Brake Discs get used often during the driving process. Because we are constantly applying the brakes when we come to a stop it is necessary to get them checked often.
Not all cars and vehicles have the ability to detect if there is damage on the brake disc. The only way to know when the discs need replacing is by listening to the sound they make whilst you are coming to a stop or if you look at the surface directly. Most worn out or damaged brake discs will have pieces missing or have an uneven surface because of the wear.
Sounds to listen out for are squeaks, squeals and grinding. Sometimes a little smudge of dirt is on the surface of the disc and can easily be cleaned. Often you can tell if there is dirt on the surface if the noise is not consistent. Consistent noise means that there is wear along a certain point where the brake pad passes.
Sometimes, to even the surface of the brake disc, a mechanic has to use a lathe to get it back to the way it was before. A lathe is a machine that has a stationary cutting piece that rotates the workpiece. By clamping the disc, the face being parallel to the clamp, the machine spins the workpiece and from there you can move the cutting piece on the surface of the disc so that it can remove any bumps sticking out. This should only be done if the grooves are small. Too much removal of the surface will make the disc too think.
The face of the brake disc has a shiny appearance and a few grooves. Irregular grooves mean that there is wearing on the disc. You can easily feel for this with your finger after the brakes have cooled down. Another texture to feel for is rough patches along the face. When you feel the irregular grooves and rough patches it is time to get new brake discs.
Some mechanics can inspect faulty brake pads through the wheel to reduce the amount of time it takes for an inspection. Of course, if you are doing it yourself and are unable to see the braking system through the wheel you should remove the tyre and the wheel for better vision of the disc.