High speed die cutting

With the increasing aesthetic and functional requirements of the product, the internal structure of the product is designed more and more complex, and the shape design of the mold is more and more complex. The proportion of free surface is increasing, and the corresponding mold structure is more and more complex. All of these put forward higher requirements for mold processing technology. High speed machining technology has been increasingly used in mold cavity processing and manufacturing.

Compared with the traditional cutting, the cutting speed and feed speed of CNC high speed cutting have been greatly improved, and the cutting mechanism is also different. High speed cutting has made an essential leap in cutting. The metal removal rate per unit power has increased by 30%~40%, the cutting force has decreased by 30%, the cutting life of the tool has increased by 70%, the cutting heat left in the workpiece has been greatly reduced, and the low order cutting vibration has almost disappeared.

In the processing of the high hardened steel parts (HRC45~HRC65) of the die, high-speed cutting can replace some EDM electric machining and grinding and polishing processes, thus avoiding electrode manufacturing and time-consuming electric machining, and greatly reducing the amount of grinding and polishing of bench workers. For some thin-walled die workpieces that are increasingly needed in the market, high-speed milling can also be successfully completed, and on the high-speed milling CNC machining center, multi step machining can  be completed by clamping the die at one time.

01 High speed milling machine tool

1. High stability machine tool support components

The bed and other supporting parts of high-speed cutting machine tools should have good dynamic and static stiffness, thermal stiffness and the best damping characteristics. Most machine tools use high-quality, high rigidity and high tensile gray cast iron as the supporting component material. Some machine tool companies also add polymer concrete with high damping characteristics to the base to increase its vibration resistance and thermal stability, which can not only ensure the stability of machine tool accuracy, but also prevent tool chatter during cutting. It is also an important measure to improve the stability of the machine tool to adopt closed bed design, integrally cast bed, symmetrical bed structure and densely distributed reinforcing ribs.

2. Machine tool spindle

The spindle performance of high speed machine tool is an important condition to realize high speed machining. The speed range of the spindle of the high-speed cutting machine tool is 10000~100000m/min, and the spindle power is greater than 15kW. The axial clearance between the tool handle and the spindle is controlled by the spindle compressed air or cooling system to be no more than 0.005mm. The main shaft is also required to have the performance of fast speed rise and accurate stop at the specified position (i.e. extremely high angular acceleration and deceleration), so the high-speed main shaft is usually of hydrostatic bearing type, aerostatic bearing type, hot pressed silicon nitride (Si3N4) ceramic bearing magnetic suspension bearing type and other structural forms. Oil air lubrication, jet lubrication and other technologies are mostly used for lubrication. The spindle is generally cooled by water or air inside the spindle.

3. Machine tool drive system

In order to meet the needs of high-speed die machining, the drive system of high-speed machining machine tool shall have the following characteristics:

(1) High feed speed. The research shows that for small diameter tools, increasing the speed and the feed rate per tooth is beneficial to reducing tool wear. At present, the commonly used feed speed range is 20~30m/min. If the large lead ball screw drive is adopted, the feed speed can reach 60m/min; If linear motor is adopted, the feed speed can reach 120m/min.

(2) High acceleration. For high-speed machining of 3D complex curved surface profiles, the drive system is required to have good acceleration characteristics. It is required to provide high-speed feed drivers (fast forward speed is about 40m/min, 3D contour machining speed is 10m/min), which can provide acceleration and deceleration from 0.4m/s2 to 10m/s2.

Most machine tool manufacturers use ball screws with small lead, large size and high quality controlled by full closed loop position servo or multi head screws with large lead.

4. CNC system

Advanced CNC system is the key factor to ensure the quality and efficiency of high-speed machining of complex mold surfaces. The basic requirements of high-speed machining of mold for CNC system are:

(1) High speed digital control loop, including 32-bit or 64 bit parallel processor and hard disk above 1.5Gb.

(2) Feedforward control of velocity and acceleration; Jerk control of digital drive system.

(3) Advanced interpolation method to obtain good surface quality, accurate size and high geometric accuracy.

(4) It is required to have a large buffer register, which can read and check multiple program segments in advance, so that when the shape (curvature) of the machined surface changes, measures such as changing the feed speed can be taken in time to avoid over cutting.

(5) Error compensation function, including thermal error compensation caused by heating of motor and spindle, quadrant error compensation, measurement system error compensation, etc. In addition, high-speed die cutting requires high data transmission speed.

(6) The transmission speed of traditional data interfaces, such as RS232 serial port, is 19.2 kb, while many advanced machining centers have adopted Ethernet LAN for data transmission, with the speed up to 200 kb.

5. Cooling and lubrication

High speed machining uses cemented carbide tools with coating, and the cutting efficiency is higher without cutting fluid at high speed and high temperature. This is because: the milling spindle rotates at a high speed. If the cutting fluid wants to reach the cutting area, it must first overcome the great centrifugal force; Even if it overcomes the centrifugal force and enters the cutting area, it may evaporate immediately due to the high temperature in the cutting area, with little or no cooling effect; At the same time, the cutting fluid will make the temperature of the tool edge change violently, which is easy to cause cracks. Therefore, the dry cutting mode of oil/air cooling lubrication should be adopted. In this way, high-pressure gas can be used to blow away the cutting generated in the cutting area rapidly, so that a large amount of cutting can be carried away. At the same time, the atomized lubricating oil can form an extremely thin micro protective film on the tool edge and the workpiece surface, which can effectively extend the tool life and improve the surface quality of the parts.

02 Tool shanks and tools for high speed machining

Due to the influence of centrifugal force and vibration during high-speed cutting, the cutting tool is required to have high geometric accuracy, clamping repeated positioning accuracy, high stiffness and high speed dynamic balance safety and reliability. Due to the characteristics of large centrifugal force and vibration during high-speed cutting, the traditional 7:24 taper tool handle system shows obvious defects such as insufficient rigidity, low repeated positioning accuracy, and unstable axial size during high-speed cutting. The expansion of the spindle causes the deviation of the tool and clamping machine structure center, which affects the dynamic balance ability of the tool. At present, HSK high-speed tool handle and the hot expansion and cold contraction fastening tool handle popular abroad are widely used. The hot expansion and cold contraction fastening tool handle has a heating system. The tool handle generally uses the cone to contact the spindle end face at the same time, which has good rigidity, but the tool interchangeability is poor. A tool handle can only be installed with one tool of connection diameter. Because this kind of heating system is relatively expensive, HSK type knife handle system can be used in the initial stage. When the number of high-speed machine tools in the enterprise exceeds 3 or more, it is more appropriate to use the hot expansion and cold contraction fastening tool handle.

Tool is one of the most active and important factors in high speed machining, which directly affects machining efficiency, manufacturing cost and machining accuracy of products. In the process of high-speed machining, the tool should bear high temperature, high pressure, friction, impact, vibration and other loads. The high-speed cutting tool should have good mechanical properties and thermal stability, that is, it has good characteristics of impact resistance, wear resistance and thermal fatigue resistance. The cutting tool technology of high speed machining has developed rapidly, such as diamond (PCD), cubic boron nitride (CBN), ceramic tools, coated carbide, (carbon) titanium nitride carbide TIC (N), etc.

Carbide is the most commonly used cutting tool material in machining cast iron and alloy steel. Hard alloy tool has good wear resistance, but its hardness is lower than that of cubic boron nitride and ceramics. In order to improve the hardness and surface finish, the tool coating technology is adopted, and the coating materials are titanium nitride (TiN), aluminum nitride titanium (TiALN), etc. The coating technology makes the coating develop from a single coating to a multi-layer coating with multiple coating materials, which has become one of the key technologies to improve the high-speed cutting ability. The cemented carbide insert with a diameter of 10~40mm and a TiCN coating can process materials with a Rockwell hardness of less than 42, while the TiN coated tool can process materials with a Rockwell hardness of 42 or higher.

When cutting steel at high speed, the tool materials shall be P-type hard alloy, coated hard alloy, cubic boron nitride (CBN) and CBN composite tool materials (WBN) with high thermal hardness and fatigue strength. For cutting cast iron, K-type hard alloy with fine grains shall be used for rough machining, and composite silicon nitride ceramics or polycrystalline cubic boron nitride (PCNB) composite tools shall be used for fine machining.

When precision machining nonferrous metals or non-metallic materials, polycrystalline diamond PCD or CVD diamond coated tools should be selected. When selecting cutting parameters, the concept of effective diameter should be paid attention to for round inserts and ball end milling cutters. High speed milling cutter shall 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. Rounding or leading angle shall be made at the connection of main and auxiliary cutting edges to increase the sharp angle of the cutter and prevent thermal wear at the cutter tip. The cutting edge length and tool material volume near the tool tip should be increased to improve tool rigidity. Under the conditions of ensuring safety and meeting processing requirements, the tool overhang should be as short as possible, and the toughness of the center of the tool body should be better. The tool handle shall be thicker than the tool diameter, and the connecting handle shall be in the shape of inverted cone to increase its rigidity. Try to leave a coolant hole in the center of the tool and tool system. The effective cutting length of ball end milling cutter shall be considered, and the cutting edge shall be as short as possible. Two spiral groove ball end milling cutter is usually used for rough milling of complex surfaces, and four spiral groove ball end milling cutter is usually used for precision milling of complex surfaces.

03 High Speed Machining Technology and Strategy of Die and Mold

High speed machining includes rough machining, residual rough machining for the purpose of removing allowance, and semi finishing, finishing and mirror machining for the purpose of obtaining high-quality machined surfaces and fine structures.

1. Rough machining

The main goal of die rough machining is to pursue the material removal rate in unit time and prepare the geometric contour of the workpiece for semi finishing. The process plan for rough machining in high speed machining is the combination of high cutting speed, high feed rate and small cutting parameters.

Contour machining is widely used by many CAM software. Two methods are widely used: spiral equal height and equal Z axis equal height. That is, only one feed is made in the processing area, and a continuous and smooth tool path is generated without tool lift. Arc cut in and out are used for tool advance and withdrawal. The characteristic of spiral contour method is that there is no tool path movement between contour layers, which can avoid the impact of frequent tool lifting and feeding on the surface quality of parts and unnecessary consumption of mechanical equipment. Steep and flat areas are processed separately to calculate areas suitable for contour and similar 3D offset, and spiral mode can be used to generate optimized tool paths with few tool lifts to obtain better surface quality.

In high speed machining, it is necessary to adopt arc cut in and cut out connection mode, as well as arc transition at the corner to avoid sudden change of the tool feed direction. It is prohibited to use the connection mode of direct cutting to avoid embedding the tool into the workpiece. When machining the mold cavity, the tool shall not be vertically inserted into the workpiece, but shall be inclined (the common inclination angle is 20 °~30 °). It is better to use the screw type to lower the tool load.

When machining the mold core, the cutter shall be cut from the outside of the workpiece and then cut into the workpiece horizontally. When the cutter cuts in and out of the workpiece, it shall adopt inclined (or circular arc) cutting in and out as far as possible to avoid vertical cutting in and out. Climbing cutting can reduce cutting heat, reduce tool force and work hardening degree, and improve machining quality.

2. Semi finishing

The main goal of die semi finishing is to make the workpiece contour shape flat and the surface finishing allowance even, which is particularly important for tool steel dies, because it will affect the change of tool cutting layer area and tool load during finishing, thus affecting the stability of the cutting process and the quality of the finishing surface.

Rough machining is based on volume model, while finish machining is based on face model. The geometric description of parts in the previously developed CAD/CAM system is discontinuous. Because there is no intermediate information describing the machining model after rough machining and before fine machining, the distribution of residual machining allowance and the maximum residual machining allowance of rough machined surfaces are unknown. Therefore, the semi finishing strategy should be optimized to ensure that the workpiece surface has a uniform residual machining allowance after semi finishing. The optimization process includes: the calculation of the contour after rough machining, the calculation of the maximum remaining machining allowance, the determination of the maximum allowable machining allowance, the partition of the profile whose remaining machining allowance is greater than the maximum allowable machining allowance (such as the areas where the transition radius of grooves, corners, etc. is smaller than the rough machining tool radius), and the calculation of the tool center path during semi finishing.

Most of the existing CAD/CAM software for die and mould high-speed machining have the function of residual machining allowance analysis, and can adopt reasonable semi finishing strategies according to the size and distribution of the residual machining allowance.

3. Finishing

The high speed finish machining strategy of the die depends on the contact point between the tool and the workpiece, and the contact point between the tool and the workpiece changes with the surface slope of the machined surface and the effective radius of the tool. For the machining of complex surfaces composed of multiple surfaces, continuous machining should be carried out in one process as far as possible, instead of machining each surface separately, so as to reduce the times of tool lifting and tool lowering. However, due to the change of surface slope during machining, if only the machining step over is defined, the actual step distance on surfaces with different slopes may be uneven, thus affecting the machining quality.

In general, the curvature radius of the finish machined surface should be 1.5 times greater than the tool radius to avoid sudden changes in the feed direction. In the high-speed finishing of the die, every time the workpiece is cut in or out, the feed direction should be changed by arc or curve transition as far as possible, instead of straight transition, so as to maintain the stability of the cutting process.

High speed finishing strategies include 3D offset, contour finishing, optimal contour finishing and spiral contour finishing. These strategies can ensure that the cutting process is smooth and stable, ensure that the material on the workpiece can be quickly removed, and obtain a high-precision, smooth cutting surface. The basic requirement of finishing is to obtain high precision, smooth surface quality of parts, and easily realize the processing of fine areas, such as small fillets, grooves, etc. For many shapes, the most effective finishing strategy is to use the three-dimensional spiral strategy. Using this strategy can avoid frequent direction changes in parallel and offset finishing strategies, thus improving processing speed and reducing tool wear. This strategy can generate continuous and smooth tool paths with few tool lifts. This machining technology combines the advantages of spiral machining and contour machining strategies. The tool load is more stable, the number of tool lifts is less, the machining time is shortened, and the probability of tool damage is reduced. It can also improve the quality of the machined surface and minimize the need for manual grinding after finishing. In many cases, it is necessary to combine the contour finishing of steep areas with the 3D equidistant finishing of flat areas.

NC programming should also consider geometric design and process planning. When using the CAM system for high-speed machining NC programming, in addition to the selection of tools and processing parameters according to the specific situation, the selection of processing methods and the programming strategy used become the key. An excellent programming engineer using CAD/CAM workstation should also be a qualified designer and technologist. He should have a correct understanding of the geometric structure of parts, and have knowledge and concepts of ideal process arrangement and reasonable tool path design.

04 High speed cutting NC programming

High speed milling requires more and more CNC programming systems, and expensive high-speed machining equipment requires higher security and effectiveness of software. High speed cutting has special process requirements than traditional cutting. In addition to high-speed cutting machine tools and high-speed cutting tools, it is also essential to have appropriate CAM programming software. The NC instructions of NC machining include all the technological processes. An excellent high-speed machining CAM programming system should have high computing speed, strong interpolation function, automatic over cutting inspection and processing capability in the whole process, automatic tool handle and fixture interference inspection, feed rate optimization processing function, track monitoring function to be machined, tool path editing and optimization function, machining residual analysis function, etc. High speed cutting programming should first pay attention to the safety and effectiveness of processing methods; Secondly, we should do everything possible to ensure that the tool path is smooth and stable, which will directly affect the processing quality and the life of machine tool spindle and other parts; Finally, the tool load should be as uniform as possible, which will directly affect the tool life.

1. The CAM system should have a high calculation and programming speed

High speed machining uses very small feed rate and cutting depth, and its NC program is much larger than the traditional NC program. Therefore, the software calculation speed is required to be faster to save the time for tool path editing and optimization programming.

2. The whole process automatic over cutting prevention processing capability and automatic tool handle interference detection capability

High speed machining is carried out at a cutting speed of nearly 10 times that of traditional machining. Once over cutting occurs, it will have disastrous consequences for machine tools, products and tools. Therefore, its CAM system must have the ability to automatically prevent over cutting in the whole process and the function of automatic tool handle and fixture interference inspection and avoidance. The system can automatically prompt the shortest clamping tool length and automatically check the tool interference.

3. Rich high-speed cutting tool path strategies

High speed machining has special requirements for the tool path of the machining process compared with traditional methods. In order to ensure the maximum cutting efficiency and the safety of machining in high-speed cutting, the CAM system should automatically optimize the feed rate according to the size of the instantaneous machining allowance, automatically edit and optimize the tool path, analyze the machining residue, and monitor the processing path, so as to ensure the stability of the high-speed machining tool stress state, Improve the service life of the tool.

Due to the strict process requirements of high-speed machining, the over cutting protection is more important, so it takes more time to simulate and check the NC instructions. In general, the high-speed machining programming time is much longer than the general machining programming time. In order to ensure the sufficient utilization rate of high-speed processing equipment, more CAM personnel should be allocated. Existing CAM software, such as hyperMILL, PowerMILL, MasterCAM, Unigraphics NX, etc., all provide high-speed milling tool path strategies with related functions.

At present, high-speed cutting technology is mainly used in the automobile industry and mold industry, especially in the field of machining complex surfaces, the processing field with high requirements for the rigidity of the workpiece itself or tool system, etc. Its efficiency and high quality are highly praised by people.