Twist drill introduction
Twist drill is a tool for drilling round holes in workpieces through its rotary cutting relative to a fixed axis. It is named because of its chip-containing groove is spiral and resembles a twist. Spiral grooves have 2, 3 or more grooves, but 2 grooves are the most common. Twist drill can be clamped in manual, electric handheld drilling tools or drilling machine, milling machine, lathe and even machining centers. The drill material is usually high-speed tool steel or carbide.
Twist drill specifications
Φ1.0, Φ1.5, Φ2.0, Φ2.5, Φ3.0, Φ3.2, Φ3.3, Φ3.5, Φ3.8, Φ4.0, Φ4.2, Φ4.5, Φ4.8, Φ5.0, Φ5.2, Φ5.5, Φ5.8, Φ6.0Φ, 6.2, Φ6.5, Φ6.8, Φ7.0 Φ7.0, Φ7.2, Φ7.5, Φ7.8, Φ8.0, Φ8.2, Φ8.5, Φ8.8, Φ9.0, Φ9.2, Φ9.5, Φ10.0, Φ10.2, Φ10.5, Φ11.0, Φ12.0, Φ12.5, Φ13.0, Φ13.5, Φ14.
Disadvantages of twist drills
Twist drills have the following geometry Although the shape is more reasonable than the flat drill, there are still the following shortcomings:
(1) The value of the leading angle at each point on the main cutting edge of the standard twist drill is too different from the inside and outside. Drill at the outer edge of the main cutting edge of the front angle of about +30 °; and close to the center of the drill, the front angle of about -30 °, near the center of the drill front angle is too small, resulting in large chip deformation, cutting resistance; and close to the outer edge of the front angle is too large, in the machining of hard materials, the strength of the cutting edge is often suspected of insufficient.
(2) the cross blade is too long, the front angle of the cross blade is a large negative value, up to -54 ° ~ -60 °, which will produce a large axial force.
(3) Compared with other types of cutting tools, the main cutting edge of the standard twist drill is very long, which is not conducive to chip separation and chip breaking.
(4)The vice rear angle of the vice cutting edge at the edge band is zero value, which causes the friction between the vice rear face and the hole wall to increase, the cutting temperature to rise, the wear at the corner of the outer edge of the drill is larger, and the roughness of the machined surface deteriorates.
Twist drill operating method
"Less grinding" first is "not grinding", get the drill bit hastily that is grinding, is certainly blind grinding. Only in the sharpening before the placement of a good position, in order to play a solid foundation for the next step of the "good grinding", this step is quite important.
Here are four phrases to guide the sharpening process, the effect is better.
The first mnemonic: "The edge is leveled by the wheel surface."
This is the first step in the relative position of the drill and the grinding wheel, and often students start sharpening by leaning on the wheel before the edge is leveled. This is definitely not good sharpening. Here the "edge" is the main cutting edge, "leveling" refers to be sharpened part of the main cutting edge in the horizontal position. The "wheel surface" is the surface of the grinding wheel. "Lean" means to lean in slowly. At this time, the drill bit can not touch the grinding wheel.
The second mnemonic: "The drill shaft is placed diagonally out of the sharp angle."
This refers to the positional relationship between the drill axis and the surface of the grinding wheel. "sharp angle" that is, half of the top angle 118 ° ± 2o, about 60 ° this position is very important, directly affecting the size of the top angle of the drill bit and the shape of the main cutting edge and cross cutting edge bevel angle. Students should be prompted to memorize the angle of 60° in a commonly used piece of 30°, 60°, 90° triangular plate, which is easy for students to grasp. Mnemonic I and Mnemonic II refers to the relative position of the drill before sharpening, the two should be coordinated, do not in order to set the edge and neglected to set the bevel angle, or in order to set the beveling axis and neglected to set the edge. In practice, these mistakes are often made. At this point the drill is ready to contact the grinding wheel in the correct position.
Mantra 3: "Grind the back from edge to back."
This refers to slow sharpening along the entire back face of the drill, starting at the cutting edge. This makes it easier to dissipate heat and sharpen. On the basis of stable consolidation of mnemonics I and II, at this time the drill bit can gently touch the grinding wheel for a smaller amount of sharpening, sharpening to observe the uniformity of the spark, to adjust the pressure size in time and pay attention to the cooling of the drill bit. When the cooling restart sharpening, to continue to set the position of the mnemonic one and two, which is often not easy to grasp at the beginning, often involuntarily change the correctness of its position.
Mnemonic four: "up and down swing tail don't cock."
This action is also very important in the process of drill sharpening, often students in the sharpening of the "up and down swing" into the "up and down rotation", so that the other main edge of the drill is destroyed. At the same time, the end of the drill can not be high above the horizontal centerline of the grinding wheel, otherwise it will make the edge blunt, can not cut.
Twist drill bit processing impact
(1) twist drill diameter by the limitations of the hole diameter, spiral groove to make the drill core more thin, the drill bit stiffness is low; only two prongs belt guided, the axis of the hole is easy to be skewed; transverse edge to make the centering difficult, the axial resistance increases, the drill bit is prone to oscillation. Therefore, the shape and position error of the drilled hole is larger.
(2) the twist drill's front and back surfaces are curved surfaces, and the front and back angles of the points along the main cutting edge are different, and the front angle of the cross edge reaches -55°. Cutting conditions are very poor; the distribution of cutting speed along the cutting edge is unreasonable, and the tip with the lowest strength has the highest cutting speed, so it wears badly. As a result, the machined hole precision is low.
(3) The main cutting edge of the drill participates in the cutting, and the cutting speed of each point on the edge is not equal, which is easy to form spiral chips, and it is difficult to remove the chips. Therefore, the chip and the hole wall extrusion friction, often scratched the hole wall, the surface roughness after processing is very low