With these nine datum target areas, we have eliminated translations in the Z- and the Y- axes, and rotation around the Y-axis (pitch) and Z-axis (yaw).Ģ Datum Target Areas on the Secondary Datum Plane Z-axis: Perpendicular to both the X- and Y-axes, directed toward the ceiling.

Y-axis: Perpendicular to the X-axis, along the plane of the fuselage passenger floor.

X-axis: Along the longest axis of the fuselag.

The directions of the axes can be defined as: In our case, these datum target areas are designated by A1 to A9. Since we have a very large surface, the use of datum target areas, as opposed to datum target points, makes it easier to measure. Therefore, an adequate fixturing system for sheet metal usually requires N>3 locators on the primary datum to properly restrain workpiece deformation.ĩ Datum Target Areas on the Primary Datum Planeįor our case, we selected 9 positions for datum targets on the largest cylindrical surface of the fuselage (Figure 1). Analysis shows that the average deflection of a sheet steel plate 400 mm x 400 mm x 1 mm under its self-weight can be between 1-3 mm with a 3-2-1 location layout.

This deformation cannot be neglected, even under the self-weight of the workpiece. However, for most sheet metal manufacturing processes, the main dimensional problem is the sheet metal deformation normal to the surface. In a previous post, we talked about using 3-2-1 precision location to create an accurate and repeatable datum reference frame for the part, while avoiding overconstraint. For more detailed information on datum targets, you may refer to the ASME standard, section 4.24.

This post will present a possible case in the aerospace industry for locating a fuselage part with 9-2-1 location, using datum target areas. Primary datum (in green) defined by nine datum target areas (A1 to A9 – 2 datum target areas not shown)

Datum aviation