Twist drill is the most widely used hole cutting tool. The diameter usually ranges from 0.25 to 80 mm. It mainly consists of a working part and a handle. The working part has two spiral grooves shaped like a twist, hence the name. In order to reduce the friction between the guide part and the hole wall during drilling, the twist drill gradually reduces the diameter from the drill tip to the shank and presents an inverted cone shape. The spiral Angle of twist drill mainly affects the size of the front Angle of the cutting edge, the strength of the blade flap and the chip discharge performance, which is usually 25° ~ 32°. The spiral groove can be processed by milling, grinding, hot rolling or hot extrusion, etc. The front end of the bit is ground to form a cutting part. The standard twist drill has a cutting part top Angle of 118, a cross edge bevel of 40° ~ 60°, and a back Angle of 8° ~ 20°. Due to structural reasons, the front Angle is large at the outer edge and gradually decreases toward the middle. The transverse edge is negative front Angle (up to about -55°), which ACTS as a squeeze during drilling. In order to improve the cutting performance of twist drill, the cutting part can be modified and ground into various shapes (such as group drill) according to the nature of the processed material. The shank form of twist drill has two kinds: straight shank and taper shank. During processing, the former is clamped in the drill chuck, while the latter is inserted in the taper hole of machine tool spindle or tail seat. Common twist drills are made of high speed steel. The twist drill with welded carbide inserts or crowns is suitable for machining cast iron, hardened steel and non-metallic materials, etc. The integral twist drill is used for machining instrument parts and printed circuit boards, etc.