![]() The manufactured part will be inspected against the datum reference frame. ![]() Figure 4: Datum C locks down the last Degree of Freedom Inspecting a Part using the Datum Reference Frame This datum feature is also a surface, resulting in another planar datum.įinally, Datum C locks down this last degree of freedom (translation: Z), as shown in Figure 4. However, it is being controlled in reference to Datum A first, then Datum B. As this datum feature is also a surface, it will also result in a planar datum.ĭatum Feature C is the right surface indicated on the top view of the drawing, and it is also being controlled with perpendicularity. It is being controlled with perpendicularity back to Datum A. ( For more information regarding Datum Feature Symbol placement, see our Datum Feature Symbol page.)ĭatum Feature B is the back surface indicated on top view of the drawing. Because of how the datum feature symbol is placed, we know that it is referencing the surface which results in a planar datum. Figure 1: Datum Feature References in Feature Control Frameĭatum Feature A is the bottom surface indicated on the front view of the drawing that is being controlled by flatness. The feature control frame for this callout references datum features A, B, and C. In the part drawing below, we will take a look at the datum reference frame being created for the position callout indicated by the red arrow. When designing a part, it is crucial that the datum reference frame mimics the part’s functionality. Lower precedent datums only control the degrees of freedom not already controlled by higher precedent datums. If possible, datum features should be selected in the order that the part would assemble in real life. The order of the datum features being referenced in a feature control frame is important because it will dictate which features take precedence when locking down the datum reference frame for inspection. This generally means that all six degrees of freedom in a coordinate system must be locked down. The datum reference frame must lock down all degrees of freedom (DOF) necessary for the part. Importance of the Datum Reference Frame in Design This is known as simultaneous requirements. Other feature control frames that list the same datums with the same order and the same modifiers will also be inspected to this datum reference frame. A datum reference frame sets up the framework that this geometric control will reference from. The datum reference frame is the coordinate system created by the datums specified within a feature control frame. ![]() Datum features are the real features of a part that will be physically measured. that are derived from the datum features and are simulated by measurement equipment. Datums are the theoretically exact points, lines, axes, etc. Datum features are real, tangible features on a part and are usually important functional surfaces. This reference system is called a Datum Reference Frame (DRF).ĭatums are derived from datum features. Datums are used in Geometric Dimensioning and Tolerancing drawings to create a reference system for inspecting a manufactured part.
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