2008 PROPOSED GD&T TIPS
Summary of Changes
by Gary Whitmire
A proposed new Y14.5 Geometric Dimensioning and Tolerancing Standard has just been officially released for comments. If past practice is any guide, this draft will become the new standard in a few months. This standard will replace the 1994 GD&T Standard. It is expected that all organizations which now use the 1994 Standard will eventually switch to this 2008 GD&T Standard. The proposed standard itself is 264 pages long, so following is a summary of a few of the changes from the 1994 Standard:
A. There are new symbols and dimensioning conventions for:
• All Over as used in Profile Control
• Continuous Features
• Independency
• Unequally-disposed Tolerance Zone
• Moveable Datum Target
• Datum Translation
B. There is a guide to the six degrees of freedom for datums
C. Non-perpendicularity of intersecting surfaces is addressed
D. There is a change under Spotface
E. Basic dimensions are clarified
F. There is a clarification on Limit of Size
G. There is a change to Rule 1
H. Tolerance of position has been reformatted
I. Dimensions defined by CAD data have been addressed
J. The effect of datum references in positional tolerances is explained in degrees of freedom
K. Control of coaxial features is addressed
L. There are new boundary methods
M. There are a number of new terms, including:
• Maximum Material Boundary (MMB)
• Least Material Boundary (LMB)
• True Geometric Counterpart (TGC)
• Regardless of Feature Boundary (RMB)
• Degrees of Freedom (DOF)
N. There are twenty (20) new definitions
O. A contoured surface can be a datum
P. There is a new datum symbol placement requirement
Q. There is a new leader line symbol
