Mechanical Drawing

Basics of GD&T : Geometric Dimension and Tolerance

Geometric dimension and tolerance ( GD&T ) is a system to define nominal and allowable variations in part and assembly geometry. They are used along with linear toleranceASME Y14.5-2009 standard has defined GD&T symbols in detail. 

Why GD&T is Required?

To ensure manufactured part quality. It is necessary to communicate design and assembly requirements in drawing. Geometric dimension and tolerance helps in accurately defining part geometry requirements. 

GD&T allows comparatively larger tolerance zone. Therefore part rejection rate and cost also decreases. 

Types of Geometric Dimension and Tolerance

Five types of GD&T tolerance are used to define a part design intent accurately. This includes 14 symbols that controls the features and geometry of a part.

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

Form Control

Foam control geometric tolerance controls the form of the feature of a part. Location and Orientation tolerances can also used to control the foam of a feature. 

Datum reference is not used with form control geometric dimension and tolerance. Form tolerance are further classified in four types.

  1. Straightness
  2. Flatness
  3. Circularity
  4. Cylindricity
Straightness 

Straightness is a type of form control geometric dimension control tolerance in. It controls a condition where all element of a surface or an axis of a feature lies in a straight line.

Straightness Tolerance Representation
Flatness 

Flatness is a type of form control GD&T tolerance. It controls the variations in flat surface, regardless of any datum feature. 

Flatness Tolerance Representation
Circularity / Roundness 
Circularity Tolerance Representation

Circularity controls the roundness of a circular feature in two dimensional tolerance zone. 

  • It’s value is independent of any datum feature.
  • Circularity tolerance value is always less than the diameter dimensional tolerance.
Cylindricity 
Cylindricity Tolerance Representation

Cylindricity tolerance in gd&t controls the roundness of a circular feature in 3-dimensional tolerance zone. 

  • It is independent of any datum feature.
  • Cylindricity tolerance ensures feature of a part is round and straight enough along its axis.

Profile Control

Profile control tolerance in “Geometric Dimension and control Tolerance” defines a uniform boundary around a surface. All points of controlled surface must lie within defined boundary. 

Profile Control Tolerance controls a feature form, size, orientation and location simultaneously. It is further classified in two types.

    Profile Of a Line Control 
    Profile of a Line Control Tolerance Representation

    “Profile of a Line Control” GD&T tolerance creates a 2-dimensional tolerance zone around any line of a feature.

    • It controls size, orientation, location and form of a feature simultaneously.
    • MMC and LMC are not applicable with profile of line control GD&T tolerance.
    • It can be used with or without datum plane.
    Profile Of a Surface Control 
    Profile of a Surface Control Tolerance Representation

    Profile of a Surface Control Tolerance creates a uniform 3D boundary tolerance zone along full length and width of the surface.

    • It controls size, orientation, location and form of a feature simultaneously.
    • MMC and LMC are not applicable with profile of surface control GD&T tolerance.
    • It can be used with or without datum plane.

    Orientation Control

    Orientation Control GD&T tolerance controls the orientation of the part feature with respect to another feature or datum. They can be classified in three categories.

    1. Parallelism
    2. Perpendicularity
    3. Angularity
    Parallelism 
    Parallelism is a type of orientation control tolerance in GD&T. It controls the parallelism between two lines, surfaces or an axis.
    Parallelism Tolerance Representation
    • Parallelism tolerance does not controls the angle of the referenced feature. But it creates a tolerance zone where all points of the feature must lie. 
    • Datum plane is required to control Parallelism Tolerance

    Parallelism tolerance is explained in detail in parallelism article.

    Perpendicularity 

    Perpendicularity tolerance in GD&T is used to control perpendicularity of a surface feature or an axis with respect to datum plane. 

    Penpendicularity Tolerance Representation
    • When perpendicularity is applied over surface, Tolerance zone will be two parallel surfaces/planes/lines perpendicular to datum plane. 
    • Whereas when applied to an axis. Tolerance zone will be a cylinder boundary around a true axis. Axis of referenced feature must lie in this cylinder boundary.
    Angularity 

    Angularity tolerance in GD&T controls the orientation of one feature with respect to datum at specified angle. 

    Angularity tolerance Representation
    • Tolerance zone will be two parallel planes /surfaces in 3D.
    • Angularity tolerance can also be used to control the axis of any feature w.r.t datum plane.
    • Datum Plane is required to define angularity tolerance.

    Location Control

    Location control in “Geometric Dimension and Control Tolerance” defines the deviation of a feature from the actual location. It is further classified in three types.

    1. Position Tolerance
    2. Concentricity
    3. Symmetry

    Concentricity and symmetry controls the center distance of feature whereas position tolerance controls coaxiality of a features.

    Position Tolerance 

    Position tolerance in gd&t controls the variation in the location of a feature from exact true position. It is the total permissible variation in feature location from its exact true position. MMC, LMC, projected tolerance, tangent planes can be used along with position tolerance.

    Concentricity Tolerance Representation

    Concentricity tolerance controls the central axis of a cylinder or sphere with respect to datum plane/axis. In other words, it controls the median points on high precision cylindrical parts such as transmission gears

    Read this article for more details on Concentricity tolerance in detail.

    Symmetry  

    Symmetry tolerance creates a 3-dimensional geometric tolerance zone w.r.t. datum. It controls how much the points between two features may deviate from a specified center plane or axis. It can only be applied to non circular features.

    Symmetry tolerance is similar to concentricity. But it controls rectangular features and involves two imaginary flat planes.

    Runout

    Runout in “geometric dimension and control” controls the variation in a feature when the part is rotated 360° around the datum axis. It also measures the wobbling of a part. Runout can be controlled with two symbols : 

    1. Circular Runout
    2. Total Runout
    Circular Runout 
    Circular Runout Representation

    Circular runout creates a 2-Dimensional circular tolerance zone defined by a datum axis.

    • It controls the total variation in controlled surface, when the part is rotated around the datum true axis.
    • Runout is used to control features of a rotating part such as drill, gears, shafts, axles and machine tool parts.
    Total Runout 
    Total Runout Representation

    Total runout creates a 3-dimensional cylindrical tolerance zone defined by datum axis. 

    It controls the total variation in the reference surface, when the part is rotated around the datum’s true axis.

    Conclusion

    To sum up, Geometric Dimension and tolerance (GD&T) has the advantage of communicating part design and assembly intent in engineering drawing. It also helps in increasing tolerance zone as well. To calculate required tolerance, tolerance stack up calculator can be used.

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