Basics of GD&T : Geometric Dimensioning and Tolerances

GD&T is a system to define nominal and allowable variations in part and assembly geometry.

ASME Y14.5-2009 standard controls GD&T symbols. 

Why to use GD&T Symbols 

  • GD&T can more accurately define part dimensional requirements, allowing larger tolerance zone compared to linear dimensions.
  • GD&T symbols can be used to communicate design intent effectively with manufacturer.
  • GD&T helps in reducing rejection rate. Therefore cost is saved.

Types of GD&T Tolerances

Five types of GD&T tolerances are used to define a part design intent. This includes 14 symbols that controls the feature of a part

Tolerances Types

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

1) Form Control

It controls the form of the feature of a part. Instead of form control tolerance, location and orientation tolerances can also be used to control the foam of a feature.

Note :- Datum reference is not required with foam tolerance. 

Types of Foam Tolerances

1.1) Straightness

1.2) Flatness

1.3) Circularity

1.4) Cylindricity

1.1) Straightness 

Straightness controls the foam of a line over a surface or part feature. LMC and MMC modifiers can be used to define straightness of a part.

Check out in detail about straight here

1.2) Flatness 

Flatness controls the flatness of a surface, regardless of any datum feature. Value of flatness tolerance is always less than the dimensional tolerance associated with  part feature.

Check out in detail about flatness here

1.3) Circularity / Roundness 

Circularity controls the roundness of a circular feature in two dimensional tolerance zone. It is independent of any datum feature. Value of circularity tolerance is always less than the diameter dimensional tolerance of the part feature.

1.4) Cylindricity 

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

2) Profile Control

Profile tolerance defines a uniform boundary around a surface within which the elements of the surface must lie. It simultaneously controls a feature‘s form, size, orientation, and location.

Types of Profile Tolerance

2.1) Profile of a line Control

2.2) Profile of a Surface Control

2.1) Profile Of a Line Control 

It makes a 2-dimensional tolerance zone around any line of a feature. It 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.

2.2) Profile Of a Surface Control 

Profile of surface control makes a 3-Dimensional tolerance zone around a surface. it makes a uniform 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.

3) Orientation Control

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

3.1) Parallelism

3.2) Perpendicularity

3.3) Angularity

3.1) Parallelism 

Parallelism controls the parallelism between two surfaces or axis. It describes a parallel orientation of one referenced feature to datum surface or line in 3D tolerance zone.

Check out in detail about parallelism here

3.2) Perpendicularity 

Perpendicularity can be used to control surface or an axis. When 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.

3.3) 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.

4. Location Control

Location Tolerance defines the deviation of a feature from the actual location. It can be controlled with following symbols

4.1) Position 

4.2) Concentricity

4.3) Symmetry

Concentricity and symmetry controls the center distance of feature whereas position controls co-axiality of a features.

4.1 Position Tolerance 

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

Concentricity controls the central axis of a cylinder or sphere w.r.t. a datum plane/axis.

It is used where high precision is required to control median points on a cylindrical part. For example in transmission gears, gears need to be concentric with  mounting.

Check out more about concentricity here

4.3 Symmetry  

Symmetry makes a 3-dimensional geometric tolerance zone w.r.t. datum that 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, it controls rectangular features and involves two imaginary flat planes.

5. 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 feature. Runout can be controlled with two symbols : 

  1. Circular Runout
  2. Total Runout

5.1) Circular Runout 

Circular runout makes a 2-Dimensional circular tolerance zone that is defined by a datum axis. It controls the total variation that the reference surface can have, 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.

5.2) Total Runout 

Total runout makes a 3-dimensional cylindrical tolerance zone defined by datum axis. It controls the total variation that the reference surface can have, when the part is rotated around the datum’s true axis.

Conclusion

To sum up, GD&T Offers the advantage of communicating part design intent in engineering drawing. In some cases it helps in increasing tolerance zone as well. Therefore rejection reduces. That results in cost saving.

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