A twist drill is a tool for drilling round holes of a workpiece by rotating and cutting relative to a fixed axis. It is named because of the spiral shape of the chip flutes, which resemble twists. Spiral grooves have 2 grooves, 3 grooves or more, but 2 grooves are the most common. Twist drills can be clamped on manual and electric hand-held drilling tools or drilling machines, milling machines, lathes and even machining centers. The bit material is generally high-speed tool steel or cemented carbide.
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Standard twist drill. Twist drill is composed of shank, neck and working part.
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(1) Helix angle β Helix angle is the angle between the outermost helix of the spiral groove of the drill bit and the axis of the drill bit after unfolding into a straight line. Because the lead of each point on the spiral groove is the same, the helix angle at different diameters of the drill bit is different. The helix angle is the largest at the outer diameter, and the helix angle is smaller as it is closer to the center. Increasing the helix angle increases the rake angle, which is conducive to chip removal, but the rigidity of the drill bit decreases. The helix angle of standard twist drill is 18°~38°. For drills with smaller diameters, the helix angle should be smaller to ensure the rigidity of the drill.
(2) Rake angle γOm [Because the rake surface of the twist drill is a spiral surface, the rake angle of each point on the main cutting edge is different. From the outer circle to the center, the rake angle gradually decreases. The rake angle at the tip of the knife is about 30°, and it is about -30° near the chisel edge. The rake angle on the chisel edge is -50°~-60°.
(3) Clearance angle αOm [The relief angle of the selected point on the main cutting edge of the twist drill is expressed by the feed relief angle αOm in the point column section. The column profile is a straight line parallel to the axis of the drill bit through the selected point m of the main cutting edge, and a cylindrical surface formed by the rotation of the straight line around the axis of the drill bit. αOm also changes along the main cutting edge, the closer to the center the αOm increases. The relief angle α at the outer circle of the twist drill is usually 8°~10°, and the relief angle at the chisel edge is 20°~25°. This can make up for the effect of the reduction of the actual working angle of each point on the main cutting edge due to the axial feed movement of the drill bit, and can adapt to the change of the rake angle.
(4) Entering angle κrm The entering angle is the angle between the projection of the tangent to the selected point m of the main cutting edge on the base plane and the feed direction. The base surface of the twist drill is the plane that passes through the selected point of the main cutting edge and contains the drill axis. Since the main cutting edge of the drill does not pass through the axis line, the base surface of each point on the main cutting edge is different, and the entering angle of each point is also different. When the vertex angle is ground, the entering angle of each point is also determined. Entering angle and vertex angle are two different concepts.
(5) Frontal angle 2φ The frontal angle is the angle between the projections of the two main cutting edges on a plane parallel to them. A small front angle is easy to cut into the workpiece, the axial resistance is small, and the working length of the cutting edge is increased, and the nominal thickness of the cutting layer is reduced, which is conducive to heat dissipation and improving tool durability; if the front angle is too small, the strength of the drill will be weakened. The deformation increases, the torque increases, and the drill bit is easy to break. Therefore, a reasonable front angle should be sharpened according to the strength and hardness of the workpiece material. The front angle 2φ of the standard twist drill is 118°
(6) Chisel edge slant angle ψ Chisel edge slant angle is the angle between the main cutting edge and the chisel edge projected on a plane perpendicular to the drill axis. When the flank of the twist drill is ground, ψ is formed naturally. It can be seen from Figure 3-5 that the chisel edge angle ψ increases, the chisel edge length and axial resistance decrease. The bevel angle of the chisel edge of a standard twist drill is about 50°~55°.
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(1) The diameter of the twist drill is limited by the hole diameter. The spiral groove makes the drill core thinner and the drill bit rigidity is low; only two ribs are guided, the axis of the hole is easy to deviate; the chisel edge makes centering difficult, and the axial resistance increases Large, the drill is easy to swing. Therefore, the shape and position error of the drilled hole is large.
(2) The rake face and flank face of the twist drill are both curved surfaces, and the rake and relief angles of each point along the main cutting edge are different, and the rake angle of the chisel edge is -55°. The cutting conditions are poor; the distribution of cutting speed along the cutting edge is unreasonable, and the cutting edge with the lowest strength has the highest cutting speed, so the wear is serious. Therefore, the precision of the processed hole is low.
(3) The full edge of the main cutting edge of the drill bit participates in the cutting, and the cutting speed of each point on the edge is not equal. It is easy to form spiral chips and difficult to remove chips. Therefore, the chips and the hole wall are squeezed and rubbed, which often scratches the hole wall, and the surface roughness after processing is very low
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