A recent survey conducted in the German metal processing industry showed that drilling machining is the most time-consuming process in a machining shop. In fact, 36% of all machining man-hours are consumed in hole machining operations. Corresponding to this, the turning process takes 25% and the milling process takes 26%. Therefore, using high-performance solid carbide drills instead of high-speed steel and ordinary carbide drills can significantly reduce the number of man-hours required for drilling, thereby reducing hole machining costs.
In the past few years, the cutting parameters (especially the cutting speed) have been continuously improved, and in particular, the cutting speed of high-performance solid carbide drills has increased significantly. Twenty years ago, typical solid carbide drills had a typical cutting speed of 60 to 80 m/min. Nowadays, it is no surprise that the machine is capable of drilling steel with a cutting speed of 200m/min, provided that the machine tool can provide sufficient power, stability, and coolant delivery capability. In spite of this, drilling machining has great potential for improvement in machining efficiency compared with the general cutting speed of turning or milling.
Solid carbide drills require high toughness of the substrate, and the wear of the drill bit is acceptable under controlled and uniform conditions. Therefore, a typical drill tool grade contains more cobalt than a turning or milling tool.
The drill material is usually made of fine-grained carbide to increase the cutting edge strength and ensure uniform wear without chipping. Water-based cutting fluids are often used when machining with cemented carbide bits, so the temperature at the cutting edge is not too high, but the bit is required to have thermal shock resistance. The best performing drill bit grade is a typical pure tungsten carbide material without the need for large amounts of tantalum carbide or titanium carbide.
For solid carbide drills, the coating must perform more than merely increase surface hardness and wear resistance. The coating must provide thermal insulation and chemical inertness between the tool and the workpiece material; the bonding between the workpiece material and the coating must be minimized to reduce friction; the coating surface must be as smooth as possible; Twist drills must also have crack propagation resistance. The dynamic characteristics of the drilling process may cause micro-cracking, and in order to maintain tool life, crack propagation must be prevented. By choosing the right coating process and producing the appropriate coating microstructure, the coating material can be placed under compressive stress, significantly extending tool life.
Use a multi-layer coating for good results. The multi-layer coating prevents the micro-cracks from diffusing between the layers of the coating. Even if individual coatings are damaged and peel off, other coatings can still protect the cemented carbide substrate. For drilling tools, the use of nano-coatings and precisely tailored coatings also has great potential for development.
For example, a new TiAlN nano-coating layer with TiN on the top layer can solve many of the problems encountered in drilling stainless steel. The smooth TiN top coating reduces the adhesion and friction between the tool and the workpiece material, while the underlying TiAlN nano coating provides the tool with hardness and wear resistance. This coating has excellent crack propagation resistance and thermal shock resistance. When drilling stainless steel, the cutting speed can reach 70~80m/min, which is almost twice that of conventional drills.
In order to give full play to the excellent properties of modern cemented carbide substrates and surface coatings, it is necessary to optimize the design of the drill bit's geometric parameters and drill patterns. The drill tip, drill angle, land shape, and cutting edge preparation must be used according to the processing application. The chip flutes, chip flutes, and the number of lands are reasonably adjusted.
High-efficiency cutting bits generally use one of four drill tip geometries. Among them, the tetrahedron drill tip with a chisel edge is easy to grind, and at the same time it is easy to control the grinding tolerance, but its center clearance is small. When the feed amount is large, the flank face will come into contact with the bottom of the hole, thus affecting the feed rate. The improvement. The other is a conical drill tip, which has a larger center clearance than a tetrahedron drill tip, so the axial thrust generated during drilling is smaller, but the geometry of the drill tip is more complex and it is not easy to ensure tool manufacturing. Consistency with management. In addition to the above two types of drill tips, there are auger tips available, which are divided into two different types: the traditional auger tip with a chip flute, the chip can be discharged from the center; new auger The tip mills the flutes and flank surfaces simultaneously, eliminating drilling steps and further improving chip flow. Because the center clearance of these two drill tip designs is larger than that of several other drill tip geometries, it has a very high feed capacity. In addition, the new auger tip also has high-speed cutting capabilities and can be drilled with a small axial thrust. The only disadvantage of this point geometry is that the grinding process required to manufacture the drill bit is complicated.
When selecting the drill, in addition to tool life and machining speed, another major factor to consider is the quality of the hole. In recent years, how to reduce glitches has become the focus of attention. Deburring is a typical manual process, the processing cost is high, and if it is not handled properly, it may cause serious problems.
The solid carbide drill bit will exert great pressure on the workpiece material during high-speed rotation and feeding. Therefore, when using a conventional drill-type design or point-of-dust angle machining, larger burrs are produced at the outlet of the through hole. To solve this problem, the simplest method is to increase the drill angle to 135° to 145°. A drill with a drill angle in this range can produce a disc at the outlet of the hole and keep the workpiece material at all times. Under tensile stress, the material is easy to cut rather than just pushing it out of the workpiece. Cutting edge preparation, chamfering and other geometries optimization measures can also play a significant role in reducing burrs.
When drilling gray cast iron and ductile cast iron, completely different problems arise. The brittleness of these materials is greater, and the material disintegration is more likely to occur at the outlet of the through-hole than to form burrs. Material chipping not only affects the quality of the workpiece, it can also lead to broken drill bits. Drilling chamfers designed for cast iron machining allow the drill to drill the workpiece in a very smooth manner and keep cutting until the final turn, which helps to prevent the material from collapsing.
The design of the drill tip requires continuous adjustment based on the geometry of the flutes. The number of cutting edges, the thickness of the chisel edge, the width of the flutes, and the width of the land are all factors to consider when designing the drill tip. In addition, the influence of the workpiece material cannot be ignored.
Two-spin twist drills are usually the best tool choice when drilling on steel. This drill is easy to use, easy to regrind, and has excellent fault tolerance enough to minimize jump errors and tolerate machine and workpiece instability.
Drills with more than two flutes have performance advantages when drilling large aspect ratio holes or drilling holes in internally stressed workpiece materials such as cast steel. The three-slot drill has three lands and three cutting edges, so it has better guiding and self-centering ability when drilling. However, because such drills cannot tolerate too much torque, they are only recommended for machining grey cast iron and non-ferrous materials. Drills with 2 cutting edges and 4 lands can also be used as an alternative tooling solution (especially if in-tool cooling is required).
The four-edged twist drill performs excellently when machining steel and cast iron because it is very fault-tolerant and can be drilled at a high feed rate that exceeds that of single-slot bits. This drill is also the preferred tool for drilling deep holes up to 30 times the depth of the hole, and its drilling speed is about 5 times that of conventional gun drills.
For the machining of aluminum alloys, straight drill bits are used to obtain the best drilling accuracy, and complex step holes can be machined in a relatively simple manner. The disadvantage of the straight slot drill bit is that it requires extremely high precision of the tool clamping. Such drill bit lacks fault tolerance to radial runout, excessive cutting speed and feed rate or low coolant pressure.
A very serious problem in drilling (especially deep hole drilling) is that if the drill bit deviates from the centerline of the hole (runaway) in the initial stage, then it will be almost impossible to correct it in the subsequent machining. Drill the pilot bit in an eccentric position until it reaches the bottom of the hole. However, because the drill has a helix angle, the drilled hole will also be spiral. To avoid this problem, the most important thing is to have a correct drill tip with good self-centering ability. In addition, improving the directionality of the drill bit also helps prevent runaway. The two-land drill can only get 25% of the support at the beginning of drilling, so even if it is under a small force, it is easy to move from the center to most directions. The four-edged drill bit can be supported in all directions so that holes with better roundness and cylindricity can be machined. 4-land drill bits also provide better support performance in non-uniform drilling or through-hole drilling, and such drilling operations are very common in the processing of hydraulic parts.
In today's drilling processes, chip evacuation has to be fully controlled, not as in the past, as long as the operator feels that the drilling force is increased, he can use the drill slamming method at any time. A crucial issue is that starting from the formation of cuttings at the point of the drill, chips and chips must be realized in such a way that the chips can be easily matched with the chip flutes, and the chips can have less friction. Smooth out of the hole.
The kinematics of the drilling process actually contributes to the control of the chip. Since the cutting speed in the center of the drill point is zero, the chip flows more or less around the chisel edge and will be fully formed in the chip flutes. As long as the chip flutes have the correct geometry, it is easy to generate swarf of the same size. In addition, the flutes are not only negatively chiseled at both ends but also polish the surface of the groove wall, which also contributes to the formation of free chip flow and enables drilling under controlled conditions.
Using the right drill and reasonable drilling parameters can increase production efficiency and reduce processing costs. But what should be the cost of cutting tools? First of all, these advanced bit geometries are more difficult to manufacture than traditional bit geometries, so in general the new drills are more expensive than conventional drills. However, this new type of bit can be reground four to five times. Although the tool life will be reduced by about 10% after each regrind, it is still possible to save more than 50% of the tool cost.
However, the reduction in bit life caused by each regrind can also cause some problems. In order to guarantee the processing safety, only the drill with high safety factor can be used for processing, so the user must use a monitoring and tracking system to replace the reground bit in time. To solve this problem, the only way is to use "disposable" products, but the use of disposable solid carbide drills is usually not economical.
A new modular drill bit design can effectively avoid the above problems. This type of drill uses a replaceable cemented carbide drill tip. Its cutting performance and tool life are comparable to high-performance solid carbide drills. There is no threaded connection between the drill tip and the steel drill shank or other difficult-to-handle connections on small diameter drill bits. Since the design of the drill tip does not have to take into account the need for re-grinding, the geometry of the drill tip can be optimized. A positive rake angle is used in the cross-edge area of ​​the drill tip to reduce the cutting force and improve the self-centering ability. As the rigidity of the alloy steel body is reduced compared to that of the solid carbide drill, it is important to use the positive rake angle to compensate for the stiffness of the drill bit.
Unlike conventional cemented carbide drills, the flutes of modular drill bits do not use the same helix angle from front to back, but instead use a right-handed helix at the front of the chip hopper to accelerate the flow of chips, and at the rear of the chip flutes. With a small negative helix angle, the negative helix angle is particularly useful for increasing the stability of the drill bit and reducing the vibration, and it is also very important for compensating the rigidity reduction of the steel drill body.
Compared with the solid carbide drill, the difference between the modular drill is that the coolant outlet is located in the chip flutes and is directly aligned with the rake face of the cutting edge (the exit of the solid carbide drill cooling hole is On the side of the drill tip, the importance of this design is that the rake face of the cutting edge is usually the area with the highest processing temperatures and it is highly desirable to provide effective cooling between the chip and the tool material. This cooling hole design optimizes the cooling effect while also creating a thermal shock effect on the chips that helps improve chip control. Since the chisel bit is not directly exposed to the impact of coolant, it is also advantageous for the chips to be formed at the cutting edge where the cutting speed is very low.
In addition to the consistency of tool life, another important advantage of using a modular drill bit is that the tool stock can be greatly reduced. When using conventional carbide drills, there is a large demand for bit stock due to the fact that a large number of drills are often in the regrind reprocessing flow. The use of disposable drills eliminates the need for a regrind reprocessing process, where the tool stock is equal to the number of drills machined (perhaps a few spare parts are prepared on the tool holder).