Making sense of derived median line ( or at least trying to....) 2

[Kedu]

On each individual cross sections, the lines deriving or defining the center points ( directly opposed points found on the actual surface) must pass thru a common center or not necessarily?

I do understand the cross section requirement to be normal / perpendicular to the axis of the UAME, but I am questioning the lines to determine those center points?

 

[pmarc]

While I think I know where the idea of the center points derived from lines comes from, this is not how the "center points of all cross sections of the feature" should be understood.

I am saying this based on the fact that 1994 version of Y14.5 referred to ANSI B89.3.1 for determination of the center points. According to that document there is only a single center point in each cross section of the feature. The center point is the center of a circle associated with the actual surface in that cross section.

The problem is that in 2009 edition the reference to B89.3.1 has been removed from the DML definition and as a result of this users have been left with exteremely muddy definition.

 

[greenimi]

Pmarc,

May I ask you if the derived median line definition is muddier than the actual local size one?  Just wondering….


Now seriously, if a single center point exist for each cross section then the original OP question is not applicable, correct ?


Also, could you, please develop a little bit how to get the center of the circle associated with the actual surface in each cross section? How to determine a center of an imperfect section ? minimum circumscribed circle? Maximum circumscribed circle? Least square circle?
How to get its center?

Also, correct me if I am wrong, these center points has little to do with the axis of UAME. Do you see any relationship with the axis of UAME? Again, the center points of the cross sections, and not the cross sections themselves (which according to the OP should be perpendicular to the UAME)

Thank you again

 

[pmarc]

greenimi,
It does not really matter which of the two definitions is muddier - in current form none of them should be in the standard (that is my personal opinion).

Kedu's question is applicable and it is good she/he asked it. The question simply proves the ambiguity of the DML definition.

As for how to establish a center of the circle associated with the actual surface in each cross section, ANSI B89.3.1, "Measurement of out-of-roundness" gives four options:
2.8 Centers for Out-of-Roundness Measurement
The centers of the measured polar profile which may be used to determine the out-of-roundness value when specified are those related to one of the following alternative methods of out-of-roundness assessment:

2.8.1 Minimum Radial Separation (MRS). This center is that for which the radial difference between two concentric circles which just contain the measured polar profile is a minimum1.
2.8.2 Least Squares Center (LSQ). This center is that of a circle from which the sum of the squares of the radial ordinates of the measured polar profile has a minimum value.
2.8.3 Maximum Inscribed Circle (MIC). This center is that of a largest circle that can be inscribed within the measured polar profile2.
2.8.4 Minimum Circumscribed Circle (MCC). This center is that of the smallest circle which will just contain the measured profile3.

1 This is also known as the center for minimum Total Indicator Reading (TIR). The British Standards Institution publication 3730:1964 refers to it as Minimum Zone Center (MZC).
2 This is also known as the plug gage center and is generally used for internal diameters.
3 This is also known as the ring gage center and is generally used for external diameters.

The standard also says that:
The center from which the out-of-roundness value shall be determined unless specified otherwise is the Minimum Radial Separation Center.

I remember I saw in some GD&T materials (I believe in ETI's online course) that their recommendation was to use MIC for internal features of size and MCC for external features of size.

As for a relation between the center points constituting the DML and the axis of UAME, I am not sure there is anything meaningful to say.

 

[Kedu]

Pmarc,

The idea of center points derived from lines comes from this discussion

https://www.eng-tips.com/viewthread.cfm?qid=334394

 

[greenimi]

Thank you pmarc for your comprehensive review.
You improved my understanding of this geometrical callout (which is not very popular / used in my world, but I need to know it nevertheless).

One interesting fact about form controls in general, from four of them (circularity, flatness, cylindricity, straightness) at least three have issues/inaccuracies with/within their definitions. Isn’t it strange?

Circularity: the axis which is intended to be used is not clearly defined. More decisive axis could be indicated. “any plane perpendicular to an axis or spine…..” Wouldn’t you want something more precise? Or maybe even no axis needed at all, in the same way as flatness is defined.

Cylindricity: “all points of the surface are equidistant from a common axis”. As discussed before on this forum, maybe that common axis is not needed and only two coaxial cylinders suffice.
Surface Straightness: from 1994 to 2009; nominal axis replaced with the axis of UAME, which, probably, is a good move to clarify things.

DMLS: as discussed above.

So, to recap, 75% of the form geometrical callouts have issues (only flatness is spared from this infamous and unfortunate group).

 

[Belanger]

greenimi -- you can also include the fourth (flatness) in that "strange" category, because flatness can be used on a feature of size (derived median plane!).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[greenimi]

You are correct J-P.
My mindset was still in 1994 (maybe because that's the current revision we are using). DMPF is not defined in 1994, hence my ignorance. Sorry about that.

 

[pmarc]

Kedu,
Yes, I thought the idea came from the concept of median points used in the concentricity definition. I hope now it is a bit clearer to you how the center points defining DML are/should be understood.

greenimi, J-P,
From my experience I would say that although form controls are relatively easy to comprehend by people learning and applying GD&T, they are quite challenging when it comes to details like provision of robust mathematical definition or proper methods of inspection. I guess most of that is a result of the fact that these controls do not use datum references - this causes a huge headache downstream.

 

[Belanger]

greenimi -- I admit that I was kind of yanking your chain a little.
But c'mon, the 2009 standard was almost 10 years ago

Pmarc -- I agree. Unlike position or profile, where there is a known nomimal, form tolerances become sort of a "best-fit" idea. Compound that with the confusion about derived median line/plane, and it can be tricky from a mathematical viewpoint.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[3DDave]

I like to consider flatness applied to a wedge.