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The twist drill is the most widely used hole machining tool. Usually, the diameter ranges from 0.25 to 80 mm. It is mainly composed of the working part of the drill bit and the shank. The working part has two spiral grooves, which are shaped like a twist, hence the name. In order to reduce the friction between the guide part and the whole wall during drilling, the twist drill gradually decreases in diameter from the drill tip to the shank to form an inverted cone shape. The helix angle of the twist drill mainly affects the size of the rake angle on the cutting edge, the strength of the blade, and the chip removal performance, usually 25° to 32°. The spiral groove can be processed by milling, grinding, hot rolling, or hot extrusion, and the front end of the drill bit is sharpened to form a cutting part. The apex angle of the cutting part of the standard twist drill is 118, the bevel angle of the chisel edge is 40°~60°, and the relief angle is 8°~20°. Due to structural reasons, the rake angle is large at the outer edge and gradually decreases toward the middle, and the chisel edge is a negative rake angle (up to about -55°), which acts as a squeeze during drilling.
In order to improve the cutting performance of twist drills, the cutting parts can be ground into various shapes (such as group drills) according to the properties of the material to be processed. The shank of the twist drill has two types: straight shank and tapered shank. The former is clamped in the drill chuck during processing, and the latter is inserted into the taper hole of the machine tool spindle or tailstock. Generally, twist drills are made of high-speed steel. Twist drills with welded carbide inserts or crowns are suitable for processing cast iron, hardened steel, and non-metallic materials, etc. Solid carbide small twist drills are used for processing instrument parts and printed circuit boards.
The cutting part of the flat drill is shovel-shaped, with simple structure and low manufacturing cost. The cutting fluid is easily introduced into the hole, but the cutting and chip removal performance is poor. There are two types of flat drill structures: integral type and assembled type. The integral type is mainly used for drilling micro-holes with a diameter of 0.03 to 0.5 mm. The assembled flat drill blade can be replaced, and internal cooling can be used. It is mainly used for drilling large holes with a diameter of 25 to 500 mm.
deep hole drill
A deep hole drill usually refers to a tool that processes holes with a ratio of hole depth to a diameter greater than 6. Commonly used are gun drill, BTA deep hole drill, jet drill, DF deep hole drill, etc. Nesting drills are also commonly used for deep hole drilling.
The reaming drill has 3 to 4 teeth, and its rigidity is better than that of the twist drill. It is used to expand the existing hole and improve the machining accuracy and finish.
The countersink has more teeth, and the hole end is processed into the desired shape by the forming method, which is used to process the countersunk head hole of various countersunk head screws or to cut the outer end face of the flat hole.
The center drill is used for drilling the center hole of the shaft workpiece. It is essentially a combination of a twist drill with a small helix angle and a countersink, so it is also called a compound center drill.
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