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CNC high-speed cutting machining is one of the most important advanced manufacturing technologies in mold manufacturing. It is an advanced manufacturing technology that combines high efficiency, high quality, and low consumption. Compared with the traditional cutting process, the cutting speed and feed rate have been greatly improved, and the cutting mechanism is not the same. High-speed cutting has made an essential leap in cutting machining. The metal removal rate per unit power has increased by 30% to 40%, the cutting force has been reduced by 30%, the cutting life of the cutting tool has increased by 70%, and the cutting heat remains in the workpiece. As the amplitude decreases, low-level cutting vibrations almost disappear. With the increase of cutting speed, the removal rate of the blank material per unit time increases, the cutting time decreases, and the processing efficiency increases, thereby shortening the product manufacturing cycle and improving the market competitiveness of the product. At the same time, the high-speed machining with a small amount of fast-forward reduces the cutting force. The high-speed discharge of the chips reduces the cutting force and thermal stress distortion of the workpiece, and improves the possibility of poor rigidity and thin-walled parts machining. Due to the reduction of cutting force, the increase of the rotational speed makes the working frequency of the cutting system away from the low-order natural frequency of the machine tool, and the surface roughness of the workpiece is most sensitive to the low-order frequency, thereby reducing the surface roughness. In the processing of the high hardened steel parts (HRC45~HRC65) of the die, high-speed cutting can replace the electrical machining and grinding and polishing processes, thus avoiding the manufacture of electrodes and time-consuming electrical machining, and significantly reducing the fitter's Grinding and polishing amount. For some thin-walled mold parts that are increasingly needed in the market, high-speed milling can also be successfully completed, and at high-speed milling CNC machining centers, the molds can be set up at a time to complete multi-step machining.
The high-speed machining technology has a huge impact on the mold processing technology, changing the complex and lengthy process flow such as “annealing→milling→heat treatment→grinding†or “electric discharge machining→hand sanding and polishing†used in traditional mold processing, and can even be used High-speed cutting replaces all the original processes. In addition to the direct processing of hardened mold cavities (especially semi-finishing and finishing), high-speed machining technology has also been widely used in EDM electrode processing and rapid prototyping. Massive production practices show that the application of high-speed cutting technology can save about 80% of the manual grinding time in the subsequent processing of the mold, saving the processing cost of nearly 30%, the mold surface machining accuracy of up to 1 m, the cutting efficiency of the tool can be increased by 1 times.
Second, high-speed milling machine tools
High-speed cutting technology is one of the major development directions of cutting technology. It has taken a higher level with the development of basic technologies such as CNC technology, microelectronic technology, new materials and new structures. Due to the special characteristics of die processing and its own characteristics of high-speed machining technology, higher-speed machining of related technologies and processing systems (processing machine tools, numerical control systems, tools, etc.) has been proposed with higher requirements than traditional die machining.
1. High stability machine tool support parts
Supporting parts such as the bed of high-speed cutting machine tools should have good dynamic and static stiffness, thermal stiffness and optimum damping characteristics. Most machine tools use high-quality, high-rigidity and high-strength gray cast iron as the supporting component material. Some machine tool companies also add high damping polymer concrete to the base to increase their vibration resistance and thermal stability. This not only ensures the stability of the machine tool accuracy, but also prevents the chattering of the tool during cutting. The use of a closed bed design, an overall casting bed, a symmetrical bed structure and a dense ribbed reinforcement are also important measures to improve the stability of the machine tool. In the design process of some machine tool companies, modal analysis and finite element structure calculations are also used to optimize the structure and make the machine tool support components more stable and reliable.
2. Machine tool spindle
High-speed machine tool spindle performance is an important condition for high-speed machining. High-speed cutting machine spindle speed range of 10000 ~ 100000m/min, spindle power greater than 15kW. The axial clearance between the shank and the spindle is controlled by the main shaft compressed air or cooling system to be no more than 0.005mm. It is also required that the spindle has the characteristics of rapid acceleration and quick and accurate stopping at a specified position (ie, having a very high angular acceleration and deceleration). Therefore, high-speed spindles often use hydrostatic bearing type, aerostatic bearing type, and hot pressing silicon nitride. (Si3N4) ceramic bearing magnetic suspension bearing and other structural forms. Lubrication mostly uses oil and gas lubrication, jet lubrication and other technologies. Spindle cooling generally uses the internal water cooling or air cooling of the spindle.
3. Machine Tool Drive System
In order to meet the need for high-speed machining of molds, the drive system of high-speed machining machines should have the following characteristics:
(1) High feed rate. Studies have shown that for small diameter tools, increasing the speed and feed per tooth helps reduce tool wear. At present, the commonly used feed speed range is 20~30m/min. If a large-lead ball screw drive is used, the feed speed can reach 60m/min; with a linear motor, the feed speed can reach 120m/min.
(2) High acceleration. High-speed machining of three-dimensional complex surface profile requires the drive system to have good acceleration characteristics, and it is required to provide a high-speed feed driver (fast forward speed is about 40m/min, 3D contour processing speed is 10m/min), which can provide 0.4m/s2 Acceleration and deceleration up to 10m/s2.
Most machine tool builders use small-lead, large-size, high-quality ball screws or large-lead multi-head screws with full-closed position servo control. With the development of motor technology, advanced linear motors have been introduced and successfully applied to CNC machine tools. The advanced linear motor drive eliminates the problems of mass inertia, advancement, hysteresis, and vibration in CNC machines, accelerating the servo response speed, and improving servo control accuracy and machining accuracy.
4. CNC system
The advanced numerical control system is a key factor to ensure the high-speed machining quality and efficiency of the complex surface of the mold. The basic requirements for the high-speed cutting of the mold on the CNC system are:
(1) High-speed digital control loops, including: 32-bit or 64-bit parallel processors and hard disks over 1.5 Gb; extremely short linear motor sampling time.
(2) Feed forward control of speed and acceleration; Jerk control of digital drive system.
(3) Advanced interpolation methods (based on NURBS spline interpolation) for good surface quality, accurate dimensions, and high geometric accuracy.
(4) Look-ahead function. Requires a large-capacity buffer register for reading and checking multiple blocks in advance (eg up to 500 blocks for DMG machines and up to 1000~2000 blocks for Simens systems) to take place on the shape of the machined surface (curvature) When changes occur, measures such as changing the feed rate can be taken in time to avoid overcutting.
(5) Error compensation functions, including thermal error compensation, quadrant error compensation, and measurement system error compensation due to heat generation from linear motors, spindles, etc. In addition, high-speed cutting of the mold requires high data transmission speeds.
(6) The traditional data interface, such as the RS232 serial port, has a transmission speed of 19.2kb, and many advanced machining centers have used Ethernet for data transmission at speeds up to 200kb.
5. Cooling lubrication
The high-speed machining uses a coated carbide cutting tool, eliminating the need for cutting fluids at high speeds and high temperatures, resulting in higher cutting efficiency. This is because: When the milling spindle rotates at a high speed, the cutting fluid must reach the maximum centrifugal force if it reaches the cutting zone. Even if it overcomes the centrifugal force and enters the cutting zone, it may evaporate immediately due to the high temperature in the cutting zone. The cooling effect is even small. No; At the same time the cutting fluid will make the temperature of the cutting edge of the tool drastically change, and it will easily lead to the occurrence of cracks. Therefore, it is necessary to adopt the dry cutting method of oil/gas cooling lubrication. In this way, high-pressure gas can be quickly blown away from the cuts generated in the cutting zone, thus allowing a large amount of cutting heat to move away. At the same time, the atomized lubricating oil can form a very thin layer of microscopic protective film on the blade edge and workpiece surface. Effectively prolongs tool life and improves surface quality of parts.
Third, high-speed cutting tool holder and tool
Due to the effects of centrifugal force and vibration during high-speed cutting, it is required that the tool has high geometric accuracy, repeatability of the positioning of the fixture, and high rigidity and high-speed dynamic balance. Due to the characteristics of large centrifugal force and vibration during high-speed cutting, the traditional 7:24 taper shank system shows obvious defects such as insufficient rigidity, low repetitive positioning accuracy, and unstable axial dimensions when performing high-speed cutting. The expansion causes the center of gravity of the tool and the clamping mechanism to deviate, which affects the dynamic balance ability of the tool. At present, the HSK high-speed tool holders and foreign popular hot-cold-and-collapse fastening tool holders are widely used. The thermal expansion and shrinkage cold type fastening tool holder has a heating system. The tool holder generally uses the taper part and the spindle end surface to contact at the same time. The rigidity is good, but the tool exchangeability is poor, and a tool holder can only be installed with a connection diameter. Tool. Since such a heating system is relatively expensive, an HSK type tool holder system can be used in the initial stage. When the company's number of high-speed machine tools exceeds three or more, the use of thermal expansion and shrinkage fastening tool holder is more appropriate.
The tool is one of the most active and important factors in high-speed machining. It directly affects the machining efficiency, manufacturing cost, and machining accuracy of the product. The cutters are subjected to high temperature, high pressure, friction, impact and vibration loads during the high speed machining process. The high speed Cutting Tools should have good mechanical properties and thermal stability, ie good impact resistance, wear resistance and thermal fatigue resistance. . Cutting tool technology for high-speed cutting is rapidly developing. It is used in many applications such as diamond (PCD), cubic boron nitride (CBN), ceramic tool, coated carbide, (carbon) titanium nitride carbide TIC (N )Wait.
Carbide is the most commonly used tool material in machining cutting tools for cast iron and alloy steel. Carbide cutters have good wear resistance but lower hardness than cubic boron nitride and ceramics. In order to improve the hardness and surface finish, tool coating technology is adopted. The coating materials are titanium nitride (TiN), titanium aluminum nitride (TiALN), and the like. The coating technology has enabled the coating to develop from a single coating to a multi-layer, multi-coating material coating, which has become one of the key technologies for improving high-speed cutting capability. Diameters in the range of 10 to 40 mm, and carbide titanium carbide coated carbide inserts can process materials with Rockwell hardness less than 42 and titanium nitride nitride coated tools can process Rockwell hardness of 42 or higher. s material. For high-speed cutting of steel materials, P-type hard alloys, coated cemented carbides, cubic boron nitride (CBN), and CBN composite tool materials (WBN) with high thermal and fatigue strengths should be used as tool materials. For cutting cast iron, fine-grained K-type hard alloys should be used for roughing, and composite silicon nitride ceramics or polycrystalline cubic boron nitride (PCNB) composite tools should be used for finishing. For precision machining of non-ferrous or non-metallic materials, polycrystalline diamond PCD or CVD diamond-coated tools should be used. When selecting the cutting parameters, the concept of effective diameter should be taken into consideration for round inserts and ball end mills. High-speed milling tools should be designed and manufactured according to dynamic balance. The rake angle of the tool is smaller than that of the conventional tool, and the back angle is slightly larger. The major and minor cutting edge connections should be rounded or chamfered to increase the tool nose angle to prevent thermal wear at the tool tip. The cutting edge length and tool material volume near the tool tip should be increased to increase the tool rigidity. Under the condition of ensuring safety and meeting the processing requirements, the tool overhang is as short as possible and the center of the cutter body is better in toughness. The shank is thicker than the tool diameter and the connecting shank has an inverted taper to increase its rigidity. As far as possible, coolant holes are left in the center of the tool and tool system. The ball end mill must consider the effective cutting length. The cutting edge should be as short as possible. The two spiral groove ball end mills are usually used for rough milling of complex surfaces, and the four spiral groove ball end mills are usually used for fine milling of complex surfaces.
Fourth, mold high-speed processing technology and strategy
High-speed machining includes rough machining for residual removal, residual rough machining, and semi-finishing, finishing, and mirror finishing for the purpose of obtaining high-quality machining surfaces and fine structures.
Rough processing
The main goal of mold roughing is to pursue the material removal rate per unit time and to prepare the geometric contour of the workpiece for semi-finishing. The process plan that should be adopted for roughing in high-speed machining is a combination of high cutting speed, high feed rate, and small cutting amount. The contour processing method is a processing method commonly used by many CAM software. More applications are the spiral height contour and the equal Z axis contour height method, that is, only one feed in the machining area, and the continuous smooth tool path is generated without lifting the tool. Cut in and cut out. Spiral contouring is characterized by the fact that there is no waiting for the tool path movement between the high level, which can avoid the effect of frequent lifting and infeed on the surface quality of the parts and the unnecessary loss of mechanical equipment. Handling steep and flat areas separately, calculating areas that are suitable for contours and suitable for using similar 3D offsets, and using spirals to generate optimized tool paths with less tooling and better surface quality. In high-speed machining, arc cutting, cutting connection, and arc transition at corners must be adopted to avoid abruptly changing the tool feed direction. It is prohibited to use the direct lower knife connection method to avoid embedding the tool in the workpiece. When machining the mold cavity, the tool should be avoided from being inserted into the workpiece vertically. Instead, use a tilt-down knife (usual tilt angle is 20° to 30°). It is best to use a spiral knife to reduce the tool load. When processing mold cores, it is best to cut the workpiece from the outside of the workpiece and cut it horizontally. When the tool cuts in or cuts out the workpiece, it should be inclined or arc-shaped as far as possible to cut in and cut out, so as to avoid vertical cut-in and cut-out. The use of climbing cutting can reduce the heat of cutting, reduce the force and work hardening of the tool, and improve the processing quality.
2. Semi-finishing
The main goal of the semi-finishing of the mold is to make the contour of the workpiece flat and the surface finishing allowance is even. This is especially important for the tool steel mold, because it will affect the change of the tool cutting layer area during the finishing and the change of the tool load, thus affecting Cutting process stability and finishing surface quality.
Roughing is based on the volume model, and finishing is based on the surface model. Previously developed CAD/CAM systems do not continuously describe the geometry of the part. Since there is no description of the intermediate information of the rough machining and the pre-finishing machining model, the remaining machining allowance distribution and the maximum remaining machining allowance of the rough machining surface are both It is unknown. Therefore, the semi-finishing strategy should be optimized to ensure a uniform residual machining allowance on the workpiece surface after semi-finishing. The optimization process includes: calculation of the profile after rough machining, calculation of the maximum remaining machining allowance, determination of the maximum allowable machining allowance, and division of the model surface where the remaining machining allowance is greater than the maximum allowable machining allowance (such as the transition of grooves, corners, etc.) The radius is smaller than the area of ​​the roughing tool radius) and the calculation of the trajectory of the shank during semi-finishing.
The existing mold high-speed machining C A D /CAM software mostly has the residual machining allowance analysis function, and can adopt a reasonable semi-finishing strategy according to the size and distribution of the remaining machining allowance. For example, MasterCAM software provides methods such as Pencil milling and Rest milling to remove the corners with a large remaining machining allowance after rough machining to ensure a uniform machining allowance for subsequent operations.
3. Finishing
The high-speed finishing strategy of the mold depends on the contact point between the tool and the workpiece, and the contact point between the tool and the workpiece changes as the surface slope of the machined surface and the effective radius of the tool change. For complex surface machining that consists of a combination of multiple surfaces, continuous machining should be performed in one process as much as possible, instead of machining each surface individually to reduce the number of times the tool is lifted or dropped. However, due to the change of the surface slope during processing, if only the step side of machining is defined, the actual step distance on the surface with different slopes may be uneven, which may affect the processing quality.
Under normal circumstances, the radius of curvature of the finished surface should be greater than 1.5 times the radius of the tool to avoid abrupt changes in the feed direction. In the high-speed finishing of the die, each time the workpiece is cut in or cut out, the feed direction should be changed as much as possible using arcs or curves to avoid the use of straight line transfer to maintain the smoothness of the cutting process.
High-speed finishing strategies include three-dimensional offsetting, contour finishing, and best-in-class finishing, spirals, and other high-precision machining strategies. These strategies ensure that the cutting process is smooth and stable, ensuring that the material on the workpiece can be quickly removed and a high-precision, smooth cutting surface can be obtained. The basic requirement for finishing is to obtain high precision, smooth surface quality of parts, and easy processing of fine areas such as small fillets, grooves, etc. For many shapes, the most effective strategy for finishing is to use a three-dimensional spiral strategy. Using this strategy can avoid frequent direction changes that occur in parallel strategies and offset finishing strategies, which can increase machining speed and reduce tool wear. This strategy can generate continuous smooth toolpaths with little tooling. This machining technology combines the advantages of spiral machining and contour machining strategies. The tool load is more stable, the number of pick-ups is fewer, the machining time can be shortened, and the probability of tool damage reduced. It also improves the quality of the machined surface and minimizes the need for manual grinding after finishing. In many occasions, it is necessary to use a combination of contour finishing in a steep area and a three-dimensional equidistant finishing method in a flat area.
The numerical control programming also needs to consider the geometric design and the craft arrangement, when uses the CAM system to carry on the high speed processing numerical control programming, except the tool and the processing parameter chooses according to the concrete situation, the choice of the processing method and the programming strategy adopted have become the key. An excellent programming engineer using CAD/CAM workstations should also be a qualified designer and technologist. He should have a correct understanding of the geometry of the part and have the knowledge and concepts of ideal process planning and reasonable tool path design. .
Fifth, high-speed cutting CNC programming
High-speed milling processing has become increasingly demanding for numerical control programming systems, and expensive high-speed processing equipment poses higher requirements for software safety and efficiency. High-speed cutting has special process requirements than traditional cutting. In addition to high-speed cutting machines and high-speed cutting tools, it is also crucial to have suitable CAM programming software. The numerical control machining numerical control instructions include all the technological processes. An excellent high-speed machining CAM programming system should have a high calculation speed, a strong interpolation function, automatic over-cut inspection and processing capabilities, and automatic tool holder and fixture interference. Inspection, feedrate optimization processing functions, trajectory monitoring functions to be processed, tool path editing optimization functions, and machining residual analysis functions. High-speed cutting programming must first pay attention to the safety and effectiveness of the processing method; Second, we must do everything possible to ensure a smooth and stable tool path, which will directly affect the quality of machining and spindle parts and other parts of the life; Finally, we must try to make the tool load uniform, This will directly affect the life of the tool.
1. The CAM system should have a very high computational programming speed
High speed machining uses a very small amount of feed and depth of cut. The NC program is much larger than the traditional CNC machining program. Therefore, it requires the software to calculate faster, so as to save the tool path editing and optimize the programming time.
2. Full automatic anti-over-cutting capacity and automatic tool holder interference check capability
High-speed machining is performed at nearly 10 times the speed of conventional machining. Once overcutting has catastrophic consequences for machine tools, products, and tools, the CAM system must have full automatic anti-cutting capability and automatic tool holders. Interference check and avoidance function with the fixture. The system can automatically prompt the shortest clamping tool length and automatically perform tool interference check.
3. Abundant high-speed cutting tool trajectory strategy
High-speed machining has special requirements on the machining process compared to the traditional way. In order to ensure maximum cutting efficiency and ensure high-speed machining safety, the CAM system should be able to automatically determine the feed rate based on the instantaneous allowance. The optimization process can automatically optimize the tool path editing, processing residual analysis and monitor the processing trajectory, so as to ensure the smoothness of the high-speed machining tool state and improve the service life of the tool.
After high-speed processing equipment is used, the demand for programmers will increase. Due to the strict requirements of high-speed machining processes, over-cut protection is more important, so it takes more time to carry out simulation tests on NC commands. In general, the high-speed machining programming time is much longer than the normal machining programming time. In order to ensure sufficient utilization of high-speed processing equipment, more CAM personnel must be deployed. Existing CAM software, such as PowerMILL, MasterCAM, UnigraphicsNX, Cimatron, etc., provide related high-speed milling tool path strategies.
Sixth, concluding remarks
High-speed cutting technology is one of the main development directions of cutting technology. Currently, it is mainly used in the automotive industry and mold industry, especially in the field of machining complex surfaces, the workpiece itself or the processing field where the tool system rigidity is high, etc. The integration of advanced processing technology is highly praised by people for their high efficiency and high quality. It not only involves high-speed machining processes, but also includes high-speed machining tools, numerical control systems, high-speed cutting tools, and CAD/CAM technology. Mould high-speed machining technology has been widely used in the mold manufacturing industry in developed countries. However, the application range and application level in China still need to be improved. Because of its unparalleled advantages in traditional processing, it will still be the inevitable development of processing technology in the future. direction.
In the modern mold production, as the appearance and function requirements of the plastic parts are getting higher and higher, the internal structure design of the plastic parts becomes more and more complicated, the shape design of the mold becomes more and more complicated, and the proportion of free curved surfaces increases continuously. The corresponding mold structure is also designed more and more complicated. All of these put forward higher requirements for mold processing technology, not only to ensure high manufacturing precision and surface quality, but also to pursue the appearance of the processing surface. With the deepening of the research on high-speed machining technology, especially with the continuous development of processing machines, CNC systems, tool systems, CAD/CAM software and other related technologies, high-speed machining technology has been increasingly applied to mold cavities. Processing and manufacturing.