The advancement of cutting tool technology, especially tool materials, has played a significant role in the development of high-speed machining. In the field of high-speed cutting, there are currently four types of tools that use cermets, ceramics, coatings, and superhard materials, each of which has its own characteristics and application range and complement each other. Among them, the coating tools have developed the fastest and have a huge potential for application in the field of high-speed cutting. At present, they account for more than 50% of all tools.
Superhard material tools have an exclusive advantage in the field of high-speed cutting, and their practical applications are increasing day by day. PCD (polycrystalline diamond) tools are the best choice for high-speed cutting of aluminum alloys and non-metal materials in such tools, while diamond-coated tools are not only practical but have grown rapidly; PCBN (polycrystalline cubic boron nitride) The product) tool is suitable for cutting materials such as cast iron and hardened steel at a higher speed, and CBN (cubic boron nitride) coated tools are also expected to make significant technological breakthroughs in the near future.
In order to make the high speed cutter have a sufficient service life and low cutting force, the optimum tool geometry should be selected according to different workpiece materials. Compared with ordinary cutting, the front angle of high-speed cutting tools is generally smaller or even negative, and the back angle should be slightly larger, and often use a rounding or chamfering tool tip to increase the tool front angle to prevent the tool tip. Heat wear. Because the high-speed cutting rotary cutter works at a very high speed, the centrifugal force problem is very prominent. Therefore, it is required that the cutter body structure and the blade clamping structure should be very reliable. At the same time, it needs to undergo strict dynamic balancing on the dynamic balance instrument. It can be further installed on the machine to perform balancing with the spindle assembly.
The 7:24 cone connection widely used between the tool and the spindle at normal rotation speed, when high speed rotation, because the solid taper shank can not be "swelled" by the centrifugal force as the spindle hole, a gap between the two will cause the tool in the The taper hole oscillates, causing an axial positioning error of the tool and disrupting the dynamic balance of the structure. In order to overcome the shortcomings of poor high-speed performance of this connection, a number of coupling methods suitable for high-speed cutting have been developed, such as the HSK tool system and the Capto tool system.
The following describes the selection of tools, tool holders, and cutting amounts in detail.
1 Tool Material
To achieve high-speed cutting, tool material is the key. High-speed cutting materials mainly include hard alloys, coated tools, cermets, ceramics, cubic boron nitride and diamond tools. Each has its own advantages for different workpiece materials and different cutting speed ranges. It must be noted that there is a fitting problem between the tool material and the workpiece material pair. That is, a tool material and workpiece material perform well, but when machining another workpiece material, it is not ideal. In other words, there is no one. This versatile tool material is suitable for high-speed machining of all workpiece materials.
High-speed cutting tool materials must be selected according to the workpiece material and processing properties. In general, ceramic cutters, coated cutters, and CBN cutters are suitable for high-speed machining of ferrous metals such as iron and steel; PCD cutters are suitable for high-speed machining of non-ferrous metals such as aluminum, magnesium, and copper. The table lists some of the workpiece materials that are suitable for machining the above tool materials.
Ceramic cutting tools have been used to process a variety of cast iron, steel, thermal spray welding materials, nickel-based high-temperature alloys.
Diamond tools are suitable for machining non-metallic materials, non-ferrous metals and their alloys. Due to the poor thermal stability of the diamond, the hardness is lost when the cutting temperature reaches 800°C. Because diamond and iron have strong chemical affinity, iron atoms easily interact with carbon atoms at high temperatures to convert them into graphite structures, and tools are easily damaged. Therefore, diamond tools are not suitable for processing steel materials when cutting nonferrous metals. , PCD tool life is tens or even hundreds of times the carbide tool.
Cubic boron carbide tools are capable of both rough and finish cars of hardened steels, bearing steels, high speed steels, and chilled cast irons, as well as high speed cutting of superalloys, thermal spray materials, hard alloys, and other difficult-to-machine materials. The CBN tool is one of the best tools for realizing the grinding machine.
2 tools
The following describes the machining of common tools in the machining center.
1, milling cutter
In face milling, size and position are important factors due to the relationship between the milling cutter and the workpiece. When selecting a tool, the width of the workpiece determines the diameter of the cutter. For machining small parts, the general tool diameter is 30% larger than the workpiece is ideal, but the power and stability of the machine in many cases play a decisive role. Face milling often takes several passes to complete.
The milling cutter blade is another important factor in optimizing the milling effect. It is an advantage if more than one blade is involved in the cutting at any one time, but the number of blades that participate in the cutting at the same time is a disadvantage. Each cutting edge cannot be cut at the same time during cutting, and the required power is related to the cutting edge that participates in cutting. In terms of chip formation process, cutting edge loading and machining results, the milling cutter plays an important role in the position of the workpiece. In face milling, using a milling cutter that is approximately 30% larger than the cutting width and positioning the milling cutter close to the center of the workpiece, the chip thickness does not change very much. The cut chip thickness is slightly thinner than the cut thickness at the center.
In order to ensure that a sufficiently high average chip thickness/feed per tooth is used, the number of teeth of the milling cutter that are suitable for this process must be correctly determined. The pitch of the cutter is the distance between the active cutting edges. According to this value, the milling cutter can be divided into three types - dense tooth cutter, spade cutter and special milling cutter.
Related to the milled chip thickness is also the leading angle of the face milling cutter. The main rake angle is the angle between the main cutting edge of the insert and the surface of the workpiece. There are mainly 45 degrees, 90 degree angles and round blades. The direction of the cutting force changes greatly with the main angle. The milling cutter with a main angle of 90 degrees produces a radial force that acts in the feed direction, which means that the machined surface will not withstand More pressure is more reliable for milling weaker workpieces.
The radial cutting force and the axial direction of the milling cutter with a 45° cutting angle are approximately equal, so the generated pressure is relatively even and the requirements for the power of the machine tool are relatively low. It is especially suitable for milling short-cut materials that generate chipping chips. Workpieces.
The milling cutter of a circular insert means that the leading angle continuously changes from 0 to 90 degrees, depending on the cutting depth. This type of blade has a very high cutting edge strength and is suitable for large feeds because of the relatively thin chips generated in the direction of the long cutting edge. The direction of the cutting force in the radial direction of the blade is constantly changing, and the pressure generated during processing will depend on the cutting depth. The development of modern insert geometries has the advantages of smooth cutting effect, low power demand, and good stability. Today, it is no longer an effective roughing cutter. It is widely used in both face milling and end milling.
There are two ways with respect to the feed direction of the workpiece and the direction of rotation of the milling cutter. The first type is straight milling. The direction of rotation of the milling cutter and the feed direction of the cutting are the same. At the beginning of the cutting, the cutter bites the workpiece and cuts out the last swarf. The second type is up-cut milling. The direction of rotation of the milling cutter and the feed direction of the cutting are reversed. The milling cutter must slip on the workpiece before starting the cutting, starting at a cutting thickness of zero, and reaching the cutting thickness at the end of cutting. maximum.
In three-edge milling cutters, some end mills or face milling, the cutting forces have different directions. When face milling, the milling cutter is just outside the workpiece, and the direction of the cutting force should be given special attention. In crush milling, the cutting force presses the work piece to the table, and the cutting force forces the work piece to leave the work table during up-cut milling.
Since crush milling has the best cutting effect, crush milling is usually preferred. Only when the machine tool has a thread clearance problem or if there is a problem that cannot be solved by crush milling, then the up-milling is considered.
Under ideal conditions, the diameter of the milling cutter should be larger than the width of the workpiece, and the axis of the milling cutter should always be a little distant from the centerline of the workpiece. When the tool is placed in the center of the cutting, it is easy to produce burrs. The direction of the radial cutting force will continue to change as the cutting edge enters and exits the cut. The spindle of the machine tool may be vibrated and damaged. The blade may be chipped and the machined surface will be very rough. The cutter is slightly off center and the cutting force direction will not fluctuate - the cutter will get a preload. We can drive the center milling ratio in the center of the road.
Each time the cutter insert enters the cut, the cutting edge is subjected to an impact load that depends on the cross section of the chip, the workpiece material and the type of cutting. When cutting into and out, whether or not the cutting edge and the workpiece can be properly engaged is an important direction.
In Fig. a, the milling cutter axis is located completely outside the workpiece width and the impact force at the time of cutting is taken by the outermost tip of the insert, which means that the initial impact load is absorbed by the most sensitive part of the tool. The end of the milling cutter is also to leave the workpiece with the tool tip, that is to say that the blade is cutting and leaving from the beginning, and the cutting force always acts on the outermost tool tip until the impact force unloads. In figure b, the centerline of the milling cutter is exactly on the edge line of the workpiece. When the thickness of the chip is at its maximum, the blade is detached and the impact load is maximized when cutting. In figure c, the axis of the milling cutter axis lies within the width of the workpiece. The initial impact load at the time of cutting in is taken along the cutting edge by the part farther away from the most sensitive tip, and the blade exits the cutting relatively smoothly when the knife is retracted.
For each blade, the way the cutting edge leaves the workpiece when it is about to exit the cut is important. The remaining material near the retraction may cause the blade clearance to decrease somewhat. A momentary tensile force is generated along the rake face of the insert as the chip disengages from the workpiece and burrs are often generated on the workpiece. This tensile force endangers the chip edge safety in hazardous situations.
When the milling cutter axis line and the workpiece edge line are coincident or close to the edge of the workpiece (figure b), the situation is very serious. Achieve better milling:
Check the power and rigidity of the machine to ensure that the required cutter diameter can be used on the machine.
2 The overhang of the tool on the spindle is as short as possible. Fig. 3 Effect of milling cutter axis and workpiece position on impact load 3 Use the correct milling cutter pitch suitable for this process to ensure that there are not too many inserts while cutting and the workpiece is engaged to cause vibration, and on the other hand milling the narrow workpiece Or when milling the cavity, make sure that there is enough blade and workpiece engagement.
4 Ensure that the feed per blade is used so that when the chips are thick enough, the correct cutting result can be achieved and the tool wear can be reduced. Positive-angled slotted indexable inserts for smooth cutting results and lowest power.
5 Select the cutter diameter that is suitable for the width of the workpiece.
6 Select the correct declination angle.
7 Place the cutter correctly.
8 Use cutting fluid only when necessary.
9 Follow the rules for tool maintenance and repair and monitor tool wear.
2, drill
Drill bit is the most widely used tool in hole cutting tools. Especially when drilling holes with a depth of less than 30 mm, the bit is structurally divided into integral and indexable insert bits. As the automotive industry pursues high production efficiency, more and more The use of shoulder and chamfer compound drills is also increasingly widespread.
Many workpieces require drilling of one hole or several holes, and today most of these holes are machined on CNC machine tools and machining centers. In principle, there are many different types of holes, and the most common difference between these holes is the fit clearance. These holes include threaded holes, holes with excellent fit requirements, pipe holes, and holes machined for weight removal. These holes are either through-holes or blind holes and have different requirements for cutting tools and methods.
In the drilling process, in order to achieve satisfactory results in an effective way, there are four main factors to consider.
1 Ratio of diameter to hole depth 2 Required precision and surface roughness of the machined hole 3 Type, quality and hardness of workpiece material 4 Machine tool, especially machining conditions and spindle speed
These factors will affect the choice and application of bit types. In all processes, the stability of the workpieces, machine tools and process systems is the most important. The drilling process plays certain constraints when considering what types of drills are suitable for the machining process. The smallest indexable blade diameter is 12.7 mm.
3, sickle
The boring knife is divided into integral type, clamping type and adjustable type according to the structure, and the adjustable type is divided into fine adjustment type and differential type. Mainly used in the processing of automotive transmission shells are single-edged fine-tuning boring tools and double-edged rough boring tools.
Rough boring tool utilizes an axial adjustment mechanism to make the two-flute height exactly the same, achieve the ideal balance and prevent vibration. The feed screw is the lifeblood of the fine boring head. In some manufacturers, the paired production method is used to minimize the backlash between the screw and the nut and obtain the highest reliability. In the hole on the back of the crucible, often need to reverse the workpiece, or turn the table, it is not only a waste of time, and it is difficult to ensure the coaxiality, in Japan BIG company's EWN fine buns only need to reverse the blade can be installed Anti-tank processing, that is to ensure that the accuracy has increased production efficiency. For holes with high precision requirements, the cutter bar has a high dynamic balance effect. In the BIG high-speed small-hole fine-boring head moving balance ring, the built-in balance block will move. According to the relevant data in the instructions, the balance ring will be transferred. To the corresponding position can make the bread head in balance.
4, tapping
There are two kinds of tapping methods on the machining center, high-precision automatic reverse tapper, the maximum speed of 6000r/min, without any rigid role of compensation. These two types of tapping methods have their own advantages and disadvantages, so they are selected in accordance with the processing requirements. In mass production, due to the pursuit of high efficiency, automatic reverse tapping tapers will be conducive to production, but it has complex mechanisms, many accessories, and it is difficult to maintain. expensive. At present, with the increase in the number of CNC machining centers, rigid tapping will become increasingly common.
When rigid tapping is used, the CNC system of the machining center controls the axial feed, so the tap itself does not need to control the task. When the tapping is rigid, the rotation speed of the tap is synchronized with the axial feed of the machine spindle, which is 100%. The tap can be synchronized. Clamps in a fixed holder without any floating function.
Tapping tools are generally used for elastic clamping.
5, composite tool
In order to maintain high efficiency, we must maximize the "cutting" time so that the relevant time that is not spent in the actual cutting mode is minimized. In terms of tools, how to make the cutting time the shortest, and that is a composite tool, a tool's working process is tool change - the rapid movement of the tool or the table - cutting speed movement - rapid return to the tool change point, if the two tools compound , can save a tool tool change time, rapid movement time and the safety distance of 3-5 mm before the start of cutting. The main compound knife has a drilling chamfer compound tool, a rough boring and a chamfer compound boring tool. However, whether the tool is compounded must be calculated or tested, with the shortest processing time as the standard.
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