Grinding is done with a wide range of machines, including: Knife turned by hand -stones for sharpening (grindstones) Angle grinders and die grinders are examples of portable power tools. Grinding machines are many types of pricey industrial machine equipment. They usually consist of a housing in which a rotary grinding wheel is mounted on a shaft that can be driven by an electric motor or another form of engine. The most common type of grinding machine is the horizontal grinder, also called a belt grinder, disk grinder, or even plane. This is the machine used to smooth wooden surfaces, such as cutting boards and furniture. The abrasive material used in these machines is called sandpaper. The grit size of the paper determines how fine a finish can be achieved. For example, 100-150-180-220-240-280-320-350-400 grit papers can be used to smooth wood surfaces. The coarser the paper, the slower it will cut. Horizontal grinders are easy to use and maintain and they provide consistent results. The main disadvantage is that they can only work on one surface at a time. Another common type of grinding machine is the vertical grinder. These machines have two heads: One head has a cylindrical cavity in which a rod called a quill is inserted. The other head holds the sandpaper roll.