There's a TV programme called "How It's Made" which often shows automation of mass-produced machined parts. Very often, it seems that metal parts are machined (especially drilled) by rotating the part against a stationary tool rather than holding the part and drilling into it.
What is the advantage of doing it this way? I've never been able to work it out.
You can be sure the resulting surface is round.
If you used a router and rotated the bit around the piece then you would get inaccuracies due to the movement being harder to control.
Using a lathe-like setup the cutting tool is just a chisel that is cheaper to replace and refurbish when it's worn down than a router bit.
Maintaining a co-linear hole/feature with the axis of rotation of the part. Automatic lathes with bar feeders have also been around for multiple decades. These machines can be "programmed" to make that particular part, in high speed, without any need for an operator. Milling or drilling that feature in a round part (which is more difficult to hold in such a set up) would require much more to set up square and maintain concentric to the outside diameter. These machines can also be set up to run autonomously (CNC) however they would require a robotic arm to load and un load parts into the machine.
Another possibility to consider is that the part is on a moving track and can then proceed to the next station / operation.
