Which practice is unsafe while the milling machine is in operation?

• A. Check the speed range before turning on
• B. Check tool clearance before turning on
• C. Adjust the spindle speed while the spindle is running
• D. Check your e-mail before turning on

Answer: (C)

Adjusting the spindle speed while the spindle is running is unsafe because changing cutting speed while the tool is engaged with the workpiece causes sudden, uncontrolled changes in cutting forces. That abrupt shift can cause the tool to grab, stall, or fracture, overload the drive system, or loosen the workholding, leading to tool breakage, damaged part, or dangerous chip ejection. The proper workflow is to set and verify the correct spindle speed with the machine completely stopped, then begin the cut. Safe pre-run checks like confirming the speed range and ensuring tool clearance help prevent crashes, while distractions such as checking email during operation can create unsafe conditions.