On a CNC machine, useful output only happens while the tool is actually cutting metal, everything else, such as swapping one tool for another, counts as idle or non productive time. Doing this by hand is slow because an operator has to stop the spindle, physically remove one tool, fit the next one, and reset it accurately. An Automatic Tool Changer carries several tools ready in a carousel or magazine and swaps them in seconds under program control, with no operator needed. Since this directly cuts down the time spent waiting between cuts, its whole purpose is to shrink tool change time and the idle time of the machine.