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ASME vs. ISO 10

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Scorch

Scorch

Mechanical
Feb 13, 2002
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Does anybody have a cheat sheet on what the differences are between the two standards? Doing a search for the differences give me many opportunities to attend classes to learn what the differences are. I just need a quick and dirty list of the major differences, any help would be appreciated.

Scorch

Don't worry about people stealing your ideas. If your ideas are any good, you'll have to ram them down people's throats.
--Howard Aiken, IBM engineer

 
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One of the biggest differences I found between ASTM-1994 and ISO is Concentricity and Symmetry. The only similarity is the symbol that used on the drawing. This is just one of the many existing differences that has cause many issues for me, in addition to the envelope principle already stated by KENAT.

According to the GD&T Pocket Guide (First Edition) by Al Neumann, Scott Neumann Copyright 1995 pg 60
"At present the ISO standards do not recognize the unique interpretation of concentricity and symmetry as defined in the ASME Y14.5M 1994 standard. The concentricity and symmetry characteristics in ISO have the same interpretations as the position characterist in the ASME Y14.5M 1994 standard"

If you are going to be using the ISO standards, I suggest buying and reviewing the following ISO Standards Handbooks:




A cheat sheet is no substitution for actually having the standards you will be referencing in your product specifications.
 
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Also, I have never found composite profile tolerance shown, stated, or explained anywhere in the ISO specifications for geometrical tolerances. I think this is a very important tolerancing tool that they are missing.
 
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SeasonLee,

That was exactly what I was looking for, a good comparison of the two standards. Thanks to everybody else for your responses.

Scorch

Don't worry about people stealing your ideas. If your ideas are any good, you'll have to ram them down people's throats.
--Howard Aiken, IBM engineer

 
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Recently I read a book mentioned the term MMR / LMR (Maximum / Least Material Requirement), after Google searching I understood it was used by ISO, is it also a difference between the two systems ISO and ASTM? Expect someone who can give more detailed information about this term.

Thanks for all inputs

SeasonLee
 
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MMR and LMR in ISO have the same meaning as MMC and LMC in the ASME Y14.5M-1994 standard. There are many changes that have been made to the ISO standards lately. My observation is that the ISO standards are becoming very metrology based to make it easy for CMMs to inspect parts. The Y14.5 standard is based on design intent to assure parts will function properly.
 
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Scorch,

The ASME Y14.5M standard is one standard that is modified once every 10 years plus or minus. Under the ASME standard, drawings are seen as individual legal documents.

The ISO standard is not ONE standard...it is a group of standards...some of which seem to conflict. Drawings are not seen as legal documents and are permitted to contain items such as manufacturing notes that are not allowed under the ASME standard.

As for callouts, the trend is to merge the two together...for example the ASME 94 picked up the ISO Datum Feature identifier. And I've been told that the next ASME release will continue that trend.

Michael
 
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ASME Y14.5M-(2009) may surprize many in how little it progressed towards unification with ISO. As stated, ISO is not really a standard, so it is really a degradation of ASME Y14.5 to try to conform to it beyond pulling in much of the symbology, which was already done in ASME Y14.5-1994.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
 
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ISO and ASME Y14.5 were getting closer together until about 2004 when a revised ISO 1101 came out. ISO is very metrology based, especially with regards to CMMs. They like to talk in terms of extracted features. Y14.5 focuses on design intent to assure that parts assemble and work. Y14.5 talks in terms of mating surfaces and boundaries of features which affest tolerance analysis.
 
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To expand on Don's posts, ISO guides you to use use a mathematical equivalent of the feature (plane, cylinder, cone, etc.) established by an algorithm processing the limited number of data points. This inevitably results in a "CMM surface" or "generated surface" which is within the material of the workpiece.

So, for a cylindrical hole, you select a probing strategy and generate your points which are fed into an algorithm; the result is a generated surface which is larger than the maximum inscribed cylinder that would fit within the hole. The mating boss would be similarly probed to establish an equivalent generated surface, which would be smaller than the diameter of the minimum circumscribed cylinder. To simplify, the hole acts larger than it actually is, and the pin acts smaller than it actually is ... the actual pin which is supposed to be able to fit according to the inspection results may not fit into the actual hole.


ASME considers a mechanical mating situation where the high points of mating parts will interact. So for our example you find the largest perfect cylinder (gage pin) that will fit within a cylindrical hole (specified at RFS), and use that cylinder's axis and size as representing the hole. The mating boss will be considered as equivalent to the smallest circumscribed perfect cylinder's axis and diameter. The ASME method, by considering the worst case, ensures that actual parts will fit together when the inspection data says they should.

Now those that have spent too much time thinking this over, recognize the reality that the ASME model isn't completely accurate either. Have you ever had the three highest points on a plate actually meet the three highest points on another plate? Theoretically, but not practically. Inevitably some of the high points on one plate will slip into the low points on the mating part ... so the mating planes for each is actually going to be within the material surface anyway. So what's the difference? The ASME model introduces a comparatively minor error and still makes sure that parts will fit. ISO introduces potentially significant errors in the datuming and individual feature inspections, which accumulate with the potential that accepted parts don't fit.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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