In recent years, five-axis linkage CNC machining centers have been increasingly widely used in various fields. In practical applications, whenever people encounter problems with efficient and high-quality processing of special-shaped and complex parts, five-axis linkage technology is undoubtedly an important means to solve such problems. More and more manufacturers tend to look for five-axis equipment to meet high-efficiency, high-quality processing. But do you really know enough about five-axis machining?
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Mechanical structure form of five-axis machine tool
To truly understand five-axis machining, we must first understand what a five-axis machine tool is. Five-axis machine tool (5 Axis Machining), as the name suggests, refers to adding two rotation axes to the three common linear axes of X, Y, and Z. Two of the three axes A, B, and C have different motion modes to meet the technical needs of various products.
In terms of the mechanical design of 5-axis machining centers, machine tool manufacturers have always been unremittingly committed to developing new motion modes to meet various requirements. Based on the various types of five-axis machine tools currently on the market, although their mechanical structures are diverse, they mainly include the following forms:
1. Two rotation coordinates directly control the direction of the tool axis (double swing head form).
2. The two coordinate axes are at the top of the tool, but the rotation axis is not perpendicular to the linear axis (sloping swing head type).
3. Two rotation coordinates directly control the rotation of space (double turntable form).
4. The two coordinate axes are on the workbench, but the rotation axis is not perpendicular to the linear axis (sloping workbench type).
5. One of the two rotation coordinates acts on the tool and the other acts on the workpiece (one swing and one rotation).
*Terminology: If the axis of rotation is not perpendicular to the linear axis, it is considered a "droop" axis.
After seeing the five-axis machine tools with these structures, I believe we should understand what and how the five-axis machine tools move. However, what characteristics can such a diverse machine tool structure exhibit during processing? Compared with traditional three-axis machine tools, what are the advantages? Next, let us take a look at the shining points of five-axis machine tools.
02
The many advantages of five-axis machining
Speaking of the characteristics of five-axis machine tools, we need to compare them with traditional three-axis equipment. Three-axis machining equipment is relatively common in production, including vertical, horizontal and gantry types. Common processing methods include end mill end edge processing and side edge processing. Profiling processing of ball nose cutters, etc. However, no matter which form and method is used, they all have a common feature, that is, the direction of the tool axis remains unchanged during the machining process. The machine tool can only realize the rectangular coordinates of the tool in space through the interpolation of the three linear axes of X, Y, and Z. movement in the system. Therefore, when facing the following products, the disadvantages of low efficiency of three-axis machine tools, poor processing surface quality or even inability to process are exposed.
Compared with three-axis CNC machining equipment, five-linkage CNC machine tools have the following advantages:
1. Maintain the best cutting condition of the tool and improve cutting conditions
As shown in the figure above, in the three-axis cutting method on the left, when the cutting tool moves toward the top or edge of the workpiece, the cutting state gradually deteriorates. To maintain optimal cutting conditions here, a rotating table is required. And if we want to completely process an irregular plane, we must rotate the workbench multiple times in different directions. It can be seen that the five-axis machine tool can also avoid the situation where the linear speed of the center point of the ball end mill is 0 and obtain better surface quality.
2. Effectively avoid tool interference
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As shown in the figure above, for parts such as impellers, blades and blisks used in the aerospace field, three-axis equipment cannot meet the process requirements due to interference. A five-axis machine tool can satisfy this requirement. At the same time, five-axis machine tools can also use shorter tools for processing, improving system rigidity, reducing the number of tools, and avoiding the generation of special tools. For our business owners, this means that the five-axis machine tool will save you money in terms of tool costs!
3. Reduce the number of clampings and complete five-sided processing in one clamping
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As shown in the figure above, the five-axis machining center can also reduce datum conversion and improve machining accuracy. In actual processing, only one clamping is required, and the processing 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 in the five-axis machining center, the number of tooling fixtures, workshop floor space and equipment maintenance costs are also reduced. This means you can use fewer fixtures, less factory space and less maintenance costs to complete more efficient and higher-quality processing!
4. Improve processing quality and efficiency
As shown in the figure, the five-axis machine tool can use the tool side edge cutting to achieve higher processing efficiency.
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5. Shorten the production process chain and simplify production management
The complete processing of five-axis CNC machine tools greatly shortens the production process chain and can simplify production management and planning and scheduling. The more complex the workpiece, the more obvious its advantages over traditional production methods with dispersed processes.
6. Shorten the new product development cycle
For companies in aerospace, automotive and other fields, some new product parts and molding molds have complex shapes and high precision requirements. Therefore, a five-axis CNC machining center with high flexibility, high precision, high integration and complete processing capabilities can It can well solve the precision and cycle problems 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.
To sum up, five-axis machine tools have too many advantages, but the tool attitude control, CNC 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 we talk about five-axis machine tools, we have to talk about the issue of true and false five-axis. We all know that the biggest difference between true and false five-axis is the RTCP function. However, what is RTCP, how is it generated and how to apply it? Next, we will take a closer look at RTCP based on machine tool structure and programming post-processing to understand its true appearance.
03
About RTCP
RTCP, in high-end five-axis CNC systems, is considered to be Rotated Tool Center Point, which is what we often call the tool tip point following function. In five-axis machining, when pursuing the trajectory of the tool tip point and the attitude between the tool and the workpiece, additional motion of the tool tip point occurs due to rotational motion. The control points of the CNC system often do not coincide with the tool tip point, so the CNC system must automatically correct the control points to ensure that the tool tip point moves according to the prescribed trajectory. The industry also calls this technology TCPM, TCPC or RPCP and other functions. In fact, the functional definitions of these names are similar to RTCP. Strictly speaking, the RTCP function is used on the double swing head structure, and the rotation center point of the swing head is used for compensation. Functions similar to RPCP are mainly used on double-turntable machine tools, and compensate for changes in linear axis coordinates caused by the rotation of the workpiece. In fact, these functions have the same goal, which is to keep the tool center point and the actual contact point between the tool and the workpiece surface unchanged. Therefore, for the convenience of expression, this article unifies this type of technology as RTCP technology.
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So how did the RTCP function come about? Many years ago, when five-axis machine tools first became popular in the market, the RTCP concept was highly publicized by machine tool manufacturers. At that time, the RTCP function was more like a gimmick for technology's sake, and more people were enthusiastic and hyped about the technology itself. In fact, RTCP function is exactly the opposite. It is not only a good technology, but also a good technology that can bring benefits and create value to customers. For machine tools with RTCP technology (also known as true five-axis machine tools 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 for the offset, greatly reducing auxiliary time and improving processing. Accuracy. At the same time, post-processing is simple, as long as the tool tip point coordinates and vector 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 rotation.
Below we will take the dual turntable high-end five-axis CNC system as an example to introduce the RTCP function in detail.
Define the concepts of the fourth axis and the fifth axis in a five-axis machine tool: in a double rotary table structure, the rotation of the fourth axis affects the attitude of the fifth axis, and the rotation of the fifth axis cannot affect the attitude of the fourth axis. The fifth axis is the rotation coordinate on the fourth axis.
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Okay, after reading the definition, let's explain it. As shown in the figure above, 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 4th axis A rotates, because the C axis is installed on the A axis, the C axis attitude will also be affected. In the same way, for the workpiece we place on the turntable, if we program the tool center cutting, changes in the rotation coordinates will inevitably lead to changes in the X, Y, and Z coordinates of the linear axes, resulting in a relative displacement. In order to eliminate this displacement, the machine tool must compensate for it. RTCP is a function created 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 previous article, we all know that the change in rotation coordinates causes the offset of the linear axis coordinates. Then it is particularly important to analyze the rotation center of the rotation axis. For double-turntable structure machine tools, the control point of the C-axis, which is the fifth axis, is usually at the rotation center of the machine tool table. For the fourth axis, the midpoint of the fourth axis is usually selected as the control point.
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In order to realize five-axis control, the CNC system needs to know the relationship between the 5th-axis control point and the 4th-axis control point. That is, in the initial state (machine tool A and C axis 0 position), the fourth axis control point is the origin of the fourth axis rotation coordinate system, and the position vector [U, V, W] of the fifth axis control point. At the same time, you also need to know the distance between the A and C axis. For double turntable machine tools, an example is shown in the figure below.
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Having said this, you can see that for machine tools with RTCP function, the control system keeps the tool center at the programmed position. In this case, programming is independent and has nothing to do with machine tool motion. When you program on a machine tool, you don't have to worry about machine motion or tool length; all you need to consider is the relative motion between the tool and the workpiece. The control system does the rest for you. for example:
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As shown in the figure above, without the RTCP function turned off, the control system does not consider the tool length. The tool rotates around the center of the axis. The tip of the knife will move out of its position and will no longer be fixed.
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As shown in the figure above, with the RTCP function turned on, the control system only changes the tool direction, and the tool tip position remains unchanged. The necessary compensating movements in the X, Y and Z axes are automatically calculated.
How to solve the problem of linear axis coordinate offset for five-axis machine tools and CNC systems that do not have RTCP? We know that many five-axis CNC machine tools and systems in China are fake five-axis. The so-called fake five-axis actually refers to machine tools without RTCP function. Whether the true or false five-axis is not judged by the appearance or whether the five axes are linked. You must know that the fake five-axis can also be linked with the five axes. The main difference between false five-axis is that it does not have a true five-axis RTCP algorithm, which means that false five-axis programming needs to consider the pendulum length of the spindle and the position of the rotary table. This means that when using false five-axis CNC systems and machine tool programming, you must rely on CAM programming and post-processing technology to plan the tool path in advance.
For the same part, if the machine tool is changed or the tool is changed, CAM programming and post-processing must be performed again. Moreover, when clamping the workpiece, the false five-axis machine tool needs to ensure that the workpiece is at the center of rotation of the worktable. For the operator, this means that a lot of clamping and alignment time is required, and the accuracy cannot be guaranteed. Even for indexing processing, the fake five-axis is a lot of trouble. The true five-axis only needs to set up a coordinate system, and only needs one tool setting to complete the processing.
The following figure takes the NX post-processing editor settings as an example to illustrate the coordinate transformation of the false five-axis:
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As shown in the figure above, the false five-axis relies on post-processing technology to indicate the center position relationship between the fourth axis and the fifth axis of the machine tool to compensate for the displacement of the rotation axis to the linear axis coordinates. The CNC program X, Y, and Z generated by it are not only the programming approach points, but also include the necessary compensation on the X, Y, and Z axes.
The result of such processing will not only lead to insufficient processing accuracy and low efficiency, the generated program will not be versatile, and the labor cost will also be high. At the same time, since each machine tool has different rotation parameters, it must have corresponding post-processing files, which will also cause great inconvenience to production. Furthermore, the generation program of the fake five-axis cannot be changed, and it is basically impossible to realize manual five-axis programming. At the same time, because there is no RTCP function, many of its advanced five-axis functions cannot be used, such as the five-axis tool compensation function.
In fact, for five-axis machine tools, it is just a tool we use to achieve processing results, and there is no distinction between true and false. The important thing is that our process determines the processing method. Relatively speaking, true five-axis machine tools are more cost-effective.
