Position GD&T

[[La...]]

Hi everyone,

[[La...]]

Hi everyone,

I'm measuring the position tolerances of the circles.

But the position tolerance manual calculation and the GOM calculation are different.

Why does this difference occur?

[[Ch...]]

Because you are comparing the RPS alignment (X' and Y') with datum system under material requirement "alignment" (position tolerance) and most properly these are not the same. You can find more information about datum system under MMR in the Zeiss Tech Guide: https://techguide.gom.com/en/gom-software-2021/article/general_mmr_background_information.html. 

Edited November 2, 2022

[[Be...]]

Additional information on datum system as view coordinate system:

• Only nominal coordinate systems are allowed as view coordinate system
• If you have a GD&T datum system with a nominal portion the nominal portion is use. The nominal portion is automatically calculated during the construction of the GD&T datum system if possible.
• Therefore, the view is not aligned to the actual datum system. You have to construct a nominal datum system to visualize this behavior and to use it as view coordinate system.
• Pure actual datum systems or coordinate systems are not supported as view coordinate system due to the open question what the customer would expect if the project contains stages. And due to the fact that to each project new stages could be easily added we prevented this possibility.

Regards,

Bernd

[[La...]]

Thank you very much for your help.
I will examine the document in the link you sent in detail. Christoph Schult
 

I've revised it as far as I understand it for now.

[[Ch...]]

An additional hint: You should compute your comparison with more significant digits. You can change this number of displayed significant digits in the preferences.

And please check if the computation parameters are in accordance with the chosen standard. As an example, if you check the GD&T position in accordance with the ISO standard, the circle has to be computed as best-fit circle without any sigma or other constraints. Otherwise, the internal circle used during GD&T computation and the constructed circle can differ. Please have a look into the Tech guide again: https://techguide.gom.com/en/gom-software-2022/article/introduction_to_gdat.html

"The construction of a fitting element is based on a selection on the actual data. The software includes this selection in the GD&T analysis of the fitting elements. Based on this selection, the software computes the tolerance or the datum. During computation, the software ignores the computation method (e.g. Gaussian, Chebyshev, constraints) that was used to create the fitting element. This procedure ensures that the software always complies with the prescribed fitting methods of the standards (Defaults)."

This recomputation ensures that the GD&T computation uses the correct computation method. Of course if somebody will compare the results with classical computation it makes the live harder.