Milling in hard materials was always a challenge for robots given their low stiffness. This lead to the robot's TCP being pushed away from the programmed path by the acting process forces.
The solution is to simulate the occuring process forces, material and tool dependent, and subsequently to compensate these forces to increase the robot's path accuracy and thus the workpiece's quality. For example a milling application on a complex contour with a robot leads to a workpiece tolerance of +-0.2mm, a good result for a robot roughing application. Check the application in the video, material is aluminium alloy EN AW-6005A: