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