Real five-axis machining center?

With the maturity of domestic CNC technology, five-axis CNC machining centers have been widely used in various fields in recent years. In practical application, whenever people encounter high-efficiency and high-quality machining problems of irregular and complex parts, five-axis linkage technology is undoubtedly an important means to solve such problems. In recent years, with the vigorous development of China's aerospace, military industry, auto parts and mold manufacturing industry, more and more manufacturers tend to look for five-axis equipment to meet high efficiency and high quality processing. But do you really know enough about five-axis machining? Please follow Bian Xiao's footsteps into the world of five-axis machining.

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Five-axis machining

To truly understand five-axis machining, the first thing we have to do is to understand what a five-axis machine tool is. Five-axis machining, as its name implies, refers to adding two rotating shafts to the three commonly used linear axes of X, Y and Z, and the two rotating shafts in the axes of A, B and C have different motion modes to meet the technical requirements of various products. In the mechanical design of five-axis machining center, machine tool manufacturers have been committed to developing new motion modes to meet various requirements. Summarize all kinds of five-axis machine tools on the market at present. Although the mechanical structures are various, they mainly have the following forms:

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The two rotation coordinates directly control the direction of the tool shaft (double swing head form)

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The two coordinate axes are at the top of the tool, but the rotation axis is not perpendicular to the linear axis (swinging head type).

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Two rotation coordinates directly control the rotation of the space (double turntable form)

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Two coordinate axes are on the workbench, but the rotation axis is not perpendicular to the linear axis (vertical workbench).

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Two rotational coordinates, one acting on the tool and the other acting on the workpiece (one pendulum and one rotation form).

* Terminology: If the rotation axis is not perpendicular to the straight axis, it is considered as a "vertical" axis.

After reading these five-axis machine tools, I believe everyone should understand what and how the five-axis machine tools are moving. However, what characteristics can such a diverse machine tool structure show in the machining process? Compared with the traditional three-axis machine tool, what are its advantages? Next, let's take a look at the luminous point of the five-axis machine tool.

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Characteristics of five-axis machine tools

When it comes to the characteristics of five-axis machine tools, it is necessary to compare them with traditional three-axis equipment. Three-axis processing equipment is common in production, including vertical, horizontal and gantry. The common machining methods are end edge machining and end milling cutter side edge machining. Copying machining of ball-end milling cutter, etc. However, no matter which form or method has the same feature, that is, the direction of the tool axis remains unchanged during the machining process, and the machine tool can only realize the movement of the tool in the spatial rectangular coordinate system through the interpolation of the three linear axes of X, Y and Z. Therefore, facing the later products, the shortcomings of the three-axis machine tool, such as low efficiency, poor surface quality and even inability to machine, are exposed.

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Compared with three-axis CNC machining equipment, five-axis CNC machine tools have the following advantages:

1. Keep the tool in the best cutting state and improve the cutting conditions.

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As shown in the above figure, in the three-axis cutting mode in the left figure, when the tool moves to the top or edge of the workpiece, the cutting state gradually deteriorates. To keep the best cutting state here, you need to rotate the table. However, if we want to completely process an irregular plane, we must rotate the table in different directions many times. It can be seen that the five-axis machine tool can also avoid the situation that the linear velocity of the center point of the ball-end milling cutter is zero and obtain better surface quality.

2. Effectively avoid tool interference

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As shown in the above figure, for the impellers, blades and integral bladed disks used in the aerospace field, the three-axis equipment cannot meet the process requirements due to interference. And five-axis machine tools can meet. At the same time, the five-axis machine tool can also use shorter tools to process, improve the rigidity of the system, reduce the number of tools and avoid the emergence of special tools. For our business owners, it means that the five-axis machine tool saves you money on tool cost!

3. Reduce the number of clamping, and complete five-sided processing at one time.

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As can be seen from the above figure, the five-axis machining center can also reduce datum conversion and improve machining accuracy. In actual machining, only one clamping is needed, and the machining accuracy is easier to ensure. At the same time, due to the shortening of the process chain and the reduction of the number of equipment, the number of fixtures, the floor space of the workshop and the maintenance cost of equipment are also reduced. This means that you can use fewer fixtures, less plant area and maintenance costs to complete more efficient and high-quality processing!

4. Improve the processing quality and efficiency

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As shown in the figure, the five-axis machine tool can use the side edge of the tool to cut, and the machining efficiency is higher.

5. Shorten the production process chain and simplify production management.

The complete machining of five-axis CNC machine tool greatly shortens the production process chain, which can simplify production management and scheduling. The more complex the workpiece is, the more obvious its advantages are compared with the traditional production method with scattered processes.

6. Shorten the development cycle of new products.

For enterprises in aerospace, automobile and other fields, some new product parts and molding dies have complex shapes and high precision requirements. Therefore, the five-axis CNC machining center with high flexibility, high precision, high integration and complete machining capability can solve the problem of precision and cycle of complex parts processing in the process of new product development, greatly shorten the research and development cycle and improve the success rate of new products.

wait for ...

To sum up, five-axis machine tools have many advantages, but the tool attitude control, numerical control system, cam programming and post-processing of five-axis machine tools are much more complicated than those of three-axis machine tools! At the same time, when it comes to five-axis machine tools, we have to talk about the problem of true and false five-axes. We all know that the biggest difference between true and false five-axis lies in RTCP function. However, what is RTCP, how did it come into being and how to apply it? Let's learn more about RTCP and its true features by combining machine tool structure and post-processing of programming.

RTCP, in the CNC GNC6 1 high-grade five-axis CNC system, thinks that RTCP is the center point of the rotated tool, which is what we often call the tool point following function. In five-axis machining, when pursuing the trajectory of the tool tip and the posture between the tool and the workpiece, the additional motion of the tool tip is generated due to the rotary motion. The control points of the CNC system often do not coincide with the tool points, so the CNC system should automatically correct the control points to ensure that the tool points move according to the given trajectory. There are also some functional departments in the industry who call this technology TCPM, TCPC or RPCP. In fact, the functional definitions of these appellations are similar to RTCP. Strictly speaking, the double swing head structure adopts RTCP function, and the swing head rotation center point is used for compensation. The function similar to RPCP is mainly applied to machine tools in the form of double turntable to compensate the change of linear axis coordinates caused by workpiece rotation. In fact, these functions achieve the same goal, which is to keep the center point of the tool and the actual contact point between the tool and the workpiece surface unchanged. Therefore, for the convenience of expression, this paper refers to this kind of technology as RTCP technology.

So how is the RTCP function generated? Many years ago, when five-axis machine tools were just popularized in the market, the concept of RTCP was widely publicized by machine tool manufacturers. At that time, RTCP function was more like a stunt of technology for technology's sake, and more people were full of enthusiasm and hype about its technology itself. In fact, RTCP has the opposite function. It is not only a good technology, but also a good technology that can bring benefits and create value for customers. With the machine tool with RTCP technology (that is, the real five-axis machine tool in China), the operator does not need to accurately align the workpiece with the axis of the turntable and clamp it casually. The machine tool automatically compensates the offset, which greatly reduces the auxiliary time and improves the machining accuracy. At the same time, the post-processing is simple, as long as the coordinates and vectors of the tip are output. As we said before, in terms of mechanical structure, five-axis CNC machine tools mainly have double swing heads, double turntables, one swing and one turn, etc. The following five-axis machine tool with double turntable and high-grade five-axis CNC system GNC6 1 are taken as examples to introduce RTCP functions in detail.

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The concepts of the fourth axis and the fifth axis are defined in the five-axis machine tool: in the double turntable structure, the rotation of the fourth axis affects the posture of the fifth axis, but the rotation of the fifth axis cannot affect the posture of the fourth axis. The fifth axis is the rotation coordinate on the fourth axis.

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Ok, I'll explain it after reading the definition. As shown in the above figure, the 4th axis of the machine tool is the A axis, and the 5th axis is the C axis. The workpiece is placed on the c-axis turntable. When the fourth axis A rotates, because the C axis is installed on the A axis, the attitude of the C axis will also be affected. Similarly, for the workpiece we put on the turntable, if we program the center cutting of the tool, the change of rotation coordinates will inevitably lead to the change of X, Y and Z coordinates of the linear axis, thus resulting in relative displacement. In order to eliminate this displacement, the machine tool must compensate it, and RTCP is the function to eliminate this compensation.

So how does the machine tool compensate for this offset? Next, let's analyze how this offset is generated.

According to the above, we all know that the deviation of linear axis coordinates is caused by the change of rotational coordinates. Then it is particularly important to analyze the rotation center of the shaft. For the machine tool with double turntable structure, the control point of the C axis, that is, the fifth axis, is usually at the rotation center of the machine tool table. The fourth axis usually selects the midpoint of the axis of the fourth axis as the control point.

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In order to realize five-axis control, the numerical control system needs to know the relationship between the fifth axis control point and the fourth axis control point. That is, the position vector [U, v, W] of the fifth axis control point in the fourth axis rotation coordinate system with the initial state (the positions of the A and C axes of the machine tool) and the fourth axis control point as the origin. At the same time, we also need to know the distance between axis A and axis C ... For double turntable machine tools, as shown in the following figure.

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At this point, we can see that for the machine tool with RTCP function, the control system always keeps the tool center in the programming position. In this case, the programming is independent and has nothing to do with the movement of the machine tool. When you use programming on a machine tool, you don't have to worry about the movement of the machine tool and the length of the tool. All you need to consider is the relative motion between the tool and the workpiece. The rest of the work control system will be done for you. For example:

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As shown above, there is no G203 RTCP function, and the control system does not consider the tool length. The cutter rotates around the center of the shaft. The tip of the knife will move out of its original position and will no longer be fixed.

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As shown above, when the G203 RTCP function is turned on, the control system only changes the tool direction, and the tip position remains unchanged. The necessary compensation motions in the X, Y and Z axes have been automatically calculated.

G203 is an RTCP open instruction in CNC system, which is usually called in the CNC program of CAM system. The CNC program only contains the X/Y/Z point to be approached and the direction vectors A, B and C describing the tool direction. In other words, the CNC program only contains geometry and tool direction data.

How do five-axis machine tools and numerical control systems without RTCP solve the problem of linear axis coordinate offset? We know that many five-axis CNC machine tools and systems in China now belong to false five-axis. The so-called fake five-axis actually refers to a machine tool without RTCP function. True and false five-axis, neither look at the appearance nor see whether the five-axis linkage, we must know that false five-axis can also do five-axis linkage. The main difference of pseudo-five-axis is that it has no real five-axis RTCP algorithm, which means that the swing length of the spindle and the position of the turntable need to be considered in pseudo-five-axis programming. This means that when programming with pseudo-five-axis CNC system and machine tool, we must rely on CAM programming and post-processing technology to plan the tool path in advance. The same part must be reprogrammed and post-processed when changing machine tools or cutting tools. Moreover, when the pseudo-five-axis machine tool clamps the workpiece, it needs to ensure the position of the workpiece in the rotation center of its workbench. For the operator, this means a lot of clamping and alignment time, and the accuracy cannot be guaranteed. Even for indexes, false five axes are troublesome. The real five-axis only needs to set a coordinate system, and only needs to set the tool once to complete the machining. The following figure takes NX post-processing editor settings as an example to illustrate the coordinate transformation of pseudo-five-axis.

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As shown in the above figure, the pseudo-five-axis is based on post-processing technology to represent the central position relationship between the fourth axis and the fifth axis of the machine tool, so as to compensate the displacement from the rotating axis to the linear axis coordinates. The generated CNC programs X, Y and Z are not only programming approach points, but also contain necessary compensation on X, Y and Z axes. The result of this processing will not only lead to insufficient machining accuracy and low efficiency, but also the generated program is not universal and the required labor cost is high. At the same time, due to the different rotary parameters of each machine tool, there must be corresponding post-processing files, which will also bring great inconvenience to production. Moreover, the generation program of pseudo-five-axis cannot be changed, and manual five-axis programming is basically impossible. At the same time, because there is no RTCP function, many advanced five-axis functions derived from it cannot be used, such as five-axis tool compensation function. In fact, for the five-axis machine tool, it is just a tool for us to achieve the processing effect, and there is no difference between true and false. The important thing is that our technology determines which processing method to choose. Relatively speaking, true five-axis machine tools are more cost-effective. For CNC GNC6 1 CNC system, it not only has RTCP function, but also supports many high-end interpolation functions such as 3D tool compensation, C-spline interpolation, NURBS spline interpolation, large arc interpolation and conical interpolation, so as to realize more efficient, concise and high-quality machining.

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Machining S-shaped specimen with five-axis machine tool

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Machining titanium alloy impeller with machine tool