On 9/20/2013 3:14 PM, Edward Maglott wrote: > This machine was turning both sides at the same time and it was easy to > stop it and see that the high areas on one side perfectly matched the low > areas on the other side. I'll have to ponder this in my head for a while > to think of ways that the disk is not "warped" but has high and low areas > that correspond....

If you stick a dime under the disk before bolting it on, the same thing happens. It's not warped, it only means that the plane of the disc isn't perfectly normal (perpendicular) to the axis of rotation. That could be either on the car, or on the lathe. A bit of rust is enough to do it, you're only talking mils. I always wire brush the hubs before installing new rotors (and inside the rotor hat if reinstalling old ones).

When it's on the car, it can cause a bit of vibration, but it usually doesn't cause much pulsation (i.e. the brakes won't seem to grab harder at one point of their rotation) - the calipers simply move sideways to accomodate.