A milling machine can execute a variety of operations such as planing, cutting, rebating, routing, die-sinking, and other complex tool-paths, making it one of the more versatile pieces of equipment in a machine shop. Milling machines provide flexible operations and low-cost maintenance. They are used for deburring, smoothing, shaping, cutting, and surface treatment work on metal, plastic, ceramic, and wood products. Milling machines can be divided into three main categories based on the method they use to perform their functions: plunge cutters, drum cutters, and rotary cutters. Plunge cutters are driven by a motor directly through a drive shaft attached to the rear of the unit. This type of cutter is useful for cutting relatively flat stock, such as sheet metal. Drum cutters have an axial-feed mechanism that drives the material under the blade. These cutters are preferred for cutting thicker materials like tubing or bars. Rotary cutters have a rotating head with blades that cut the material as it passes over them. These cutters are most effective for cutting thick materials like pipes and beams. Other types of milling machines include horizontal machining centers (HMCs) and vertical machining centers (VMCs). HMCs and VMCs are used for repetitive tasks such as drilling holes in large numbers at fixed locations.