These are used for polishing metal surfaces by grinding away the marks left by the file or by machine-cutting tools. They are frequently seen in the forms known as emery, carborundum, and crocus cloths. These are made up of a cloth of considerable strength, covered on one side with fine particles of the abrasive material, held by a coating of glue. These cloths are numbered according to the coarseness of the grains glued thereon. Crocus cloth is covered with red oxide of iron, and is used for fine polishing.

Fig. 214.   Surface Plates.

Fig. 214. - Surface Plates.

Powdered grinding materials are also frequently used mixed with oil for grinding between two metal surfaces in contact.