To cut a flat, smooth seating surface for a bolt head or nut on the inaccessible side of a workpiece. A specialized arbor is passed through the hole, a spotfacing cutter is attached to it on the far side, and the spindle pulls the cutter into the material. It provides a square bearing surface on rough castings or angled faces where standard machining cannot reach.
Heard on the job
The casting is too awkward to flip, so we'll just back-spotface the mounting bosses.