The angle needed for the stress transformation equations is the angle between the face and the inclined plane.
The orientation of a plane surface is defined by its normal.
• The normal to the face is the axis.
• The normal to the inclined plane is the axis.
To find the angle between two planes, we simply find the angle between their respective normals.
In our Top-Drop-Sweep the Clock method, the angle that we sweep from the vertical line to the inclined plane is the same magnitude and direction as the angle between the axis and the axis...always.