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GD&T(Geometrical Dimensions and Tolerances)

learn what is GD&T .

Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describe nominal geometry and its allowable variation. It tells the manufacturing staff and machines what degree of accuracy and precision is needed on each controlled feature of the part. GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, to define the allowable variation in form and possible size of individual features, and to define the allowable variation between features.

specifications define the nominal, as-modeled or as-intended geometry. One example is a basic dimension.

Tolerancing specifications define the allowable variation for the form and possibly the size of individual features, and the allowable variation in orientation and location between features. Two examples are linear dimensions and feature control frames using a datum reference (both shown above).

There are several standards available worldwide that describe the symbols and define the rules used in GD&T. One such standard is American Society of Mechanical Engineers (ASME) Y14.5. This article is based on that standard, but other standards, such as those from the International Organization for Standardization (ISO), may vary slightly. The Y14.5 standard has the advantage of providing a fairly complete set of standards for GD&T in one document. The ISO standards, in comparison, typically only address a single topic at a time. There are separate standards that provide the details for each of the major symbols and topics below (e.g. position, flatness, profile, etc.).

There are some fundamental rules that need to be applied (these can be found on page 7 of the 2009 edition of the standard):

  • All dimensions must have a tolerance. Every feature on every manufactured part is subject to variation, therefore, the limits of allowable variation must be specified. Plus and minus tolerances may be applied directly to dimensions or applied from a general tolerance block or general note. For basic dimensions, geometric tolerances are indirectly applied in a related Feature Control Frame. The only exceptions are for dimensions marked as minimum, maximum, stock or reference.

  • Dimensions define the nominal geometry and allowable variation. Measurement and scaling of the drawing is not allowed except in certain cases.

  • Engineering drawings define the requirements of finished (complete) parts. Every dimension and tolerance required to define the finished part shall be shown on the drawing. If additional dimensions would be helpful, but are not required, they may be marked as reference.

  • Dimensions should be applied to features and arranged in such a way as to represent the function of the features. Additionally, dimensions should not be subject to more than one interpretation.

  • Descriptions of manufacturing methods should be avoided. The geometry should be described without explicitly defining the method of manufacture.

  • If certain sizes are required during manufacturing but are not required in the final geometry (due to shrinkage or other causes) they should be marked as non-mandatory.

  • All dimension and tolerance should be arranged for maximum readability and should be applied to visible lines in true profiles.

  • When geometry is normally controlled by gage sizes or by code (e.g. stock materials), the dimension(s) shall be included with the Gage or code number in parentheses following or below the dimension.

  • Angles of 90° are assumed when lines (including center lines) are shown at right angles, but no angular dimension is explicitly shown. (This also applies to other orthogonal angles of 0°, 180°, 270°, etc.)

  • Dimensions and tolerances are valid at 20 °C (68 °F) and 101.3 kPa (14.69 psi) unless stated otherwise.

  • Unless explicitly stated, all dimensions and tolerances are only valid when the item is in a free state.

  • Dimensions and tolerances apply to the length, width, and depth of a feature including form variation.

  • Dimensions and tolerances only apply at the level of the drawing where they are specified. It is not mandatory that they apply at other drawing levels, unless the specifications are repeated on the higher level drawing(s).

GD&T Symbols

Symbols or Geometric Characteristics are what most often come to mind when people think about GD&T. There are a total of fourteen GD&T characteristics, and the symbols that represent them are shown in the symbol “cheat sheet” below.

These symbols are placed in the first compartment of a feature control frame and define the type of tolerance that is to be applied to the feature. The characteristics are grouped together into types of tolerance: form, orientation, location, runout, and location of derived median points. The primary use and description of each characteristic is also shown.

GD&T is a feature-based system, and parts are composed of features. Geometric tolerances are applied to features by feature control frames. The most frequently used tolerance categories are form, orientation, and location; therefore, the ten associated symbols are the most utilized of the fourteen total GD&T symbols.

Form tolerances control the “shape” of features and are often used as a refinement of size.

Orientation tolerances control the “tilt” of feature and are always associated with basic angle dimensions, often used as a refinement to location. If applied to surfaces, orientation tolerances also control form.

Location tolerances control location and are always associated with basic linear dimensions. Position locates and orients the median plane or axis of features of size. Profile locates feature surfaces. Profile is the most powerful characteristic of all, and also controls orientation and form.

Feature Control Frame

The feature control frame states the requirements or instructions for the feature to which it is attached. Simply put, the feature control frame controls features. Each feature control frame contains only one message (requirement); if two messages for a feature are necessary, two feature control frames are required.

The first compartment of a feature control frame contains one of the fourteen geometric characteristic symbols. Only one of the symbols can be placed in a feature control frame; if there are two requirements for a feature, there must be two feature control frames or a composite tolerance. The symbol will specify the requirement for the feature, such as, “this feature must be flat,” or “this feature must be positioned.”

The second compartment of a feature control frame contains the total tolerance for the feature. The feature tolerance is always a total tolerance, never a plus/minus value.

If the tolerance is preceded by a diameter symbol (⌀), the tolerance is a diameter or cylindrical shaped zone, as in the position of a hole. If there is no symbol preceding the tolerance, the default tolerance zone shape is parallel planes or a total wide zone, as in the position of a slot or profile of a surface.

Following the feature tolerance in the feature control frame, a material condition modifier, such as MMC or LMC (see Material Condition Modifiers) may be specified if the feature has size, such as a hole. If the feature has size, and no modifier is specified, the default modifier is RFS. If the feature has no size, such as a plane surface, then the modifier is not applicable.

The third and following compartments of a feature control frame contain the datum feature reference(s) if they are required. For example, if a form tolerance, such as flatness or straightness, is specified, then no datum feature reference is allowed. However, if a location tolerance like position is specified, the datum feature references are usually specified.

The alphabetical order of the datum references has no significance—the significance is their order of precedence, reading from left to right as primary, secondary, and tertiary. The primary is the first feature contacted (minimum contact at 3 points), the secondary feature is the second feature contacted (minimum contact at 2 points), and the tertiary is the third feature contacted (minimum contact at 1 point). Contacting the three (3) datum features simultaneously establishes the three (3) mutually perpendicular datum planes or the DRF. The DRF is created by so-called Datum Simulators which are the manufacturing, processing, and inspection equipment, such as surface plate, a collet, a three jaw chuck, a gage pin, etc.

In certain situations, the datum feature modifiers Maximum Material Boundary (MMB) or Least Material Boundary (LMB) may be applied to the datum feature. The default modifier is Regardless of Material Boundary (RMB). Since the datum feature has size (it can get larger and smaller), information is necessary on the size condition of the datum feature to which the datum feature reference applies. The modified condition of the datum feature (MMB, LMB, RMB) defines the size or condition of the datum feature simulaton

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