3M™ Disc Pad Face Plate backs a same size fibre disc (sold separately) to provide firm support and the force required for grinding applications. It is used in conjunction with a disc pad hub (sold separately), which attaches to the power tool, and held into place along with the appropriate disc using a retainer nut (sold separately). This assembly allows 3M™ Disc Pad Face Plate to be easily changed without removing the disc pad hub, allowing the operator to use a single tool for multiple applications from heavy grinding to finishing a workpiece. Some face plates have ribbing on the bottom face. Ribbed face plates provide additional force when needed for optimal fracturing of the abrasive mineral. By reducing the area of contact, or pressure per square foot (PSI), ribbed face plates increase the grinding pressure of the abrasive with the same amount of operator effort, and extend disc life when dulling is a problem. Choose a plate with the level of firmness needed for the application—a lighter density face plate for lighter weight grinding applications, and a harder density face plate for more heavyweight grinding applications. Using the correct face plate for the application ensures that the abrasive mineral will fracture as it should, maximizing the life of the abrasive. The plate comes in one of four different colors that indicate the density so the correct plate can be easily identified in the workroom: red = extra hard (for heavy grinding with the coarsest grades), black = hard (for heavy metal removal, large welds, etc.), gray = medium (for general purpose grinding and finishing), and white = soft (for finishing operations on flat or contoured surfaces).