In order to reduce the friction between the guide part and the hole wall when drilling, the straight shank twist drill gradually decreases in diameter from the drill tip to the shank and assumes an inverted cone shape. The helical angle of the straight shank twist drill mainly affects the size of the rake angle on the cutting edge, the strength of the blade flap and the performance of chip evacuation, usually 25 ° ~ 32 °.
The spiral groove can be processed by milling, grinding, hot rolling or hot extrusion. The front end of the drill bit is sharpened to form a cutting part. The cutting angle of the standard straight shank twist drill is 118, the oblique angle of the transverse blade 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 negative rake angle at the transverse edge (up to about -55°) acts as a squeeze during drilling. In order to improve the cutting performance of straight shank twist drills, the cutting part can be ground into various shapes (such as group drills) according to the nature of the processed material.
Straight shank twist drills have two types of shank: straight shank and taper shank. The former is clamped in the drill chuck while the latter is inserted in the taper hole of the machine tool spindle or tailstock. Generally straight shank twist drills are made of high speed steel. Straight shank twist drills with welded carbide blades or crowns are suitable for processing cast iron, hardened steel and non-metallic materials, and solid carbide small straight shank twist drills are used for processing instrument parts and printed circuit boards.
What are the materials of straight shank twist drill
Straight shank twist drill material: alloy steel, alloy cobalt, alloy tungsten, pure tungsten steel, pure gold steel, etc.
Straight shank 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 working part and handle. The working part has two spiral grooves, shaped like a twist, hence the name.
Twist drill structure
Twist drills can be divided into three parts according to their functions:
1. Drill shank: the part of the drill bit used for clamping, and used to transmit the power (torque and axial force) required for drilling.
2. Drill neck: located at the transition between the cutter body and the drill shank. It is usually used as an empty slot for retraction of grinding wheels.
3. Drill body: the working part of the drill bit. It consists of cutting part (ie drill tip) and guide part.
Generally, taper shank twist drills are used for diameters above 13mm, and straight ones are used for the following. Mainly consider the convenience of clamping and sufficient clamping force. It is obviously not cost-effective to make a small drill bit into a cone, so it is made into a straight shank.
The difference between straight shank twist drill and taper shank twist drill
The performance of these two kinds of drills is the same, but the clamping part of the drills is different. Generally, the drills below 13 mm are straight shank drills, which is convenient for bench drills or hand drills. The drills larger than 13 mm are tapered shanks The drill bit is directly used for vertical drilling but the small size drill bit (greater than 13 mm) should be used with a reducing sleeve.