In mechanical processing, the proportion of the drilling process is about the proportion of the hole and the processing of the hole system, and there are some practical problems with the processing of smaller diameter holes, among which the smaller drill bit is very easy to break. Cause a lot of waste, and affect the processing accuracy, processing quality and production efficiency.
1. The drilling of small holes on a lathe requires relatively high machining accuracy and surface roughness. Used as a matching hole, the general hole diameter accuracy is (IT7—IT8), and the surface roughness is (Ra3.2— 0.2um), the radial runout is within 0.3mm. On the one hand, due to the small size of the drill bit, it is easy to break, causing a lot of waste and affecting the processing accuracy, processing quality and production efficiency. On the other hand, there are many problems in the use of small-diameter drills. only by clarifying the problems that small-diameter drills are prone to when drilling small holes can we take the necessary measures in a targeted manner to ensure the smooth progress of the drilling.
2. The main factors of drill bit breaking
The drill bit has a small diameter and insufficient strength, and the helix angle of the small diameter drill is relatively small, which is not easy to remove chips, so the small diameter drill is easy to break during use. The cutting speed of drilling small holes is high, the cutting temperature generated by the drill bit is high and it is not easy to dissipate heat, especially the higher temperature of the contact part of the drill bit and the workpiece, which intensifies the wear of the drill bit. In the process of drilling, manual feed is generally used, and the feed force is not easy to control uniformly, and the drill will often be damaged if you are not careful. Due to the poor rigidity of the small-diameter drill bit, it is easy to damage and bend, causing the drilling to tilt.
(1) The geometric angle of the drill changes
The geometrical angle change of the drill bit is the main reason for the breakage of the drill bit. The most important factor is the change of the drill tip angle of the drill bit. The so-called drill tip angle is the angle between the two main cutting edges of the drill bit. Generally, the tip angle of a standard twist drill is 118°. When the tip angle of the drill is greater than 118°, the two main cutting edges are concave curves. When the tip angle of the drill is less than 118°, the two main cutting edges are convex curves. When the drill tip angle is equal to 118°, the two main cutting edges are straight. However, the smaller the diameter of the drill bit, the more difficult it is to control the tip angle of the drill edge, which leads to an imbalance of drilling force and torque, forcing the drill bit to deviate when drilling and causing the drill bit to break.
(2) Radial runout or deflection of the drill bit
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 radial runout or deflection of the drill bit is too large, it is easy to break the drill bit.
(3) Changes in drilling axial force and feed
When the drill bit 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 feeling, so it is difficult to guarantee the uniformity of the feed and the axial force. A little carelessness will cause a sharp change in the axial force and the feed, which will cause the drill to break. Therefore, the smaller the diameter of the drill, the excessive feed rate will cause the drill to break.
(4) The influence of lathe speed
When drilling, its lathe speed selection 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 bit, the higher the lathe speed.
5) Influence of operator and drilling material
When drilling, the operator's concentration and energy dispersion are also one of the reasons for the drill bit to break. Secondly, the impact of processing materials is also great, especially for materials with strong toughness, which is difficult to remove chips and easy to block, and it is easy to break the drill.
(6) Other factors
a. Due to the excessive wear of the drill bit, the geometric angle of the drill bit changes. At this time, if the operator forcibly drills into the workpiece, the drill bit is likely to break.
b. The drill is not properly centered, and the end face of the workpiece before drilling is not flat.
c. The tailstock of the lathe is offset, and the center of the drill bit is not aligned with the rotation center of the workpiece, which not only enlarges the drilling diameter, but also breaks the drill bit.
d. Because the drill bit was extended too long, radial runout occurred, causing the drill bit to break.
3. Solution
(1) Before drilling, the end face of the workpiece must be flattened, without a convex head at the center, and the drill bit must be installed in the tailstock sleeve, and the axis of the drill bit must be aligned with the axis of rotation of the workpiece.
(2) In order to prevent radial runout of the drill bit, a stop iron can be added to the tool post to support the head of the drill bit and help the drill bit to center.
(3) When drilling small and deep holes, use a center drill to drill the center hole first to avoid drilling the hole crookedly. During the drilling process, the drill must be withdrawn from time to time to remove iron chips.
(4) When drilling small and deep holes, in order to avoid high resistance during drilling, which may cause the hole position to deviate and the drill bit to break, a higher lathe speed should be selected. Generally, the lathe speed is 700-1000r/min.
(5) Due to the low strength and poor rigidity of the small-diameter drill bit, it is easy to break, so when starting to drill, the feed force should be light to prevent the bit from bending and slipping, so as to ensure the correct position of the drill hole. Pay attention to the strength and feel 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 required. If the feed force is too small, the manual feed is not easy to feel. At this time, a small weight can be installed on the feed mechanism to achieve the feed purpose by its weight.
(6) When the drill bit just touches the end face of the workpiece and the through hole is about to drill through the workpiece, because the chisel edge penetrates first, the axial resistance increases and the drill bit is easily broken, so the feed rate must be slowed down. Under normal circumstances, drill steel When feeding, the feed rate is 0.15—0.35 mm/r; when drilling castings, the feed rate is slightly larger, generally 0.15—0.4 mm/r.
(7) During the drilling process, pay attention to frequent withdrawal of the drill and lift the drill in time. Because small-diameter drills are working, the chip flutes are narrow and the chip removal is not smooth, so the drilling and chip removal should be carried out in time. The number of back drilling is proportional to the depth of the hole. At the same time, you can also take this opportunity to input coolant or Cool in air. By adopting the above method, the breakage of the drill can be reduced, so as to save material, improve production efficiency and improve the processing quality of the workpiece.
(8) When small-diameter drills are drilling, due to poor chip removal, the drill and temperature rise quickly. In order to reduce the cutting temperature, reduce the friction coefficient between the chip, the workpiece and the tool contact surface, so as to increase the life of the small-diameter drill For the purpose of this, sufficient cooling must be carried out, and it is generally better to use transparent anti-rust water as the cooling liquid. In addition, a layer of molybdenum disulfide can be coated on the groove of the drill bit, or lubricated with low-viscosity mechanical oil or vegetable oil, which can get better results.
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