The wheels are very durable hardened steel bearings. When you roll the nut or saddle on sandpaper on a flat surface, the nut and saddle sander keeps the nut or saddle perfectly vertical.  

 Adjustable Height sander for bone, plastic and composite nuts and saddles.  

the Guyker version has a more secure fixing of the clamping bolts

 This tool is typically used after shaping a saddle or nut to its final shape.