Different results in the same measurement

[pimigas]

Hello

I´m measuring the distance between two surfaces using the distance between two probed planes.

If i choose the distance between plane A and plane B i have the result X.

If i choose the distance between plane B and plane A i have the result Y.

But X and Y are very differents.

Why it's happens?

[kieran.gill]

This could be a happening for a couple of reasons:

1. Your distance between two planes could be referencing a different alignment

2. Your two inspected planes may individually be referencing different alignments

3.The reason why you may be seeing the discrepancy could also be because you may have the a different alignment set as the active alignment.

Please check your alignments for individual items and check active alignment and let me know if this solves your issue ?

If this does please mark this case as solved.

Kind Regards

Kieran

[alexey.zorin]

I'd like to wide Kieran's answer.

The distance between two planes calculates along the normal vector of the Datum Plane.

There are no two perfectly parallel planes in the real world.

So each plane has individual normal vector.

Moreover two planes possibly have different size (bounding limits).

Expectedly bigger plane can be calculated as shortest distance to the the datum plane.

Even expectedly distance between two planes of different size can give different result if swap datum plane.

Please click on the Accept as Solution button if my post solves your issue or answers your question.

[pimigas]

@kieran.gill

The alignment is the same. This is not the problem.

@alexey.zorin

I understood your explanation but now i do not feel comfortable with this tool. The result depends the on the side of where I am observing. We need to study the  Heisenberg's uncertainty principle to use  the distance between two probed planes? 

And now other doubt comes up: how to measure the distance between two surfaces?

[alexey.zorin · Accepted solution]

To be honest it is really important to know which plane is datum one.

From metrology principals it is required to measure plane with maximum bounding points if plane's position and orientation is important, and measure as much as possible points if flatness is to be going to be controlled.

So, probably you agree that distance between two planes may be controlled if these planes are about parallel.

(if not so should talk about angular displacement/position between two planes).

In most cases controlled plane have flatness and parallelism parameters relative to the Datum plane.

The Datum plane here is most trusted plane produced in more rigid technological cycle comparative to controlling plane.

What I'm talking about. Only one plane can be called as Datum. So it is not correct mix two planes in the distance two planes tool/item in PowerInspect.

Regarding two surfaces.

Please give more details. Which type of surfaces you are going to measure and even find distance between them.

Best Regards,

Alexey

[alexey.zorin]

Here is very good description of relations between parallelism and distance tolerance.
http://www.gdandtbasics.com/parallelism/

[d.yeager]

I avoid this tool as well.  Not only will the choice of your datum affect the measure, but the actual area you probe for the feature plane I think will change your results.  I do not think the PI takes into account the nominal area of the plane.  For example, if you have a 10x10 (inch) surface, with one side lifted .50 inches, but only probe half the surface in the direction of the lift, then you result will only show .25 in difference from low to high..

Depending on the part, I find a way to create a point (or points) at the location(s) I need to measure the distance.  Most often in my case, the points I try to measure would be at the location that supports the featured surface (example, fixture pins).