A great discussion you two have going. First, I want to give a plug for Alex's new “ISO Geometrical Tolerancing Reference Guide” that will be published soon. It will be the most exhaustive coverage of the ISO standards with lots of great examples to help you understand the standards. It will answer all of your ISO questions.
I must first preface my answer with the statement that my answer is dependent on the information provided in your question. Several factors that could affect my answer are:
1. The drawing release date, this affects which version of the ISO standard applies. ISO 8015 was revised in 1985, and ISO 2768 was revised in 1989.
2. Additional standards referenced on the drawing, such as ISO 1101. This standard was revised in 2004. The drawing should reference this standard also and its revision date to avoid any confusion.
3. The standards listed above also reference other ISO standards that affect drawing interpretation. There are approximately 20 other ISO standards invoked by the three standards mentioned above.
Nothing in these three ISO standards, or their related standards, states that we should interpret the four diameters as one. The “4x” in front of the size dimensions means there are four diameters with the same nominal size and tolerance. You must inspect each diameter separately for size. Therefore, you will have four separate size measurements shown in your inspection report.
The coaxiality tolerance applied to the size dimensions means there is a cylindrical tolerance zone of diameter 0.1 centered about the A-B datum axis. Whether you interpret coaxiality as a single tolerance zone for all four diameters or as four separate tolerance zones does not affect how coaxiality is measured. This is because ISO 1001:2004 states that coaxiality applies to the extracted actual median line. This is an imperfect line (not straight) passing through all the median (center) points of each cross section. Therefore, practically speaking, the center points of each cross section must be in the tolerance zone. You will have to measure at least four cross sections to know if the center points of each diameter lies in the tolerance zone. If the diameters were longer, you would need several cross sections for each one.
Interpreting the four diameters as one would only affect the measurement report. If interpreted as one diameter, the measurement report could show coaxiality only once. This is not a good practice because if a part failed coaxiality and you reported it just once, you would not know where it is out. By reporting coaxiality four times, if three diameters were good and one was out, you will know which one failed.
If the drawing referenced ASME Y14.5-2009 and used the CF modifier, then the four diameters become one feature of size. This is significant because Rule #1 (envelope principle) would apply to the full length of the four diameters. This means the size tolerance controls the straightness of the axis established by all four diameters for their full length. There is no equivalent symbol in ISO. Even if ISO had an equivalent symbol, the envelope principle would not apply because ISO 8015 invokes the independency principle. This means that the size and form are independent. Size is only a two-point measurement at each cross section. This means the straightness tolerance may exceed the size tolerance and the drawing must specify the straightness tolerance. To invoke the envelope principle in ISO, an encircled “E” symbol is placed next to the size dimension or a capital letter “E” suffix is shown after ISO 2768, (i.e. ISO 2768:1989-mK-E). However, the envelope principle will apply individually to each diameter.
Referencing ASME Y14.5-2009 and using the CF symbol also affects how coaxiality is measured. With the CF symbol, the axis of the actual mating envelope established by all four diameters must lie in the tolerance zone. Without the CF symbol, the axis of the actual mating envelope of each individual diameter must be in the tolerance zone. If the diameters were significantly off center and skewed and two diameters were diametrically opposite the other two, then the individual axes may fail, but the one axis established from all four may pass.
There is a CZ (Common Zone) symbol in ISO used with geometric tolerances applied to multiple surfaces when you want all the surfaces to be in one common zone. ISO 1101 shows it placed inside a flatness tolerance frame applied to multiple co-planar surfaces. It is equivalent to profile of a surface applied to multiple co-planar surfaces in the ASME standard. However, using CZ with coaxiality will not affect the interpretation either because, as stated above, whether you have one zone or four the measurement is the same per ISO.
If you invoke the envelope principle in ISO and specify a zero straightness tolerance at MMC (MMR in ISO) with the CZ modifier on the “4x” diameters, then you will have an interpretation similar to ASME with the CF symbol applied to the size dimension and Rule#1. However, this will only control the straightness of the four diameters within one tolerance zone, but still does not affect the measurement of the coaxiality tolerance.
I hope this is helpful. Check our website often, so you will know when Alex’s new ISO reference guide is available. If you need further assistance with this, feel free to contact us directly.
ASME GDTP Senior Level S-0488
Dimensional Engineering Mentor
Effective Training, Inc.
2118 S. Wayne Rd.
Westland, MI. firstname.lastname@example.org://www.etinews.com
Last edited by RoyC on Fri Aug 14, 2009 10:27 am, edited 1 time in total.