Symbols of GD&T (in detail)

SREELAKSHMY V.U (B.E)

There are 14 geometrical characteristics symbol which controls the features in GD&T. They are classified under five controls and are given:

1. FORM • Straightness

• Flatness

• Circularity

• Cylindricity

2. ORIENTATION• Angularity

• Perpendicularity

• Parallelism

3. LOCATION• Position

• Concentricity

• Symmetry

4. PROFILE• Profile of a line

• Profile of a surface

5. RUN OUT• Circular Run out

• Total Run out

Straightness

• In this feature, all elements are proposed to be in the straight line

• Straightness error is the distance between two parallel lines which include all elements of a line

• Straightness control is a geometric tolerance that when directed to a surface that limit the amount of straightness error

• Straightness control requires no datum reference and modifiers (MMC,LMC, P, T)

• It controls each line independently and separately

• If tolerance zone for straightness is applied to a FOS is a cylinder, diameter modifier is used

• If modifier is given in the frame, then the condition is used to describe the axis of FOS

Flatness

• In this feature, all elements are supposed to be in one plane

• Flatness error is the distance between two parallel planes which include all elements of a surface (High and low points)

• Flatness control is a geometric tolerance that limits the amount of flatness error

• Flatness feature requires no datum and no modifiers. And it must be applied to a planar surface

• The distance between the parallel plane is determined by the flatness control tolerance value

Circularity

• It is a condition where all points of a surface of revolution at any section perpendicular to an axis are equidistant from the axis

• Circularity error is the radial distance between two co-axial diameters which include all elements of a circular feature

• Circularity control limits the amount of circularity error

• Circularity feature requires no datum reference, modifiers (M,L,P,T)

• It controls each circular element independently

• Tolerance zone for a circularity control applies to a diameter in two co-axial circles

• Circularity control do not over ride Rule #1

Cylindricity

• It is the condition of a surface of revolution in which all points of the surface are equidistant from a common axis

• Cylindricity error is the radial distance between two co-axial cylinders which includes all elements of a cylindrical surface.

• Cylindricity control limits amount of cylindricity error

• Cylindricity feature requires no datum reference and modifiers

• Cylindricity feature controls the form of diameter of a perfect cylinder

Angularity

• It is used to control the angle of the surface

• Angularity requires datum reference and an angle must be specified between the tolerance feature and datum reference

• When angularity is applied to a plane, the tolerance zone will be two parallel planes oriented by a basic angle and all the elements will lie within the tolerance zone

• When angularity is applied to FOS, it controls the orientation of the axis of FOS

• When diameter is specified before the angular tolerance value, it indicates that the tolerance zone is cylindrical

Perpendicularity

• When the perpendicularity is applied to a surface, the tolerance zone is two parallel planes which are 90 degree to the datum

• When the perpendicularity is applied to a planar FOS, its tolerance zone is applied to both the surface of FOS

• When the perpendicularity control is applied to a planar surface, it controls the orientation of flatness of the surface

• When the perpendicularity control is applied to a FOS, it control the orientation of axis of the FOS

• If MMC is indicated with perpendicularity control, then bonus tolerance is permissible

• When the diameter is specified before the tolerance value, then the shape of the tolerance zone is cylindrical

Parallelism

• When parallelism is applied to a planar surface, the tolerance zone is two parallel planes which are parallel to the datum plane.

• When parallelism is applied to a FOS, its tolerance zone is applied to the axis or center plane of the FOS.

• When the diameter is specified before the tolerance value, then the shape of the tolerance zone is cylindrical

• When the parallelism control contains a diameter symbol in front of the tolerance value, the shape of tolerance value will be cylindrical

• The axis of the tolerance diameter must be within the tolerance zone

• the tolerance zone may float within the allowable location tolerance zone

Position

• Tolerance of position (TOP)

• It is used to control the location of the FOS or a pattern of FOS

• True position is the theoretically exact location of a FOS

• TOP control is a geometric tolerance that defines the location tolerance of FOS from its true position.

• Modifiers and datum references are used when TOP is used

• When Modifier is given for a part with TOP, then the tolerance is applied only to that modifier condition

• When TOP is applied on MMC, bonus tolerance and datum shift is permitted

Concentricity• It is the condition where the

mid points of all diametrically opposed elements of a surface of revolution are congruent with the axis of a datum feature.

• All the median points of the tolerance diameter must be within the tolerance zone.

• A concentricity control, datum references are always applied at RFS

• Concentricity requires datum reference and must be applied to a cylindrical FOS and at RFS

• The tolerance zone for a concentricity control is a cylinder that is co axial with the datum

Symmetry

• It is the condition where the median points of all opposed elements of two or more feature surfaces are congruent with the datum axis or datum center plane of a datum feature

• The symmetry control requires datum reference and it must be applied to a planar FOS and RFS

• The symmetry tolerance value determines the size of tolerance zone

• The tolerance zone for a symmetry control is two parallel planes centered about the datum center plane

• The distance between the plane is equal to the symmetry control tolerance value

• Median points of tolerance feature must be within the tolerance zone

Profile of a line

• Profile of a line establishes a two-dimensional tolerance zone that controls individual line elements of a feature or surface. Profile of a line is usually applied to parts with varying cross-sections, or to specific cross sections critical to a part's function.

• Profile control is a geometric tolerance which specifies a uniform boundary along the true profile that elements of a surface must lie within

• When a profile of a line control is specified the tolerance zone is two uniform lines. The tolerance zone applies for each line element of the surface

• It has 2D tolerance zone

• It can be used with the datum feature as a related feature control and without a datum reference as a form control

Profile of a surface

• It is a powerful geometric tolerance which is used to control the part surface

• It controls the size, location, orientation and form

• When the profile of a surface control is specified, tolerance zone is uniform boundary. The boundary applies for the full length and width of the surface

• It must have a datum reference and should be applied to a true profile

• It has 3D tolerance zone and it can be used to tolerance a polygon, conical feature

Circular Run out

• It is a composite control that affects the form, orientation, location of circular elements of a part feature relative to a datum axis

• It is applied to each circular element of the tolerance feature independently

• Tolerance zone exist for each circular element of the tolerance diameter

• The shape of the tolerance zone is two co-axial circles whose centers are located on the datum axis.

• It is known as composite control because it limits the circularity, orientation and axis offset of a diameter.

• Circular run out tolerance zone is the radial distance between circles equal to the run out tolerance value

Total Run out

• It is a composite control that affects the form, orientation, location of all surface elements of a diameter relative to a datum axis

• Total run out requires a datum reference and it must be applied at RFS

• The run out tolerance value is equal to the radial distance between the two cylinders

• The tolerance zone of a total run out is the two co-axial cylinders whose centers are located on the datum axis

• It is also a composite control as it limits the location, orientation and cylindricity

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