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The application of high-speed cutting in the processing of cast iron and alloy steel

this paper briefly introduces the progress of high-speed cutting technology and the processing of cast iron and alloy steel with high 90 mm performance, which are currently used in mold manufacturing. The main contents of this paper include: (1) theoretical and Experimental Research on tool collapse and tool life in high-speed milling of high hardness materials; (2) When machining sculptured surfaces, the NC program is optimized by changing the spindle speed and feed rate to maintain almost constant chip load; (3) Predict the chip flow, stress and temperature on the cutting tool and the residual stress of the machined surface layer

I. preface

with the progress of machine tools, cutting tools and high-speed rotating end mills, "high speed machining (HSM)" has become an efficient processing process, which can complete the processing of high-precision and high surface quality parts. Until recently, high-speed milling was applied to the processing of complex aluminum alloy parts in the aviation industry. With the significant improvement of machine tools, spindles and CNC systems or control units, high-speed machining has been successfully used. Recently, with the progress of cutting tool technology, high-speed machining has been applied to machining alloy steel (hrc>30), widely used in stamping dies for automotive and electronic components products, as well as plastic mold parts. The definition of high-speed machining depends on the type of workpiece material being processed. Figure 1 shows the cutting speed commonly used in high-speed machining of different materials. For example, the cutting speed used for high-speed machining of alloy steel is 500m/min, and this speed is often used for milling aluminum alloys

the main advantages of high-speed machining are: high material removal rate, short development cycle, low cutting force, and the heat taken away by chip removal makes the deformation of the workpiece small. However, the problems related to the application of high-speed machining mainly depend on the workpiece material and the required product geometry. The disadvantages of high-speed machining are: excessive tool wear requires particularly expensive machine tools, and these machine tools must have good spindles, controller units and fixtures, tool handles with dynamic balance, and the most important thing is to have advanced tool materials and coating materials

Figure 1 speed range of different materials in high-speed machining

with the expansion of the application range of high-speed machining, the research on new tool materials, the improvement of tool design structure, the generation of new NC tool path strategy and the improvement of cutting conditions have also been improved. Moreover, computer-aided simulation of cutting process has also appeared. This technology is of great significance for predicting tool temperature and stress and prolonging tool life. The application of casting, stamping and mold processing represents the recent expansion of the application scope of high-speed cutting in China, where cast iron, cast steel and alloy steel are guided by the comprehensive, harmonious and sustainable scientific concept of development. Industrial leading countries spend most of their development time on machining and polishing processes in stamping and mold manufacturing. As shown in Figure 2, the machining and polishing of dies or molds account for about 2/3 of the total processing cost, and high-speed milling is just used to shorten the development cycle and processing cost

Figure 2 Comparison of mold manufacturing cycle

II. Overview of main theories

1 Processing technology in high speed cutting

using advanced machine tools and cutting tools to process alloy steel (hrc>30) is a cost saving technology. Moreover, high-speed cutting of hard alloy steel has the following advantages: reducing the finishing time, the deformation of finished parts disappears after heat treatment, high material removal rate, low processing cost and high surface quality. In the processing of tool steel dies and molds, the high-speed machining of hard materials replaces the slow EDM process, and the high-speed machining of hard steel parts will produce high temperature and stress on the contact surface between the workpiece and the tool. Therefore, the application of high-speed machining requires a fundamental understanding of the relationship between cutting parameters, tool life and machined surface quality. It is necessary to master how the temperature and stress generated in high-speed cutting affect tool wear, tool premature edge collapse and residual stress on the machined surface

the experimental data show that when machining hard steel parts, the microstructure and thermal properties of the workpiece material affect the cutting flow. Generally speaking, the cutting force produced by workpieces with high hardness is also large, and tool materials with different thermal properties can reduce the cutting force. In order to better grasp the cutting process and improve the performance of cutting tools, the application of deformation theory and digital technology has emerged

in processing high hardness materials, when the cutting speed is from low to high and the appropriate feed rate, continuous chip deformation can be found, as shown in Figure 3a. Figure 3B shows serrated chips produced by higher feed rates. The form of sawtooth chip formation changes constantly between cutting force and punching force, and produces high-frequency vibration at the same time, which affects the tool life and edge collapse

Figure 3 legend of chip formation in hard steel machining

after studying the micro diagram of intermittent cutting, it is shown that the cause of serrated chip formation is the periodic fracture deformation of the tool rake face, as shown in Figure 3B. The fracture of the workpiece surface is transmitted to the chip until it becomes a high stress state area

in the machining of hardness parts, continuous chips are thin undeformed chips, and sawtooth chips are thick undeformed chips. According to recent observations, the frequency of generating sawtooth chips is very high, and the chip edge is subject to high-frequency force vibration. The influence of chip formation on tool wear and surface quality has not been well understood, but chip formation certainly affects cutting force

2. Machine tool system in high-speed cutting

(1) machine tool and controller

high speed milling machine tools have many different configurations, and three-axis vertical and horizontal machining centers are the most commonly used configurations. Although vertical machining centers have shortcomings in chip removal, they are still more economical choices and are more widely used than horizontal machining centers. In the investigation of high-speed machining, it is found that most people use horizontal machining centers. The four axis machining center can tilt the milling cutter to improve the cutting conditions; The five axis machine tool has exchangeable spindle units, which can realize rough machining, semi precision machining and precision machining

there are many high-performance machining centers on the market (spindle speed: 1000 ~ 50000r/min, spindle power: 7.5 ~ 40kW, feed rate: 10 ~ 60m/min). High speed machining requires a rigid spindle with high stability and small vibration, and the tool handle is balanced with a thermal fit. The servo system and control unit must be advanced, capable of supporting pre calculation and rapid response, and have the program capability of large transmission capacity without data loss. The computer manufacturing system and pre calculation system must allow the machine tool to accelerate and decelerate most effectively to realize tool compensation. The current machine tool technology gradually adopts high-speed linear motor drive. The 3-D contour feed rate is about 12m/min, and the increase and decrease rate is close to 9.8m/s2. Figure 4 shows the four axis horizontal machining center (Makino A55 delta) used in the high-speed machining laboratory

Figure 4 Makino A55 delta

(2) cutting tools

cemented carbide is the most commonly used tool material in cutting tools for machining cast iron and alloy steel. Cemented carbide tools have good wear resistance, but their hardness is lower than cubic boron nitride and ceramics. In order to improve the hardness, cemented carbide tools are coated with hard coating materials, such as titanium nitride, titanium aluminum nitride and titanium carbonitride. Recently, double-layer soft base coating materials, such as movic, are also used. Other cutting tool materials used in processing include ceramics, cermet and polycrystalline diamond

in general, cemented carbide blades with diameters ranging from 0.5 inches to 1.5 inches and titanium carbonitride coating can process materials with Rockwell hardness less than 42; The tools coated with titanium aluminum nitride can process materials with Rockwell hardness of 42 or higher. Different cutting tool materials and coating materials can be selected according to the use requirements. Table 1 shows the performance characteristics of cutting tool materials. The cutting tools and coating materials used in high-speed cutting can be divided into: cubic boron nitride and silicon nitride machining cast iron, titanium nitride and titanium carbonitride coated alloy cutting tools machining alloy steel with Rockwell hardness of 42, titanium nitride aluminum and aluminum titanium nitride coated alloy cutting tools machining alloy steel with Rockwell hardness of 42 or even higher. In special applications, especially high hardness turning tools (HRC) and polycrystalline cubic boron nitride blades with suitable cutting edges have also been successfully applied. Table 1 Characteristics of advanced cutting tool materials and coatings

3 High speed cutting of molds

in injection molds, casting molds, forging molds and panel stamping molds, molds are composed of functional components and support components. Punch and die are usually machined from die steel. However, large stamping dies are often cast close to the final size and retain machining allowance. Support components are standard parts to ensure the overall function of tooling components, such as positioning, part injection, heating or cooling. Through the application of standard die components, the time of processing dies can be reduced, and machining mainly produces punch, die and other parts

(1) mold materials

according to a recent survey, 50% of mold manufacturers process injection molds. In the United States, the most commonly used die materials are 3Cr2Mo die steel (hrc30), 4Cr5MoV1Si steel, hrc45 ~ HRC60 forging die and hrc46 ~ HRC50 casting die. Table 2 lists the most commonly used mold materials. Table 2 the most widely used mold materials in the United States

(2) surface quality

finishing needs to occupy the largest share of the development cycle, which is about 25% - 30% of the whole development cycle for injection molds, molds and forging molds. For large stamping dies, finishing takes up most of the whole manufacturing time, and also affects the processing time of fitters (grinding and polishing). The processing time of mold and injection mold is about 15%, and that of sheet metal forming mold is about 20%. With small knife spacing, the residual height will be reduced, and the processing time of fitter will also be reduced

the surface accuracy of injection mold is higher than that of forging die and stamping die. Table 3 shows the average values of dimensional errors and form and position errors of various molds. In mold manufacturing, the main purpose of high-speed milling is to reduce or eliminate manual polishing and reduce finishing time. Low surface roughness can be obtained by increasing the finishing path or using a large diameter milling cutter. The step distance AE and the tool diameter D determine the theoretical surface roughness rth; Table 3 tolerance requirements of the mold

because the maximum tool diameter is often limited by the part geometry, the theoretical surface roughness value can only be reduced by reducing the tool spacing. If the tool spacing is reduced by 50%, the tool path will automatically increase by 100%, which means that the finishing time will be doubled. To compensate for the increased time, a high feed rate is necessary. High feed rate requires high spindle speed to ensure constant chip thickness, and also requires high cutting speed. Accordingly, temperature and tool wear will not be avoided

III. application of high-speed milling

high speed milling of aluminum alloy has been well known and has been used in the aviation industry for more than 10 years. Recently, high-speed machining is mainly used in turning of hard materials, processing of molds and castings. The workpiece materials used in high-speed milling are listed in Table 2: cast iron, Cr12MoV (hrc59), 3Cr2Mo (hrc30) and 4Cr5MoV1Si (hrc46). GM alloy cast iron gm241 (hrn210) is mainly used for processing stamping dies, and 3Cr2Mo die steel is the most commonly used steel for processing injection molds. Due to the low carbon content, it is usually processed when heat treated to hrc30 in advance, and then quenched to HRC50 ~ HRC55. In the application of die-casting die, when hot forging die steel 4Cr5MoV1Si is in hrc46 state