May 8, 2009

# GD&T – Datum – A Better Insight – ASME Y14.5

Datums are the reference surfaces or the starting points for the location and  orientation of features. They are essential for appropriate and complete tolerancing of a part.

Datums are theoretically perfect points, lines, and planes. They establish the origin from which the location or geometric characteristics of features of a part are established. These points, lines, and planes exist within a structure of three mutually perpendicular intersecting planes known as a datum reference frame.

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. It can also rotate around the X-axis, the Y-axis, and the Z-axis. A part is oriented and immobilized relative to the three mutually perpendicular planes of the datum reference frame in a selected order of precedence.

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. Parts are relatively imperfect. 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 secondary datum feature is pushed up against a second plane of the datum reference frame with a minimum of two points of contact. 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. Thus eliminating all six degrees of freedom.

Datums 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. 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.

## How To Select Datum Feature In The Order of Precedence

Datum features are selected to meet design requirements. When selecting datum features, the designer should consider the following characteristics:

Functional surfaces
Mating surfaces
Surfaces of sufficient size to allow repeatable measurements

Datum features must be easily identifiable on the part. If parts are symmetrical, or have identical features making identification of datum features impossible, the datums features must be physically identified. Selecting datums is the first step in dimensioning a part.

Figure shows a part with four holes. 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. 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 hole locations are dimensioned from the bottom and left edges of the part. Datum B is specified as the secondary datum, and datum C is specified as the tertiary datum in the feature control frame. Datum surfaces for location are selected because of their relative importance to the controlled features. The bottom edge of the part was selected as the secondary datum because it is larger than the left edge. The left edge might have been selected as the secondary datum if it were a mating surface.

## Datum Facts

• Datums are theoretically perfect points, lines, and planes.
• Datums exist within a structure of three mutually perpendicular intersecting planes known as a datum reference frame.
• A part is oriented and immobilized relative to the three mutually perpendicular planes of the datum reference frame in a selected order of precedence.
• Since measurements cannot be made from theoretical surfaces, datums are assumed to exist in and be simulated by the processing equipment.
• Datums are specified in order of precedence as they appear in the feature control frame.
• Datum features are selected to meet design requirements. Functional surfaces, mating surfaces, readily accessible surfaces, and surfaces of sufficient size to allow repeatable measurements make good datum features.
• A datum feature symbol is used to identify physical features of a part as datum features. Datum feature symbols should not be applied to centerlines, center planes, or axes.
• Plane, flat-surface features not subject to size variations make the best datums.
• When a cylinder is specified as a datum, the entire surface of the feature is considered to be the datum feature.