Lathe, mill and drill press spindle speeds must be adjusted while the machines are OFF.

• A. True
• B. False
• C. Only if calibrating
• D. Optional

Answer: (B)

Spindle speed control depends on the drive system. If a machine uses a fixed belt/pulley arrangement, changing speeds is a mechanical operation that should be done with the machine stopped to avoid entanglement or belt damage. However, many modern machines have electronic variable-speed spindles or VFDs that allow safe speed adjustments while the spindle is running, as long as you follow the proper procedure and keep clear of moving parts. Because of this variability, the idea that speeds must always be adjusted only with the machines OFF isn’t universally true. Always follow the manufacturer’s procedure and use power-off only for mechanical belt/gear changes, while on-the-fly changes are allowed only when supported and performed safely.