4t6ford, that's exactly what it is, an on the car brake lathe. The hub looking part attaches to the hub of the car, and spins the disc, hub and all. The arm at the bottom of the photo has two lathe bits that face the disc both sides at once. Various companies made them, Grizzly Grinder was a popular brand.
A lot of cars that need that machine usually never get the rotors replaced EVER. I had a Nissan that needed that machine in the past and it was a bit*h to replace the rotors, but it was worth it in the long run, but it was pricey. Floating rotors are so much less of a h***le...
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