Molds are technical products and typical non-fixed products. Each set of molds must undergo creative design, CNC programming, production preparation, machining, assembly, and tryout. The cycle is longer, especially its machinery. Processing is time-consuming. Therefore, how to increase production efficiency, shorten the development cycle, increase the level of mold manufacturing, and reduce production costs have always been problems faced by mold manufacturers.
Whether it is a stamping die or a forging die, the materials making up the die cavity are generally made of high-strength, wear-resistant materials (such as various alloy tool steels and stainless steels, etc.). The hardness of these materials after quenching heat treatment is very high, generally above 50HRC, it is difficult to use conventional machining methods for cutting and finishing.
For a long time, the best way to deal with such difficult-to-machine materials is to use EDM special machining methods. There are two obvious shortcomings in EDM: First, the production efficiency is low, and second, the processing quality is difficult to guarantee.
The emergence of high-speed cutting technology has brought a new approach to mold manufacturing technology. Compared with EDM machining, high-speed machining has advantages such as high production efficiency, good product quality, and the ability to process hard parts and thin-walled parts with complex cavity shapes. Therefore, since the 1990s, the foreign mold industry began to use high-speed cutting methods for the mold cavity finishing.
According to statistics, currently 85% of the molds in industrialized countries have been replaced by high-speed cutting processes. High-speed cutting has been established in the mainstream of international mold manufacturing processes. In the 1990s, China also began to pay attention to the research and application of high-speed cutting technology. The key technologies for high-speed cutting tools are tool technology and machine tool technology. This article analyzes the related technology of high-speed milling tools for hardened die.
2 The advantages of high-speed milling hardened molds
The most commonly used machining method in high-speed machining is high-speed milling. The use of a high-speed milling die instead of an EDM tool has the following advantages.
(1) Good processing quality When the die is processed by the traditional EDM method, the physical-mechanical properties of the surface of the workpiece material will be damaged in the process of local high-temperature discharge ablation on the surface of the workpiece, often causing fine cracks on the surface of the cavity. , it is difficult to guarantee the quality of the workpiece processing. High-speed milling processes parts at cutting speeds that are about 10 times higher than conventional milling speeds, and the stock of the blank is momentarily cut away from the workpiece before it is sufficiently deformed. The residual stress on the workpiece surface is very small. At the same time, in the high-speed milling process, the spindle speed of the machine tool is extremely high (8000r/min to 100000r/min), and the excitation frequency of the machine tool-fixture-workpiece-tool technology system is much higher than its natural frequency range. Impact. Therefore, parts have high machining accuracy and good surface quality. After high-speed milling mold cavity, the surface quality can reach the level of grinding, it can save the subsequent grinding process.
(2) High production efficiency Since EDM is based on "micro-cutting" by discharge ablation, the machining process is very slow. At the same time, the surface roughness of the mold cavity does not meet the requirements of the mold, often after EDM. Time-consuming hand-grinding and polishing of the mold are required, so this process is extremely inefficient. Machining molds in machining centers or high-speed milling machines, the process itself is several times more efficient than EDM. At the same time, the roughing and finishing of the cavities and the machining of the other parts of the dies can be performed in one clamping of the workpiece, ie the so-called One Pass Machining. In addition, this technique does not require electrodes and subsequent manual grinding and polishing, and it is easy to automate the process. Therefore, the application of high-speed machining technology has greatly increased the speed of mold development.
(3) The ability to machine hard parts and thin-walled parts with complex shapes It can be seen from the high-speed cutting mechanism that the cutting force during high-speed milling is greatly reduced and the cutting process becomes easier. High-speed cutting can process hardened steel with a material hardness of 60HRC or more without the use of cutting fluids, so-called Hard Machining and Dry Machining. What is even more remarkable is that the transverse cutting force is very small in high-speed milling, which is extremely advantageous for machining some fine ribs and thin walls (with a wall thickness of less than 1 mm) in complex mold cavities. Of course, high-speed milling tools also have some limitations. When the workpiece material hardness is more than 60HRC, and there is a narrow and deep cavity with a very small chamfer and fillet, high-speed milling is very difficult, and the organic combination of high-speed milling and EDM can achieve better The economic effect.
3 Cutting Tool Technology for High Speed ​​Milling Hardened Dies
In the high-speed milling tooling, the tool technology is the most critical, and involves many aspects, mainly in the selection of tool materials and geometric parameters, tool damage and detection, tool and machine tool connection technology, tool safety and other issues.
3.1 Selection of tool materials and geometric parameters (1) High-speed milling of tool materials High-speed tool materials and processed materials must have low chemical affinity, good thermal stability, impact resistance, wear resistance, and thermal fatigue resistance. And has excellent mechanical properties.
At present, the tool materials used for high-speed hard milling include polycrystalline cubic boron nitride (PCBN), ceramics, new type hard alloys, and coated hard alloys. The three major factors should be based on the mold material, tool geometry, and cutting conditions. Select tool material.
Because polycrystalline cubic boron nitride (PCBN) tools have high hardness and wear resistance, they are suitable for high-speed cutting of hardened steels. When processing workpieces with a hardness lower than 50HRC, the chips formed by the PCBN cutters are long strips and wear on the surface of the tool, which shortens the tool life. Therefore, PCBN tools are suitable for processing materials with hardness higher than 55-65HRC.
Ceramic cutters cost less than PCBN tools and have good thermochemical stability, but their toughness and hardness are not as good as PCBN tools. Therefore, ceramic tools are more suitable for machining relatively soft materials (HRC ≤ 50). The new carbide and coated carbide tools cost less, but the cutting hardness is not as good as the PCBN tool and the ceramic tool, and is generally between 40 and 50HRC.
From the current research situation, the performance of polycrystalline cubic boron nitride (PCBN) tools is better among all tool high-speed cutting tool materials. It is the main tool material for machining hardened steel molds.
(2) Selection of tool geometry parameters When the tool material is selected, the choice of tool geometry parameters has a greater influence on the tool life and cutting speed. Generally, the blade shape with the greatest possible strength should be selected, and the radius of the tool tip arc should also be used. May be big.
In comparison with ordinary milling, the rake angle should be 10° smaller for high-speed milling and 5°-8° larger for lower relief angles. During high-speed milling, different parameters of the tool are different depending on the machining material. When machining hardened steel, the tool geometry that plays an important role is the rake angle γ0 and the relief angle α0. The empirical values ​​of reasonable rake angle γ0 and relief angle α0 during high-speed milling are shown in the following table.
Table Reasonable γ0, α0 values ​​of high-speed cutting tools for different materials Workpiece material - γ0-α0
Aluminum alloy -12°～15°-13°～15°
Steel -0° to 5°-12° to 16°
Cast iron -0°-12°
In addition, the cutting force of hard cutting is large. In addition to the requirement of blade strength, the strength and rigidity of the cutter bar are also required to be high.
3.2 Tool Damage and Detection (1) Tool Damage Because of the high price of high-speed milling tools, the damage of the tool severely shortens the service life of the tool and increases the cost of high-speed milling. Therefore, controlling the damage of the tool and enhancing the detection of the tool are important for high-speed milling. There are two conditions of wear and damage to the cutter. Wear is the phenomenon of surface material consumption caused by contact and friction between the tool and the workpiece during machining. Breakage is the phenomenon that the tool is chipped, fractured, and plastically deformed, causing the tool to lose its cutting ability. It includes brittle failure and plastic damage.
Tool wear is a difficult problem to solve in high-speed milling. The wear of the tool during high-speed milling mainly includes flank wear, front crater wear, boundary wear, micro chipping, exfoliation, and plastic deformation. The main wear patterns of different machining materials and high-speed tool materials are different. Back flank wear is the most common form of high-speed tool wear, and it is also the normal wear of the tool. Generally, the width VB of the flank wear area is used as the wear limit of the tool. The increased width of the flank wear area will rapidly reduce tool cutting. The wear of crater craters mainly occurs in the high-speed cutting of plastic metal, and often occurs under cutting conditions where the cutting temperature is high and the tool is hard-red. The boundary wear often occurs at the edge of the cutter's flank face and the workpiece's contact edge. The shape is a narrow groove. The high speed cutting of stainless steels and high temperature alloys tends to cause boundary wear. Micro chipping is a small gap that occurs on the cutting edge of the tool and usually occurs during intermittent high speed cutting.
Exfoliation mainly occurs on the front and back surfaces of the tool due to the contact fatigue of the tool-chip, tool-workpiece contact area or thermal stress fatigue.
(2) Tool detection At present, the tool detection mainly adopts three forms of manual detection, off-line detection, and on-line detection. Manual detection is based on the experience of the worker during the processing of the state of the tool to detect; off-line detection is a special inspection of the tool before processing, and predict its life, to see if the current processing tasks can be completed; online detection is also called real-time detection, The tool is detected in real time during the machining process and the corresponding processing is performed based on the detection result.
3.3 Tool and machine tool connection technology In the high-speed cutting conditions, the connection system between the tool and the machine tool is an important aspect affecting the machining accuracy and tool safety. The traditional standard 7:24 taper solid long tool shank structure can not meet the requirements of high-speed cutting. New types of tool holders must be developed and developed to connect tools and machine tools. Under high-speed cutting conditions, the tool system (tools, chucks and holders) is required to have the following characteristics:
(1) Higher tool system accuracy Tool system accuracy includes system positioning and clamping accuracy and tool repeat positioning accuracy. The former refers to the connection accuracy between the tool and tool holder, tool holder and machine tool spindle; the latter refers to the tool system after each tool change. Accuracy consistency. The high system accuracy of the tool system can guarantee the static and dynamic stability of the tool system under high-speed machining conditions.
(2) Rigid tool system rigidity The static and dynamic rigidity of the tool system is an important factor that affects the machining accuracy and cutting performance. Insufficient rigidity of the tool system can cause the tool system to vibrate, which can reduce the machining accuracy and exacerbate the wear of the tool and reduce the service life of the tool.
(3) Better Balancing Under high-speed machining conditions, the unbalance of small masses can cause huge centrifugal forces, causing rapid vibrations in machine tools and machining processes. Therefore, the balance of high-speed tool systems is very important.
In order to meet the requirements of cutting tool systems for high-speed cutting, in the past decade, various industrialized countries have successively developed and developed a variety of new types of tool holders. At present, the most representative ones are the German HSK holders, the American KM holders and the Japanese BIG-PLUS holders.
The HSK tool shank completes both radial and axial double-sided positioning from the taper and flange faces to achieve a rigid connection to the spindle. When the tool holder is installed on the spindle of the machine tool, the hollow short tapered shank with a taper of 1:10 is in complete contact with the taper hole of the spindle, and it functions as a centering to realize the coaxiality between the tool and the spindle. At this time, there is also a gap of about 0.1 mm between the HSK shank flange and the spindle end face. Under the action of the tensioning mechanism, the pull rod moves to the left so that the front end cone expands the elastic jaws radially. At the same time, the outer cone surface of the jaws acts on the 30° cone surface of the hollow short taper shank, and the short hollow cone The handle is elastically deformed, and its end surface is tightly contacted with the end surface of the main shaft, so that the function of the holder and the spindle cone surface and the spindle end surface can be simultaneously positioned and clamped.
The KM tool holder is a 1:10 short-cone hollow tool holder cooperating with the HSK tool holder. The length of the taper shank is only 1/3 of the standard 7:24 taper solid long tool holder. Due to the shorter fitting taper, the interference problem caused by the simultaneous positioning of the end face and the taper face is partially solved. On the other hand, the fit interference between the KM tool holder and the spindle taper hole is higher, which can be 2 to 5 times that of the HSK tool holder structure, and the connection rigidity is higher than that of the HSK tool holder. At the same time, compared with other types of hollow taper shank connections, the taper shank used for the same outer diameter of the flange has a smaller diameter, so that the spindle taper hole expands little at a high speed, and the high speed performance is good.
3.4 Safety of tools Safety requirements for milling cutters are required for high-speed milling of dies. Tests have shown that the structure and strength of ordinary milling cutters can not meet the requirements of high-speed cutting. When the milling cutter rotates at a high speed, the centrifugal force exerted on each part of the cutter far exceeds the effect of the cutting force itself and becomes the main load of the cutter. When the centrifugal force reaches a certain degree, the tool may be deformed or even broken, resulting in serious consequences. Therefore, the research on the safety technology of high-speed milling cutters and the prevention of tool damage caused by centrifugal force are extremely important for the tool technology of high-speed milling dies.
Germany's research achievements in high-speed cutting tool systems have made important contributions to the promotion and application of high-speed cutting technology. As early as in the early 1990s, Germany began to study the safety technology of high-speed milling cutters and achieved a series of results. It also formulated the draft DIN 6589-1 "Safety Requirements for High-speed Rotary Milling Cutters", which specifies the high speed The test methods and standards for the failure of milling cutters have technically presented guiding opinions on the design, manufacture and use of high-speed milling cutters and stipulated a unified safety inspection method. The draft standard has become a guiding document for the safety of high-speed milling cutters in various countries.
There are two safety failures for high-speed milling cutters: deformation and cracking. Different types of milling cutters have different safety test methods. For machine-mounted indexable milling cutters, a test method is that the permanent deformation of the tool or displacement of the part does not exceed 0.05 mm at 1.6 times the maximum operating speed; the other test method is at twice the maximum operating speed. The tool is not broken, including screws that clamp the blade being sheared, blades or other clamping elements being flung away, and the body bursting. For integral mills, it must be tested at 2 times the maximum operating speed without bending or breaking.
In combination with the safety standards of high-speed milling cutters, through the analysis of the finite element calculation model, in order to meet the safety requirements of high-speed milling cutters, the following aspects can be started:
(1) Tool Quality and Tool Structure Reducing the tool quality, reducing the number of tool system components, and simplifying the tool system structure are effective ways to increase the tool breaking limit. Comparing the relationship between the fracture limit and the tool quality, the number of tool components and the number of contact surfaces of the tool with the same diameter tool, it can be found that the lighter the tool quality, the fewer the number of component parts of the tool system and the contact surface of the components, and the limit speed of tool fracture. The higher. In order to reduce the tool quality, the tool body material should be selected according to the ratio of the material tensile strength to the density and the speed range of the tool application. At present, high-strength aluminum alloys are used to manufacture high-speed milling cutter bodies.
In the tool structure, care should be taken to avoid and reduce the stress concentration. Grooves on the cutter body (including pockets, chip pockets, and keyways) can cause stress concentration and reduce the strength of the cutter body. Therefore, it should be avoided to have a sharp corner at the bottom of the slot and groove. At the same time, the structure of the tool should be symmetrical to the axis of rotation so that the center of gravity passes through the axis of the cutter. The clamping and adjusting structure of the blade and the tool holder should eliminate the play as much as possible and require good repeatability.
(2) Clamping mode of tool (blade) Simulation calculation and cracking test studies show that the clamping method of high-speed milling cutter blades does not allow ordinary friction clamping, but use a blade with a center hole to clamp with a screw. Or use a specially designed tool structure to prevent the blade from flying. The direction of the clamping force of the tool holder and the blade is preferably the same as the direction of the centrifugal force. At the same time, the pretightening force of the screw must be controlled to prevent the screw from being damaged prematurely due to overload. There are two kinds of high-precision, high-rigidity clamping methods for small-diameter shank cutters: hydraulic chucks and thermal expansion and shrinkage chucks.
(3) Balance of the tool During high-speed rotation, the imbalance of the tool will not only cause vibration of the spindle system of the machine tool, affect the machining accuracy, but also generate additional radial load on the tool system. The size of the tool is proportional to the square of the tool speed.
It can be seen that increasing the balance of the tool can greatly reduce the centrifugal force and improve the safety of high-speed tools. Therefore, in accordance with the requirements of the Draft Standard for Safety Requirements for High-Speed ​​Rotary Milling Cutters, milling cutters for high-speed cutting must undergo dynamic balance testing and should meet the G4.0 balanced quality level requirements specified in ISO1940-1.
4 Conclusion
Through the analysis of the tool material and geometric parameters, tool damage and detection, tool and machine tool connection technology, tool safety in four aspects of the high-speed milling hardened mold tool requirements, only solve the problems of high-speed milling tools, only It is conducive to the popularization and application of high-speed milling technology for molds.
Hard Tooling High Speed ​​Milling Tool Technology Analysis
1 Introduction