What Is a Twist Drill?

Twist drill is a tool for drilling circular holes of a workpiece by its rotary cutting relative to a fixed axis. It is named because its chip groove is spiral and shaped like a twist.

twist drill

Twist drill is a tool for drilling circular holes of a workpiece by its rotary cutting relative to a fixed axis. It is named because its chip groove is spiral and shaped like a twist. Spiral grooves have 2 grooves, 3 grooves or more, but 2 grooves are the most common. Twist drills can be clamped in manual, electric hand-held drilling tools or

Standard twist drill. Twist drill consists of shank, neck and working part.

(1) Helix angle The helix angle is the angle between the helix of the outermost circle on the helical groove of the drill and the straight line of the drill. 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 at the outer diameter is the largest, and the closer to the center the helix angle is smaller. Increasing the helix angle increases the rake angle, which is conducive to chip removal, but the drill bit stiffness decreases. The standard twist twist angle is 18 ° ~ 38 °. For drills with smaller diameters, the helix angle should be smaller to ensure the stiffness of the drill.

(2) Rake angle Om [Because the rake face of the twist drill is a spiral surface, the rake angle of each point on the main cutting edge is different. The front angle gradually decreases from the outer circle to the center. The rake angle at the tip is about 30 °, and it is about -30 ° near the side.

(1) The diameter of twist drills is limited by the hole diameter. The spiral groove makes the drill core thinner and the drill bit low in rigidity. With only two edges to guide, the axis of the hole is easy to deflect. Large, the drill bit 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 curved surfaces, and the rake angle and rake angle of each point along the main cutting edge are different. 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 severe. Therefore, the accuracy of the machined hole is low.

(3) The full cutting edge of the main cutting edge of the drill participates in cutting, and the cutting speeds of the points on the cutting edge are not equal, and it is easy to form spiral chips, and it is difficult to remove chips. Therefore, the chips and the hole wall are squeezed and rubbed, often scratching the hole wall, and the surface roughness after processing is very low.

"Less grinding" means "not grinding". If you get a drill in a hurry, it must be a blind grinding. Only by setting the position before sharpening can we lay a solid foundation for the next "sharpening", this step is very important.

Here four guidelines are used to guide the sharpening process, the effect is better.

Tip one: "The edge of the blade is flat against the wheel."

This is the drill bit with

Although the geometry of twist drills is more reasonable than flat diamonds, it still has the following disadvantages:

(1) The value of the rake angle at each point on the main cutting edge of a standard twist drill is too large. The rake angle of the main cutting edge at the outer edge of the bit is approximately + 30 °; while near the core, the rake angle is approximately -30 °; the rake angle near the core is too small, resulting in large chip deformation and high cutting resistance; and near the outer edge The front angle is too large, and the cutting edge strength is often insufficient when processing hard materials.

(2) The horizontal blade is too long, and the rake angle of the horizontal blade is a large negative value, which is -5 4 ° ~ -6 0 °, 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 rake angle of the secondary cutting edge at the blade is zero, which causes increased friction between the secondary flank and the hole wall, increases the cutting temperature, causes greater wear at the corners of the outer edge of the drill, and deteriorates the roughness of the processed surface. .

The above defects often make twist drills wear quickly, which seriously affects the drilling efficiency and the quality of the processed surface.