During normal operation, a CNC program will cause the machine to make tool changes. With double-arm automatic tool changers, a T word in the program will place the cutting tool in the ready position of the automatic tool changer magazine, and an M06 will actually make the tool change.

With single-arm tool changers, the T word may do everything. Regardless of how tool changes are actually made, it is imperative that the cutting tool currently in the spindle matches the program segment that uses the tool. During normal operation—when the program runs from the first tool to the second, the third and so on—there is little chance of a mismatch. However, when operators must rerun a tool, which is essentially running tools out of sequence, there is a chance that they may not restart the program at the appropriate command. If they start the program from a command after the tool change, and if a different tool is in the spindle, the machine will attempt to run the cutting tool with the wrong series of commands. The results could be disastrous.