Final answer:
Changing spindle speeds on a milling machine typically does not require the spindle to be running, but always check the machine's manual for specific instructions to avoid damage and ensure safety.
Step-by-step explanation:
When changing spindle speeds on a milling machine, it is generally not necessary for the spindle to be running if changing only the speed within the same range. However, it is important to consult the machine's manual since procedures can vary depending on the milling machine model. For a common variable speed milling machine, speed changes are usually made while the spindle is stopped to prevent damage to the machine's gears and spindle. Always prioritize safety and manufacturer guidelines when operating machinery.