The history of high-speed cutting can be traced back to the concept of high-speed cutting first proposed by Dr. Carl Salomon of Germany in the 1930s. Dr. Salomon's research breaks through the traditional cutting theory's understanding of cutting heat, and believes that cutting heat is only a monotonically increasing function relationship with cutting speed in the traditional cutting speed range. When the cutting speed exceeds a certain limit, the cutting temperature no longer increases with the increase of the cutting speed, but decreases with the increase of the cutting speed, that is, the cutting speed is monotonically decreasing in the range of higher speed. Dr. Salomon's research was interrupted by the Second World War. Beginning in the late 1950s, high-speed cutting tests began to enter various experimental studies, and the mechanism of high-speed cutting began to be recognized by scientists. In 1979, a research team led by the German Government Research and Technology Department, led by the PTW Institute of the University of Darmstadt, Germany, and a research institute consisting of university research institutes, machine tool manufacturers, tool manufacturers, and users began a systematic study on high-speed milling. In addition to the high-speed cutting mechanism, the research team simultaneously researched and solved various application solutions for high-speed milling of machine tools, tools, process parameters, etc., so that high-speed milling in the processing mechanism has not been fully agreed, first in aluminum alloy processing and hard material processing The fields have been applied to solve the processing needs in the fields of mold, automobile, aviation, etc., and thus have achieved great economic benefits.
From the current test, as the cutting speed is gradually increased, the deformation law during cutting changes somewhat. The shear deformation in the chip is gradually intensified, and the slippage in the shear zone is gradually strengthened. Even the chip shape of the plastic material is gradually changed from the ribbon chip to the jagged chip, and further it is possible to further convert into a unitary chip. The figure below shows the shape of the chip at different cutting speeds for nickel-base superalloys.
v=106m/min v=125m/min v=160m/min v=200m/min
Since the chips will change from strip-shaped chips to unit chips under high-speed cutting conditions, the friction between the chips and the rake face will no longer be one of the main sources of cutting force and cutting heat; also due to the increase in cutting speed, the flank The elastic deformation of the workpiece material will also decrease as the deformation speed gradually fails to follow the cutting speed, and the friction of the flank face is also reduced, thereby having a favorable influence on reducing the cutting force and the cutting heat. Therefore, at high-speed cutting, the main cutting heat will be derived from the chips, and the temperature rise of the workpiece and the tool is very small, and the high-speed cutting is also called "cold cutting".
The German high-speed cutting research team believes that the speed range of high-speed cutting should be 5-10 times that of conventional cutting speed. The realization of high-speed cutting may involve many problems such as machine tools, tools, workpieces, and process parameters. This article will introduce some of the ideas about high-speed cutting tools and other related factors related to the tool.
First, the interface between the tool and the machine tool
In traditional boring and milling, we usually use a variety of 7:24 tool interfaces. The main shaft end faces of these interfaces have a gap with the tool. Under the action of the high-speed rotation of the main shaft and the cutting force, the large-end aperture of the main shaft expands, which causes the axial and radial positioning accuracy of the tool to decrease. At the same time, the axial size and weight of the taper shank are large, which is not conducive to rapid tool change and miniaturization of the machine tool.
For high-speed machining, we usually recommend a new interface standard called HSK. HSK is a new knife-machine interface developed by the Machine Tool Research Institute of Achen University in Germany for high-speed machine tools. It has formed six forms for automatic and manual tool change, central cooling and end face cooling, common and compact. . HSK is a small taper (1:10) hollow short taper shank. When used, the end face and the taper face are simultaneously contacted to form a high contact rigidity. After analysis, although the HSK connection will also expand when the high-speed rotation, but still maintain good contact, the speed has little effect on the connection rigidity of the interface.
Second, the balance of the tool
The principle of physics indicates that the centrifugal force of a particle in rotation is proportional to the mass of the particle, the distance between the particle and the axis of rotation, and the square of the angular velocity (or rotational speed) of the rotation. In other words, if the speed is increased by 1 time, the centrifugal force will increase by four times. This means that at high rotational speeds, the machining accuracy and longevity of the tool can be severely affected by centrifugal forces.
A precision boring tool manufacturer provides the following figures to illustrate this problem. They chose two boring tools to test, and only one fine boring knife was pre-balanced. There is no difference in the roundness of the holes processed by the two files at 5000r/min, which are 1.1μm. These errors are mainly caused by the accuracy of the machine tool system; when the speed is increased by 1 to 10000r/min. The situation is obviously different. The roundness of the hole processed by the balanced boring tool is slightly increased at 1.25 μm compared with 5000 r/min, and the roundness of the hole processed by the unbalanced boring tool is much higher than that at 5000 r/min. 6.30μm, which is more than 5 times that of the balance-adjusted file.
Therefore, the tools used for high-speed rotation must be balanced.
According to the balance theory, the imbalance of the rotor can be divided into three types:
Static imbalance:
The static imbalance is only one unbalanced mass and the unbalanced mass is located in the middle of the two supports, so that the centrifugal force in rotation is equal in magnitude and direction of the reaction forces on the two supports. Short overhanging tools (such as disc cutters) in machining can be approximated as static imbalance.
Even imbalance:
There are two unbalanced masses distributed at a 180° position symmetrical to the midpoint of the support, so that the centrifugal force during rotation is equal in the opposite direction of the reaction forces on the two supports, forming a couple.
Dynamic imbalance:
There are two or more unbalanced masses, and the distribution does not conform to the above rule. The centrifugal force in the rotation is different in the magnitude and direction of the reaction forces on the two supports. It can be said that the dynamic imbalance is a superposition of static imbalance and even imbalance, and most of the rod cutters are such imbalances.
There are two or more unbalanced masses, and the distribution does not conform to the above rule. The centrifugal force in the rotation is different in the magnitude and direction of the reaction forces on the two supports. It can be said that the dynamic imbalance is a superposition of static imbalance and even imbalance, and most of the rod cutters are such imbalances.
Unbalanced elimination has three methods of weighting, de-duplication and adjustment. The factory pre-balance of the tool adopts the method of drilling and weight-removing. That is, a hole of a specified size and depth is drilled at a position measured and calculated by the balancing machine so that the tool is statically balanced in the positional section, or one hole is drilled in each of the two positions to achieve dynamic balance. The indexable tool will generate a new micro-unbalance due to the replacement of the blade and the fitting. The overall tool will also form a certain micro-unbalance after the tool holder is loaded. We often use the adjustment method to remove the imbalance to achieve Balance the purpose. There are three main ways to adjust the method:
Balance adjustment ring: This is the main method used to install the overall tool in high-speed machining. Usually there are two balance adjustment rings on the tool holder. By rotating the balance adjustment ring separately, a resultant force and a balance torque can be generated to achieve dynamic balance.
Balance adjustment screw: This type of cutter can be used in this way. This method typically achieves a balance adjustment by changing the centroid position within the section by radially moving the two screws in one (or two) sections.
Balance adjustment block: A large-size single-edged tool (such as a single-edged file) usually has a balance adjustment block. The adjustment block is in the same section as the single-blade cutter head and can move radially. The balance is achieved by the movement of the adjustment block.
In addition to the balancing method described above, reducing the weight of the tool is also an effective method to reduce tool imbalance. The picture on the right is Walter's tool for high-speed cutting of aluminum alloys. The tool body of the tool is made of high-strength aluminum alloy. Since the density of aluminum is only 34.6% of steel, the centrifugal force at the same manufacturing precision is greatly reduced. Taking a 200 mm diameter milling cutter as an example, the weight of the tool is reduced from 9.8 kg to 3.7 kg compared to the same diameter steel cutter body milling cutter, and the maximum allowable speed is also increased from 4200 r/min to 13200 r/min.
Aluminum alloy body milling cutter with balance adjustment screw
For tools used under high-speed cutting conditions, disc type cutters can generally only perform static balance due to the relatively small axial dimension; while the rod-type cutters have a longer overhang, and there may be clips between the mass axis and the axis of rotation. The corners cannot be ignored, so dynamic balancing is necessary. It must be made clear that only the balance adjustments made in two or more sections may be dynamic, and the balance carried out in one section should be static.
(a) (b)
Central asymmetric three-toothed structure
In terms of general law, a centrally symmetrical structure is more suitable for high speed machining. In the right picture (a) is a center-asymmetric three-tooth structure in which only one tooth of the three teeth is over the center, the tool is generally not suitable as a high-speed cutting tool; and (b) is a centrally symmetric two-tooth structure, two of which When the teeth are over the center, it is more suitable for high speed cutting. Similarly, the flattened cylindrical shank removes some of the material on one side of the tool because it is flattened, which also causes tool imbalance, which is also disadvantageous for high-speed cutting. In addition, it is usually pressed from the side with screws, so that the gap between the mounting hole on the shank and the tool shank becomes an asymmetrical gap during the clamping process, and the imbalance after installation may be intensified, which is more suitable for high-speed cutting. Therefore, we must fully consider the influence of the structure in the tool selection process of high-speed cutting, and avoid the imbalance of the tool structure in principle. Because the amount of imbalance that can usually be removed by adjustment is rather limited, the principle imbalance often far exceeds the range of imbalances that can be eliminated by the adjustment of the shank.
We believe that tool suppliers should be clear about whether the tools they provide can be used for high-speed cutting. Nowadays, many European and American tool makers have indicated on their promotional materials such as the symbol "HSC" for high-speed cutting or the symbol "HSM" for high-speed machining. Therefore, it is generally not suitable to mark such symbols. High speed cutting.
Third, security
Since high-speed cutting usually requires a higher rotational speed, the possibility of the tool being broken and disassembled under the action of large centrifugal force is greatly increased. The set of photos on the left shows that this problem exists not only, but also with no small risks.
The bulletproof glass of the indexable milling machine with the blade flying off is crushed
In the figure, the indexable milling cutter with the blade flying off is a 100 mm diameter face milling cutter produced by a well-known domestic manufacturer. Shandong University used the milling cutter to install a single blade for cutting test. When the speed increased to 5000r/min, only one blade flew away from the cutter body when the centrifugal force exceeded the friction of the block, hitting the machine tool. On the protective Steel Plate. One of the users in Suzhou was unfortunate. Their broken pieces hit the bulletproof glass of the machine and the glass was completely crushed.
So, why do high-speed machining tools have such a large damage capability? We made the following brief analysis. If a 40 mm diameter indexable milling cutter is machined at 40,000 r/min, the linear speed is 5024 m/min, or 83.7 m/s, and the momentum is 1.26 kg·m/s, calculated as the blade weight of 0.015 kg. This result is comparable to the momentum of a famous micro-sound pistol bullet (the pistol bullet has a weight of about 0.005 kg and an initial velocity of 230 m/s, so the momentum is 1.20 kg·m/s).
The structure and safety of the blade cutter have a lot to do with it. The milling cutter that the blade used in Shandong University is a clamping method when it is commonly used in China. This clamping method relies entirely on friction to resist centrifugal force, and the blade is relatively easy to fly out, while the tool using the screw clamping method needs to cut the screw to cause the blade to fly out, and the safety is greatly improved. Studies have shown that with the gradual increase of the speed of the milling cutter clamped by the screw, the screw will be elongated under the action of the centrifugal force, and the cutter body will also slightly expand. At about 30,000 to 35,000 r/min, the critical stress has reached a permanent tensile deformation, and before the critical speed is reached, the screw has been bent, causing the clamping force to decrease and the blade to be displaced.
Therefore, we must pay full attention to the safety of high-speed machining tools to prevent safety accidents during high-speed cutting. The high-speed machining tool safety specification proposed by the German high-speed cutting working group has long been recommended as a German standard and an international standard. The specification states that for blade-type tools, the manufacturer must ensure that the test is performed at 1.6 times the maximum service speed (np=1.6nmax), the permanent deformation of the tool or the displacement of the part does not exceed 0.05 mm, and is twice as large as the maximum use. At the speed (np=2nmax), the tool does not burst; for a monolithic tool, it should be tested under np=2nmax without bending or breaking. At the same time, tool manufacturers are required to indicate the maximum speed of use for tools that can be used for high speed cutting. The safety of the tool used under these conditions will be fully secured. Of course, for non-high speed cutting tools (ie in the green part of the left picture), this is not required. Therefore, we should strictly follow the approved maximum speed regulations provided by the manufacturer when selecting high-speed cutting tools. However, since there is no safety standard for high-speed cutting tools in China, we need to be more cautious in the selection.
For high speed cutting tools, we should take certain measures to ensure their safety. For example, Walter requires the use of original blade locking screws for high-speed cutting blade cutters to ensure safety; the specified torque is guaranteed when the blade is installed; and after the blade is replaced 5 times, the new blade locking screw is used to prevent the screw The clamping force is reduced due to fatigue, which affects the safe use of the tool.
Milling cutter with centrifugal force unloading
Similarly, tool manufacturers should take steps to enhance the safety of tools in high-speed machining. In recent years, some famous tool manufacturers in foreign countries have adopted technical measures to enhance the safety of cutting tools in the development of blade cutters. One of them is to increase the centrifugal force unloading structure between the blades and the cutter body. The blade on the left has a protrusion on the blade, and the corresponding position of the blade has a hole. The centrifugal force of the blade can be shared by the friction between the blade and the blade, the support force of the screw and the support force of the centrifugal unloading structure. The ability to fly out of the blade has been strengthened.
Fourth, the material of high-speed cutting tools
Although we always hope to obtain high hardness to ensure the wear resistance of the tool and high toughness to prevent cracking of the tool, the current technological development has not found such a superior performance tool material, fish in the bear's paw Can't have both. Therefore, we will choose more suitable tool materials in practice as needed:
The toughness of the tool material is prioritized during roughing;
The hardness of the tool material is prioritized during finishing.
The technology of diamond, cubic boron nitride and ceramics has a long history. Their common features are high hardness, high brittleness and high impact.
Tool material performance
In recent years, the development of coating technology has enabled fine-grained cemented carbide to improve its high temperature and impact resistance while maintaining its high toughness, providing technical guarantee for more economical cutting tools for high-speed cutting applications.
For example, the Tiger·Tec technology created by Walter (known in Chinese as “Tiger Bladeâ€) uses special plating technology to make the front and back knives of the blade appear in black and gold, respectively, and improve the rake surface. The connection structure between the alumina coating and the base plating enhances the strength of the coating, enhances the thickness of the aluminum oxide layer, and greatly improves the high temperature resistance of the blade. Due to the unitization of the chip state during high-speed cutting, the tiger blade is mainly suitable for machining cast iron, and is expanded to be suitable not only for cast iron machining but also for steel machining.
Walter's Quar·Tec line of products introduced at the European Machine Tool Show in 1999 has a blade material named WQM35, which uses a 100-layer coating technology. The total thickness of the 100-layer coating is 6-8 μm, and the thickness of the single layer is mostly 20-50 nm. Due to the high-speed cutting of the coating, especially the high-speed milling, under the impact of cutting force and cutting heat, which is not large but the alternating frequency is very high, it is easy to expand the tiny notch to form cracks, thereby reducing the film layer and the film layer. The bonding force between the film layer and the substrate. Therefore, we recommend the choice of coated cemented carbide (especially fine-grained cemented carbide) during roughing, and the choice of diamond or cubic boron nitride for finishing.
100-coated WAP35
Five, high speed processing strategy
High-speed machining can result in high metal removal rates, so it is important to check that the required torque and power exceed the torque and power that the machine can provide during roughing. Especially when processing aluminum parts, due to their extremely high processing speed, their requirements may lead to values ​​beyond our imagination. Taking a 32 mm diameter corn milling cutter as an example, if the groove is milled at a line speed of about 1000 m/min (the corresponding speed is about 10000 r/min), when the depth of cut is 10 mm, the power consumption is about 19 kW. When the depth is increased to 30mm, the power consumption will increase to about 56kW. Therefore, our recommendations are:
In high-speed machining, increase the proportion of cutting time in the whole working time as much as possible, and reduce the non-machining time (such as tool change, adjustment, idle stroke, etc.);
Face milling and end milling: machining strategies with small or medium sized tools and layered cutting;
In slot milling, layered cutting, small depth of cut, medium feed is used. Due to the high cutting speed, a high feed rate can also be obtained.
There are also some specific processing considerations:
When using the ball end mill, it should be noted that the cutting speed near the center is extremely small, close to "zero", so the cutting conditions are relatively bad. Therefore, if possible, it should be that the axis of the milling cutter and the normal direction of the workpiece are inclined by an angle. According to the test, when the angle is about 15°, the life of the tool will reach a maximum value.
The overhang of the tool will have some effect on the tool life in high speed machining. As a result of the limited test, when the 6mm end mill overhang is increased from 30mm to 60mm, the tool life is expected to increase to 140% to 230%.
In profiling, milling along a cross-sectional profile (also known as contour milling) is preferred over milling along a longitudinal profile (also known as hill-climbing). Because the cutting conditions of contour milling are relatively consistent, the cutting is smooth, the surface roughness of the workpiece is also better, and the cutting conditions of the climbing milling are extremely unstable. The cutting edge is prone to chipping near the center, and the surface roughness of the workpiece Not ideal. In the case of hill-climbing, experiments have shown that the tool life of the down-milling is better than that of the up-cut milling, and the tool life of the up-milling is better than that of the down-milling.
Change of cutting force when milling internal cavity
When milling the internal cavity, when the tool is fed to the corner, the radial force will increase sharply due to the sudden increase of the cutting wrap angle, and the peak value will reach about 170% of the normal cutting value. Therefore, we recommend implementing a so-called "cycloidal cutting" at the corner so that a sudden increase in cutting force can be avoided, resulting in smooth cutting and longer tool life.
In terms of cooling, we should pay special attention. Oil mist cooling (also known as quasi-dry cutting) is ideal. Jet cooling, high pressure and large flow internal cooling are also acceptable, but low pressure, external cooling should be avoided.
Concerned about surprises
Tag: High Speed ​​Cutting Tool Material High Speed ​​Machining Cutting Speed ​​Chip
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