如图,在平面直角坐标系\(xOy\)上,点\(A(1,0)\),点\(B\)在单位圆上,\(∠AOB=θ(0 < θ < π)\).
\((1)\)若点\(B(- \dfrac {3}{5}, \dfrac {4}{5})\),求\(\tan (θ+ \dfrac {π}{4})\)的值;
\((2)\)若\( \overrightarrow{OA}+ \overrightarrow{OB}= \overrightarrow{OC}\),\( \overrightarrow{OB}\cdot \overrightarrow{OC}= \dfrac {18}{13}\),求\(\cos ( \dfrac {π}{3}-θ)\).