This is an example of a component made out of a high grade, high tensile material which is somewhat difficult to tap. We have had issues in the past about getting a good surface-finish.
There are key differences between thread-milling and normal tapping.
Tapping, it’s like a screw thread with teeth in it. We just drive the tap down and come back out. It’s taken a fairly heavy cut because it has to form those teeth in one pass.
Contrast this against, thread-milling, which is another way of forming a thread. We spin this at high speed, and then what we do, is you go down into the hole which through a process of helical interpolation spiral back out of that hole which is what cuts the thread.
The advantage being is we are only cutting one tooth at a time. This gives us much more control over the thread. We can control the size, as well as the finish.
Overall we get a much better finished product and a more reliable process.
A thing to note is we can only do this on a CNC Machine, because you have to be able to spiral that out.
You can do this on a manual machine, but longevity, reliability and quality, thread-milling is the way to go.