Rotatable cutting tool with hard cutting member

A rotatable cutting tool (30) for impinging a substrate while rotatably retained within a holder bore The rotatable cutting tool (30) includes a hard cutting member (70, 90, 110) affixed in a socket (44) at the axial forward end (34) of a cutting tool body (32)The hard cutting member (70, 90, 110) has an axial forward functional portion (80, 100, 120) which has a starting transverse functional dimension (C, C, C"), and further has a base portion (86, 98, 118) which has a maximum transverse base dimension (D, D', D")There is a ratio of the starting transverse functional dimension (C, C, C") to the maximum transverse base dimension (D, D', D") ranging between equal to 0 42 and less than 0 48