Short tool five-axis milling technology

The use of short cutting tools is a main feature of five-axis machining. Short tools will significantly reduce tool deviation, thereby obtaining good surface quality, avoiding rework, reducing the use of welding rods, and shortening EDM processing time. When considering five-axis machining, the goal of using five-axis machining of molds must be to use the shortest cutting tool possible to complete the processing of the entire workpiece, including reducing programming, clamping and machining time while obtaining a more perfect surface quality. .

As long as the workpiece cavity is not very deep (relative to the tool diameter), a three-axis tool path (2, 3, 5) is sufficient. If the workpiece has deep pockets and narrow areas, it is not sufficient to use a pure three-axis toolpath to complete the entire finishing operation. In this case, poor surface quality and long machining times ensue. Figure 1 shows the situation of a three-axis tool path. Here, the shortest tool must be very long in order to be able to process all areas of the workpiece in the vertical direction.

Figure 1 Three-axis machining path

When using shorter tools, the spindle should be tilted to ensure that special areas of the workpiece can be machined. 3+2 axis machining is usually thought of as setting a constant angle to the spindle. Complex workpieces may require many tilted views to cover the entire workpiece, but this results in overlapping tool paths, which increases machining time.

Additionally, all the tilted views are difficult to combine accurately, which increases the amount of hand-sanding work and greatly increases in and out motions, often resulting in surface quality issues and more tool movement.

Figure 2 3+2 axis tool path

Finally, programming in this manner is interfering and time-consuming, and the sum of all views often does not cover the entire geometry. Figure 2 demonstrates four workpiece views, but there is still an area in the center of the workpiece that is not covered, and this area still requires an additional oblique view.

Figure 3 Five-axis tool path

In order to overcome the shortcomings of 3+2-axis machining, five-axis simultaneous machining may be a better choice, not to mention that some five-axis machine tools also have some functions specially designed for the mold industry. Five-axis linkage machining can coordinate three linear axes and two rotary axes so that they move simultaneously, solving all the problems of three-axis and 3+2-axis machining. The tool can be very short, and there will be no overlapping of views or omission of the processing area. Even less likely, processing can be carried out continuously without additional import and export (see Figure 3).

Vertical five-axis machining center

There are two types of rotary axes for this type of machining center. One is the table rotary axis. The worktable set on the bed can rotate around the X-axis, which is defined as the A-axis. The general working range of the A-axis is +30 degrees to -120 degrees. There is also a rotary table in the middle of the workbench, which rotates around the Z-axis at the position shown in the figure, which is defined as the C-axis. The C-axis rotates 360 degrees. In this way, through the combination of the A-axis and the C-axis, except for the bottom surface of the workpiece fixed on the workbench, the remaining five surfaces can be processed by the vertical spindle.

The minimum indexing value of the A-axis and C-axis is generally 0.001 degrees, which allows the workpiece to be subdivided into any angle and to process inclined surfaces, inclined holes, etc. If the A-axis and C-axis are linked with the XYZ three-linear axis, complex spatial surfaces can be processed. Of course, this requires the support of high-end CNC systems, servo systems and software. The advantage of this arrangement is that the spindle structure is relatively simple, the spindle rigidity is very good, and the manufacturing cost is relatively low. However, the general workbench cannot be designed to be too large, and the load-bearing capacity is also small. Especially when the A-axis rotation is greater than or equal to 90 degrees, the workpiece will bring a large load-bearing moment to the workbench during cutting.

The other one relies on the rotation of the vertical spindle head (picture). The front end of the spindle is a rotary head, which can circle the Z-axis 360 degrees on its own to become the C-axis. The rotary head also has an A-axis that can rotate around the X-axis, generally up to ±90 degrees, to achieve the same function as above. The advantage of this setup is that the spindle processing is very flexible and the workbench can be designed to be very large. The huge fuselage of the passenger aircraft and the huge engine casing can be processed on this type of machining center.

This design also has a major advantage: when we use a spherical milling cutter to process a curved surface, when the tool center line is perpendicular to the processing surface, since the vertex linear speed of the spherical milling cutter is zero, the surface quality of the workpiece cut out by the vertex will be very poor. The spindle rotation design is used to rotate the spindle at an angle relative to the workpiece, so that the spherical milling cutter avoids vertex cutting, ensuring a certain linear speed and improving surface processing quality. This structure is very popular for high-precision curved surface processing of molds, which is difficult to achieve with table rotary machining centers. In order to achieve high rotation accuracy, high-end rotary axes are also equipped with circular scale feedback, and the indexing accuracy is within a few seconds. Of course, the rotation structure of this type of spindle is relatively complex and the manufacturing cost is also high.

Vertical five-axis machining center with rotating spindle

The gravity of the spindle of the vertical machining center is downward, and the radial force on the bearings during high-speed idling operation is equal, and the rotation characteristics are very good, so the speed can be increased. Generally, the high speed can reach more than 1,2000r/min, and the practical maximum speed has reached 4,0000 rpm. The spindle system is equipped with a circulating cooling device. The circulating cooling oil takes away the heat generated by high-speed rotation, reduces it to the appropriate temperature through the refrigerator, and then flows back to the spindle system. The three linear axes of X, Y and Z can also use linear grating feedback, and the bidirectional positioning accuracy is within the micron level. Since the rapid feed reaches more than 40~60m/min, most of the ball screws of the X, Y, and Z axes adopt central cooling. Like the spindle system, the refrigerated circulating oil flows through the center of the ball screw to take away the heat. .

Comparison with three-axis simultaneous milling

Milling can produce a good curved approximation of the surface. When using a ball-end tool for three-axis simultaneous milling, the tool can be guaranteed to cut to any coordinate point on the workpiece through linear feed motion in the x, y, and z axes, but the direction of the tool axis cannot be changed. The actual cutting speed at the point on the tool axis is zero, and the chip space in the center of the tool is also very small. If these points are involved in cutting, unfavorable cutting conditions will lead to a decrease in the quality of the machined surface, increased blade wear, and extended processing time, so that high-grade tool materials cannot be fully utilized.

Compared with three-axis linkage milling, five-axis linkage milling has a series of advantages. At this time, through the movement of the two rotating axes, the direction of the tool axis can be adjusted at any time, so that the angle between the milling cutter axis and the workpiece surface and the actual cutting speed remain unchanged. The tool path can be set more flexibly to meet the requirements for the given peak and valley depth of the workpiece surface. When using a ball-nose tool for machining, no matter what orientation the tool is relative to the workpiece, the chips are always separated on the hemispherical surface. Chips with the same geometry and dimensions are therefore always cut every time. What changes is the motion trajectory of the cutting edge when separating chips, as well as the cutting edge contact conditions and cutting geometric motion conditions determined thereby. In other words, the cutting process and geometric motion parameters can be influenced by purposefully changing and determining the orientation of the tool, and both can be optimized from aspects such as tool wear, surface quality and machining process stability.

Figure: Geometric motion relationship of five-axis milling of ball-end milling cutter

Of course, the CNC programming of five-axis linkage milling is relatively complex and requires higher computing power and speed of the computer numerical control (CNC) system. It requires each linear feed axis of the machine tool to make large compensation movements while avoiding interference and collisions. Therefore, in mold manufacturing, only the advantages of five-axis simultaneous milling can be used to process workpieces within a certain range.

Five-axis machining with trunnion and planetary structures

Five-axis machining machines with trunnion and planetary structures usually add a two-axis rotary table to a standard three-axis machine tool. The advantage of this structure is that the parts are fixed on the center line of rotation. Depending on the size, when the rotary axis changes its angle during linked machining, the linear coordinate compensation stroke is the shortest. During this operation and at the end of each machining cycle, the feed is limited by the operating status of any one of the five coordinate axes. Usually the compensation stroke is too large and is limited by the linear axis. Due to its large weight and offset effects, it cannot effectively increase the acceleration of linear coordinates. In contrast, thanks to direct drive technology, high dynamics can easily be achieved with rotary coordinates. Combining this with short strokes helps shorten machining cycle times and improve machining accuracy.

Short compensation stroke usually has nothing to do with five-axis positioning. It only causes problems during high-speed machining, when linear coordinates and rotary coordinates are required to reach the target position almost simultaneously. As five-axis machining parts become more and more complex and require more types of tools, a large amount of tool change time will be generated during the machining cycle.

On the other hand, because the casting of the machined part is very close to the actual shape and the application of high-performance cutting technology, the cutting time is shortened. As shown in the figure, when processing aluminum automobile steering knuckles, an innovative five-axis machining concept is used, and the chip-to-chip time is only 1.9s. Since all tools only need to be changed once when processing two parts on the left and right sides, the tool change time is reduced by 50%.

Adopts more efficient loading and unloading technology. The standardized robot unit cooperates with the overall raw material and finished parts storage device to effectively reduce non-production time. In addition to loading and unloading materials for the two machines, the robot can also be responsible for cleaning, marking, measuring and other operations. Adjustment times between different machining applications are short thanks to the planetary table, which can be equipped with seamless clamping. Using high-speed direct drive, the two worktables can be mirrored for processing, which is very meaningful for the processing of symmetrical parts, such as processing symmetrical parts on the left and right sides of cars, trucks, or airplanes.

Motion simulation of Vericut five-coordinate high-speed milling machine tool

Since the tool path is relatively complex during five-coordinate high-speed milling, and the tool axis vector changes frequently during processing, especially when performing high-speed cutting, the tool movement speed is very fast. Therefore, the CNC program must be edited before actual product processing. Proofreading and review are very necessary.

Due to the large amount of programs in five-coordinate linkage high-speed cutting, many programs use manual methods or are simulated in CAM software. It is difficult to effectively check whether there are problems with the CNC program and the actual output of the machine tool. Using Vericut software can save proofreading time and perform real simulated processing. Vericut software can very realistically simulate interference, overcutting, advance and retreat, etc. during machine tool processing. It can especially simulate five-axis machining and its RTCP very well. Function. Vericut provides many functions, including complete graphical display of blank size, position and orientation, and can simulate 2 to 5-axis linkage milling and drilling.

UGII/Vericut cutting simulation module is a third-party module integrated in UGII software. It uses human-computer interaction to simulate, check and display NC machining programs. It is a convenient method to verify CNC programs. By eliminating the need for trial cutting of sample parts, machine tool debugging time can be saved, tool wear and machine tool cleaning work can be reduced. By defining the blank shape of the part to be cut and calling the NC tool position file data, the correctness of the tool path generated by the NC can be verified. UGII/Vericut can display the processed and colored part model, so users can easily check incorrect processing conditions.

As another part of the inspection, the module can also calculate the volume of the processed part and the amount of blank removal. The digital model in UGII can be directly transferred to Vericut software for simulation, including digital information such as blank, product, CNC tool path and tool. Figure 9 shows the Vericut interface provided in the UGNX environment. The simulation situation in Vericut software when processing an integral impeller machine tool provides a better detection process to ensure the quality of the product.

Key points of five-axis tool path design

在设计刀具路径之前,应将CAD三维模型的系统精度设置得尽可能高。特别是在不同 CAD 系统之间转换模型时,首选 CATIA (*.model) 格式和 Parasolid (*.x_t) 格式。进行数据转换,然后使用IGES格式进行数据转换。采用IGES格式时,系统精度一般应不小于0.01mm。尤其是在对精密零件进行五轴高速切削时,模型的精度和刀具插补的精度对刀具路径有重要影响。产量有重要影响。

空间面轴加工涉及的内容很多,尤其是五轴加工。五轴加工涉及加工导向面、干涉面、轨迹限制区域、刀具进退、刀轴矢量控制等关键技术。四轴、五轴加工的基础是了解刀轴的矢量变化。四轴、五轴加工的关键技术之一是刀轴矢量(刀轴的轴矢量)如何在空间变化,而刀轴矢量的变化是通过刀具的摆动来实现的。回转台或主轴。对于矢量不变的定轴铣削情况,一般可以采用三轴铣削来加工产品。五轴加工的关键是控制刀轴矢量在空间位置上的连续变化或使刀轴矢量构成机床原始坐标系。空间中的一定角度是通过铣刀的侧刃或底刃切削来完成的。刀轴的矢量变化控制一般有如图3所示的几种方法:

①直线:刀轴矢量方向与空间直线平行的定角法;

②PatternSurface:曲面的法线表达是刀轴的向量始终指向曲面的法线方向;

③起点:该点控制刀轴远离空间某一点的矢量;Topoint:刀轴的向量指向空间中的某一点;

④SwarfDriver:刀轴矢量沿空间曲面的直纹方向变化(曲面具有直纹属性);

⑤刀轴矢量连续插补控制。从上述刀轴矢量控制方法来看,五轴数控铣削的切削方法可以根据实际产品的加工来设计和规划合理的刀具路径。

UGII/ContourMilling 三轴高速和高速分层粗铣时刀具路径之间的圆弧过渡。支持高速铣削加工:系统提供的等高分层加工用于高速铣削场合。拐角处采用圆角形式过渡,避免90度急转弯(高速情况下容易损坏导轨和电机)。同时采用螺旋进退。,系统还提供环绕等多种方式支持高速加工刀具路径生成策略。UGII/VariableAxisMilling变轴铣削模块支持定轴和多轴铣削功能,可以处理UGII建模模块中生成的任何几何形状,同时保持主模型相关性。该模块提供经过多年工程使用验证的3~5轴铣削功能,并提供刀轴控制、刀具进给模式选择和刀具路径生成功能。刀轴矢量控制方法及加工策略。

UGII/SequentialMilling顺序铣削模块可以控制刀具路径生成过程中的每一步,支持2至5轴铣削编程,与UGII主模型完全相关。以自动化的方式,可以获得与APT直接编程相同的绝对精度。控制,允许用户以交互方式逐段生成刀具路径,并保持对过程中每个步骤的控制。提供循环功能,允许用户仅定义曲面上最内部和最外部的刀具路径,这些路径会自动生成中间步骤。该模块是UGII专用模块,与UGII数控加工模块中的自动清根功能相同。适用于高难度的数控编程。图4分别为三轴联动和五轴联动加工的刀具路径和实际产品加工示意图。

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