Abstract: A cutter assembly has an adapter, an outer collar, a recoil spring, a head, a pushing assembly and a central drill. The adapter has an L-shaped guiding slot and multiple balls. The outer collar is slidably and rotatably mounted around the adapter and has an annular protrusion and a positioning rod. The annular protrusion selectively abuts against the balls. The positioning rod is mounted on the outer collar and extends slidably into the guiding slot. The head is mounted detachably on the adapter and has a saw and an inserting rod. The inserting rod is inserted into the adapter and has an annular engaging groove. The annular engaging groove is formed around the inserting rod at a position corresponding to the balls to selectively hold the balls in the engaging groove.

Abstract: A cutter assembly has an adapter, an outer collar, a recoil spring, a head, a pushing assembly and a central drill. The adapter has an L-shaped guiding slot and multiple balls. The outer collar is slidably and rotatably mounted around the adapter and has an annular protrusion and a positioning rod. The annular protrusion selectively abuts against the balls. The positioning rod is mounted on the outer collar and extends slidably into the guiding slot. The head is mounted detachably on the adapter and has a saw and an inserting rod. The inserting rod is inserted into the adapter and has an annular engaging groove. The annular engaging groove is formed around the inserting rod at a position corresponding to the balls to selectively hold the balls in the engaging groove.