These straight flute thread mills have the root radius that is required for the external "UNJ" thread.
External - UNJ
SPTM Thread Mills - Internal Stub Acme
SPTM Thread Mills - Internal Acme
TM - Metric Thread Mills -30 degree Helical Flutes - Solid Carbide - Full Profile
TM - Metric Thread Mills -15 degree Helical Flutes - Solid Carbide - Full Profile
TM - Metric Thread Mills -Straight Flute - Solid Carbide - Full Profile