When should you threadmill instead of tapping?

Threadmilling is advised when the Thread Diameter is above 6mm and the Thread depth is above 2xD.
Also, when the component is valuable or the material is exotic, we would look at Threadmilling as a much safer option.

On the contrary to Threadmills, we would look at Threadwhirling when the size is below 6mm Diameter (Although our range does go upto M20) and the depth is above 1.5xD.