GD&T in Aerospace Assembly: Use of Multiple Datum Targets for N-2-1 Location of Fuselage
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. For more detailed information on datum targets, you may refer to the ASME standard, section 4.24 [1].
N-2-1 Location with Datum Target Areas
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. However, for most sheet metal manufacturing processes, the main dimensional problem is the sheet metal deformation normal to the surface [2]. This deformation cannot be neglected, even under the self-weight of the workpiece. 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. Therefore, an adequate fixturing system for sheet metal usually requires N>3 locators on the primary datum to properly restrain workpiece deformation.