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.

Here are some samples of nested parts i did recently

In both cases back of all pieces were machined at the same time, dowel holes milled so that there would be a way to align top and bottom.

Ealot of material was lost, but it was a scrap anyways, so all i gained was alot of saved man-hours.

And here are 4 more pictures: