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High-speed CNC technology of mold manufacturing

High-speed CNC technology, which is one of the most important common technologies in modern advanced manufacturing technology, represents the development direction of cutting, and has gradually become the mainstream technology of cutting. “High speed" in high-speed CNC is a relative concept. For different processing methods and work piece materials, the cutting speed used in high-speed cutting is different. Generally, the cutting speed used for high-speed cutting is 5-10 times or more than the conventional cutting speed. Using high-speed cutting technology can significantly improve processing efficiency and accuracy, reduce cutting forces and the impact of cutting heat on the work piece, even more achieve process intensification. Therefore, it has been widely used in aerospace, mold manufacturing, automobile manufacturing, precision machinery and other fields to achieved good technical and economic benefits.

In the manufacturing of modern molds, due to the increasingly complex mold surface design, the proportion of free surface is increasing, so the mold CNC technology has put forward higher requirements, not only to ensure high manufacturing accuracy and surface quality, but also to pursue the beauty of the CNC surface. With the development of high speed machining technology, especially in the machine tool, numerical control system, tool system, CAD/CAM software and other related technology, high speed machining technology has been more and more applied to the mold manufacturing and processing. High speed machining technology has a huge impact of the mold processing technology which change the complicated and tedious process flow such as "annealing → milling→ heat treatment → grinding" or "EDM process →  grinding and polishing"  even more replaces all previous processes. In addition to high-speed processing technology, which can be applied to the direct processing of hardened mold cavities (especially semi-finishing and finishing), it has also been widely used in EDM electrode processing and rapid prototype manufacturing. Large-scale production practice shows that the application of high-speed cutting technology can save about 80% of the manual grinding time and reduce nearly 30% of the processing cost, moreover the mold surface processing accuracy can reach 1µm, and the cutting efficiency of the tool can be doubled.

First, the requirements to the CNC system of high-speed machining

Due to the special characteristics of mold processing and the characteristics of high-speed CNC technology, higher technology and technology systems (CNC machine, CNC systems, tools, etc.) have put forward higher requirements than traditional mold processing.

1. CNC spindle

It’s very important for high-speed machine tool spindle performance to high-speed cutting. Speed range to high-speed cutting machine spindle is 10,000~100,000 r/min, also require it to have the performance of rapid speed up and quick exact stop at the specified position which means it has extremely high angular acceleration and deceleration, therefore, high-speed spindles often adopt structural forms such as hydrostatic bearing type, air hydrostatic bearing type, and magnetic suspension bearing type.

2. CNC drive system

In order to meet the needs of high-speed mold processing, the drive system of the processing machine should have the following characteristics.

(1) High feed speed

Research shows that for small diameter tools if increasing the speed and feed per tooth will be beneficial to reduce tool wear. By now, The common feed speed range is 20 ~ 30m / min, but if using large lead ball screw drive, the feed speed can reach 60m/min; using linear motor can make the feed speed reach 120m / min.

(2) High acceleration

As for 3D complex curved surfaces requires the drive system to have good acceleration characteristics. The acceleration of the drive system should reach 20~40m / s ^ 2

(3) High velocity gain factor KV

To achieve high 3D contour dynamic accuracy and minimal lag, velocity gain factor required KV=20~30(m/min)/mm in a general way.

3. CNC system

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

(1) High-speed digital control loop include 32-bit or 64-bit processor and hard disk above 1.5Gb;extremely short linear motor sampling time (<500µs).

(2) Feed forward control of speed and acceleration;jerk control of digital drive system.

(3) Advanced interpolation method (NURBS based spline interpolation) which obtain good surface quality, precise dimensions and high geometric accuracy.

(4) Look-ahead function. Requires a large-capacity buffer register, which can read and check multiple program segments in advance (such as DMG machine tools up to 500 program segments, Simens system can reach 1000 to 2000 program segments) in order to occur in the shape (curvature) of the processed surface When changing, you can take measures such as changing the feed rate in time to avoid over cutting, etc.

(5) Error compensation function which including thermal error compensation caused by linear motor, spindle, quadrant error compensation, measurement system error compensation and other functions.

  In addition, the high-speed cutting of molds has high requirements for data transmission speed. The transmission speed of traditional data interfaces such as RS232 serial port is 19.2kb, and many advanced processing centers have adopted Ethernet local area network (Ethernet) for data transmission, the speed can reach 200kb.

4. High-speed cutting tool

The main development trend of high-speed cutting tool systems is the dual positioning tool holders (such as HSK tool holders from OTT in Germany and KM tool holders from Kennametal in the United States), which have axial positioning accuracy of up to 0.001mm.Under the action of high-speed rotating centrifugal force, the tool holder is locked more firmly, and its radial pulsation does not exceed 5µm. The tool materials used for high-speed cutting are mainly carbide, ceramics, cermet, cubic boron nitride (PCBN), polycrystalline diamond, etc. In order to meet the requirements of high-speed machining of molds, the development of tool technology is mainly focused on the research of new coating materials and coating methods, and the development of new tool structures.

Second, the mold high-speed processing technology

1. Rough process

The main goal of mold roughing is to pursue the material removal rate per unit time and prepare the geometric contour of the work piece for semi-finishing. During the cutting process, due to the change in the metal area of the cutting layer, the load on the tool changes, which makes the cutting process unstable, the tool wear speed is uneven, and the quality of the machining surface decreases. Many currently developed CAM software can keep the cutting conditions constant through the following measures to obtain good machining quality.

(1) Constant cutting load. A constant cutting layer area and material removal rate are obtained through calculation, so that the cutting load and the tool wear rate are balanced to improve tool life and machining quality.

(2) Avoid sudden change of tool feed direction.

(3) Avoid burying tools in the work piece. For example, when processing the mold cavity, avoid inserting the tool vertically into the work piece. Instead, use a tilting method (usually an inclination angle of 20 ° to 30 °). It is best to use a spiral type to reduce the tool load. When processing the mold core,  As far as possible, first cut down from the outside of the work piece and then cut into the work piece horizontally.

(4) When cutting in and out of the work piece, the tool should be slanted (or arc) as much as possible to avoid cutting in and out vertically.

(5) Climbing cutting can reduce cutting heat, reduce tool stress and work hardening, and improve machining quality.

2. Semi-finished

  The main goal of semi-finishing of the mold is to make the contour of the work piece flat and the surface finishing allowance uniform. It is particularly important for tool steel molds because it will affect the change in the area of the cutting layer of the tool and the change in the load of the tool during finishing. Stability of the cutting process and finish surface quality.

  Roughing is based on the volume model, while finishing is based on the surface model. The previously developed CAD / CAM system's geometric description of the parts is discontinuous. Because there is no intermediate information describing the machining model after roughing and before finishing, the remaining machining allowance distribution and the maximum remaining machining allowance on the rough machining surface are unknown. Therefore, the semi-finishing strategy should be optimized to ensure that the work piece surface has a uniform remaining 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, and the partitioning of the profile (such as grooves, corners, etc.) for the remaining machining allowance greater than the maximum allowable machining allowance. Area with a radius smaller than the radius of the roughing tool) and calculation of the tool trajectory during semi-finishing.

  Most existing CAD / CAM software for high-speed machining of molds has a residual machining allowance analysis function, and can adopt a reasonable semi-finish machining strategy according to the size and distribution of the residual machining allowance. For example, Open Mind's Hyper Mill and Hyper Form software provide methods such as Pencil milling and Rest milling to clear the corners of the larger remaining machining allowance after roughing to ensure uniform machining allowance in subsequent processes. Local milling in Pro / Engineer software has similar functions. For example, the remaining machining allowance of the local milling process is equal to that of rough machining. In this process only uses a small-diameter milling cutter is used to clear the uncut corners of rough machining before semi-finishing. If the value of the remaining machining allowance of the local milling operation is taken as the remaining machining allowance of the semi-finishing, this step can not only remove the corners that are not cut in the roughing, but also complete the semi-finishing.

3. Finish machining

  The high-speed finishing strategy of the mold depends on the contact point between the tool and the work piece, and the contact point between the tool and the work piece changes with the slope of the curved surface and the effective radius of the tool. For complex curved surface processing composed of multiple curved surfaces, continuous machining should be performed in one operation as much as possible, instead of processing each curved surface separately to reduce the number of times of lifting and lowering. However, due to the change in the surface slope during processing, if only the side step over is defined, the actual step may be uneven on surfaces with different slopes, which will affect the processing quality.

  Pro / Engineer solved the above problem by redefining the scallop machine while defining the amount of knife on the side; Hyper Mill provides an equidistant machine method, which can ensure a uniform amount of side cutting between the cutting paths, without being limited by the surface slope and curvature, and ensure that the tool always bears a uniform load during the cutting process.

In general, the radius of curvature of the finished surface should be greater than 1.5 times the tool radius to avoid sudden changes in the feed direction. In the high-speed precision machining of the mold, every time the work piece is cut in and out, the feed direction should be changed as much as possible by arc or curve transfer, and straight transfer is avoided to maintain the smoothness of the cutting process.

Third, optimization of the feed rate

At present, many CAM software have the function of optimizing and adjusting the feed rate: in the semi-finishing process, when the cutting layer area is large, the feed speed is reduced, and when the cutting layer area is small, the feed speed is increased. Optimized adjustment of the feed rate can stabilize the cutting process and improve the quality of the machined surface. The size of the cutting layer area is completely calculated automatically by the CAM software. The adjustment of the feed rate can be set by the user according to the processing requirements.

Fourth, concluding remarks

Die high-speed machining technology is an integration of multiple advanced machining technologies, which not only involves high-speed machining processes, but also includes high-speed machining machine tools, CNC systems, high-speed cutting tools, and CAD / CAM technology. High-speed mold processing technology has been widely used in the mold manufacturing industry in developed countries, but its application scope and application level in China still need to be improved, so vigorously developing and promoting the application of high-speed mold processing technology is of great significance to promote the overall technical level and economic benefits of China's mold manufacturing industry.

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