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, allowable variation in form and possible size.

Definitions

Following are terms used while working with GD&T

Dimension: 

A numerical value (with units of measure) indicated on a drawing  along with lines, symbols, and notes to define the size or geometric characteristic part feature.

Basic Dimension: 

Basic dimension is a numerical value 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.

True Position :  

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

Reference Dimension :

 Reference dimension is a dimension  for information purposes only, that means it’s auxiliary information that does not govern production or inspection operations. Reference dimension can be a repeat of a dimension or driven from other values shown on the drawing.

Datum:

Datum is theoretically exact point, axis or plane.

Location or geometric characteristics of features of a part are established from Datum.

Datum Target : 

Datum target is used to establish a datum on specified point, line or area on a part.

Feature:

Feature is a general term used for a physical portion of a part, such as a surface, hole etc

Feature of Size:  

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

Datum Feature:

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

Actual Size:  

Measured size is known as actual size.

Limits of Size:

Specified maximum and minimum sizes are known as limit of size.

Maximum Material Condition (MMC) : 

MMC is a condition in which a feature of size contains the maximum amount of material within the stated limits of size. For example minimum hole diameter or maximum shaft diameter.

Least Material Condition (LMC) : 

LMC is a condition in which a feature of size contains the least amount of material within the stated limits of size. For example maximum hole diameter or minimum shaft diameter.

Regardless of Feature Size (RFS) : 

RFS is 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: 

Virtual condition is a boundary generated by the combination of  the specified MMC limit of size of a feature given geometric tolerances.

Tolerance: 

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

Unilateral Tolerance:

Tolerance in which variation is permitted in one direction from the specified dimension

Bilateral Tolerance:

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

Geometric Tolerance:

Category of tolerances used to control form, profile, orientation, location and runout of a feature.

Form Control

Form tolerances are applicable to single (individual) features or elements of single features, that are not related to datum. We can classify foam tolerances in four types. 

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

Straightness 

  • Straightness is a condition where an element of a surface or an axis is a straight line.
  • It 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

Flatness

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

Circularity

  • Circularity/Roundness is used to control the circularity of a round feature in 2D tolerance zone.
  • It 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.

Cylindricity

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

Profile Control

Profile tolerance controls the profile of a feature of a part. It can be classified in two categories.
  • Profile of a line Control
  • Profile of a Surface Control

Profile Of a Line Control 

  • Profile of a line control, describes a two dimensional tolerance zone around any line of a feature. 
  • It 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.

Profile Of a Surface Control

  • Profile of surface control describes a 3-Dimensional tolerance zone around a surface. 
  • It controls the size, location, orientation and foam of any surface.
  • When the profile of surface control is specified, tolerance zone is a uniform boundary along full length and width of the surface.
  • Its not possible to control profile of a surface using general tolerances. 
  • Datum reference is required to control Profile of a Surface.

Orientation Control

  • Parallelism
  • Perpendicularity
  • Angularity

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.

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.
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.

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.

Location Control

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

  1. Position Control
  2. Concentricity
  3. Symmetry

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, position tolerance zone is typically a cylinder within which the axis of the feature must lie.

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.

Symmertry

  • 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.

  1. Circular Runout
  2. 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.

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