解：?$(1)$?∵?$AC$?平分?$∠BAD$?
∴?$∠BAC=∠CAD$?
∵?$BC⊥AC，$??$CD⊥AD$?
∴?$∠ACB=∠ADC=90°$?
∴?$△ABC∽△ACD$?
∴?$\frac {AB}{AC}=\frac {AC}{AD}$?
∵?$AB=18，$??$AC=12$?
∴?$\frac {18}{12}=\frac {12}{AD}$?
∴?$AD=8$?
?$(2)$?∵?$△ABC∽△ACD$?
∴?$\frac {DE}{CF}=\frac {AC}{AB}=\frac 23$?
