Major Update on Geometric Dimensioning and Tolerancing (GD&T)
ASME Standard Y14.5 2009: Overview of the Most Important Changes:


Geometric Dimensioning and Tolerancing (GD&T) Y14.5 is the American standard (ASME) used to specify and interpret dimensional requirements for mechanical components and assemblies. In March 2009, ASME has released a new Y14.5 standard; ASME Y14.5 – 2009, the first major revision to this language in 15 years. In general, the major change to the standard is that users will find it much easier to be used and understand, but the fundamental geometric concepts have not changed.

Generally the major updates in ASME Y14.5 – 2009 Standard can be divided into 3 groups:

1) New Concepts
2) A more complete definition of Datum
3) New Symbols and definition

1) New Concepts

1a) New Names for Material Condition Modifiers & Loss of the RFS Modifier

The Y14.5 2009 standard separation of the Modifiers into two Groups: "Geometric Tolerance Value" and "Material Boundary Modifiers", plus removal of the explicit Modifier (S). The main purpose is to clearly differentiate between the effects and use of these modifiers when applied to tolerance values versus datum features. When the modifiers symbol applied to tolerance values, it will impact the tolerance zone size. The tolerance zone mobility or datum shift will occur when the modifier applied to datum features frames. Please refer to the figure 1.0 below for detail.

Figure 1.0: New Concepts of Material Condition Modifier - ASME Y14-5 (2009)

1b) Actual Mating Envelopes

Actual Mating Envelope (AME) is a theoretically perfect boundary fit to a Feature of Size. For example, a cylindrical AME is fit to a cylindrical Feature of Size.

Figure 2.0: Illustration to clarify the Concept of "Actual Mating Envelope"

The new 2009 standard defines two types of Actual Mating Envelopes: i) Related Actual Mating Envelope and ii) Unrelated Actual Mating Envelope. Under Related Actual Mating Envelope, the part size is first to oriented and/or located to a Datum Reference. Unrelated Actual Mating Envelope is using the AME to oriented and fit to the feature. This may be used to establish a primary Datum and to obtain an axis or center plan to verify conformance to a Geometric Tolerance as well.

Figure 3.0: Difference between i) Related Actual Mating Envelope and
ii) Unrelated Actual Mating Envelope.

1c) Features of Size (FOS)

The 1994 standard defined a feature of size as a single entity. Mean FOS must establish from one cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces required.

In the 2009 standard, the "features of size" had been divided into two categories: i) regular FOS and ii) irregular FOS. The regular FOS remains the definition given above. The irregular FOS is new concept and it is defined as a directly tolerance feature or collection of features that may contain or be contained by an actual mating envelope. Therefore, a grouping of objects can now form a "feature of size".

Figure 4.0: i) Regular Feature of Size and ii) Irregular Feature of Size.

2) Definition of Datum

2a) Datum Feature Simulator

Datum Feature Simulator is the concept update to clarify the establish process of the Datum Reference Frame. The 2009 Standard replaces the concept of the Datum Feature Simulator from "a theoretically perfect" or "physically almost perfect" to the "True Geometric Counterpart" concept for a much more visceral understanding of the Datum Reference Frame establishment process. Now, the new Datum Feature Simulator concept is clearly understood as entity: i) from which we extract Datum, ii) in which we first establish Datum Reference Frames, and iii) with which we transfer Datum Reference Frames to actual parts by matching their Datum Features to their simulators. Example: If the Datum Reference Frame showing " ". Mean, the datum A must first establish by marrying the surface 'A' (Datum features) to the flat surface (simulator) and following by datum B with the same set up method. Please see the figure below for detail.

Figure 5.0: Datum Feature Simulator Concept use to establish the Datum Reference Frame.

2b) Datum

Under standard Y14.-5 2009, it also improve users' understanding of these concepts in order to ensure better implementation in design and application in manufacturing and coordinate metrology measuring (CMM). The new 2009 Standard significantly clarifies the concept of Datum showing for the first time that Datums are extracted from Datum Feature Simulators and not from Datum Features. There are six possible Datums: 1) a single point, 2) a single line, 3) a point on a line, 4) a single plane, 5) a line in a plane and 6) a point on a line in a plane. The new Standard further shows which degrees of freedom each Datum type constrains when acting as a primary Datum. Please see the Table 2.0 below for detail.

Table 1: Six Possibility of Datums and it Degree of Constraint Capability

2c) Composite Feature Control Frames - Increased numbers of Tiers

New 2009 standard permit Composite Feature Control Frames tool up to total of four tiers. The 2009 Standard states the rule also expands the applicability to a third tier referencing only one Datum Feature, and even allows a fourth tier referencing no Datum Feature. (See figure 6.0 below) They provide special function encoding power by limiting the Datum Features in the second and all lower tiers to constraining only rotational degrees of freedom.

Figure 6.0: Example of Feature Control Frames with fourth tier.

2d) Degrees of Constraint Modifiers

There are 6 degrees of Rotational and Translational freedom between two mating parts during an assembly process. A new tool "Degrees of Constraint Modifiers" was introduced by the new 2009 standard. It used to explicit specification of the degrees of freedom on each Datum Feature is permitted to constrain. Degrees of Constraint Modifiers is the bracketed lower case letters [u,v,w,x,y,z], when placed behind a Datum Feature label in a Feature Control Frame. In actual case, these modifiers are highly recommended to be used in place of the second or third tier of a composite Feature Control Frame to make Composite Feature Control Frames easier to understand. Please refer to the Figure 7.0 below for all the 6 symbols of Degrees of Constraint Modifiers and how it constrain the parts movement.

Figure 7.0: Definition of Degree of Constraint Modifier Symbols.

2e) Datum Reference Frame Axis Identifiers:

Datum Reference Frame Axis Identifiers is another new tools introduced by the standard 2009. This tools is use to incorporate the coordinate system axes into the CAD models and drawings by using X, Y and Z axis labels. The benefit for this new tools is able accelerate the decoding of Feature Control Frames by stated clearly the Datum reference Frame axes by allowing the addition of a bracketed list of Datum Features to the label: X[A,B,C] in CAD models or drawings. Nevertheless, these tools also help to standardize the measurement data report when using coordinate measuring machine. Figure 8.0 below are showing the how the Datum Reference Frame Axis Identifiers are stated on engineering 2d drawing.

Figure 8.0: Examples of the Datum Reference Frame Axis Identifiers stated on the drawings.

3) New Symbols

3a) Enhancement on Profile Control

There are three symbols were added to the profile control; i) Unequal Profile Tolerance (Circle U), ii) Nonuniform Profile Zone and iii) All Over. Before the new 2009 standard, a chain line and two arrows were shown to displace the tolerance zone in the applicable area to specify an unequal profile tolerance. In new 2009 standard, a Unequal Profile Tolerance- Circle U symbol follows the tolerancedtolerance value in the profile feature-control frame is introduced to replace the graphical sketching. The second new symbolssymbol that was introduced in the 2009 standard is Nonuniform profile zone. This new symbol is the option of creating a "non-uniform" tolerance zone for either of the two profile . The third enhancement on profile control is to providesprovide a symbol as a replacement for the note "ALL OVER". However, the note "ALL OVER" is still permitted.

3b) Enhancement on Datum

There are two new symbols were added and impacting the datum concept. First, a new datum feature called "Translation Modifier" was added to allow the simulator to translate from its basic location. This symbol follows a datum feature referenced in a feature-control frame. The datum-feature is required along with the translation modifier to clarify the process of establishing a datums and datum-reference frames from datum features. It is also able to clarify an establishment of a secondary or tertiary datum relative to a higher precedence of datum axis.

The second new symbol added is "Movable Datum Target". This symbol is a standard to resembles the concept of the translation modifier, where the simulator translates to make contact with the datum feature. It is used to explicitly specify a particular Datum Target. Its associated Simulator are not only free to move, but required to move when the part is inserted into the simulator set, and also the direction in which it must move in order to eliminate play between the part and the simulator.

3c) New Symbols Impacting Tolerance Zone (Size Control)

Under new 2009 standard, a new graphic symbol is provided to override the Envelope Rule and eliminate the need for a note. In the past, the drawing must specify the words "PERFECT FORM AT MMC NOT REQD" to the size control in order to say so. The addition of the "Independency modifier" (I) is a much more efficient way to accomplish the same purpose.

Continuous Feature Modifier; "<CF>" is a new symbol to explicitly turn what might be individual features into a single (continuous) feature. When applied to an interrupted collection of Features of Size, the "Continuous Feature" modifier imposes the Envelope Rule on the entire set as a group.

4) Counterbore/Spotface Symbol This new symbol use to indicate a counterbore or a spotface. The symbol precedes the dimension of the counterbore or spotface, with no space. Please refer to the Table 2 and Figure 9.0 below for the summary of all the new symbols available on ASME Standard Y14.5 2009.

Table 2: All the New Symbols on the ASME Standard Y14.5 (2009)

Figure 9.0: Examples of the New Symbols -ASME Standard Y14.5 (2009).

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