Drills are cutting tools used to remove material to create holes, almost always of circular cross-section. Drills come in many sizes and shapes and can create different kinds of holes in many different materials. In order to create holes drill bits are usually attached to a drill, which powers them to cut through the workpiece, typically by rotation. The drill will grasp the upper end of a bit called the shank in the chuck. Drills come in standardized drill bit sizes. A comprehensive drill bit and tap size chart lists metric and imperial sized drills alongside the required screw tap sizes. There are also certain specialized drill bits that can create holes with a non-circular cross-section. Most drill bits for consumer use have straight shanks. For heavy duty drilling in industry, bits with tapered shanks are sometimes used. Other types of shank used include hex-shaped, and various proprietary quick release systems. The diameter-to-length ratio of the drill bit is usually between 1:1 and 1:10. Much higher ratios are possible (e.g., "aircraft-length" twist bits, pressured-oil gun drill bits, etc.), but the higher the ratio, the greater the technical challenge of producing good work. The best geometry to use depends upon the properties of the material being drilled.http://www.daotian-tech.com/grinding-wheel/