Mandrels are a BITCH! Buy NEW mandrels, as you will seldom be able to remove one without snapping off the bolts. Use anti-seeze liberally on the bolt threads of the new one.
PS Always keep the old belt for the blades, new belts will not slip, and cause damage when you hit something. Nice worn , old belts slip enough to let the blades survive when you hit a rock or a dog toy.