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Twist Drill Bit (837)

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Twist drill is a tool for drilling round holes of workpieces by rotating cutting relative to a fixed axis. It is named because of the spiral shape of the chip flute, which resembles a twist.
Twist drill is a tool for drilling round holes of workpieces by rotating cutting relative to a fixed axis. It is named because of the spiral shape of the chip flute, which resembles a twist. Spiral grooves have 2 grooves, 3 grooves or more, but 2 grooves are the most common. Twist drills can be clamped on manual or electric hand-held drilling tools or drilling machines, milling machines, lathes and even machining centers. The material of the drill bit is generally high-speed tool steel or cemented carbide.
Standard twist drill. Twist drill is composed of shank, neck and working part.

Basic angle of twist drill:

(1) Helix angle β Helix angle is the angle between the outermost helix on 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 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 profile. 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 varies 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°. In this way, it can compensate for the effect of the reduction of the actual working clearance angle of each point on the main cutting edge due to the axial feed movement of the drill bit, and it 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 axis of the drill bit. 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 projected angle of the two main cutting edges on a plane parallel to it. 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 improved 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 bevel angle ψ Chisel edge bevel angle is the angle between the main cutting edge and the chisel edge projected on a plane perpendicular to the axis of the drill bit. 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°.
Twist drill processing impact:

(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; there are only two ribs to guide, the axis of the hole is easy to deviate; the chisel edge makes the 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 relatively 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 very 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 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.

Twist drill operation method:

"Small grinding" is first of all "no grinding". When you get the drill bit, you can sharpen it in a hurry. It must be blind grinding. Only by placing a good position before sharpening can we lay a solid foundation for the next step of "sharpening". This step is very important.
Here are four phrases to guide the sharpening process, the effect is better.
Formula 1: "The edge of the blade is flat against the surface of the wheel."
This is the first step in the relative position of the drill bit and the grinding wheel. Often students start sharpening by leaning on the grinding wheel before the cutting edge is leveled. This is definitely not good. Here, the "cutting edge" is the main cutting edge, and "swing flat" means that the main cutting edge of the sharpened part is in a horizontal position. "Wheel surface" refers to the surface of the grinding wheel. "Kai" means to move closer slowly. At this time, the drill bit cannot touch the grinding wheel.
Mantra 2: "The drill shaft releases the front angle obliquely."
This refers to the positional relationship between the axis of the drill bit and the surface of the grinding wheel. The "front angle" is half of the apex angle of 118°±2o, which is about 60°. This position is very important and directly affects the size of the drill bit's apex angle, the shape of the main cutting edge, and the bevel angle of the chisel edge. It is necessary to remind students to memorize the angles of 60° in a 30°, 60°, and 90° triangle plate, which is easy for students to master. Formula 1 and Formula 2 both refer to the relative position of the drill bit before sharpening. Both should be considered. Do not neglect the bevel angle for the purpose of leveling the cutting edge, or neglect the leveling of the cutting edge for the purpose of arranging the inclined axis. These errors often occur in actual operation. At this time, the drill bit is ready to contact the grinding wheel when it is in the correct position.
Mantra three: "grind the back from the edge to the back."
This refers to the slow sharpening along the entire flank surface from the cutting edge of the drill. This facilitates heat dissipation and sharpening. On the basis of stabilizing and consolidating formulas one and two, the drill can lightly touch the grinding wheel to perform a smaller amount of sharpening. When sharpening, observe the uniformity of sparks, adjust the pressure in time, and pay attention to the cooling of the drill. When the sharpening is restarted after cooling down, the positions of formula one and two must be continued. This is often not easy to grasp when beginners, and often involuntarily change the correctness of its position.
Formula four: "Do not swing the tail up and down."
This action is also very important in the process of sharpening the drill. Often students change the "swing up and down" to "turn up and down" during sharpening, causing the other main cutting edge of the drill to be destroyed. At the same time, the tail of the drill bit cannot be raised above the horizontal centerline of the grinding wheel, otherwise the cutting edge will be blunt and cannot be cut.
Disadvantages of twist drills:
Although the geometry of twist drill is more reasonable than flat drill, it still has the following shortcomings:
(1) The rake angle value at each point on the main cutting edge of a standard twist drill is too different from the inside and outside. The rake angle of the main cutting edge at the outer edge of the bit is about +30°; and near the core, the rake angle is about -30°. The rake angle near the core is too small, resulting in large chip deformation and high cutting resistance; and near the outer edge When the rake angle is too large, the cutting edge strength is often insufficient when machining hard materials.
(2) The chisel edge is too long, and the rake angle of the chisel edge is very negative, reaching -54°~-60°, which will generate a large axial force.
(3) Compared with other types of cutting tools, the main cutting edge of standard twist drills is very long, which is not conducive to chip separation and chip breaking.
(4) The secondary clearance angle of the secondary cutting edge at the edge zone is zero, which causes the friction between the secondary flank face and the hole wall to increase, the cutting temperature rises, the wear at the corners of the outer edge of the drill bit is greater, and the surface roughness of the processed surface deteriorates. .

The above defects often cause the twist drill to wear quickly, which seriously affects the efficiency of drilling and the improvement of the quality of the processed surface.

Twist Drill Bit


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