True Position vs X and Y Dimensions



Hello All,
This is my first post on Elsmar so please bare with me.

I am measuring a part using my Romer Arm and Polyworks.
The problem I am getting is the the X dimension is in tolerance (Basic), the Y dimension is in tolerance (Basic), but then the true position is 14mm out of tolerance.
I have some knowledge of True Position...I get that X and Y can be in and True Position can still be out, but the X and Y are close to perfect and the True Position is out by 0.5".

How can this be?
Am I doing something wrong with the alignment or coordinate system in PolyWorks?
FYI: Alignment was based of Datum A Plane and Datum B Hole, and Coordinate System is also based off both of these.


Forum Moderator
The reason you could be in spec for the X and Y and out of spec for Position can be shown in the attached drawing. The red areas clearly show how this may occur. However, in this scenario the X Y coordinates would not be "nearly perfect". If your X/Y coordinates are reasonable, the datums 'should' be correct. This would lead me to believe that it may be how you are specifying the Position tolerance. Have you verified that you have only specified one coordinate system? Could the X/Y be based on the part coordinate system and the Position based on the machine coordinate system?


  • True Position vs X and Y Dimensions
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Thanks so much Miner. After evaluating this further, it was found that the Datums were out of tolerance which may be why the true positions were thrown out. Once we clamped the part down and re-scanned using our Romer Arm, the true position dimensions came out good and the X and Y coordinates were around the same.


The customer will be drilling the parts down so it will be "clamped" during installation...But I agree that if they do not call out the part to be in a clamped state on the print, it automatically means free state...
The customer is not the best at making prints, so I will be writing a deviation to change the print to a clamped state.
Thanks for the reply!


Involved In Discussions
Is the part clamped by the customer? Unless otherwise stated, the part should be within specification in the "free state".
Technically true but if you're measuring on a touch probe CMM the part has to be held in place so free state is usually a myth.


Going to add to this: You should also consider the math behind what you are asking. True position on a CMM involves having an alignment referred to as level-axis-origin. If you are measuring your feature as a circle, the math finds the center using a least squares routine. You have a choice at this point: How do you report this calculated center point in your level-axis-origin coordinates? It won't physically lie on your alignment planes. The CMM can do this in two (primarily used ways).

1) It can take the plane created by all the points you touched, use this plane to establish a normal vector, and project the center point along this vector until it intersects your level-axis-origin. It then reports this location.
2) It can take the center point it determined from your probe hits, and then calculate a vector normal to your level-axis-origin that goes through the center point and report THIS intersection as your true position.

These can give you very different results. In most cases, #2 is the "default" in CMM languages. But it may not be what you want. Also, if your programmer is weak, they may not even understand this.

Proud Liberal

Quite Involved in Discussions
Can you post sections of the print showing the datums and the feature in question (including the feature control frame) ?

Also, what software package is your CMM using?
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