This is a brake rotor removed from the vehicle. Also known as the disc of “disc brakes.”
Sometimes when applying the brakes you get a shudder or vibration, it’s the result of a brake rotor that needs to be resurfaced or machined. The rotor above is mounted on a brake lathe in the shop and we resurface them here on site. Also known as machining the rotors.
This is after the first pass of the lathe. Do you see how there is a light and dark spot on the face of the rotor, that’s how warped it is, the lathe only cut about half the face or surface of the rotor on the first pass.
See how shiny the complete surface of the rotor is? This what a resurfaced or machined rotor looks like. Now the surface is nice and flat. This will give the new brake pads a nice surface to break into.
And this will also eliminate the vibration or shudder when applying the brakes. But, rotors must be thick enough to resurface. All rotors come with a number usually cast into the back side that tells what the minimum thickness is.
You don’t want to machine a rotor below this specification, if you do, the brakes tend to run hotter and don’t last as long. I use a micrometer to measure the rotors when inspecting the brakes so I can tell if the vehicle needs new ones or if they can be machined.