In recent years, five-axis linkage CNC machining centers have been more and more widely used in various fields. In practical applications, whenever people encounter the problem of high-efficiency and high-quality processing of special-shaped 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 and high-quality processing. But, do you really know enough about five-axis machining?
01
The mechanical structure of the 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 rotary axes to the three common linear axes of X, Y, and Z. The two rotation axes in the A, B, and C three-axis have different motion modes to meet the technical requirements of various products.
In terms of the mechanical design of the 5-axis machining center, 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 there are various types of mechanical structures, there are mainly the following types:
1. Two rotation coordinates directly control the direction of the tool axis (double swing head form).
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2. The two coordinate axes are at the top of the tool, but the rotation axis is not perpendicular to the linear axis (sagging head type).
3. The two rotation coordinates directly control the rotation of the space (double turntable form).
4. The two coordinate axes are on the table, but the rotation axis is not perpendicular to the linear axis (plumbed table type).
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5. One of the two rotation coordinates acts on the tool, and the other acts on the workpiece (one pendulum and one revolution).
*Term: If the axis of rotation is not perpendicular to the linear axis, it is considered a "plunging" axis.
After seeing the five-axis machine tools with these structures, I believe we should understand what and how the five-axis machine tools are moving. However, what characteristics can such a diverse machine tool structure show during processing? Compared with traditional three-axis machine tools, what are the advantages? Next, let's take a look at the luminous points of the five-axis machine tool.
02
The many advantages of five-axis machining
Speaking of the characteristics of five-axis machine tools, it is necessary to compare them with traditional three-axis equipment. Three-axis processing equipment is relatively common in production, and there are several forms such as vertical, horizontal and gantry. Common processing methods include end mill end edge processing and side edge processing. Profiling of ball-end knives, etc. But no matter which form or method has a common feature, that is, the direction of the tool axis remains unchanged during the machining process, and the machine tool can only achieve the tool in the space Cartesian coordinates through the interpolation of the three linear axes of X, Y, and Z. movement in the department. Therefore, when faced with the following products, the disadvantages of three-axis machine tools, such as low efficiency, poor surface quality and even inability to process, are exposed.
Compared with three-axis CNC machining equipment, five-linkage CNC machine tools have the following advantages:
1. Keep the tool in the best cutting state and improve cutting conditions
As shown in the figure above, in the three-axis cutting mode in the left figure, when the cutting tool moves to the top or the edge of the workpiece, the cutting state gradually deteriorates. To maintain optimum cutting conditions here too, a rotary table is required. And if we want to completely process an irregular plane, the table must be rotated multiple times in different directions. It can be seen that the five-axis machine tool can also avoid the situation that the center point line speed of the ball end milling cutter is 0, and obtain better surface quality.
2. Effectively avoid tool interference
As shown in the figure above, for parts such as impellers, blades and blisks used in the aerospace field, the three-axis equipment cannot meet the process requirements due to interference. The five-axis machine tool can be satisfied. At the same time, the five-axis machine tool can also use shorter tools for processing, improve the rigidity of the system, reduce the number of tools, and avoid the production of special tools. For our business owners, it means that in terms of tool costs, the five-axis machine tool will save you money!
3. Reduce the number of clamping times, and complete five-sided processing in one clamping
As shown in the figure above, it can be seen that the five-axis machining center can also reduce benchmark conversion and improve machining accuracy. In actual processing, only one clamping is required, and the processing accuracy is easier to be guaranteed. 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 fixtures, workshop area and equipment maintenance costs are also reduced. This means that you can use less fixtures, less workshop area and 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 be cut with the side edge of the tool, and the processing efficiency is higher.
5. Shorten the production process chain and simplify production management
The complete machining of five-axis CNC machine tools greatly shortens the production process chain, which can simplify production management and planning and scheduling. The more complex the workpiece, the more obvious its advantages over traditional production methods with decentralized processes.
6. Shorten the new product development cycle
For enterprises in aerospace, automotive and other fields, some new product parts and molding molds have complex shapes and high precision requirements. Therefore, five-axis CNC machining centers with high flexibility, high precision, high integration and complete processing capabilities can be used. It can well solve the precision and cycle problems of complex parts processing in the process of new product development, greatly shorten the development cycle and improve the success rate of new products.
To sum up, the five-axis machine tool has too many advantages, but the five-axis machine tool attitude control, CNC system, CAM programming and post-processing are much more complicated than the three-axis machine tool! At the same time, when we talk about five-axis machine tools, we have to talk about the problem 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, let's take a look at RTCP in detail by combining the structure of the machine tool and the post-processing of programming to understand its true face.
03
About RTCP
RTCP, in the high-end five-axis CNC system, thinks that RTCP is the 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 is generated due to the rotary motion. The control points of the CNC system often do not coincide with the tool tip points, so the CNC system must automatically correct the control points to ensure that the tool tip points move according to the prescribed trajectory. In the industry, this technology is also called TCPM, TCPC or RPCP. In fact, the function definitions of these names are similar to RTCP. Strictly speaking, the RTCP function is used in the double swing head structure, and the center point of the swing head rotation is used for compensation. The function similar to RPCP is mainly applied to the machine tool in the form of double turntable, and it compensates the change of linear axis coordinates caused by the rotation of the workpiece. In fact, these functions have the same goal by different routes, all of which are to keep the center point of the tool and the actual contact point between the tool and the surface of the workpiece unchanged. Therefore, for the convenience of expression, this article unifies this type of technology as RTCP technology.
So how did the RTCP function come about? Many years ago, when five-axis machine tools first became popular in the market, the concept of RTCP was hyped by machine tool manufacturers. At that time, the function of RTCP was more like a gimmick for the sake of technology, and more people were enthusiastic and hyped about the technology itself. In fact, the function of RTCP is just 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 (that is, the so-called true five-axis machine tools in China), the operator does not need to accurately align the workpiece with the axis line of the turntable, and clamp it casually. The machine tool automatically compensates for the offset, greatly reducing auxiliary time and improving processing. precision. At the same time, the post-processing is easy to make, as long as the coordinates and vectors of the tool tip point are output. As we said before, in terms of mechanical structure, five-axis CNC machine tools mainly have structures such as double swing heads, double turntables, and one swing and one turn.
Below we will take the high-end five-axis CNC system with double turntable as an example to introduce the RTCP function in detail.
Define the concept of the fourth axis and the fifth axis in the five-axis machine tool: the rotation of the fourth axis affects the attitude of the fifth axis in the double rotary table structure, 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.
Well, 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-axis rotates, because the C-axis is installed on the A-axis, the posture of the C-axis will also be affected. In the same way, for the workpiece we put on the turntable, if we program the tool center cutting, the change of the rotation coordinate will inevitably lead to the change of the X, Y, Z coordinates of the linear axis, resulting in a relative displacement. In order to eliminate this displacement, the machine tool must compensate it, and RTCP is a function produced 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 offset of the linear axis coordinates is caused by the change of the rotation coordinates. Then it is particularly important to analyze the rotation center of the rotation axis. For a machine tool with a double turntable structure, the control point of the C-axis, that is, the fifth axis, is usually at the center of rotation of the machine table. For the 4th axis, the midpoint of the 4th axis is usually selected as the control point.
In order to realize five-axis control, the numerical control system needs to know the relationship between the control point of the fifth axis and the control point of the fourth axis. That is, the initial state (0 position of the A and C axes of the machine tool), the position vector [U, V, W] of the fifth axis control point in the fourth axis rotating coordinate system where the fourth axis control point is the origin. At the same time, it is also necessary to know the distance between the A and C axes. For a double turntable machine tool, an example is shown in the figure below.
Speaking of this, you can see that for machine tools with RTCP function, the control system keeps the tool center at the programmed position all the time. In this case, programming is self-contained and independent of machine kinematics. When you program on a machine tool, you don't have to worry about machine motion and tool length, all you need to think about is the relative motion between tool and workpiece. The rest of the job control system will do it for you. for example:
As shown in the figure above, when the RTCP function is not 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 position and will no longer be fixed.
As shown in the figure above, when the RTCP function is turned on, the control system only changes the direction of the tool, and the position of the tool tip remains unchanged. The necessary compensation movements in the X, Y, Z axes are automatically calculated.
And 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. The true and false five-axis is neither based on the appearance nor whether the five axes are linked. You must know that the fake five-axis can also be used for five-axis linkage. The difference between the fake five-axis is that it does not have the real five-axis RTCP algorithm, which means that the fake five-axis programming needs to consider the pendulum length of the spindle and the position of the rotary table. This means that when programming with a fake five-axis CNC system and machine tool, it is necessary to 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. And the false five-axis machine tool needs to ensure that the workpiece is at the center of rotation of the worktable when clamping the workpiece. For the operator, this means that a lot of time is required for clamping and alignment, and the accuracy cannot be guaranteed. Even for indexing machining, 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 machining.
The figure below takes the NX post-processing editor settings as an example to illustrate the coordinate transformation of the fake five-axis:
As shown in the figure above, the false five-axis relies on post-processing technology to display 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 not only program the approach point, but also include the necessary compensation on the X, Y, and Z axes.
The result of such 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 also high. At the same time, because the rotation parameters of each machine tool are different, there must be corresponding post-processing files, which will also cause great inconvenience to production. Furthermore, the generated 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 advanced five-axis functions derived from it cannot be used, such as the five-axis tool compensation function.
In fact, for five-axis machine tools, it is just a tool for us to achieve processing results, and there is no distinction between true and false. The important thing is that our technology determines which method to choose for processing. Relatively speaking, true five-axis machine tools are more cost-effective.
