Obtaining different dimensions with the same reference planes.

[[La...]]

-The planes on the STL data were created manually.
-The planes in the CAD and STL data were matched with the "Link to Actual Element..." command.

-The measurement was made with the "Construct Projected Point Distance" command.

-The STL data value was 16.78mm. (When over the plane on the CAD data.) (image 1)

-However, if this measurement is made only on STL data, the result is different. 16.542mm (image 2)

What is the reason for this difference?

Image 1

Image 2

[[Ja...]]

I think a base answer here is that it sadly does not make sense to have a projected point distance between 2 planes.   The point you are projecting in fig 2 is an arbitrary point in space on the infinitely large plane - the result could be anything!

I would suggest that youd use an inner caliper or similar to assess this dimension

[[Ch...]]

The reason behind that is the distance 16.72 mm is based on the nominal planes which are created through CAD Data. When you use projected point distance on nominal planes and select one plane as a projection point and another plane as a plane, behind the box (During calculation) it translate that distance as an actual distance based on one nominal plane (blue) as a point and another actual (green) plane as a plane which gives us a different value than the distance completely based on actual planes. So, if you want to use projected point distance then create it using actual planes (16.54 mm) and try to use inner disc caliper distance to verify it.

[[Ja...]]

Sorry - the at function here doesnt work well....

Christian - you are on the right kind of explanation , but the ' point' the software creates for the location of a plane is effectively random.

It would be like doing a 2 point distance between 2 planes - this fundamentally doesnt make sense.

Measurement between cylinders is an analogous topic for which there is a nice FAQ article here :- https://connect.gom.com/display/GKB/How+to+measure+the+distance+between+cylinders

 

measuring between planes is no simple task and for robust spc you need to think very carefully about what you construct.

The caliper style is just one , simple, way of achieving an answer for what looks to be intent of smallest gap.