There doesn't seem to be significant changes in the standard, you have three new terms for datums and some stronger wording. Here's what the forward says.
This issue is a revision of ASME Y14.5M-1994, Dimensioning and Tolerancing. The main object for this revision has
been to rearrange the material to better direct the thought process of the user when applying Geometric Dimensioning
and Tolerancing. The subject matter of Sections 1 through 4 remains the same as in the previous revision. Sections 5
and 6 were formerly titled "Tolerances of Location" and "Tolerances of Form, Profile, Orientation, and Runout." The
new order following Section 4, Datums, is Section 5, Tolerances of Form; Section 6, Tolerances of Orientation; Section 7,
Tolerances of Location; Section 8, Tolerances of Profile; and Section 9, Tolerances of Runout. When applying GD&T the
first consideration is to establish a datum reference frame based on the function of the part in the assembly with its mating
parts. After the datum reference frame is established, the form of the primary datum feature is controlled, followed
by the orientation and/or location of the secondary and tertiary datum features. After the datum features are related
relative to each other, the remaining features are controlled for orientation and location relative to the datum reference
framework. Further rearrangement has occurred within each section so that the basic concepts are presented first and
then the material builds to the more complex. The subcommittee believes this will aid the user of the Standard to better
understand the subject of Dimensioning and Tolerancing.
Three new terms that are introduced are used only with datums. The terms are "maximum material boundary
(MMB)," "least material boundary (LMB)," and "regardless of material boundary (RMB)." These terms better describe
that there is a boundary defined when applying datums. MMB and LMB may be a maximum material or least material
boundary, respectively, or the applicable virtual condition. The MMB would be an actual maximum material boundary
if the tolerance (location or orientation) for that datum feature was zero at MMC. The LMB would be an actual least
material boundary if the tolerance (location or orientation) for that datum feature was zero at LMC. In the case of a feature
of size as a primary datum feature, the MMB or LMB would be the actual maximum or least material boundary if
the form of the feature of size was controlled by Rule #1, or a zero at MMC or LMC straightness of the axis or flatness of
the center plane was applied. RMB indicates that the datum features apply at any boundary based on the actual size of
the feature and any geometric tolerance applied that together generate a unique boundary.
Since many major industries are becoming more global, resulting in the decentralization of design and manufacturing,
it is even more important that the design more precisely state the functional requirements. To accomplish this
it is becoming increasingly important that the use of geometric and dimensioning (GD&T) replace the former limit
dimensioning for form, orientation, location, and profile of part features. This revision contains paragraphs that give
a stronger admonition than in the past that the fully defined drawing should be dimensioned using GD&T with limit
dimensioning reserved primarily for the size dimensions for features of size. Additionally, recognizing the need to
automate the design, analysis, and measurement processes, and reduce the number of "view dependent tolerances,"
additional symbology has been introduced for some more common tolerancing practices.
David Haigh
