Sensing micrometer-thick irregularities
Hard metallic materials are sometimes affected by a phenomenon known as "sinking" when they are machined on a milling machine. This is characterized by an unevenness of a few micrometers on the underside of the surface that is being worked on. This lessens the precision of the machining.
Mr. Igarashi took out an oil-based marker pen and colored part of the reverse side with it. The extremely thin layer of ink from the pen adhering to the metal is enough to smooth out this unevenness. If the opposite side is flat, "sinking" will not occur. If he perceives an unevenness of a few micrometers using his fingertips, he applies the marker pen.
As Mr. Igarashi says, "With concentration and experience, it is possible to sense tiny protrusions with your fingertips, but although we all possess this sensitivity, it requires effort to develop it and turn it into a skill."