Figure 1 The basic structure of the drill
The drill bit will have different degrees of wear during the hole machining process. The drill bit material and wear condition will be analyzed. When the drilling process is improved, the drill bit edge will be ground and strengthened, which can effectively improve the drill bit during processing. Wear conditions improve drill performance and service life.
Hole machining occupies an important position in metal cutting processing, and generally accounts for about 1/3 of the machining volume. The drill holes account for about 22% to 25%, and the remaining holes account for about 11% to 13%. Because of the harsh hole processing conditions, the technology development of hole machining tools is slower than that of cars and milling tools. In recent years, with the continuous improvement of production efficiency, automation, and machining center performance in medium and small batch production, the development of tool grinding technology and multi-axis CNC tool sharpening equipment has led to the development of hole machining tools. The drill bit grinding technology that has been used for many years in the production of machinery and the most widely used overall structure has gradually matured. Through the improvement of drilling processing conditions for the grinding and strengthening of the drill edge, it is necessary to seek a solution from the structural characteristics of the drill and the actual use.
Drill characteristics
1. The material of the drill is classified into high-speed steel and hard alloy. The high-speed steel is mainly made of high-speed steel W-series and Mo-type materials. The cemented carbide is made of tungsten-titanium (YG), tungsten-titanium-cobalt (YT) materials. More representative are W18Gr4V, YG6 and YT14 as listed in Table 1.
2. The basic shape and structure of the twist drill did not change much (see Figure 1).
3. There are certain characteristics and correlations between the geometric angles of the twist drill cutting edges. As shown in FIG. 2 , the main declination angle is Kr, the blade inclination angle is λs, the rake angle is λs, the relief angle is αf, and the front angle is 2φ (traditionally 118°).
Table 1 Physical and mechanical properties of high-speed steel and hard alloy materials
Among them, the drill spiral structure has the following characteristics:
(1) The main declination angle Kr is also determined after the front angle 2φ is determined.
(2) Since the cutting edge (bit diameter) of the cutting edge of the drill is the lowest point of the cutting edge, it can be seen from the structure that the cutting edge inclination λs of the cutting edge of the cutting bit is negative.
(3) Under the influence of the helical groove shape of the drill bit, the blade front angle λs gradually decreases from the ligament of the drill outer diameter to the direction of the drill core.
(4) The rake angle λs of the rake angle of the cutting edge increases with the increase of the main declination angle Kr.
Figure 2 The geometry of the cutting edge
4. The twisted edge of a twist drill is also an important part of the cutting edge. As shown in Fig. 2, the rake angle γom, relief angle αf, and angle φ of the chisel edge also change with the cutting edge of the drill bit.
Drill bit wear during processing
1. Drill bit wear mainly occurs at the cutting edge (see Figure 3)
Figure 3 Drill bit wear during processing
2. The analysis of the force applied on the drill bit in the actual machining process mainly focuses on the cutting edge of the drill bit, in which the cutting edge is subjected to the largest torque and the axial force of the chisel edge is more concentrated (see Table 2 and Figure 4).
Table 2 Distribution of cutting force in cutting part during bit processing
3. The distribution of cutting heat generated by the drill during machining is shown in Figure 5. In the processing, the drill core has a small cutting angle and is always at the cutting edge of the machined part. The axial force taken by the drill bit accounts for about 57%. The heat generated during the cutting process cannot be discharged in time. It is the highest temperature of the drill bit. section. Figure 5 shows the highest temperature in the red area of the core and the drill bit edge, and the most concentrated heat is at the horizontal edge of the core.
Figure 4 Force analysis of the cutting edge
Improve the drilling process
1. Improve the cutting edge of the drill
A new type of sharpening method is used to improve the geometry of the drill tip and chisel edge. In the past, the sharpening of the drill bit was performed by the ordinary sharpening method. After sharpening the front angle of 2φ, the sharp part of the bit was manually ground with the 90° forming edge of the grindstone circumference. Due to the limitation of the traditional sharpening method, the symmetry of the drill after grinding is poor and the precision is low. Only using the traditional 118° front angle can ensure that the cutting edge is a straight edge. In recent years, our company has introduced a CNC universal tool grinder. The machine adopts a more advanced five-axis CNC system, which can realize the grinding and grinding of the cutting edge of the drill, and can change the shape of the cutting edge of the drill and still ensure a high Tool accuracy. Therefore, we have tried to improve the service life of the drill bit by improving the geometry of the drill bit, improving the drill bit and improving the drilling processing conditions.
Fig. 5 Distribution of cutting heat
According to the structural characteristics of the drill bit, we first changed the front angle (2φ angle) of the twist drill, and used a front angle of 118° to 140° to perform the test. After tracking and mastering the processing conditions at the production site, we found that when machining cast iron, the use of a drill bit with a sharp angle has a certain effect: during drilling, the processing becomes lighter, the sound and vibration are significantly reduced, and the hole surface is rough. The degree is improved. Judging from the shape of the chips, the machining process is stable. However, as the bit's sharpness increases, the bit wear increases. After several attempts, it was found that the processing is most stable when the front angle is about 130°, and the number and quality of processing are significantly improved.
In order to improve the axial force of the drill's chisel section during machining, it is necessary to overcome the adverse cutting conditions such as the negative rake angle at the chisel edge. In the case of cross-cutting, we use a large cut to grind the chisel edge and shorten the width of the chisel edge so that the cutting edge's chisel edge crosses the main cutting edge, reducing the axial force during drilling and turning during drilling. Moment (see Figure 6). After inspection in practice, the accuracy of centering has been greatly improved after the axial force of the drill has been improved. The use of this type of drill bit on the housing machining center can eliminate the center drill under certain conditions, improve the machining efficiency and shorten the production cycle. The drill has been gradually tested and promoted in our company's production.
Fig. 6 Improvement of the drill bit
Compared with high-speed steel drills, the machining conditions of hard alloy drills are more demanding. The carbide drills used in the screw hole process that our company processes on the flanges have lower original machining quantity and processing efficiency, and we have also tried some improvements:
According to the advantages of high hardness of hard alloy materials, the use of a large front angle of 140 ° to increase the cutting rake angle, change the force of the drill, reduce the cutting force, and make the processing more smooth. According to the characteristics of the processed material, the chisel edge part of the drill bit is improved, and the R-shaped cutting edge transition is adopted. The rake rake angle is increased on the basis of the R-shaped cutting edge, and the core part is drilled and positioned before the drilling to achieve self-adjustment. Centering, cancel the center drilling process, to meet the requirements of the location, and at the diameter of the edges of the edge cutting process, the formation of protective edge, so that the drill bit is not easy to cause a chipping when drilling, greatly improving the life of the drill .
This type of structure is particularly suitable for the improvement of small diameter drills. Now our company synchronizer workshop processing two-speed synchronizer pin hole diameter φ7.5mm, tolerance range 0 ~ +0.016mm, a total of six holes in each part, relative position requirements 0.05mm.
Figure 7 Improved drill cutting edge
Coating treatment is also an important method to improve the service life of the drill. According to the processing conditions, different coatings can improve the surface hardness and oxidation temperature of the drill, reduce the friction coefficient, and greatly increase the service life of the drill. TiN coating (yellow coating color) has greatly improved the performance of high-speed steel drills, which can effectively improve the hardness of high-speed steel drills, improve the surface roughness of drills and reduce friction, and improve drilling conditions. The TiCN (coating color is grayish black) and TiALN (coating color purple brown) coatings are widely used on cemented carbide drills. The performance of these several coating materials is shown in Table 3.
Table 3 Performance comparison of several typical coating materials
The combination of edge strengthening (passivation) and coating can greatly improve the use of the drill. The edge strengthening (passivation) of the drill bit is also prepared for the coating, and the passivation allows the coating material to have a sufficient bonding surface. Practice has shown that the service life of drills reinforced by cutting edges is 40% to 50% longer than that of untreated drills.
Conclusion
Because the drill bit must be limited to the size of the hole and limited by its own structure, it is difficult to improve the technology. However, with the advancement of science and technology, CNC sharpening equipment has been continuously developed, and the design and manufacturing level of twist drills have been greatly improved over the past. As one of the most basic and extensive tools in hole machining tools, drill bits have achieved great development and progress in the field of machining.
