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True Position – Position Tolerance

Position is one of the most useful and most complex of all the symbols in GD&T. The two methods of using Position discussed on this page will be RFS or Regardless of Feature Size and under a material condition (Maximum Material Condition or Least Material Condition). Position is always used with a feature of size.

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Datum Feature

A datum is theoretical exact plane, axis or point location that GD&T or dimensional tolerances are referenced to. You can think of them as an anchor for the entire part; where the other features are referenced from. A datum feature is usually an important functional feature that needs to be controlled during measurement as well.

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Profile of a Surface

Profile of a surface describes a 3-Dimensional tolerance zone around a surface, usually which is an advanced curve or shape...

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Maximum Material Condition (MMC)

Maximum Material Condition (MMC), is a feature of size symbol that describes the condition of a feature or part where the maximum amount of material (volume/size) exists within its dimensional tolerance.

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Flatness

GD&T Flatness is a common symbol that references how flat a surface is regardless of any other datum’s or features. It comes in useful if a feature is to be defined on a drawing that needs to be uniformly flat without tightening any other dimensions on the drawing. The flatness tolerance references two parallel planes (parallel to the surface that it is called out on) that define a zone where the entire reference surface must lie.

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Concentricity

Concentricity, is a tolerance that controls the central derived median points of the referenced feature, to a datum axis. Concentricity is a very complex feature because it relies on measurements from derived median points as opposed to a surface or feature’s axis.

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Perpendicularity

Perpendicularity is a fairly common symbol that requires the referenced surface or line to be perpendicular or 90° from a datum surface or line...

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Runout

Runout is how much one given reference feature or features vary with respect to another datum when the part is rotated 360° around the datum axis.

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Parallelism

Parallelism is a fairly common symbol that describes a parallel orientation of one referenced feature to a datum surface or line...

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Circularity

The circularity symbol is used to describe how close an object should be to a true circle...

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Straightness

The standard form of straightness is a 2-Dimensional tolerance that is used to ensure that a part is uniform across a surface or feature. Straightness can apply to either a flat feature such as the surface of a block, or it can apply to the surface of a cylinder along the axial direction. It is defined as the variance of the surface within a specified line on that surface.

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Total Runout

Total Runout is how much one entire feature or surface varies with respect to a datum when the part is rotated 360° around the datum axis...

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Cylindricity

The Cylindricity symbol is used to describe how close an object conforms to a true cylinder...

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Feature Control Frame

In GD&T, a feature control frame is required to describe the conditions and tolerances of a geometric control on a part's feature...

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Symmetry

GD&T Symmetry is a 3-Dimensional tolerance that is used to ensure that two features on a part are uniform across a datum plane...

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Angularity

Angularity is the symbol that describes the specific orientation of one feature to another at a referenced angle...

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Least Material Condition (LMC)

Least material condition is a feature of size symbol that describes a dimensional or size condition where the least amount of material (volume/size) exists within its dimensional tolerance...

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Profile of a Line

Profile of a line describes a tolerance zone around any line in any feature, usually of a curved shape...

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Regardless of Feature Size

Regardless of feature size simply means that whatever GD&T callout you make, is controlled independently of the size dimension of the part. RFS is the default condition of all geometric tolerances by rule #2 of GD&T and requires no callout.

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GD&T Rule #1: Envelope Principle

GD&T Rule #1, also known as the Envelope principle, states that the form of a regular feature of size is controlled by its “limits of size." Limits of size, or otherwise known as size tolerances, can be seen in many forms. A few of them are symmetric, unilateral, and bilateral.

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Unequally Disposed Profile

The unequally disposed profile tolerance symbol is used to apply unilateral or unequal tolerance zones to a profile of a part.

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Independency

The Independency Symbol is used on drawings to declare that the requirement for perfect form at MMC or LMC is removed and the form tolerance may be larger than the size tolerance. This symbol only exists in the ASME Y14.5 standards, not the ISO GPS standards.

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Envelope Requirement (E Symbol) – ISO Only

The Envelope Requirement Symbol is used on ISO drawings to declare that size is to control form with respect to perfect form at MMC.

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Datum Target

The Datum Target Symbol is used to define a specific point, line, or area to be used in order to establish a datum on a drawing.

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Continuous Feature

The Continuous Feature Symbol is used to indicate that a group of disjointed features or surfaces are to be considered as one continuous feature.

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Projected Tolerance Zone

The Projected Tolerance Symbol is in used GD&T to indicate the tolerance zone of a feature is to be assessed beyond the surface extents of the feature.

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Free State Symbol

The Free State Symbol is in used GD&T to indicate that a specific dimension and its associated tolerance are to be assessed in an unrestrained state.

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Restrained Condition Note

Some parts, such as those made of rubber or sheet metal, may fail inspection due to deformation from gravity if left in their "free state." This can be prevented by invoking a "restrained state."

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Tangent Plane

The Tangent Plane Symbol is used in conjunction with GD&T surface controls to indicate that a specific control is applied to a theoretical tangent plane simulated by the high points of irregular tangible surface rather than the actual elements of the surface themselves.

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Counterbore

The counterbore symbol is used on a drawing to indicate that a counterbore hole feature is required. A counterbore is a flat-bottomed cylindrical hole that is larger than and coaxial to another cylindrical hole.

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Spotface

A spotface is a very shallow flat-bottomed cylindrical hole that is larger than and coaxial to another cylindrical hole. Its purpose is to provide a flat mounting surface for mating parts, such as washers or pan head screws.

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Countersink

A countersink consists of a conical hole that is coaxial to a cylindrical hole, where the angle of the cone is determined by the fastener to be used. The purpose of a countersink is to allow a fastener, typically a flathead screw, to sit slightly below the surface of the part.

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Diameter

The diameter symbol is used to indicate that the size of a circular feature is being dimensioned using the diameter of that feature.

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Square

The Square symbol is used to indicate a square feature on a drawing.

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Radius

The Radius symbol is used when a circular feature is dimensioned using the radius length.

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Controlled Radius

The Controlled Radius symbol is a variation of the Radius, and is used when the surface of the contour is required to be a "fair" curve.

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Spherical Radius

The Spherical Radius is used to indicate that the Radius is of a spherical, rather than circular, feature.

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Spherical Diameter

The Spherical Diameter symbol is used to indicate that the Diameter is of a spherical, rather than circular, feature.

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