Datum Axis MMC Secondary When a Part Is Oriented by a Surface and Located by a Diameter, It Is Common to Have the Surface and Diameter Designated As Datum Features

Alessandro Anzalone, Ph.D.

Hillsborough Community College, Brandon Campus Sections: 1. FOS Datum Features 2. FOS Datum Feature Applications (RFS) 3. FOS Datum Feature Referenced at MMC 4. FOS Datum Feature Applications (MMC) Specifying an Axis or Centerplane as a Datum A FOS is specified as a datum feature by associating the datum identification symbol with the FOS. When a FOS is specified as a datum feature, it results in an axis or centerplane as a datum. There are five common ways to specify an axis or centerplane as a datum. They are: 1. The datum identification symbol can be touching the surface of a diameter to specify the axis as a datum. 2. The datum identification symbol can be touching the beginning of a leader line of a FOS to specify an axis datum. 3. The datum identification symbol can be touching a feature control frame to specify an axis or centerplane as a datum. 4. The datum identification symbol can be in line with a dimension line and touching the extension line on the opposite side of the dimension line arrowhead of a FOS to specify an axis or a centerplane as a datum. 5. The datum identification symbol can replace one side of the dimension line and arrowhead. When the dimension line is placed outside the extension lines of a FOS dimension, it specifies an axis or centerplane as a datum.

Datum Terminology Referencing FOS Datum Features

When referencing FOS datum features, the following items apply: 1. The datum sequence must be specified. 2. The material condition (MMC or LMC) must be specified. 3. If no material condition is specified, RFS is the default condition. Primary datum axis RFS Primary datum centerplane RFS Datum Axis RFS Secondary When a part is oriented by a surface and located by a diameter, it is common to have the surface and diameter designated as datum features.

When referencing the datums with the face primary and the diameter secondary (RFS), the following conditions apply: 1. The part will have a minimum of three-point contact with the primary datum plane. 2. The secondary datum feature simulator will be adjustable in size. 3. The datum axis is the axis of the datum feature simulator. 4. The datum axis is perpendicular to the primary datum plane. 5. A second and third datum plane will be associated with the datum axis.

If an angular and/or rotational relationship is important, a tertiary datum is necessary.

Datum Axis RFS Tertiary When a part is oriented by a surface, located by a diameter, and has an angular relationship relative to a FOS, it is common to have the surface, diameter, and FOS designated as datum features.

When referencing the datums with the face primary, diameter secondary (RFS), and slot tertiary (RFS), the following conditions apply: 1. The part will have a minimum of three points of contact with the primary datum plane. 2. A datum axis perpendicular to the primary datum plane will exist. 3. A datum centerplane that will pass through the datum axis and be perpendicular to the primary datum plane will exist.

Effects of Specifying the MMC Modifier

Where a FOS datum is referenced at MMC, the gaging equipment that serves as the datum feature simulator is a fixed size. The datum axis or centerplane is the axis or centerplane of the gage element. The size of the true geometric counterpart of the datum feature is determined by the specified MMC limit of size or, in certain cases, its MMC virtual condition.

Referencing a FOS datum at MMC has two effects on the part gaging: 1. The gage is fixed in size. 2. The part may be loose (shift) in the gage.

A special-case FOS datum is when a FOS datum feature is referenced at MMC, but simulated in the gage at a boundary other than MMC. There are two cases where special-case FOS datums apply:

1. Where a straightness control is applied to a FOS datum feature 2. Where secondary or tertiary datum features of size in the same datum reference frame are controlled by a location or orientation control with respect to this higher ranking datum

Datum Shift Whenever a FOS datum feature is referenced at MMC, the gage element (datum feature simulator) that simulates the perfect feature counterpart is fixed in size. Since the gage is fixed in size, but the part datum feature may vary within its size limits, there may be some looseness between the part and the gage. Datum shift is the allowable movement, or looseness, between the part datum feature and the gage. Datum shift may result in additional tolerance for the part.

Datum Axis MMC Primary When a diameter is designated as a datum feature and referenced in a feature control frame as primary at MMC, a fixed-gage element may be used as the datum feature simulator. The size of the fixed-gage element is equal to the MMC (or in certain cases, worst-case boundary) of the datum feature. The datum axis is the axis of the datum feature simulator. Depending upon the actual mating size of the datum feature, a datum shift may be available.

Datum Centerplane MMC Primary When a FOS that consists of parallel planes is designated as a datum feature and referenced in a feature control frame as primary at MMC, a fixed- gage element may be used as the datum feature simulator. The size of the fixed-gage element is equal to the MMC (or in certain cases, worst-case boundary) of the datum feature. The datum centerplane is the centerplane of the datum feature simulator. Depending upon the actual mating size of the datum feature, a datum shift may be available.

Datum Axis MMC Secondary When a part is oriented by a surface and located by a diameter, it is common to have the surface and diameter designated as datum features.

When referencing the datums with the face primary and the diameter secondary (MMC), the following conditions apply: 1. The part will have a minimum of three-point contact with the primary datum plane. 2. The datum feature simulator will be a fixed size, in this case, the worst-case boundary of datum feature B. 3. The datum axis is the axis of the datum feature simulator. 4. The datum axis is perpendicular to the primary datum plane. 5. Depending upon the datum feature actual mating size, a datum shift may be available.

If angular relationship is important, a tertiary datum is necessary.

Datum Axis Secondary, Datum Centerplane Tertiary (MMC) When a part is oriented by a surface, located by a diameter, and has an angular relationship relative to a FOS, it is common to have the surface, diameter, and FOS designated as datum features.

When referencing the datums with the face primary, diameter secondary (MMC), and slot tertiary (MMC), the following conditions apply: 1. The part will have a minimum of three points of contact with the primary datum plane. 2. The datum feature simulators will be fixed-size gage elements. The datum axis is the axis of the datum feature simulator. 3. The datum axis is perpendicular to the primary datum plane. 4. Depending upon the datum feature’s actual mating size, a datum shift may be available. 5. Second and third datum planes are to be associated with the datum axis. 6. The tertiary datum centerplane is the centerplane of the tertiary datum feature simulator.

Datum Axis from a Pattern of Holes, MMC Secondary When a part is oriented by a surface and located by a hole pattern, it is common to have the surface and the hole pattern designated as datum features. When referencing the datums with the face primary and a hole pattern secondary (MMC), the following conditions apply: 1. The part will have a minimum of three points of contact with the primary datum plane. 2. The datum feature simulators will be fixed-size gage elements equal to the virtual condition of each hole diameter. In this case, individual datum axes are established at the basic location of each hole. Datum axis B is at the theoretical center of the pattern. A second and third datum plane exist from the datum axis. 3. When the part is mounted on the primary datum surface, the pattern of holes establishes the second and third datum planes of the datum reference frame. A tertiary datum reference is not necessary since all six degrees of freedom are controlled.

Datum Sequence When interpreting datum reference frames that involve both feature of size datum features and planar datum features, the datum sequence plays a major role in the final part tolerances. In the top section of the following figure, the datum portion of the feature control frame is left blank. In the bottom portion of the same figure, the datum reference frame is completed in three different possible sequences.

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