default fitting method of toleranced feature and datum feature

[[pe...]]

hello 

I'm newbie of Atos Scan. when i calculate the true position of a circle, i can only choose method- ISO 1101/ASME Y14.5/Geometry.

i don't know how ISO standard treat toleranced feature and datum feature.

I checked ISO 5459, if datum feature is a plane, fitting method is Chebyshev without constraint；if datum feature is inner circle, fitting method is maximum inscribed circle. Right?

I checked ISO 1101, 8.2.2.2 states “By default, the specification applies to the indicated extracted integral or derived feature itself.  The associated toleranced feature specification element is an optional specification element(such as Chebyshev, Gauss, maximum inscribed element...)”.         With associated toleranced feature specification element, i understand a ideal circle(associated toleranced feature) will be created from real feature profile，and center of the ideal circle shall be in the tolerance zone(circle).        But it's not default situation, “By default, the specification applies to the indicated extracted integral or derived feature itself. "  How to understand it?  the tolerance zone can be still a circle?

Looking forward to your reply. thanks!

[[Di...]]

Hi Pei,

Question 1)

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Just search in the direct help for General information about GD&T. Inside of this article you will find all computation methods that are used for GD&T. It's at the bottom of the article

Question 2)

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Yes, you are right. And the GOM software uses this in the background automatically for you in case you select ISO or ASME. Geometry is used for exceptions from the defaults or specific cases only.

Question 3)

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Yes, you are right it is not the default situation. It is an exception coming from the drawing if the designer wants to have the computation based on an associated element like (C), (G), (N), (T) or (X). It will only be used if you set the correct association method for the fitting element and select geometry for the check.

You will find in-depth information about this for example in the parallelism direct help.

And yes, the tolerance zone can result in a single circle if the base element is a circle as well.

 

Hope this helps

Best regards

Dinko

Edited October 8, 2021

[[pe...]]

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Dear Dinko

thank you for your explanation.

And last question, if no associated element indicated in location specification in the drawing， which fitting method is used by default or commonly？Gauss?

[[Di...]]

Hi Pei,

several defaults apply depending on the standard ISO or ASME, the usage and the needed parameters.

The most basic approach in ISO is the determination of the extracted axis which has to be fully enclosed by the given tolerance zone.

The extraction of the axis is a multi step process described as follows on a cylinder:

1) A gaussian cylinder is computed using all selected points.

2) The axis of the cylinder is used to carry out sections, perpendicular to this gaussian axis.

3) On those sections gaussian circles are fitted.

3) The center points of those circles are then connected to the so called extracted axis which, according to the used method, includes also form deviation.

4) Then the software uses Chebychev to compute the smallest cylinder, restricted by the TED's for the location, that encloses the extracted points. The diameter of this cylinder is then visible in the result label as deviation.

All of these theoretical backgrounds are covered by the AUKOM GD&T training where all the methods that are used by the software packages are explained in detail.

Hope this helps

Best regards

Dinko

[[pe...]]

Dear Dinko

thanks again for your further explanation. i really need to learn more on GD&T.

Have a nice day!