Straightness tolerance specifies a tolerance zone within which the line element of a feature must lie.

Used to control flatness for a gasket or seal, to attach a mating part or for better contact with a datum simulation surface. Applies without regard for the size of the feature (RFS).

Used to limit the lobing (out of round) of a shaft diameter, which can cause bearing or bushing failure. Control applies to only one cross-section at a time.

Used to limit the surface conditions (out of round, taper, and straightness) of a shaft diameter.

Used to limit the amount of error for line elements relative to their true profile.

Used to control the size, location, orientation and form of planar, curved, or irregular surfaces, polygons, cylinders surfaces of revolution, or cones and coplanar surfaces.

Used to control the amount a surface, axis, or centerplane is permitted to vary from its specified angle.

Used to control the amount a surface, axis, or centerplane is permitted to vary from 90° to the Datum.

Used to limit the amount a surface, axis, or centerplane is permitted to vary from being parallel to the datum.

Tolerance of position (TOP) is used to define the variation of location of a feature of size (FOS) with respect to true position (theoretically exact location).

Used on rotational parts to ensure balanced distribution of mass and consistent wall thickness. Alternative: TOP and runout.

Used on planar FOS to ensure balanced distribution of mass and consistent wall thickness. Alternative: TOP.

Used to control the form, orientation, and circularity of individual circular elements of a part feature relative to one or more datum axis.

Used to control the form, orientation, and cylindricity of a diameter relative to one or more datum axis.

• Rule #1: Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extend to which variations in its form, as well as its size, are allowed. No element of a feature shall extend beyond the MMC boundary of perfect form. The form tolerance increases as the actual size of the feature departs from MMC toward LMC. There is no perfect form boundary requirement at LMC.
• Note that rule #1 does not apply to Stock size parts or parts made of flexible materials and, in a part with multiple FOS, rule #1 does not control location, orientation or relationship between them.
• Rule #2: Regardless of Feature Size (RFS) is the default condition of all geometric tolerances. Otherwise MMC or LMC must be specified on the Feature Control Frame. RFS means that the geometric tolerance applies regardless of the actual produced size of the feature.
• MMC is very useful when designing a part destined for mass production (reduce rejection rates). However, if you're only designing few or one-of-a-kind part specify your tolerances RFS.

• A datum is a theoretically exact plane, point, or axis from which a dimensional measurement is made.
• A datum feature is a part feature that contacts a datum.
• A true geometric counterpart is the theoretical perfect boundary or best fit tangent plane of a specified datum feature.
• Datum Features are selected on the basis of part function and assembly requirements.
• Datum Features are often the features that orient (stabilize) and locate the part in its assembly.
• A Datum reference frame is a set of three mutually perpendicular datum planes, which provide direction and origin of measurements.
• A part may have as many datum reference frames as needed to define its functional relationships.
• The 3-2-1 Rule defines the minimum points of contact, with the primary datum as 3, the secondary datum as 2, and the tertiary datum as 1.
• Coplanar surfaces are two or more surfaces that are on the same plane.
• Coplanar datum features are two or more datum features that are on the same plane.

• Datum targets are symbols that describe the shape, size, and location of gage elements that are used to establish datum planes, axes, and points.
• Datum targets are shown on a drawing, but they actually do not exist on a part.
• Datum targets should be used whenever...
  • It is not practical to use the entire surface as a datum plane.
  • The designer suspects the part may rock or wobble when the datum feature contacts the datum plane
  • Only a portion of the feature is used in the function of the part.
• When a datum target symbol is used on a drawing, it is often accompanied by the datum feature symbol.
• Basic Dimensions should be used to describe the location of datum targets, which assures that there will be minimum variation between gages.

• When a Feature Of Size (FOS) is specified as a datum feature, it results in an axis or centerplane as a datum.
• When a Diameter (FOS) is designated as a datum feature, the datum axis is derived from placing the part in a datum feature simulator (gage element).
• When referencing FOS datum features, the following items apply:
  • the datum sequence must be specified.
  • the material condition (MMC or LMC) must be specified.
  • if no material condition is specified, RFS is the default condition.
• Using a FOS as a primary datum feature at RFS is Expensive. Only specify when there is no other choise.

• A composite position feature control frame has one position symbol that applies to the four horizontal segments that follow.
• The first segment [.010] governs the location of the hole with respect to datums A & B.
• The second segment [.007] governs the size, form, and orientation of the holes in the pattern with respect to A & B.
• The third segment [.005] governs the 3-holes coaxial parallelism with respect to datum A and also to one another.
• The fourth segment [.000] governs the 3-holes coaxial orientation with respect to one another.

• A composite profile feature control frame has one profile symbol that applies to the two horizontal segments that follow
• The upper segment governs the location of the FOS with respect to its datums
• The lower segment governs the size, form, and orientation of the FOS or pattern of features
• The smaller tolerance zone need not fall entirely inside the larger tolerance zone
• Any portion of the smaller tolerance zone that lies outside the larger tolerance zone is unusable
• The actual FOS or profile must fall completely inside both profile tolerance zones.