What's New in ASME Y14.5 2018
The definitive guide to the latest Geometric Dimensioning and Tolerancing standard.
Why the Update?
The 2018 revision is not just an update; it’s a refinement for the digital age. With a focus on Model-Based Definition (MBD) and clearer definitions, it bridges the gap between design and metrology.
Deep Dive: The 2018 Standard
The Problem (2009 & Earlier)
When a physical part has an imperfection (like a convex surface) that is used as a datum, it can “rock” on a surface plate. The 2009 standard allowed for a “Candidate Datum Set,” meaning there were multiple valid ways to stabilize the part. This led to different inspection results depending on how the inspector set it up.
The 2018 Solution
The new standard mandates a single solution: the part must be adjusted to minimize the separation between the feature and the True Geometric Counterpart. This is mathematically defined as “Constrained Least Squares.”
Real-World Impact
Imagine a table with one short leg. In 2009, you could measure it while it leaned left OR right. In 2018, there is a specific mathematical algorithm that determines exactly how that table sits, ensuring that a CMM in Tokyo measures the exact same datum reference frame as a CMM in New York.
Why were they removed?
Concentricity and Symmetry controlled the “derived median points” of a feature. To verify this, an inspector had to map opposing points of a feature and calculate their midpoints. This is:
- Extremely time-consuming for CMMs.
- Often confused with Axis-to-Axis control (Position).
- Rarely functionally necessary.
What to use instead?
Position (⌖), Runout (↗), or Profile (⌓). These symbols control the surface or axis directly and are much easier to verify reliably.
- Previously, Profile of a Surface controlled Size, Form, Orientation, and Location simultaneously. But what if you want a shape to be perfect, but you don’t care how big it is?
The Dynamic Profile modifier (a triangle symbol) allows the tolerance zone to expand or contract. The offset between the boundaries remains fixed (controlling form), but the size of the boundaries is variable.
Example Application
Think of a hexagonal key for a bolt. You need the hexagon to be a perfect hexagon so it fits the bolt head (Form), but you might have a looser tolerance on whether the key is slightly larger or smaller overall (Size). Dynamic Profile handles this “Hexagonicity” perfectly.
The “CF” symbol allows multiple surfaces (like two faces separated by a slot) to be treated as a single feature of size. In 2018, the rules were clarified:
- If you use CF, you do NOT need to specify the number of places (e.g., “2X”). The symbol implies they act as one unit.
- If you do NOT use CF, you must specify “nX” to indicate they are separate features sharing a dimension.
In 2009, you could indicate an unequal profile tolerance (e.g., more material allowed on the outside) by drawing a graphical “phantom line” offset from the surface. This was visually intuitive for humans but terrible for CAD/CAM software.
2018 Update: The graphical method is removed. You MUST use the Ⓤ modifier in the Feature Control Frame to specify the amount of tolerance zone that adds material.