// --- function --
public static double AngleBetweenZaxes(UCS ucs1, UCS ucs2)
{
double ang = 0;
quaternion q1 = new quaternion(ucs1.ToACS(new quaternion(0, 0, 0, 100)) - ucs1.ToACS(new quaternion(0, 0, 0, 0)));
quaternion q2 = new quaternion(ucs2.ToACS(new quaternion(0, 0, 0, 100)) - ucs2.ToACS(new quaternion(0, 0, 0, 0)));
ang = q1.angTo(q2);
return(ang);
}