What is typically measured to check for “runout” in a milling operation?

Runout in a milling operation typically refers to the precision with which the cutter spins around its intended axis. This measurement is crucial because any deviation from a perfectly concentric rotation can lead to issues such as uneven cutting, poor surface finish, and accelerated tool wear. Measuring the rotational accuracy of the cutter's axis helps ensure that the tool is aligned correctly, minimizing the potential for errors in the milling process.

Other aspects such as the alignment of the vise or the milling machine's vibration play important roles in the overall setup and operation, but they do not directly assess the runout of the cutter's axis. Similarly, measuring the depth of the last cut made provides information about the machining process but does not relate to the cutter's rotational accuracy. Therefore, the most relevant and direct measure for checking runout is indeed the rotational accuracy of the cutter's axis.