GD&T Cylinder Position - difference between ASME & Geometry Standard

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Dear GOM Experts

I need to prepare measurements of holes position and to understand how gom functionality works i prepared simple test scenario. And i dont understand obtained results.

Below is view 1 of this scenario. I expected calculated value shown under "CYLINDER_NOM" label shall be == PT_1_DIST * 2

(0.479*2 = 0.958)

but calculated value is 0.946

PT_1_DIST is a longest distance between fitting cylinder axis and nominal cylinder axis.

below is another view of the same scenario :

From my point of view , expected value is 0.958

This is how Position tolerance was set :

 

 

I am obligated to use ASME standard. I put tolerance value == 0.946 mm to have visualisation of area of tolerance only.

 

What i discovered, when i changed standard to "geometry" i received value what i expected for ASME (0.958)

So my question is : does GOM works correctly in this case ?

In GOM documentation, i found for ASME standard GOM will use "maximum inscribed element" for fitting cylinder.

When i tried to use "maximum inscribed element" for this fitting cylinder, it looks i do not have correct result. Below is a screenshot :

I tested this scenario with GOM inspect 2016 and 2019.

I will attach project.

 

conclusions:

1) is in this case calculation of position value (0.946) correct ?

2) if calculated value is correct for ASME standard, what caused the difference comparing to Geometry standard?

3) Maximum inscribed method for fitting cylinder seems to be incorrect, is it a bug ?

 

Best Regards

Adam B.

CYLINDER_1.ginspect

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Hello,

after a short look onto your project I discovered some problems.

1) First the tesselation of your artificial cylinder is too rough. GOM Software takes his computation on points only. Therefore a descent number of points is needed. Therefore please refine your mesh until you get something like this:

Afterwards the maximum inscribed cylinder should delivers the correct result. 

2) And according to ASME Y14.5 for a position tolerance RFS you have to build the axis of envelope element in your case a maximum inscribed cylinder. Afterwards you check if the whole axis is inside the tolerance zone that position is restricted from the used datum system. That means yes, GOM is using the maximum inscribed cylinder because the standard demands it (at least for this case and for the ASME evaluation).
Of course if you have a ISO position tolerance we extract the "middle-point-axis" according to the ISO standards

3) Geometry ignores the requirements of the standards are using the axis of the cylinder. Due to the fact that there is a difference between Best-Fit-Cylinder as in your project and maximum inscribed cylinder, there is also a difference in result. Please have a look into the warning into at tech guide/online-help that state that you may get non-standard conform evaluation if you are using the geometry mode.

 

Hope this helps

Christoph Schult

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After surface refining inscribed fitting cylinder looks correct.

Calculated position value is almost equal to the value i expected. The difference is 0.001 mm.

 

Best Regards.

Adam .B.