Originally Posted by MisterLister

fix the brake disk into a drill and with a file; file down the edge while the drill rotates the disk.

or

i used a machine screw and a nut to lock the disk onto the screw, then placed the assembly into a drill. glued a piece of wet and dry to a block, fixed the block into a vice. started the drill and rubbed the disk against the wet and dry until the correct diameter was reached.

remember safty first, a good clear head visor will stop any dust/metal from hitting your face.