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Basics of GD&T : SMLease Design

Geometric Dimension & Tolerance (GD& T) is a system for defining engineering tolerances. GD&T is very important part of mechanical product design.

GD&T defines degree of accuracy and precision required on controlled feature of part and used to define the allowable variation in form and possible size of individual features allowable variation between features. 

 


Form Control

Form tolerances are applicable to single (individual) features or elements of single features. Form tolerances are not related to datum.

 


Straightness

  • Straightness is a condition where an element of a surface or an axis is a straight line.
  • Straightness is used to control the foam of a line on the surface/feature or straightness of an axis.
  • No Datum plane is required to define straightness and while defining axis straightness LMC and MMC modifiers can be used

Read in detail about Straightness Here

 


Flatness

  • Flatness is a condition that defines the flatness of a surface regardless of any datum feature.
  • Flatness is used to make use required surface is flat without tightening any other dimension.
  • Value of Flatness tolerance is always less than the dimensional tolerance associated with a part feature.
  • No Datum Plane is required.
  • LMC and MMC can be used.

Read in detail about flatness here 

 


Circularity

  • Circularity/Roundness is used to control the circularity of a round feature in 2D tolerance zone.
  • Circularity tolerance is independent of any datum feature.
  • Value of circularity tolerance is always less than the diameter dimensional tolerance of the part.
  • Circularity tolerance can be applied to any part (external or internal surfaces) which is circular in cross-section. 
  • MMC and LMC conditions are not applicable with circularity tolerance.
  • Circularity tolerance can be applied to any part (internal as well as external) which is circular in cross-section

Read in detail about circularity here

 


Cylindricity

  • Cylindricity is a 3-Dimensional tolerance that controls the overall form of a cylindrical feature to ensure that it is round enough and straight enough along its axis.
  • Cylindricity is independent of datum feature the tolerance needs to be less than the diameter dimensional tolerance of the part.
  • Cylindricity essentially forms a perfect cylindrical boundary around the object that the entire 3-Dimensional part must lie in.+

Read in Detail About Cylindricity Here 

 


Profile Control

 


Profile of a Line Control

  • Profile of a line control describes a tolerance zone around any line in any feature.
  • Profile of a line is a 2-Dimensional tolerance range that can be applied to any linear tolerance.
  • Profile of line control : controls size, orientation, location & foam of any feature.
  • Line elements of the surface along the profile must lie within the profile tolerance zone and within a size limiting zone.
  • MMC and LMC are not applicable with profile of line control.
  • Datum Plane can be used to control the profile of a line control.

Read in detail about profile of line control here

 


Profile of a Surface Control

  • Profile of surface control is used to control part surface. It makes a 3-Dimensional tolerance zone around a surface.
  • Profile of a surface tolerance: Controls the size, location, orientation and foam of any surface.
  • When the profile of surface control is specified, tolerance zone is uniform boundary along full length and width of the surface.
  • Usually when surface profile is required, there are no tolerances on the dimensions that describe the surface and use the GD&T callout to give the acceptable range.
  • Datum reference is required to control Profile of a Surface.

Read More on profile of surface control here

 


Orientation Control

 


Parallelism

  • Parallelism can be used to control parallelism between two surfaces or the parallelism of two axis.
  • Parallelism describes a parallel orientation of one referenced feature to a datum surface or line in 3D tolerance zone.
  • Parallelism does not control the angle of the referenced feature, but it creates a tolerance zone in which the feature must lie.
  • Tolerance zone will be two parallel planes  that are parallel to the datum feature or surface.
  • MMC and LMC conditions are applicable with parallelism tolerance.
  • Parallelism is required to make sure two surfaces/features work in sync with each other and constant distance between them is maintained.

Read in detail about parallelism tolerance here:

 


Perpendicularity

 

Perpendicularity on a Surface

 

  • When applied on surface it controls the perpendicularity of a given surface with referance to datum plane.
  • Tolerance zone will be two parallel surfaces/planes/lines perpendicular to datum plane. Entire feature should lie between these parallel surfaces/lines/planes.
  • Perpendicularity does not control the angle of the referenced feature, tolerance zone will be a envelop

 

Read in detail about perpendicular tolerance here

 

 

Perpendicularity On Axis

  • When applied on axis perpendicularity controls perpendicularity of the axis of cylinder with referance to the datum.
  • Tolerance zone will be a cylinder boundary around a true axis. Axis of referenced feature must lie in this cylinder boundary.
  • When applied to a cylindrical feature diameter (Ø) symbol is used in control frame.
  • Perpendicularity is called out on the center axis of a hole to make sure shaft goes inside hole

Read in detail about perpendicular tolerance here

 
 

Angularity

  • Angularity is used to constrain the orientation of  one feature w.r.t. datum at specified angle.
  • Angularity tolerance zone will be two parallel planes /surfaces in 3D. All points on controlled feature should lie within defines planes.
  • Maximum material condition can be used along with angularity tolerance.
  • Datum plane is required to define angularity tolerance.
  • Angularity tolerance can also be used to control the axis of any feature w.r.t datum plane.
  • Tolerance zone for angularity for an axis will be a cylinder around theoretically exact axis w.r.t. datum plane.
  • Angularity can be used to control the critical feature at an angle where assembly is happening at an angle.

Read in detail about Angularity tolerance here

 

 


Location Control

Location Tolerance defines how much a feature can vary from the actual location. This can be defined by three tolerance zone.

Concentricity and symmetry controls the centre distance of feature elements while Position is used to control coaxiality of 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 the location of a feature about its exact true position.
  • Material Condition (MMC & LMC), Projected Tolerance, Tangent Planes can be used along with position tolerance.
  • Tolerance zone for position tolerance can be two parallel planes, Cylinder or a sphere.
  • Positional tolerances is used to locate features of size from datum planes such as a hole or key-way and used to locate features coaxial to a datum axis.
  • In cylindrical features, the position tolerance zone is typically a cylinder within which the axis of the feature must lie.

Read in Detail About Position Tolerance Here

 


Concentricity

  • Concentricity  tolerance is used to control the central axis of a cylinder or sphere w.r.t. a datum plane/axis.
  • Datum Plane /Axis is required to control Concentric tolerance.
  • Concentric tolerance is used where high precision is required to control median points on a cylindrical part such as transmission gears where gears need to be concentric with  mounting.
  • Runout or position tolerance is used instead of concentric tolerance as it is very difficult to measure concentric tolerance.
  • Concentricity tolerance makes a 3-Dimensional cylindrical tolerance zone.
  • LMC and MMC is not applicable with concentric tolerance.‚Äč

Read In Detail About concentric tolerance here 

 


Symmetry

 

  • Symmetry tolerance is used to control two features on a part across a datum plane.
  • Symmetry Tolerance controls the central points of a feature of size. Symmetry Tolerance is a three-dimensional geometric tolerance that controls how much the points between two features may deviate from a specified centre plane or axis.
  • Symmetry tolerance is applied to non circular features.
  • Datum is required for symmetry Tolerance.
  • Symmetry is similar to concentricity, it controls rectangular features and involves two imaginary flat planes.
  • All points on a feature of the part must be within specified tolerance limit.
  • Tolerance zone for symmetry tolerance will be two parallel planes at a distance equal to specified symmetry tolerance that are placed symmetrically to the middle plane w.r.t datum axis / plane.

 


Runout

Runout is variation in feature with respect to another datum when the part is rotated 360° around the datum axis. Runout controls  a circular feature, and how much variation it has with the rotational axis. Runout can be called out on any feature that is rotated about an axis. Runout measures the wobbling of a feature. Runout measures the wobbling of a feature.

  • Circular Runout
  • Total Runout

 


Circular Runout

  • Circular runout makes a 2-Dimensional circular tolerance zone that is defined by a datum axis where all points on the called surface must fall into.
  • Runout is the total variation that the reference surface can have, when the part is rotated around the datum’s true axis.
  • Runout is used in any rotating components such as drills, gears, shafts, axles and many machine tool parts.
  • Runout is required when oscillations or vibrations need to be controlled on a fast rotating part, like an engine or transmission.
  • Runout is regardless of feature size( MMC or LMC can not be used with runout tolerance)

 


Total Runout

  • Total runout makes a 3-Dimensional cylindrical tolerance zone that is defined by a datum axis. When datum is fixed and part is rotated, all points on measured surface should come within tolerance zone.
  • Total runout is the total variation that the reference surface can have, when the part is rotated around the datum’s true axis.
  • Total Runout controls conentricity, perpendicularity/paralellism (feature of size axis), cylindricity, circularity, straightness and of course normal Circular Runout

 


Definitions

The following terms are defined as their use applies in this Standard

Dimension A numerical value expressed in appropriate units of measure and indicated on a drawing and in other documents along with lines, symbols, and notes to define the size or geometric characteristic, or both, of a part or part feature.

Basic Dimension. A numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or datum target. It is the basis from which permissible variations are established by tolerances on other dimensions, in notes, or in feature control frames.

True Position. The theoretically exact location of a feature established by basic dimensions.

Reference Dimension. A dimension, usually without tolerance, used for information purposes only. It is considered auxiliary information and does not govern production or inspection operations. A reference dimension is a repeat of a dimension or is derived from other values shown on the drawing or on related drawings.

Datum. A theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature, A datum is the origin from which the location or geometric characteristics of features of a part arc established.

Datum Target. A specified point, line, or area on a part used to establish a datum.

Feature. The general term applied to a physical portion of a part, such as a surface, hole, or slot.

Feature of Size. One cylindrical or spherical surface, or a set of two plane parallel surfaces, each of which is associated with a size dimension.

Datum Feature. An actual feature of a part that is used to establish a datum.

Actual Size. The measured size.

Limits of Size. The specified maximum and minimum sizes.

Maximum Material Condition (MMC). The condition in which a feature of size contains the maximum amount of material within the stated limits of size-for example, minimum hole diameter, maxi-mum shaft diameter.

Least Material Condition (LMC). The condition in which a feature of size contains the least amount of material within the stated limits of size - for example, maximum hole diameter, minimum shaft diameter.

Regardless of Feature Size (RFS). The term used to indicate that a geometric tolerance or datum reference applies at any increment of size of the feature within its size tolerance.

Virtual Condition. The boundary generated by the collective effects of the specified MMC limit of size of a feature and any applicable geometric tolerances.

Tolerance. The total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits.

Unilateral Tolerance. A tolerance in which variation is permitted in one direction from the specified dimension

Bilateral Tolerance. A tolerance in which variation is permitted in both directions from the specified dimension.

Geometric Tolerance. The general term applied to the category of tolerances used to control form, profile, orientation, location, and runout.


Related Articles

Straightness

Parallelism

Flatness

Circularity

Cylindricity

Perpendicularity

Angularity

Symmetry

Profile of a Line Control

Profile of a Surface Control

MMC

Concentricity

 

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very nice article, whenever you want to do revision before going to an interview it really helps you.

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This is very helpful

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This is very helpful, can you please write an article about each tolerance with example?

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