DML and Fig. 1-1 ASME Y14.5-2009

[Burunduk]

Does anyone know what relevance does the Derived Median Line have to fig. 1-1 where it is shown? Specifically, what does it have to do with the other concepts this figure illustrates - the Actual Mating Envelope and Actual Local Size?

On the other hand, the paragraph that defines DML - 1.3.31, doesn't point to that figure, and the figure doesn't seem to illustrate the content of that paragraph other than showing a curved line in the "middle" of the feature (provides no useful information for the DML definition as far as I can tell).

 

[greenimi]

Just my guess: To emphasize that per Y14.5 standard (2009) (but they kept it also in 2018) DML must be refinement of the orientation ---5.4.1.2."Where the straightness tolerance is used in conjunction with an orientation tolerance or position tolerance value, the specified straightness tolerance value shall not be greater than the specified orientation or position tolerance value."

That is regardless if geometrically make sense or not.

 

[pmarc]

Not that I know why they decided to do this, but one of the benefits of having feature DML and axis shown together in one figure is easy comparison of the nature of their shapes - for a real imperfect part DML is never straight, axis always is. From there this can be used to explain why things like 'straightness tolerance of an axis' and 'flatness tolerance of a center plane' don't really exist in Y14.5, and that the nomenclature to use is 'DML straightness tolerance' and 'DMP flatness tolerance'.

 

[chez311]

Y14.5 describes measurement of "actual local size" which is included in Y14.5-2009 fig 1-1 and is approximately tied to the DML (the figure would suggest possibly that they are directly linked though).

Y14.5.1-1994 defines the measurement of actual value of size by only the swept spheres method. The Y14.5.1-20xx draft includes the swept spheres as the de-facto definition for size and conformance to size limits, however it includes methods to determine "actual local size" by two different methods (opposing points and circular elements) with the caveat that conformance to "actual local size" is neither required nor guarantees conformance to the swept spheres definition.

Per the Y14.5.1-20xx draft the "actual local size" is derived using a "local size spine" - which is described as a smoothed DML.

 

[pmarc]

Fig. 1-1 is about related an unrelated actual mating envelopes. I guess one could ask pretty much the same question about actual local size as Burunduk asked about DML.

 

[chez311]

pmarc,

Section 1.3.54 for the definition of actual local size refers to fig 1-1. As to why this is not reflected in the title of the figure I cannot say.

I should say I agree with your previous assertion about the use of fig 1-1 as an illustrative example of the differences between the concepts and nomenclature of DML (imperfect) and axis (perfect).

 

[Burunduk]

Thank you everyone for the input.
chez311, the "local size spine" from the new draft of Y14.5.1, you say it's described as a "smoothed DML" - does it mean that for a precise determination of ALS it will be needed to derive a median line first? (and "smooth" it?)
Where I see this might be meaningful is, for example, an external cylindrical part which is specified with a very accurate size tolerance and a relatively generous derived median line straightness, overriding rule #1. Since the form of the part is less controlled than per Rule #1, the exact way the cross sections for ALS are taken might be influential for the measurement.

 

[chez311]

Burunduk,

That section is left somewhat open. It says a well-behaved DML "possibly obtained" through smoothing and it mentions a possible method but unfortunately what method they mean is rendered unreadable DRAFT watermark. It mentions a requirement for the local size spine to be tangent-continuous.

All in all I'm not sure why they included the reference to Actual Local Size, maybe for completeness or perhaps so it can be utilized in reference to other tolerances such as AVG diameter. As far as conformance to the limits of size the swept spheres is still the final word.

 

[Burunduk]

All in all I'm not sure why they included the reference to Actual Local Size, maybe for completeness or perhaps so it can be utilized in reference to other tolerances such as AVG diameter. As far as conformance to the limits of size the swept spheres is still the final word.

I think that a more precise definition of actual local size could be useful, as in the industry, 2-point checks are still the main (if not the exclusive) way to verify conformance to the LMC limit. Such definition could be based on the geometrical principles of measurements made with devices such as calipers and micrometers.