Datums

The datum is a reference point, surface, or axis on a part that controls the geometric feature measurements are made from. Dimensions and geometric tolerances can reference datums, indicating that the measurement is with respect to that reference frame. Each part or feature is controlled by a coordinate system that introduces degrees of freedom onto the feature. The primary, secondary, and tertiary reference datums control the feature by fully defining the location and orientation.

Primary Datum – Datum feature oriented with respect to the location of the form tolerance zone (or the surface where the feature is controlled) and relative to the surface making up the geometric feature the datum is taking form of, such as a plane.

Secondary Datum – Oriented perpendicular to the primary datum with similar location controls.

Tertiary Datum – Oriented perpendicular to both the primary and secondary datum and located on a surface relative by translation to the location of the form tolerance zone.

Definition of Geometric Tolerances

Each geometric tolerance is defined by a geometric element, and therefore, each part or component feature can be broken up into geometric elements.

Planes

Cylinders

Cones

Spheres

Etc.

The geometric elements are relative to the degrees of freedom determined by the datums and are considered the nominal, or theoretical exact, form of the feature. Geometric tolerances and standard tolerances allow for controlled deviation from the nominal values, creating a design and manufacturing buffer zone where the part will continue to work properly.

Form Tolerance

The permitted maximum value of form deviation on a line or a surface. Form deviation can be measured on a 2d or a 3d geometric feature, and is controlled by the reference datum.

Orientation Tolerance

The permitted maximum value of orientation deviation including angularity, parallelism, and perpendicularity with respect to a reference datum.

Location Tolerance

The permitted maximum value of location deviation including position, coaxiality, and symmetry with respect to a feature or a reference datum. The difference between the theoretical location and actual location cannot exceed half of the location tolerance.

Run-Out Tolerances

The permitted tolerance zone is between two geometric elements, such as a plane, that are parallel or concentric and equidistant apart from the nominal location. The spacing of the geometric elements is the run-out tolerance value.

Line Profile Tolerances

The line profile is a 2d representation of a 3d feature. The deviation of the cross-section is can be controlled by the line profile tolerance, which imposes an upper and lower limit to the deviation, equidistant from the nominal line profile. The upper and lower limits are equal in profile to the nominal profile. The space between the equidistant line profile tolerance limits is considered the tolerance zone.

Straightness Tolerance

The maximum deviation of line straightness, where the nominal line profile is a straight line and the limits are composed of parallel and equidistant lines with respect to the nominal location/orientation.

Roundness (Circularity) Tolerance

Each cross-section of a circular profile must fit between two concentric circles.

Surface Profile Tolerance

The whole surface profile deviation is permitted to be between two surface profiles of equal properties to and with parallel tangent lines to the nominal surface profile. The area between the two profiles is the tolerance zone. The surface profile is the 3d representation of the line profile cross-section.

Cylindricity Tolerance

The surface deviation is contained between two coaxial cylindrical geometric features.

Flatness Tolerance

The surface deviation is contained between two parallel planer geometric features.

Angular Tolerance

The deviation is permitted between two parallel planes angled equally with respect to the nominal geometric element, based on the called out datum. The angular tolerance controls for the angle over the entire feature length, but does not control for flatness or straightness within the angular tolerance zone.

Perpendicularity Tolerance

The deviation is permitted between two parallel planes perpendicular to the called out datum.

Parallelism Tolerance

The deviation is permitted between two parallel planes parallel to the called out datum.

Positional Tolerances

The deviation from the nominal position is contained within a cylinder whose axis is on the nominal positional location. The geometric tolerance is with respect to one or multiple datums.

For example, designing a positional tolerance zone for a through-hole that will contain a bolt or a dowel pin is controlled by the minimum size of the hole and the maximum size of the bolt or pin. In this example, the location tolerance for the threaded hole and the clearance hole will be equal (in many cases, the tolerance will be disproportional and the clearance hole will be given a larger tolerance zone) and the tolerance can be calculated by the following: