Best practice for plane measurement

[[Sa...]]

Hello,

Could someone from you help me in below topic?

I am measuring a plane which is a surface using Profile of Surface tolerance. It gave me some value. Now I am trying to measure the same plane using Position tolerance. It is giving some other value which almost 50% high. What is the best practice for measuring a plane? for me the location of this plane from the datums is important. Should I go for position tolerance then? Is Profile of Surface really good then for free form surfaces? I am trying to follow ISO 5459 and ISO 1101 for this information but did not find it. Any comment is appreciable and thanks for your time. 

 

Kind Regards

Samkit

[[Ch...]]

In theory, a position tolerance and a surface profile on a plane in respect to the same datum (system) should be identical. Practically, there are slight differences in the implementation: For the position tolerance, the tolerance zone is constructed from the datum system and some theoretical exact dimension, that is in usual application taken from the nominal geometry of the checked element. We then check how well the actual points fit into that zone. For the profile tolerance, we compute the best possible transformation between actual points and nominal geometry under the constraints set by the datum system. If the checked nominal plane is perfectly parallel to the main datum plane, this yields the same results as the position tolerance. My guess is that your situation is along the lines of: Datum is a plane, the checked element is nearly, but not perfectly parallel to this plane. The surface profile can now "slide" along the datum plane, effectively changing the distance between checked plane and datum plane arbitrarily. This gives a smaller result than the position tolerance, where the distance to the datum plane is prescribed in the dialog.

For this reason, I would recommend to always use the position tolerance on planes.

[[Sa...]]

Thanks for your comments. I appreciate it.

Kind Regards

Samkit