ASME Y14.5 vs. ISO; concept of applying dimensions

[dtmbiz]

ASME Y14.5 vs. ISO

I was recently informed that the philosophy of applying dimension according to ASME Y14.4m is completely opposite from the ISO version.

In ASME dimensions are applied to component features to define function and mating requirements, with “noted” process and mfg information as necessary .

I was informed yesterday that ISO dimensions are applied for inspection, mfg, and process of the component, while function takes a back seat.
That ISO is a completely different "mind set".

I dont have access to the standard.

Is anyone familiar with enough ISO to say what I was told about ISO is true?


Thank you

 

[CheckerHater]

Have you been told that by the guy selling ASME training / reference materials?

 

[CheckerHater]

Sorry, just couldn't resist.

On the more serious note, from “The ISO Geometrical Product Specification Handbook” by Dr. Henrik S. Nielsen, Chair of ISO technical committee ISO/TC 213

Chapter 1.1 The GPS philosophy:
“The basic ideas behind GPS are laid out in ISO/TS 17450-2 and are described below.
The most basic idea of GPS is that the function of a component can be controlled by one more GPS specifications on the drawing. This means that the designer can use the GPS language to control the function of the component by expressing requirements for its geometry in the drawing.”

 

[dtmbiz]

CheckerHater

Thank you for commenting.

The person is a GDT-SP (I am told) and checker.

I really appreciate the book name and quote from it.

Thx again

 

[greenimi]

"Geometrical Dimensioning and Tolerancing for Design, Manufacturing and Inspection"
"A Handbook for Geometrical Product Specification using ISO and ASME Standards
Second edition
Georg Henzold"

16 _______________________
General Geometrical
Tolerances

"16.2 Concept of general tolerances
Before applying general tolerances, their concept must be agreed upon. This concept
has been developed in the International Organization for Standardization (ISO), as follows
(ISO 2768):
(a) Each feature requires limits for its deviations determined by its function.

 

[CheckerHater]

@dtmbiz: You are welcome and sorry for the typo, it's "one or more GPS specification"

@greenimi: Also from ISO 2768: "Exceeding the general tolerance should lead to a rejection of the work piece only if the function is impaired"
The document says that function is the boss, but for some reason it drives ASME people mad

 

[pmarc]

dtmbiz,
The situation you are facing with is unfortunately quite common. There are many folks extremely knowledgeable in Y14.5 (GDTP Seniors, authors of books) who keep claiming exactly what you wrote, that ISO GPS specifications/standards mainly focus on inspection and manufacturing, and place function in a second row. This is false myth.

CH and greenimi already offered some quotes. Here is another one taken from ISO 1101:2012 - one of the most important documents for entire ISO GPS system:
"4. Basic concepts
4.1 Geometrical tolerances shall be specified in accordance with functional requirements. Manufacturing and inspection requirements can also influence geometrical tolerancing.

To me this clearly means that function is the main driver when it comes to geometric tolerancing. Manufacturing and inspection aspects are important and in some cases can/will influence dimensioning schemes, but for sure are not number one.

 

[CheckerHater]

Pmarc brings in important aspect: both systems are function-based, but differ in the way they treat manufacturing and inspection.
It looks to me like Y14.5 is trying too hard to distance itself from them, while ISO treats them with more respect as important considerations.
I personally was always leaning towards “real life” approach that one cannot be proficient in GD&T without proper understanding how it will be used on the shop floor.

 

[KENAT]

dtmbiz, while I'd say it sounds like the guy you were talking to was perhaps exaggerating a bit I'd say there is some truth to it.

For instance, iso 2768 implicitly does this by being essentially a 'process capability' based tolerance system.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[CheckerHater]

For instance, iso 2768 implicitly does this by being essentially a 'process capability' based tolerance system.

That's a misconception. ISO 2768 is no more "process capability" than ISO 286.

The only difference is ISO 286 has 20 "tolerance grades" and ISO 2768 has 4 "tolerance classes"

They are based on the same formula and are generally compatible:

ISO 2768 vs. ISO 286
fine = IT11
medium = IT13
coarse = IT15
very coarse = IT17

 

[KENAT]

ISO 2768 is clearly based on some supposed process capability, not sure how there can be any argument on that.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[dtmbiz]

The particular component is an airfoil (blade) that has a complex shape, however the outer platforms at
tip and hub are mounting features with more common geometry that actually mate to other components.
The current drawing is for a European company. The airfoil is forged, chem. etched and then machined.

The previous drawings use “gauge points” on the drawing relative to forging requirements; delineated with Target symbol balloons.
There are duplicate points in some cases. For example, notes that state F6 is coincident with point “AP”.
F6 is a forging target and then gets chem. etched away , which is then referred to as point “AP”.
Target points D1, D2, D3, E4, E5, F6 are delineated in the same views of Datums D, E, and F which
are not even the same feature!!! The Target points are on the airfoil surface used for the forging; while
Datums D, E, F are entirely different machined platform features (tip and hub).

Asked to at least change the target point letters to something other than the machined Datum letters and
it’ an emphatic no! The manufacturer has these letters embedded in processing sheets which they refuse to change.

It’s a mess. Very confusing to interpret the drawing.

They also use a combination of basic dimensions and “gauge” dimensions. The word “gauge” is text with the dimension.
Their standard defines ‘gauge” dims as what the definition is for ASME definition for a basic dimension. (theoretical exact…)
Their standard definition for a basic dimension is:



To justify the chaos of finding drawing intent is covered by what they claim as a ISO drawing that is supposed to
Be by definition a component dimensioned for process, mfg, and inspection requirements.

Blame it on ISO is their cover story.

Oihvee!!!

 

[CheckerHater]

Amen to that!