In mechanical processing, the proportion of drilling process accounts for about 100%. The processing of holes and hole systems is already common, but there are some practical problems in the processing of smaller diameter holes, among which the smaller drill bits are very easy to break. It causes a lot of waste and affects the processing accuracy, processing quality and production efficiency.
The drilling of small holes on lathes has relatively high processing accuracy and surface roughness requirements. For matching holes, the general aperture accuracy is (IT7-IT8), the surface roughness is (Ra3.2-0.2um), and the radial runout is within 0.3mm. On the one hand, because the drill bit is small, it is very easy to break, causing a lot of waste, and affecting the processing accuracy, processing quality and production efficiency. On the other hand, because there are still many problems in the use of small diameter drill bits, only by clarifying the problems that small diameter drill bits are prone to when drilling small holes can we take necessary measures to ensure the smooth progress of drilling.
Main Factors of Drill Bit Breakage
Drill bits are small in diameter and lack strength. The helix angle of small-diameter drill bits is relatively small, making it difficult to remove chips, so small-diameter drill bits are prone to breakage during use. The cutting speed of drilling small holes is high, and the cutting temperature generated by the drill bit is high and difficult to dissipate heat. In particular, the temperature at the contact point between the drill bit and the workpiece is higher, which aggravates the wear of the drill bit. During the drilling process, manual feeding is generally used, and the feed force is not easy to control evenly. Often, the drill bit will be damaged if you are not careful. Due to the poor rigidity of small-diameter drill bits, they are easily damaged and bent, causing the drilled hole to tilt.
Changes in Drill Bit Geometry
The changes in drill bit geometry are the main cause of drill bit breakage, among which the most influential is the change in the drill bit blade angle, which is the angle between the two main cutting edges of the drill bit. The standard twist drill has a blade angle of 118°. When the blade angle is greater than 118°, the two main cutting edges are concave curves. When the blade angle is less than 118°, the two main cutting edges are convex curves. Only when the blade angle is equal to 118°, the two main cutting edges are straight lines. However, the smaller the drill bit diameter, the more difficult it is to control the blade angle, which leads to an imbalance between the drilling force and torque, forcing the drill bit to deviate during drilling and causing the drill bit to break.
Changes in Drill Bit Radial Runout or Runout
The rotation accuracy of the drill bit mainly depends on the clamping of the drill bit, the manufacturing accuracy of the drill chuck and the rotation accuracy of the machine tool spindle. If the drill bit has too much radial runout or runout, it is easy to break the drill bit.
Changes in Drilling Axial Force and Feed Rate
When the drill is drilling on a lathe, the feed rate is too small, generally only about 0.001mm per revolution. It is completely controlled by the operator’s hand feel, so it is difficult to ensure that the feed rate and axial force are uniform. The slightest carelessness will lead to a sharp change in the axial force and feed rate, causing the drill to break. Therefore, the smaller the diameter of the drill, the larger the feed rate, which will cause the drill to break.
Influence of Lathe Speed
When drilling, the lathe speed should be: n=1000V/ЛD n——spindle speed, r/min. D——drill diameter, mm. V ——cutting speed, m/min. In other words, the smaller the drill, the higher the lathe speed should be.
Influence of Operator and Drilling Material
During drilling, the operator’s concentration and distraction are also one of the reasons for the drill bit to break. Secondly, the influence of the processing material is also great, especially for materials with strong toughness, which are difficult to remove chips and easy to block, and also easy to break the drill.
Other Factors
- Due to excessive wear of the drill, the geometric angle of the drill changes. At this time, if the operator forces the drill into the workpiece, it is easy to break the drill.
- The drill is not properly centered, and the end face of the workpiece before drilling is not flat.
- The lathe tailstock is offset, and the center of the drill is not aligned with the rotation center of the workpiece, which not only expands the diameter of the hole, but also breaks the drill.
- Because the drill extends too long, radial runout occurs, causing the drill to break.
Solution to Drill Bit Breakage
Before drilling, the end face of the workpiece must be turned flat, leaving no convex head in the center, and the drill bit must be installed in the tailstock sleeve to align the drill bit axis with the workpiece rotation axis.
To prevent the drill bit from radial runout, a stopper can be added to the tool holder to support the head of the drill bit and help the drill bit to center.
When drilling small and deep holes, the center hole should be drilled with a center drill first to avoid drilling the hole crooked. During the drilling process, the drill bit must be frequently withdrawn to remove iron cuttings.
When drilling small and deep holes, in order to avoid large resistance during drilling, the hole position is deflected and the drill bit is broken. A higher lathe speed should be used. Under normal circumstances, the lathe speed is 700-1000r/min.
Due to the low strength and poor rigidity of small diameter drill bits, they are easy to break. Therefore, when drilling, the feed force should be light to prevent the drill bit from bending and slipping to ensure the correct position of the drilling start. Pay attention to the hand strength and feeling when feeding. When the drill bounces, let it have a buffer range to prevent the drill from breaking. Sometimes only a small feed force is needed. If the feed force is too small, it is not easy to feel the manual feed. At this time, a small weight can be installed on the feed mechanism to achieve the feeding purpose by its weight.
When the drill just touches the end face of the workpiece and the through hole is about to drill through the workpiece, the cross edge will penetrate first, the axial resistance will increase, and it is easy to break the drill, so the feed rate must be slowed down. Under normal circumstances, when drilling steel, the feed rate is 0.15-0.35 mm/r. When drilling castings, the feed rate is slightly larger, generally 0.15-0.4mm/r.
During the drilling process, it is necessary to pay attention to frequent drill withdrawal and timely lifting of the drill. Because the chip removal groove is narrow when the small diameter drill is working, the chip removal is not smooth. Therefore, the drill should be withdrawn and chip removal should be carried out in time, and the number of drill withdrawals is proportional to the depth of the hole. At the same time, you can also take this opportunity to input coolant or cool in the air. The above method can reduce the breakage of the drill bit, so as to save materials, improve production efficiency and improve the processing quality of the workpiece.
When drilling with a small diameter drill bit, the drill bit and temperature rise quickly due to poor chip removal. In order to reduce the cutting temperature and reduce the friction coefficient between the chips, workpiece and tool contact surface. In order to achieve the purpose of increasing the life of the small diameter drill bit, sufficient cooling must be carried out. Generally, transparent anti-rust water is better as the coolant. In addition, a layer of molybdenum disulfide can be applied to the groove of the drill bit, or lubricated with low-viscosity machine oil or vegetable oil, which can achieve better use results.
