"What is an expanding drum?" you say. An expanding drum is exactly what it says it is; the rubberized drum has slots cut into its outer edge at a certain angle. You slip a belt over the drum and turn your bench grinder on. When the drum begins to spin the outer edge of the drum expands due to the centrifugal force. The slots in the edge move to a perpendicular relationship to the outer edge of the drum essentially making it larger and keeping the belt stable, yet pliable for your use. Great tool for those not interesting in purchasing a full fledged belt grinder. Arbor hole is bushed to 1" , 3/4", 5/8" and 1/2" Diamond, silicon carbide, cork, smoothing, and felt belts are available.