Mark, Had a crown prep today that looked like your first one. Premolar with one cusp fractured off. Patient hated the sound of the drill soooooo fiqured I give it a try.

Did not have the time to take photos and truthfully I don’t think I wanted to post them if I did. Talk about ugly!
One thing I noticed is that the interproximals preped quickly coming from the occlusal. Trying to cut depth marks at the incisal was a lost caused. Seem to take forever . Coming from the facial was easier. I finally dragged out the highspeed to finish the prep off. Mainly for the margin placement and to get a second plane of reduction on the facial. My total time was the same as using just the highspeed.
Now I have questions.
Since poor reduction is a major problem in crown preps. How are you gauging your reduction? With conventional you just sink a depth cutting bur on the occlusal then the buccal in two planes and finally the lingual. Reduce the occlusal to your depth cuts then the facial, the lingual and finally the interproxmal .
How are you placing your margins? What are you doing to smooth them? Hand instruments?
Are you doing anything to protect the adjacent teeth in the interprox area?

This is pretty wild. My assistant still cannot believe it.

David