Last time I changed my brake pads (Toyota Tacoma v6 4WD), I did not get my rotors turned and was told I should have taken them to a mechanic to get it done. I was wondering if there is any alternative to outsourcing the job and why a lathe is needed. E.g., why couldn't I mount some sort of a cup brush to a 4.5" grinder and just polish them to achieve a similar effect?
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The braking surfaces of a rotor require precision machining. The tolerances are in the .002" range. You would never come close without a quality lathe. Each manufacturer also has specifications for minimum total thickness, side to side variance for vented rotors, the smoothness of the finish and run-out which is the difference between the highest and lowest spot on the brake surface. A wire brush won't remove enough material to make any improvement.
You may find that an automotive machine shop may have better prices than your local garage. I have found that in many cases the cost of the labour for the machine work is close to the cost of a new rotor. The abrasive nature of the brake pad material means that many times the rotors will be close to the minimum thickness after they are machined. When the rotor is this thin it is more susceptible to warping. The warping is what causes the pedal pulsation during braking.
In a word, accuracy!
A lathe as used by a mechanic will have the precision required, and the instrumentation available, to ensure the rotors are turned evenly.
You'd be surprised at how much vibration you can get at 70mph from the smallest inconsistencies in rotor height - and how much you lose in stopping power!