Which of the following tools is critical for checking alignment in CNC milling?

An indicator is a precision measuring tool that plays a crucial role in checking alignment during CNC milling operations. It is utilized to detect any misalignment or runout in the machine setup, such as the spindle or workpiece alignment. The indicator has a calibrated scale and a fine needle that moves in response to small changes in the measurement, allowing the operator to identify minute discrepancies that could affect the quality of the milling operation.

For instance, when setting up a CNC mill, it's essential to ensure that the workpiece is aligned correctly with the cutting tool. Using an indicator to touch off against the workpiece can help achieve this precise alignment, ensuring the milling process is efficient and results in the desired tolerances. This tool is particularly valuable because even slight misalignments can lead to significant errors in the machined part, making its use fundamental in professional machining practices.

While a drill bit is primarily used for creating holes and a caliper is useful for measuring dimensions, and a wrench is for tightening or loosening fasteners, none of these tools provide the precision measurement capability required to assess alignment as effectively as an indicator does. This precision is key in high-quality CNC operations.