Speed: As a rule, tapping is slightly faster than thread milling.
Depending on the workpiece material, thread depth, and machine tool
speed
the 1/4"-20 thread discussed so far might take 4-5 seconds to tap,
and perhaps twice that to mill with a full-profile cutter
What are the benefits of thread milling?
The primary advantage of thread milling is the ability to control
the fit.
A threaded hole is milled at a high RPM and the tool helixes into a
previously-milled hole.
So, the machine operator has the ability to adjust thread size
using a strategy similar to using an end mill, rather than a drill
bit to make a hole.
What is the difference between a thread mill and tap?
Thread milling achieves a better thread quality than tapping
because the threads are being properly machined with the clearance
needed to evacuate the chip, using a tool that is smaller than the
threads themselves.
Tapping uses a tool the same size as the thread, forcing the chip
through the thread form for evacuation.