a range of rotor hub widths and locking nut 40 is threadably engaged with threads 25 of arbor shaft 20 and drawn down against spacer rings 28 to compress rings 28 collets 26 and rotory hub 32 axially along arbor shaft 20 against arbor shaft base shoulder 23 such that hub 32 is centered about arbor shaft 20 by conical surfaces 27 of collets 26 tightening lock nut 40 also frictionally locks spacer rings 28 collets 26 hub 32 and lock nut 40 to rotate as a unit with arbor shaft 20 a motor not shown is located within lathe body 12 to rotate arbor shaft 20 and rotor 30 when mounted thereon referring to the plan view of fig3 after rotor 30 is mounted on shaft 20 tool holder 16 is arranged to hold cutting tools 18 in cutting engagement with braking surfaces 36 of disk 34 to cut material from surfaces 36 as rotor 30 is rotated by the lathe motor when lathe 10 is used to restore braking surfaces 36 tool holder 16 is first positioned to hold tools 18 close to arbor shaft 20 inboard or braking surfaces 36 and tools 18 are set to the desired cutting depth tool holder 16 then moves radially outward away from arbor shaft 20 as arbor.