The Bosch 25 pc. 4-1/2" 120 Grit X-LOCK Fine Grit Abrasive Fiber Discs (FBX45120) are made to work with a backing pad with an X-LOCK mount that offers a wheel change that's up to five times faster than conventional interfaces. There’s an audible snap when mounting an X-LOCK wheel on an X-LOCK grinder. The wheel mounts properly without the need for a spanner wrench or flange nuts. The wheel is released from the grinder with the pull of a lever. Bosch fiber discs are made with high-quality Zirconia grit for exceptional speed and durability in tough environments for abrasive sanding or blending applications. They are ideal for metal surface finishing, weld blending and rust removal.