Twist Drill is the most widely used hole machining Tool. The usual diameter range is 0.25 to 80 mm. It is mainly composed of bit working part and shank. 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 when drilling, twist drill from the drill tip to the handle direction gradually reduce the diameter of the inverted cone. The screw Angle of twist drill mainly affects the size of cutting edge Angle, edge edge strength and chip removal performance, which is usually 25° ~ 32 °. Spiral grooves can be processed by milling, grinding, hot rolling or hot extrusion, and the front end of the Drill Bit forms a cutting part after grinding. The cutting Angle of the standard twist drill is 118, the bevel Angle of the cross edge is 40° ~ 60 °, and the back Angle is 8° ~ 20°. Due to structural reasons, the rake Angle is large at the outer edge and gradually decreases to the middle, and the transverse edge is negative rake Angle (up to about -55°), which acts as extrusion when drilling.
In order to improve the cutting performance of twist drill, the cutting part can be ground into various shapes according to the properties of processed materials (such as group drill). Twist drill shank form has straight shank and cone shank two kinds, processing the former clip in the drill chuck, the latter inserted in the machine tool spindle or tail seat of the cone hole. The general twist drill is made of high speed steel. Insert welding carbide blade or crown twist drill for the processing of cast iron, hardened steel and non-metallic materials, such as the whole cemented carbide small twist drill for the processing of instrument parts and printed circuit boards.
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