When locating work-pieces that receive external forces, location datum surfaces must be provided in locations to take account for such forces. Errors in datum location provisions will result is the following problems.

Therefore, locating of work-pieces with applicable external forces must take in consideration the sizes and the directions of the allocable forces.

Example

Case of drilling fixtures

In case of [Fig.1], there are two forces that apply.

1. Rotation force of the drill bit (Torque)
2. Plunging force of the drill bit (Thrust)

[Fig.1-a shows a datum structure that can withstand these two forces. [Fig.1-b] shows an example with different locating pin layout, and it can be seen that the work-piece cannot be securely located against the torque.