1 . Introduction
Making small holes on a lathe requires high machining precision and surface roughness, especially when used for mating holes. The general aperture accuracy is IT7 to IT8 and the surface roughness is Ra3.2 to 0.2um. The radial deviation is within 0.3InN.
On the one hand, due to the small size of the drill, it is prone to breakage, resulting in significant waste and affecting machining precision, quality and productivity.
On the other hand, there are many problems with using small diameter drill bits in the drilling process.
Only by understanding the problems that can arise when drilling small diameter holes can the necessary measures be taken to ensure the smooth progress of drilling.
two . Main factors of drill breakage
The drill's small diameter and insufficient strength, combined with its small helix angle that makes chip removal difficult, make small diameter drills prone to breaking during use.
The high cutting speed when drilling small holes generates high cutting temperatures, which are not easily dissipated, especially in the contact area between the drill and the workpiece, aggravating drill wear.
During the drilling process, manual feed is often used, and the feed force is not easy to control uniformly. Slight carelessness can cause damage to the drill.
Due to the low rigidity of small diameter drills, they are easily damaged or bent, resulting in slanted holes.
(1) Changes in the geometric angle of the drill
Changes in the geometric angle of the drill are the main causes of drill breakage, among which the most significant influence is the change in the drill tip angle, which refers to the angle included between the two main cutting edges of the drill. The standard twist drill has a point angle of 118°.
When the drill tip angle is greater than 118°, the two main cutting edges are concave curves; when the drill tip angle is less than 118°, the two main cutting edges are convex curves. Only when the drill tip angle is equal to 118° are the two main cutting edges straight.
However, the smaller the drill diameter, the more difficult it is to control the drill tip angle, leading to an imbalance in drilling force and torque that results in drill breakage due to drilling deviation.
(2) Changes in radial runout or amount of bit displacement
The rotational accuracy of the drill mainly depends on the drill clamping accuracy, the manufacturing accuracy of the chuck and the rotational accuracy of the machine tool spindle. If the radial runout or displacement amount of the bit is too large, it is easy to break the bit.
(3) Changes in Axial Force and Feed Rate during Drilling
When drilling on a lathe, the feed rate is usually only about 0.001 inch per revolution, depending entirely on the operator's sensitivity to control.
Therefore, it is difficult to ensure uniform axial force and feed rate, and a small error may cause a sudden change in axial force and feed rate, resulting in drill breakage.
Therefore, the smaller the diameter of the drill, the more likely it is to break due to excessive feed.
(4) Impact of lathe speed
When drilling, the appropriate lathe speed should be selected based on the formula: n = 1000V/D, where n is the spindle speed in revolutions per minute, D is the drill diameter in millimeters, and V is the cutting speed in meters per minute. minute.
This means that the smaller the drill diameter, the higher the lathe speed must be.
(5) Influence of Operator and Drilling Material
During drilling, the concentration or dispersion of the operator's energy can also be one of the causes of bit breakage.
Furthermore, the properties of the material being drilled also have a significant impact, especially for materials with high toughness that make chip removal difficult and are prone to clogging, leading to drill breakage.
(6) Other factors
The. Excessive wear on the drill causes changes in its geometric angle, and if the operator drills into the part with force, the drill may break.
B. The drill is not centered correctly and the end face of the workpiece before drilling is not machined flat.
w. The tailstock of the lathe produces displacements, causing the center of the drill to deviate from the center of rotation of the workpiece, which not only increases the diameter of the hole, but also increases the likelihood of drill breakage.
d. The drill is extended for too long, resulting in radial runout and breaking the drill.
3. Solutions
(1) Before drilling, it is necessary to machine the end face of the workpiece flat without any protrusions, and insert the drill bit into the tailstock sleeve to align the drill axis with the rotation axis of the workpiece.
(2) To prevent radial runout of the drill, a stop can be added to the tool holder to support the drill head and help center it.
(3) When drilling small and deep holes, it is best to first use a center drill to drill a center hole to avoid drilling off-center. During drilling, the drill must be retracted frequently to remove chips.
(4) When drilling small and deep holes, to avoid excessive resistance during drilling that may cause hole position deviation or bit breakage, a higher lathe speed should be selected, generally in the range of 700-1000 rpm.
(5) Due to the low strength and low rigidity of small diameter drills, they are prone to breakage.
Therefore, when starting drilling, the feed force should be light to prevent the bit from bending or slipping and ensure that it starts drilling in the correct position. When the feed force is too small, it may be difficult to feel with your hand, so a small weight can be added to the feed mechanism to achieve the desired feed force.
(6) When the drill is about to touch the end face of the workpiece or is about to penetrate the through hole, the axial resistance increases due to the tip's first contact with the material, making the drill more susceptible to breakage.
Therefore, the feeding rate must be reduced. Generally, for drilling steel, the feed rate should be between 0.15-0.35mm/rev, and for drilling castings, the feed rate should be a little higher, generally selected at 0.15-0.4 mm/rev.
(7) During the drilling process, frequent drill retraction and timely lifting should be observed. Due to the narrow chip flutes of small diameter drills, chip removal is not smooth, so it is necessary to frequently retract the drill to remove chips, and the number of retractions should be proportional to the depth of the hole.
This is also an opportunity to introduce coolant or cool the bit in air. By adopting these methods, drill breakage can be reduced, saving materials, improving production efficiency and improving part quality.
(8) When drilling with small diameter drills, chip removal is not smooth and the temperature of the drill increases quickly. To reduce the cutting temperature, decrease the coefficient of friction between the chips, the workpiece and the tool contact surface, and improve the service life of small diameter drills, sufficient cooling must be carried out.
Generally, rust-proof clear water is used as coolant. In addition, a layer of molybdenum disulfide can be applied to the drill grooves, or low-viscosity mechanical oil or vegetable oil can be used for lubrication to achieve better results.
4. Conclusion
In conclusion, to obtain satisfactory drilling results, attention should be paid to the above aspects when using small diameter drill bits.
However, due to the limitations imposed by different part materials, quality requirements and drilling positions, the corresponding technical measurements must be changed accordingly.
