Twist drill is the most widely used hole machining tool. Usually in the range of 0.25 to 80 mm in diameter. It consists mainly of working parts and handles. The working part has two spiral grooves, shaped like twist, hence the name. In order to reduce the friction between the guide part and the hole wall during the drilling, the twist drill is gradually tapered from the drill tip toward the handle. The helix angle of the twist drill mainly affects the size of the rake angle, the flap strength and the chip removal performance, usually 25 ° ~ 32 °. The helical groove can be machined by milling, grinding, hot rolling or hot extrusion. The front end of the drill bit is sharpened to form a cutting part. Standard twist drill cutting part of the vertex angle of 118, horizontal blade angle of 40 ° ~ 60 °, the rear angle of 8 ° ~ 20 °. Due to structural reasons, the rake angle at the outer edge of the large, gradually reduced to the middle, the horizontal edge at the negative rake angle (up to -55 °), when drilling from the extrusion effect. In order to improve the cutting performance of the twist drill, the cutting part can be ground into various shapes (such as group drills) according to the nature of the material being processed. Twist drill handle in the form of two kinds of straight handle and taper shank, the former is caught in the drill chuck, the latter inserted in the machine tool spindle or tailstock cone hole. General twist drill made of high speed steel. Inlaid carbide or blade crown twist drill for processing cast iron, hardened steel and non-metallic materials, etc., the overall carved carbide twist drill for processing instrument parts and printed circuit boards and so on.