Drilling small holes on a lathe requires high machining precision and surface roughness, especially when used for mating holes. The precision of the general aperture is IT7 to IT8, and the surface roughness is Ra3.2 to 0.2um. The radial runout is within 0.3InN.
On the one hand, due to the small size of the drill bit, 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 may arise when drilling small diameter holes can necessary measures be taken to ensure the smooth progress of drilling.
2. Main Factors of Drill Bit Breakage
The small diameter and insufficient strength of the drill bit, combined with its small helix angle that makes it difficult to remove chips, makes small diameter drill bits prone to breakage 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 bit and the workpiece, exacerbating the wear of the drill bit.
During the drilling process, manual feed is often used, and the feeding force is not easy to control evenly. Slight carelessness can cause damage to the drill bit.
Due to the poor rigidity of small diameter drill bits, they are easily damaged or bent, resulting in inclined hole drilling.
(1) Changes in Drill Bit Geometric Angle
Changes in the geometric angle of the drill bit are the main causes of drill bit breakage, among which the most significant influence is the change in the drill point angle, which refers to the included angle between the two main cutting edges of the drill bit. The standard twist drill has a drill point angle of 118°.
When the drill point angle is greater than 118°, the two main cutting edges are concave curves; when the drill point angle is less than 118°, the two main cutting edges are convex curves. Only when the drill point angle is equal to 118° are the two main cutting edges straight lines.
However, the smaller the diameter of the drill bit, the more difficult it is to control the drill point angle, leading to an imbalance in drilling force and torque that results in the drill bit breaking due to drilling deviation.
(2) Changes in Radial Runout or Offset Amount of Drill Bit
The rotational accuracy of the drill bit mainly depends on the precision of the drill bit clamping, the manufacturing precision of the drill chuck, and the spindle rotation accuracy of the machine tool. If the radial runout or offset amount of the drill bit is too large, it is easy to cause the drill bit to break.
(3) Changes in Axial Force and Feed Rate during Drilling
When drilling on a lathe, the feed rate is generally only about 0.001 inch per revolution, relying entirely on the operator’s feel to control.
Therefore, it is difficult to ensure uniform axial force and feed rate, and a slight mistake can cause a sharp change in axial force and feed rate, resulting in drill bit breakage.
Therefore, the smaller the diameter of the drill bit, the more likely it is to break due to excessive feed rate.
(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 bit diameter in millimeters, and V is the cutting speed in meters per minute.
This means that the smaller the diameter of the drill bit, the higher the lathe speed should 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 drill bit breakage.
In addition, the properties of the material being drilled also have a significant impact, especially for materials with high toughness that make chip removal difficult and prone to clogging, leading to drill bit breakage.
(6) Other Factors
a. Excessive wear of the drill bit causes changes in its geometric angle, and if the operator forcibly drills into the workpiece, it may cause the drill bit to break.
b. The drill bit is not correctly centered, and the end face of the workpiece before drilling is not machined flat.
c. The tailstock of the lathe produces offsets, causing the center of the drill bit to deviate from the rotation center of the workpiece, which not only enlarges the diameter of the drilling hole but also increases the likelihood of the drill bit breaking.
d. The drill bit is extended too long, resulting in radial runout and causing the drill bit to break.
(1) Prior to 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 axis of the drill bit with the rotation axis of the workpiece.
(2) To prevent radial runout of the drill bit, a stopper can be added to the tool holder to support the head of the drill bit 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 bit should be frequently retracted to remove chips.
(4) When drilling small and deep holes, to avoid excessive resistance during drilling that may cause the hole position to deviate or the drill bit to break, a higher lathe speed should be selected, generally in the range of 700-1000 rpm.
(5) Due to the low strength and poor rigidity of small diameter drill bits, they are prone to breakage.
Therefore, when starting drilling, the feed force should be light to prevent the drill 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 by hand, so a small weight can be added to the feeding mechanism to achieve the desired feed force.
(6) When the drill bit is about to touch the end face of the workpiece or is about to penetrate through the through hole, the axial resistance increases due to the point first contacting the material, making the drill bit more susceptible to breakage.
Therefore, the feed rate must be slowed down. Generally, for drilling steel, the feed rate should be between 0.15-0.35 mm/rev, and for drilling castings, the feed rate should be slightly larger, generally selected at 0.15-0.4 mm/rev.
(7) During the drilling process, frequent retraction of the drill bit and timely lifting should be noted. Due to the narrow chip flutes of small diameter drill bits, chip removal is not smooth, so it is necessary to frequently retract the drill bit to remove chips, and the number of retracts should be proportional to the depth of the hole.
This is also an opportunity to introduce coolant or cool the drill bit in the air. By adopting these methods, drill bit breakage can be reduced, thus saving materials, improving production efficiency, and improving the quality of workpieces.
(8) When drilling with small diameter drill bits, chip removal is not smooth, and the temperature of the drill bit increases rapidly. In order to reduce the cutting temperature, decrease the friction coefficient between chips, workpiece and tool contact surface, and improve the lifespan of small diameter drill bits, sufficient cooling must be carried out.
Generally, transparent rust-proof water is used as a coolant. Additionally, a layer of molybdenum disulfide can be applied to the grooves of the drill bit, or low-viscosity mechanical oil or vegetable oil can be used for lubrication to achieve better results.
In conclusion, to achieve satisfactory drilling results, attention should be paid to the above aspects when using small diameter drill bits.
However, due to limitations imposed by different workpiece materials, quality requirements, and drilling positions, corresponding technical measures must be changed accordingly.