The BMW brake rotors and discs are the recipients of the action portion of your braking system that brings your Bavarian vehicle to a safe stop. For the smoothest stopping motion, the surfaces of the brake rotors and discs should be flat with no gouges or other imperfections on them. The reason a technician might inform you that your braking rotors and discs have to be turned on a lathe is because damage has occurred to them. This only occurs if the surface of this brake part has been damaged by not replacing the pads soon enough. If you are applying the brakes on your BMW and hear a grinding noise, then rest assured your brake rotors and discs will have to be turned to remove the imperfections and make the surface smooth again. If by chance this is not the first time you have heard the grinding noise coming from your wheels, have the technician measure the thickness of your BMW brake rotors and discs. There is a minimal thickness these parts have to be so sufficient cooling will occur during the braking procedure on your BMW. If they are too thin the brake rotors and discs will have to be replaced. You can also always tell if the braking rotors and discs are exposed to excessive heat be just looking at the surface where the pads are being applied. If you see a discoloration to the metal, chances are there was a period of time when excessive heat was generated. If they are of sufficient thickness then the discoloration can be removed with Emory cloth. All brake work should be performed by a qualified technician to ensure the job is done correctly. While it is always fun to drive your BMW fast down the road, it is essential that you can bring it to a controlled stop.