Originally Posted by
JaredP
the machine doesn't do "everything"
the operator is responsible for setups, including fixturing work and indicating and zeroing tools. There is plenty of room for error here.
before that, the operator is responsible for maintaining the machine. Making sure the mill is trammed and square. Making sure the vice is trammed and square. Zeroing tool heights. Maintaining tools, measuring wear, etc.
The CAM programmer also needs to be good at his job. He needs to set feeds & speeds, making sure he is cutting within the constraints of his tooling and machine to ensure sufficient surface finish. He needs to use appropriate toolpaths for appropriate cuts, in the appropriate order, with realistic setups.
All of these considerations are subject to human error, and are exacerbated by trying to manufacture faster than you staff and machines are capable of.
i don't know what your understanding of CNC is, but it's nowhere near as simple as clamping stock in a machine and pressing go.