Before we start milling away our stock we first need to get down to the required depth.
This is not a problem with external features when we can plunge outside.
When machining closed pockets, however, we need to find a way to get down to the machining depth first.
As usual there are several ways to get the job done. The plunging methods listed here are not ordered by their preference.
For various machining operations on different materials some may be more preferable than others.
Straight Plunging into a larger Pre-Drilled hole
This is one the best ones in my opinion.
Very few machining modes can compete in effectiveness with drilling and this method will get you the best combined tool life on most materials and (in case of many deep pockets) the least machining time, even when tool change time is factored in.