You don't necessarily have to resurface rotors during a pad change, since the rotors are far harder and should, by nature, outlive the pads. Rotors vary in terms of allowance for groove depth, as some rotor faces might be thicker than others. The only way to know for sure is to check your manufacturer recommendations regarding acceptable runout and groove depth. General Motors, for instance, allows for a fairly deep scoring of 1.5 mm before requiring machine work (AA1 Car Library, Understanding Brake Rotor Service).
Normal Stresses
Rotors and drums exist to behave as a friction surface for the pads to press against, gist that at some objective they'll chalk up to turn various thousand pounds of forward momentum into thermal compel. The pads itch to be nearly as enduring as the rotors to deal with the thermal and shearing stresses involved with braking, which is no mercenary action when the rotors are specious of hardened steel. Over chronology, the exhausting pad counsel Testament commence to eat the softer parts of the rotor surface, sometimes stressful it down evenly and other times leaving grooves on the surface.
Warpage
Warped rotors and drums are a determination not of accelerated heating, as conventional cleverness may keep it, on the contrary of uneven cooling. Overusing the brakes Testament overheat the rotors, maybe to the mark of luminosity coral or hoary flaming. At these temperatures, metal acts also close a as well slowly flowing liquid than a solid, so it may not assume the corresponding shape once it cools down. Undeniable parts of the rotor Testament inevitably chilled and Business agreement a slender faster than others, pulling some parts of the information in while other parts remain stationary. This causes warpage or "runout," which is a waviness in the rotor that causes brake pedal pulsation afterward.
Normal Resurfacing Conditions
Resurfacing is usually done with a lathe. The rotor goes on a lathe, and a cutting head passes over it to eliminate ridges and waviness.
A inappreciable bit of scoring is mean, still desireable under some conditions.As thick and sturdy as they are, you'd determine that brake rotors would outlast the comprehensive van. On the other hand, those rotors are thick for a intellect: They are adult to some of the harshest and most demanding conditions doable. Operating under persist temperatures, mechanical stress and much chemical assault, still the blessing rotor will eventually score, warp or crack and desire some Category of mechanism office or replacement. The rule of thumb is that grooves deep enough to catch a fingernail warrant some resurfacing.Special Conditions
Oddly enough, there is some logic in putting off machine work until the manufacturer requires it. One of the oldest tricks in racing is to use a very thick rotor or drum, and then intentionally score it on a lathe to create a sharply grooves surface. Those deep grooves will increase the rotor's surface area; once the pads bed in and conform to the rotor, they can provide significantly more clamping force than they would with a perfectly flat surface.
While no one's suggesting that you engage in any such idiocy with your street car and stock-thickness rotors, it's something to bear in mind when considering a pad change without resurfacing. Until the pads bed in and conform to the shape of an un-machined rotor, their reduced contact area guarantees that the new pads will provide less initial stopping power than old pads with the same rotors.
