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 GD&T 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

It 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 required in this tolerance. Foam tolerance are further classified in four types.

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

Straightness is a type of foam control tolerance in GD&T. It controls a condition where all element of a surface or an axis lies in a straight line. Datum plane is not required to define straightness. LMC and MMC modifiers can be used to define straightness of an axis. It can defined to control the line on surface or feature or an axis. This article explains Straightness Tolerance in detail.

Flatness is a type of foam control. It controls the variations in flat surface, regardless of any datum feature. Flatness value is always less than the dimensional tolerance associated with a part feature. Flatness Tolerance is explained  in detail in this article.

Circularity / Roundness 

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 controls the roundness of a circular feature in 2-dimensional tolerance zone. It is independent of any datum feature. It ensures feature is round and straight enough along its axis.

Profile Control

Profile tolerance defines a uniform boundary around a surface. All points of controlled surface must lie within defined boundary. It controls a feature form, size, orientation and location simultaneously. It is further classified in two types.

    Profile Of a Line Control 

    It makes a 2-dimensional tolerance zone around any line of a feature and controls size, orientation, location and foam simultaneously.

    MMC and LMC are not applicable with profile of line control and can be used with or without datum.

    Profile Of a Surface Control 

    It creates a uniform 3D boundary tolerance zone along full length and width of the surface.

    MMC and LMC are not applicable with profile of line control and can be used with or without datum.

    Orientation Control

    It controls the orientation of the part feature w.r.t. another feature or datum. Orientation tolerances can be classified in three categories.

    1. Parallelism
    2. Perpendicularity
    3. Angularity

    Parallelism is a type of orientation control tolerance in GD&T. It controls parallelism between two lines, surfaces or axis. Parallelism does not controls the angle of the referenced feature. it creates a tolerance zone where all points of the feature must lie. 

    Parallelism tolerance is explained in detail in parallelism article.

    Perpendicularity 

    Perpendicularity is used to control perpendicularity of a surface or an axis with respect to datum. 

    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 controls the orientation of one feature with respect to datum at specified angle. 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.

    Location Control

    Location 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 controls co-axiality of a features.

    Position Tolerance 

    Position tolerance 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 controls the central axis of a cylinder or sphere with respect to datum plane/axis. Or it controls the median points on high precision cylindrical parts. For example transmission gearsThis article explains 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 applied to non circular features.

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

    Runout

    Runout controls the variation in a feature w.r.t. datum 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 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’s 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 makes 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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