Position in ISO versus position in ASME 2

greenimi:

I looked for comparative books a few year ago and found Alex Krulikowski's reference guide to ISO geometrical tolerancing from (his business which he since sold to SAE). It is a good place to start with ISO.

Certified Sr. GD&T Professional

Well, I am most interested to get the nitty-gritty details and less the high overview these books treat the differences. Also, some of them (books and authors) do not treat the subject as written in the standard, but per the author perspective or interpretation.

pmarc's the guy for this question, but as long as ISO uses the idea of "unrelated actual mating envelope" as the basis for position, it's the same as ASME when dealing with features of size.

However, there is one big difference: ISO permits the position symbol to be used on surfaces, which is not permitted in ASME.

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

Okay. So I am waiting for pmarc to respond, and bombard him with a lot of questions

Another difference: ISO also permits the position symbol can be used on lines as well, which is not allowed in ASME.

Season

Here are the main differences between position tolerances in two systems. I also added a little bit about coaxiality, concentricity and symmetry:

1. Type of features the position tolerance can be applied to:
- ASME - as already said, features of size only;
- ISO - as already said, features of size and nominally flat surfaces.

2. Shape of the tolerance zone when applied to a feature of size
- no significant difference between both standards.

3. Type of derived features controlled by the position tolerance when applied to a feature of size:
- ASME - position tolerance controls location of axis, center plane or center point derived from the unrelated actual mating envelope of the feature. Since by definition axis and center plane are perfect in form, position tolerance does not control any aspect of form of the considered feature of size;
- ISO - position tolerance controls location of extracted median line, extracted median surface or extracted (actual) center of the sphere. Since by definition extracted median line and extracted median surface are imperfect (here is the link to a picture showing how the extracted median line of a cylinder is obtained), position tolerance has control over form of these derived features, thus over certain aspects of form of the feature of size.

4. Different possible interpretations of position tolerance applied to a feature of size:
- ASME - when applied at RFS position tolerance is interpreted in terms of axis of the feature. When applied at MMC and LMC it can be interpreted in terms of axis or in terms of surface (the draft of the future version of Y14.5 standard puts much more emphasis on surface interpretation being the "better" one for position at MMC and LMC).
- ISO - no difference for position tolerance applied at RFS, but when applied at MMC or LMC ISO standards use only the surface interpretation.

5. Coaxiality tolerance:
- ASME - coaxiality is nothing but position tolerance applied to nominally coaxial features of size. There is no extra symbol for that characteristic.
- ISO - coaxiality is also nothing but position tolerance applied to nominally coaxial features of size, but it has a separate symbol (the same as concentricity symbol in ASME).

6. Concentricity tolerance:
- ASME - this geometric characteristic has very special definition (control of feature of size's median points relative to datum axis), so it is never a special case of position.
- ISO - concentricity has the same symbol as coaxiality, but in addition the ACS modifier (Any Cross Section) is associated with the tolerance frame. This characteristic controls relationship between the toleranced feature and the datum feature in each cross section individually.

7. Symmetry tolerance:
- ASME - this geometric characteristic has very special definition (control of feature of size's median points relative to datum plane), so it is never a special case of position.
- ISO - symmetry tolerance has the same symbol as in ASME, but it is just a special case of ISO position applied to width-type features of size that are nominally in line with a datum.

pmarc:

Thanks for the fantastic synopsis. You need to write a book with your comparative knowledge. Might I ask how you came to understand both systems so well - work, academia, etc ...?

Certified Sr. GD&T Professional

Pmarc,
Thank you very much for your extensive comparison between the standards.

I do have some follow up questions and specifics:

For clarity –see print screen below- It is a figure from the standard Y14.5-2009, but resemble very close to some parts we are measuring on the CMM.
Again, just to refresh a little bit, the print/ concept is defined based on the Y14.5-1994 and is measured on a Calypso CMM (which references ISO 1101 and ISO 5459—at least these two ISO standards I can find when I searched in the help section, might be other ones too, but were not revealed at the quick search I conducted)

In order to qualify the parts per the print requirements the part is to be measured and a CMM report to be generated.
The main question for everyone is: how the IDEAL CMM report should look like/ what should contain in order to get as close as possible to the print requirements:

Let’s say we would like to measure the size of the holes Ø 3.30 - 3.32 and holes location. To make the things a little “more complicated” let’s pretend that the location of some holes are modified at MMC (as shown on the print) and some of the holes are RFS.

I will try first and please correct me / feel free to add anything I am missing:
- Size limit shall be measured (2 point measurements). Per ISO, the default condition is LSQ. How to extract this 2 point measurement from the CMM cloud points, I do not know. From a previuos thread: LOCAL SIZE OF AN EXTRACTED CYLINDER, LOCAL DIAMETER OF AN EXTRACTED CYLINDER
distance between two opposite points on the feature, where
— the connection line between the points includes the associated circle centre; and
— the cross-sections are perpendicular to the axis of the associated cylinder obtained from the extracted surface.

- UAME (rule#1) shall be qualified. Maximum circumscribed algorithm/ outer tangential should be used.

- When the size is reported on the CMM report: which size is the one needed to qualify for ASME requirements: local size, global size and which algorithm should be used.

- Location should be measured based on the RAME. Not sure about the CMM algorithm (LSQ, outer tangential, maximum circumscribed circle, etc). As far as I understood there is NO DEFAULT in ISO for this. I am not sure I am correct on this. Most likely not. Also, I guess that the axes of the cylinder are the same regardless of the modifiers used (RFS or MMC)

Let’s start the conversation and if needed break them one by one to be easy to follow.

greenimi, mkcski,

My apologies for late response, but there has been a lot going on in my personal life these days and I simply struggled to find enough time to write a post.

Trying to answer to greenimi's questions first, to qualify the part like this against requirements defined on an ASME drawing following would have to be found/measured using CMM:

1. Unrelated actual mating envelope (UAME):
- UAME is needed to find each hole axis. Once all axes are established, it can be verified if they lie within respective position tolerance zone or not. In reality, by default, many CMMs use LSQ entities (circles or cylinders) to establish the axes. While this is not fully compliant with the theory, it is often a reasonable approach (especially in case of as-produced features for which significant form errors, like straightness or roundness, are unlikely to happen);
- The axis approach can be used to verify both types of position tolerances, RFS and MMC. Although, as you well know, if in conflict the surface interpretation takes precedence, and that is why it is better to verify RAME of a FOS in case of position tolerance specified at MMC basis.

2. Size of UAME:
- This is needed to verify compliance with Rule #1;
- It is also needed if one wants to use axis interpretation of position tolerance at MMC and needs to know how much bonus tolerance is available.

3. Actual local (two-point) size:
- This, in my opinion, is the most problematic part of the job. First of all, the definition of actual local size given in Y14.5 is really muddy (I believe one of the muddiest definitions given in the whole book). The math standard - Y14.5.1 - does not really offer more help, especially from CMM perspective. That is why I am not surprised that it is so popular amongst CMM operators to report actual local size as the size of LSQ cylinder or circle exctracted from the actual feature (even the ISO definition of actual local size size has some weak points, although looks much more robust at first glance).
- With that being said, I am afraid I don't have a solution for checking actual local size per ASME requirements that would be fully in line with the theory offered by Y14.5.

mkcski,
Thank you for kind words.

Work and self-study - this is how I came to understand both systems. Not saying that I understand everything, but I know enough to know where to look for things that I don't know

Pmarc,
Thank you very much for your answer. I really appreciate your time spent writing your valuable opinions and knowledge about this subject.

What surprise me is that I thought that would be a very common subject in US: parts defined based on ASME and measured on CMM (software written based on ISO standard) . Considering the number of answers (not too many), looks like is not (common subject).
That drive me to conclude that people are not very aware about their own measurements or take the numbers received from the CMM as a gospel (CMM is an excellent number generator)---or maybe I am on a wrong forum. Most likely the latter.....

Anyway, thank you again.

greenimi:

There are many discrepancies between the theoretical of/in the Standard and the "practice" of GDT. One of my "favorite" CMM issues is establishing a datum plane from a point cloud of a flat surface. The CMM software will present a best-fit plane whereas a plane through the high-spots (as if it was sitting on a granite plate) should be used.


Certified Sr. GD&T Professional