expression, godan verb (-tsu)
temper a sword; forge a sword
Refers to the traditional process of forging and tempering a Japanese sword, involving repeated heating, hammering, and quenching.
刀匠が刀を打つ様子を見学した。
I observed a swordsmith forging a sword.
鍛える means to forge or temper metal in general, or to train/discipline a person. 刀を打つ specifically refers to the act of making a sword by hammering and tempering.
The phrase literally means 'to strike a sword', from 刀 (katana, 'sword') + を (object marker) + 打つ (utsu, 'to strike, hit'). It refers to the traditional swordsmithing process of hammering heated steel to shape and temper the blade.