A few years ago, a series of videos were posted to YouTube on the
subject of "Building Prototypes", by Dan Gelbart. I found them quite
interesting: I'd thought I basically knew how things were made, but
this was a whole 'nother set of tricks.
Now he's come out with another video in the series, on metal 3D
printing. Or, well, it starts out as a general video on the subject,
but halfway through it turns into something of an advertisement for
his own new metal 3D printer ("Rapidia" brand). The tone, though, is
still that of a professorly explanation:
https://www.youtube.com/watch?v=nyYcomX7Lus