arbors are made from hardened tools steel and precision ground between centers.  The arbors are manufactured to very close tolerances so there is no change in the straightness of the lap when it is expanded.  The Lap is mounted on the Arbor by inserting the Arbor through the hole in the Lap.  The Lap and Arbor have locking tapers to provide rotation and expansion.  Expansion is achieved by moving the Lap laterally on the arbor.  When moved the slot allows the Lap to expand.Expanders are made of hardened tool steel.  The Expander is used by butting the end with the hole against the Lap and tapping with a hammer.