Basics of GD&T : Geometric Dimensioning and Tolerances

GD&T stands for Geometric Dimension & Tolerance. It is a system to define nominal and allowable variations in geometry of a part and assembly. 

ASME Y14.5-2009 standard is used to define GD&T.

Why to use GD&T Symbols 

  • GD&T can more accurately define the dimensional requirements for a part, allowing over 50% more tolerance zone than linear dimensioning.
  • To save cost and time, Big manufacturers outsource their work. GD&T symbols are used to communicate design intent effectively with manufacturer.
  • By using GD&T we can reduce rejection. That results in cost saving.

Types of GD&T Tolerances

Four types of GD&T tolerances are used to define design intent of a part. These four types uses fourteen symbols:

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. Form control tolerances are not used very frequently because 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

  • Straightness
  • Flatness
  • Circularity
  • Cylindricity

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 independent of any datum feature. Value of circularity tolerance is always less than the diameter dimensional tolerance of the part feature.

Check out about circularity in detail here

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.

Check out in detail about cylindriciity Here

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 sometimes location.

Types of Profile Tolerance

  • Profile of a line Control
  • 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. It 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. It can be used with or without datum.

3. Orientation Control

It is used to control the orientation of the feature of a part with respect to another feature or datum. Orientation tolerances can be classified in three categories.   
  • Parallelism
  • Perpendicularity
  • Angularity

It is used to control parallelism between two surfaces or two 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 

It 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

  1. Position 
  2. Concentricity
  3. Symmetry

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

4.1 Position Tolerance 

It 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.

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

Concentric tolerance 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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