Does anyone understand "spines" of a regular FoS per ASME Y14.5.1M-1994 (R2012)?

[Tarator]

Hi all,

I am trying to understand "spines" as defined in ASME Y14.5.1M-1994 (R2012).

My question is simple: How do you construct your 2 spines (1 for MMC, and 1 for LMC)?

Without the spines, you can't measure your actual local sizes and find the actual MMC and LMC size, in addition to the actual mating size.

I would appreciate it if anyone could shed some light. Thank you.

 

[axym]

Burunduk,

The references to circular elements aren't in the Size section, they're in the Form section on derived median line straightness at MMC. In 2009 it's in 5.4.1.2, and in 2018 it's 8.4.1.3. The text reads as follows:

"Each circular element of the surface (that is, actual local size) must be within the specified limits of size."

There are also references in the figure for straightness at MMC (Fig. 5-3 in 2009, Fig 8-4 in 2018):

"As each actual local size departs from MMC, an increase in the local diameter of the tolerance cylinder is allowed that is equal to the amount of such departure. Each circular element of the surface shall be within the specified limits of size."

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

I was told that I have a tendency to use other peoples arguments (copy and paste ) from other discussions, but I think here worth it (argument about ALS definition posted by Evan J)

But the wheels on the bus go round and round.....We're all going to go round and round in circles.....

"I'm not sure what will happen with the definition of actual local size - to me, this is one of the most difficult problems in GD&T. There is not even agreement on the meaning of the current definition in Y14.5 applied to cylinders, as "any individual distance at any cross section" is interpreted differently by different people. Some interpret it as a distance extracted from 2 opposed points (as one would measure with a mic), and some interpret it as a diameter extracted from a cross-sectional circular element. Unfortunately, the standard contains text supporting both interpretations and no figures that clarify the meaning (only side views are shown, obscuring what is really happening within a given cross section). So we don't even know for sure whether a cross section of a cylindrical feature has one actual local size or many. The committee is in a difficult position, because it is now impossible to choose one or the other without contradicting past practices in some way. The Y14.5.1M-1994 mathematical definitions standard created a novel definition based on an LMC sphere, but this has been largely ignored in industry (partly because it conflicts with the idea of 2-point opposed diameters, and partly because the mathematical definitions standard was itself largely ignored).

The fact that there are camps favoring different interpretations makes it likely that different types of local size are needed for different applications. The ISO GPS standards define several different types of size, but so far Y14.5 has not embraced this approach."
Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca"

 

[axym]

greenimi,

That looks like something I would write, but I don't remember when it was. It could have been 10 weeks ago or 10 years ago ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Burunduk]

axym,
The problem with circular elements is that they aren't truly circular on a real feature. If the interpretation that each circular element is characterized by a single diameter as a "local size" is adopted, then there should have been a definition how to measure the diameter of an imperfect circle; for example an external feature - is it the diameter of a circumscribed circle on the high points? The inscribed circle on the low points? Some average? Not only does it lack a clear meaning, it also lacks support in the ALS definition because this is clearly not what "any individual distance" from the ALS definition suggests. What about width type features - how to derive a single width per cross-section? The camp that doesn't accept the opposed points interpretation has very little to offer instead.

 

[axym]

Burunduk,

I agree with all of the points you raised. I believe that I've made all of the same comments in Y14.5.1 meetings at some time or another. The circular elements are referred to in several places in the form section, but are not defined or illustrated. How they relate to actual local size is not clear.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

It could have been 10 weeks ago or 10 years ago ;^).

Isn't it sad that nothing has been done for so long?
You could easy triple "your" 10 years and the statement above is still applicable.