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
Metric Straight Flute Thread Mills - Solid Carbide - Full Profile
UN / Metric - Single Profile Thread Mills - Starting at .120 to .488 inch diameter
Acme Internal SPTM Thread Mills
Metric 30 degree Helical Flute Thread Mills - Solid Carbide - Full Profile