Section 80 of the IPC states that "Nothing is an offence which is done by accident or misfortune, and without any criminal intention or knowledge in the doing of a lawful act in a lawful manner, by lawful means, and with proper care and caution." 'A' was engaged in a lawful act (working with a tool) in a lawful manner ("with full precautions" - implying proper care and caution). The death was caused by an accident or misfortune (hatchet head flying off) without criminal intention or knowledge. Therefore, under Section 80, 'A' has committed no crime.