In engineering and manufacturing, the Geometric Dimensioning and Tolerancing (GD&T) tolerance symbols or system defines and communicate the design intent about a part’s geometry and tolerances. On engineering drawings, GD&T uses symbols to express these specifications. 

What is GD&T?

• Geometric Dimensioning and Tolerancing is an International language that uses symbols and standards.
• GD&T symbols are used in drawings to describe a part accurately.
• The GD&T language comprises of symbols, dimensions, tolerances, rules, and definitions for defining a geometry of part features.
• Geometric Dimensioning and Tolerancing were developed and started by ISO organizations and ASME – American Society of Mechanical Engineers.
• Geometric Dimensioning and Tolerancing, or GD&T, is defined by ISO 1101 and ASME Y14.5-2009.

History of GD&T 

• Geometric Dimensioning and Tolerancing originated by Stephen Stanley Parker who worked in munition under Britain.
• He developed the concept of position or true position (Tolerance) in 1938.
• Parker later 1956 published his book with the title of Drawing and Dimensions.
• Subsequently, additional ideas have expanded to encompass runout, profile, and more.
• Following World War II, the military adopted GD&T in 1950, which is currently used in many sectors to improve product quality.

GD&T Tolerance Symbols

GD&T Tolerance Symbols are a set of fourteen Symbols used in the language of geometric tolerancing.

The symbols are divided into 5 categories:

1. Form
2. Profile
3. Orientation
4. Runout
5. Location

GD&T Definitions

Understanding the basic definitions of geometric dimensioning and tolerancing is most important for better clarity.

Some important GD&T definitions based on ASME Y14.5M are:

Feature: A general term applied to a physical portion of a part, such as a surface, hole, or slot.

Feature of Size (FOS): It is one cylindrical or spherical surface, or set of two opposed elements, or opposed parallel surfaces, associated with size dimension.

Datum: A theoretically exact plane, point, or axis from which a dimensional measurement is made.

Datum symbol is attached to a plane or size feature that must be contacted for machining and Inspection

The Datum plane, datum axis, and datum centre plane are used to make measurements of a part.

Datum Feature: A part feature that contacts a datum.

Feature Control Frame: It is a rectangular box that is divided into compartments within which the geometric characteristic symbol, tolerance value, modifiers, and datum references are placed.

Free State: The parts must not be restricted during the Inspection. In other words, the free state symbol applies only when the part is otherwise restrained.

LMC- Least Material Condition: That condition where a feature of size contains the least amount of material within the stated limits of size. For example – Maximum hole size and minimum shaft size.

MMC- Maximum Material Condition: That condition where a feature of size contains the maximum amount of material within the stated limits of size. For example – Minimum hole size and maximum shaft size.

RFS- Regardless of Feature Size: This is the default condition for all geometric tolerances. No bonus tolerances are allowed. Functional gages may not be used.

Datum Tangent: A  symbol that describes the shape, size, and location of gage elements that are used to establish datum planes or axes.

Here are a few typical GD&T tolerance symbols and their short definitions:

 

GD&T Straightness

The straightness symbol is used to control the form of a feature so that it remains within two parallel lines.

Flatness

Flatness is used to control the form of a surface, ensuring that it remains within a specified tolerance zone.

GD&T Circularity

Circularity is used to control the roundness of a feature, constraining it within a specified tolerance zone.

GD&T Cylindricity

Cylindricity is used to control the overall form of a cylindrical feature, ensuring that it remains within a specified tolerance zone.

Profile of a line 

The profile of a Line is used to control the form of a feature along a straight path.

GD&T Surface Profile | Profile of a Surface

The profile of a Surface is used to control the form of a surface within a specified tolerance zone.

Surface Profile tolerance specifies a uniform boundary that the surface must lie within. It controls the shape, rather than the size or location, of a surface.

Surface Profile is often used when the surface needs to fit with another surface or when a specific form is critical for functionality.

It can be applied independently to a surface or in conjunction with other GD&T symbols to specify more complex geometric requirements. 

GD&T Angularity

Angularity refers to the permissible deviation of a surface or feature from a specified angular orientation. It quantifies the allowable amount of tilt or inclination that a surface can have concerning a datum reference plane or axis. Angularity can be applied to various features, including surfaces, holes, slots, and axes.

Perpendicularity

Perpendicularity defines the allowable deviation of a surface, axis, or line from a true perpendicular orientation relative to a specified reference datum. In other words, it ensures that a feature is oriented at a 90-degree angle (i.e. perpendicular) to a reference plane or axis within a specified tolerance zone.

Parallelism

Parallelism defines the allowable deviation of a surface, axis, or line from a true parallel orientation relative to a specified reference datum. In simple words, it ensures that two features or surfaces maintain a constant separation distance, remaining parallel to each other within a specified tolerance zone.

RUNOUT

Runout is used to control the location of a circular part features relatives to its axis.

Types of runout

There are two types of runout tolerance:

• Circular Runout
• Total Runout

Runout gd&t | Circular Runout

Circular runout is a three-dimensional tolerance and can apply only to cylindrical parts. Circular runout of part can only be inspected when it is rotating.
The Instrument is placed on the surface of the rotating parts to detect the maximum and minimum readings.

GD&T Total Runout

Total runout is similar to circular runout except that involves tolerance control along the entire length.

Position | True Position 

GD&T True Position tolerance is used to control the location of a feature’s center or axis within a specified tolerance zone.

GD&T Concentricity

Concentricity is used to control the location of a feature’s axis relative to another feature’s axis. It is a three-dimensional tolerance similar to the position.

The shaft is measured in multiple diameters to ensure that they share a common center axis.

GD&T Symmetry

Symmetry tolerance is used to ensure that a feature is symmetrically disposed about a datum or a specified axis. It is much similar to concentricity. The only difference is that it controls the rectangular features and involves two imaginary flat planes.

TOLERANCE ZONES IN GD&T

FEATURE CONTROL FRAME

In GD&T (Geometric dimensioning and tolerancing), feature control frames are required to describe the conditions and tolerances of a geometric control on a part’s feature.

The feature control frame has four parts: 

• GD&T symbol
• Tolerance zone type and dimensions
• Modifiers – such as features of size
• Datum references (if required by the GD&T Symbol)

MODIFIERS IN GD&T

Other GD&T Tolerance Symbols