Replaceable cutting head having external thread with concavely curved root and rotary cutting tool

A replaceable cutting head (22), having a head longitudinal axis (A) extending in a forward (DF) to rearward direction (DR), comprising

• a forward portion forming a cutting portion (26) comprising at least one cutting edge (30);
• and
• a rearward portion forming a mounting portion (28), the mounting portion (28) comprising a male coupling member (38) having an external thread (42) and protruding rearwardly from a head base surface (40), the head base surface (40) extending transversely with respect to the head longitudinal axis (A), and defining a boundary between the cutting portion (26) and the mounting portion (28), wherein:
• the external thread (42) comprises an external thread ridge (44) extending helically about an external thread axis (B) and comprising forward and rearward external flank surfaces (46, 48) and an external top surface (50) extending therebetween;
• the forward and rearward external flank surfaces (46, 48) generally face in opposite axial directions (DF, DR) and delimit a helical external thread groove (52) that comprises an external bottom surface (54);
• the external thread (42) has a constant external thread pitch PE;
• the external thread (42) has a constant external thread height HE;
• the external thread (42) of the male coupling member (38) is a straight thread defined by external inner and outer cylinders (EC1, EC2), the external inner cylinder (EC1) having a minor thread diameter d2 and the external outer cylinder (EC2) having a major thread diameter d1;
• and
• in a cross-sectional view taken in an axial plane containing the external thread axis (B):
• the forward and rearward external flank surfaces (46, 48) form a plurality of straight external loaded surfaces (59a) and a plurality of external non-loaded surfaces (59b) respectively;
• the external bottom surface (54) forms a plurality of concavely curved external thread roots (58), each external thread root (58) extending between first and second external root points (P1, P2) and merging with a respective external loaded surface (59a) at the first external root point (P1), the first external root point (P1) is spaced apart from the external inner cylinder (EC1) by a first external radial distance ERD1;
• and
• the first external radial distance ERD1 is greater than a third of the external thread height HE and less than two thirds of the external thread height HE.