DATUM FEATURE SYMBOL HOW TO

Plane, flat-surface features not subject to size variations make the best datums.

Datum feature symbols should not be applied to centerlines, center planes, or axes.

A datum feature symbol is used to identify physical features of a part as datum features.

Functional surfaces, mating surfaces, readily accessible surfaces, and surfaces of sufficient size to allow repeatable measurements make good datum features.

Datum features are selected to meet design requirements.

Datums are specified in order of precedence as they appear in the feature control frame.

Since measurements cannot be made from theoretical surfaces, datums are assumed to exist in and be simulated by the processing equipment.

A part is oriented and immobilized relative to the three mutually perpendicular planes of the datum reference frame in a selected order of precedence.

Datums exist within a structure of three mutually perpendicular intersecting planes known as a datum reference frame.

Datums are theoretically perfect points, lines, and planes.

The left edge might have been selected as the secondary datum if it were a mating surface. The bottom edge of the part was selected as the secondary datum because it is larger than the left edge. Datum surfaces for location are selected because of their relative importance to the controlled features. Datum B is specified as the secondary datum, and datum C is specified as the tertiary datum in the feature control frame. The hole locations are dimensioned from the bottom and left edges of the part. Datum A makes a good primary datum for the four holes because the primary datum controls orientation, and it is desirable to have bolt holes perpendicular to mating surfaces. The designer selected the back of the part as the primary datum, datum A, because the back of the part mates with another part, and the parts are bolted together with four bolts. Selecting datums is the first step in dimensioning a part.įigure shows a part with four holes. If parts are symmetrical, or have identical features making identification of datum features impossible, the datums features must be physically identified. Surfaces of sufficient size to allow repeatable measurementsĭatum features must be easily identifiable on the part.

When selecting datum features, the designer should consider the following characteristics:

DATUM FEATURE SYMBOL HOW TO

How To Select Datum Feature In The Order of Precedenceĭatum features are selected to meet design requirements. Datum A is the primary datum, datum B is the secondary datum, and datum C is the tertiary datum because this is the order in which they appear in the feature control frame.

Thus eliminating all six degrees of freedom.ĭatums are specified in order of precedence as they appear from left to right in the feature control frame they need not be in alphabetical order. Finally, the part is slid along the first two planes of the datum reference frame until the third datum feature contacts the third plane of the datum reference frame with a minimum of one point of contact. The secondary datum feature is pushed up against a second plane of the datum reference frame with a minimum of two points of contact. In order to properly place an imperfect, rectangular part in a datum reference frame, the primary datum feature sits flat on one of the planes of the datum reference frame with a minimum of three points of contact that are not in a straight line. The datum reference frame is not absolutely perfect, but it is made sufficiently accurate with respect to the part to consider it to be perfect. A part is oriented and immobilized relative to the three mutually perpendicular planes of the datum reference frame in a selected order of precedence. It can also rotate around the X-axis, the Y-axis, and the Z-axis. A part can move back and forth in the X direction, in and out in the Y direction, and up and down in the Z direction.