Geometric Dimensioning and Tolerancing (GD&T) Symbols Explained

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 describes 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.

All Around Symbol:

This indicates that a tolerance is applied to all surfaces around the part.

All Over Specification:

As well as surface tolerance noted above, this symbol specifies that the tolerance all over on the field of the drawing.

All Around This Side of Parting Line:

To apply a requirement to all features all around one side of a parting line. This symbol will be indicated on the leader line.

All Over This Side of Parting Line:

To apply a requirement to all features all over one side of a parting line. This symbol will be indicated on the leader line.

Angularity:

This is the condition of a surface, axis, or centerplane, which is at a specified angle from a datum plane or axis.

Arc Length:

This symbol will be placed above a dimension and indicates that a dimension is an arc length measured on a curved outline.

Basic Dimension:

This symbol is used to describe the exact size, orientation, profile, or location of a feature. The basic dimension will always be associated with a feature control frame or datum target.

Between:

This indicates that a profile tolerance applies to several contiguous features and letters may designate where the profile tolerance begins and ends. These letters are referenced using the between symbol (post 1994), or the word “between” (pre 1994).

Concentricity:

This describes a condition in which two or more features, in any combination, have a common axis.

Conical Taper:

This symbol is always shown with the vertical leg to the left, and indicates the taper for conical tapers.

Continuous Feature:

This can be in either symbol or note form, and is used to identify a group of two or more features of size, where there is a requirement that they be treated geometrically as a single feature of size.

Controlled Radius:

This creates a tolerance zone defined by two arcs (min max radii) that are tangent to the adjacent surfaces. Where a controlled radius is specified, the part contour within the crescent-shaped tolerance zone must be a fair curve without flats or reversals. Also, radii taken at all points on the part contour shall neither be smaller than the specified minimum limit nor larger than the maximum limit.

Counterbore/Spotface:

This symbol is used to simply indicate a spotface or a counterbore. This symbol will precede the dimension of the spotface or counterbore, with no space.

Cylindricity:

This symbol describes a condition of a surface of revolution in which all points of a surface are equidistant from a common axis.

Datum Feature:

This is the actual component feature used to establish a datum.

Datum Target

This is a specified line, point, or area on a part that is used to establish the Datum Reference Plane for manufacturing and inspection operations.

Depth/Deep:

This symbol is used to indicate that a dimension applies to the depth of a feature. This symbol will precede the depth value with no space in between.

Diameter:

This symbol indicates a circular feature when used on the field of a drawing. If a feature control frame is used then this symbol will indicate that the tolerance is diametrical.

Dimension Origin:

This symbol signifies that the dimension originates from the plane that is established by the shorter surface, and dimensional limits apply to the other surface.

Feature Control Frame:

This is a rectangular box containing the geometric characteristics symbol, and the form, runout, or location tolerance. Datum references and modifiers applicable to the feature or the datums are also contained in this box if necessary.

Flatness:

This is the condition of a surface having all elements in one plane.

Free State Variations:

This is a term used to describe distortion of a part after removal of forces applied during the manufacturing process.

Least Material Condition (LMC):

This implies that condition of a part feature of size wherein it contains the least amount of material, examples, largest hole size, and smallest shaft size. This is also the opposite to Maximum Material Condition.

Independency Symbol:

This is applied to the size dimension in order to invoke the principle of independency to regular features of size and override Rule #1.

Maximum Material Condition (MMC):

This is a condition of a part feature wherein it contains the maximum amount of material within the stated size limits. That is; minimum hole size and maximum shaft size.

Moveable Datum Targets:

This symbol may be used to indicate movement of the datum target datum feature simulator.

Number of Places:

The ‘X’ is used along with a value to indicate the number of times a dimension or feature is repeated on the drawing.

Parallelism:

This is the condition of a surface, line, or axis, which is equidistant at all points from a datum plane or axis.

Parting Lines:

These are depicted on forging/moulded/casting part drawings as a phantom extending beyond the part in applicable views, with the parting line symbol added.

Position Tolerance:

This defines a zone within which the axis or centre plane of a feature is permitted to vary from true position.

Profile of a Line:

This is the condition permitting a uniform amount of profile variation, either unilaterally or bilaterally, along a line element of a feature.

Profile of a Surface:

This is the condition permitting a uniform amount of profile variation, either unilaterally or bilaterally on a surface.

Projected Tolerance Zone:

This applies to a hole in which a pin, stud, or screw etc is to be inserted. It controls the perpendicularity of the hole to the extent of the projection from the hole and as it relates to the mating part clearance. This tolerance zone extends above the surface of the part to the functional length of the pin, stud, or screw relative to its assembly with the mating part.

Radius:

This creates a zone defined by two arcs (min and max radii). The part surface must lie within this zone.

Reference Dimension:

A dimension usually without a tolerance which is used for informational purposes only. It does not govern production or inspection operations.

Regardless of Feature Size (RFS):

The condition where the tolerance of form, runout, or location must be met irrespective of where the feature lies within its size tolerance.

Roundness:

This describes the condition on a surface of revolution (cone, cylinder, or sphere) where all points of the surface intersected by any plane.

Runout:

This is the composite deviation from the desired form of a part surface of revolution through a full rotation of the part on the datum axis.

Slope:

This symbol is used to indicate slope for flat tapers. This symbol is always shown with the vertical leg to the left.

Spherical Diameter:

This shall precede the tolerance value where the specified tolerance value represents spherical zone. Also, a positional tolerance may be used to control the location of a spherical feature relative to other features of a part.

Spherical Radius:

This precedes the value of a dimension or tolerance.

Square:

This symbol is used to indicate that a single dimension applies to a square shape. The symbol precedes the dimension with no space in between.

Statistical Tolerance:

This is the assigning of tolerances to related components of an assembly based on sound statistics. By applying statistical tolerancing, tolerances of individual components may be increased or clearances between mating parts may be reduced.

Straightness:

A condition where an element of a surface or an axis is a straight line.

Symmetry:

This is the condition in which a feature (or features) is symmetrically disposed about the centre plane of a datum feature.

Tangent Plane:

This indicates that a tangent plane is shown. The symbol is placed in the feature control frame following the stated tolerance.

Target Point:

This indicates where the datum target point is dimensionally located on the direct view of the surface.

Total Runout:

This is the simultaneous composite control of all elements of a surface at all circular and profile measuring positions as the part is rotated through 360 deg.

Datum Translation Symbol:

This symbol indicates that a datum feature simulator is not fixed at its basic location and shall be free to translate.

Unilateral and Uequally Disposed Profile Tolerance:

This symbol indicates that a profile of a surface tolerance is not symmetrical about the true profile. The first value in the feature control frame is the total width of the profile tolerance. The value following the symbol is the amount of the tolerance that is in the direction that would allow additional material to be added to the true profile.