Supplies needed to build a bat rolling machine include the shell, upper carriage, bearings, shafts and rollers. Handles are also necessary to build a bat rolling machine. Bat rolling machines are designed to roll either perpendicular to the bat or parallel. Perpendicular rollers only roll a bat one way and may leave flat spots. For this reason, parallel bat rollers are recommended. Parallel rollers usually allow the bat to roll both ways as well as the entire "sweet spot."
The shell is typically made of steel or aluminum and is often welded or screwed together. The shell is what holds the "guts" of the bat rolling machine together. The upper carriage moves up and down, and it holds the upper rollers. The three types of bearings needed include roller and thrust bearings and bushings. Rollers are made of plastic, steel, wood or some other material and are machined on a metal lathe. The tolerances are very tight, and the roller's center hole must be precise.
The aim of bat rolling is to target the bat's "sweet spot." While the "sweet spot" of a bat doesn't have a specific location on every bat, it's defined as optimum place on a bat where the contacted ball speed is the highest and vibrations felt in the hands are minimized.