Do you really know enough about five-axis machining? Five-axis machining knowledge!

 

 

In recent years, five-axis CNC machining center has been more and more widely used in various fields. In practical application, whenever people encounter the problem of high efficiency and high quality processing of special-shaped complex parts, the 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 the high efficiency, high quality processing. But do you really know enough about five-axis machining?

true and false five axis summary of the advantages and disadvantages:

 

01 The mechanical structure of the five-axis machine tool

Want to really understand the five axis machining, first of all we have to understand what is the five axis machine tool. Five-axis machine tools (5 Axis Machining), as the name implies, refer to the X, Y, Z, three common linear axes plus two rotating axes. The two rotating shafts in the three axes A, B, and C have different movement modes to meet the technical requirements of various products.

In the mechanical design of the 5-axis machining center, machine tool manufacturers are constantly working to develop new motion modes to meet various requirements. At present, all kinds of five axis machine tools on the market, although its mechanical structure in a variety of forms, but mainly in the following forms:

 

1. Two rotating 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 (pendulum type).

 

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

 

 

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

 

 

5. One of the two rotation coordinates acts on the tool and the other on the workpiece (one swing and one rotation form).

 

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

 

Having seen the five-axis machine tools with these structures, I believe we should understand what the five-axis machine tools are moving and how they move. However, such a variety of machine structure, in the processing can show what characteristics? Compared with the traditional three axis machine, what are the advantages.

 

The many advantages of five-axis machining

 

Speaking of the characteristics of five-axis machine tools, it is necessary to compare with the traditional three-axis equipment. Production of three-axis processing equipment is more common, there are vertical, horizontal and gantry and other forms. Common processing methods are end milling cutter edge processing, side edge processing. Ball end knife copying processing and so on. But no matter what kind of form and method have a common feature, is in the process of tool axis direction always remain unchanged, the machine tool can only through the X, Y, Z three linear axis interpolation to achieve the movement of the tool in the space Cartesian coordinate system. Therefore, in the face of the following products, the three-axis machine tool efficiency is low, the processing surface quality is poor or even unable to process the drawbacks are exposed.

 

 

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

 

1. Maintain the best cutting tool state, improve cutting conditions

 

 

As shown in the figure above, in the three-axis cutting method in the left figure, when the cutting tool moves to the top or the edge of the workpiece, the cutting state gradually deteriorates. And to maintain the best cutting condition here, you need to rotate the table. And if we want to complete the processing of an irregular plane, it is necessary to rotate the table in different directions many times. It can be seen that the five-axis machine tool can also avoid the ball end milling cutter center point line speed of 0, to obtain a better surface quality.

 

2. Effectively avoid tool interference

 

 

As shown in the above figure, the three-axis equipment cannot meet the process requirements due to interference for parts such as impellers, blades and integral bladed discs used in the aerospace field. The five-axis machine tool can be satisfied. At the same time, the five-axis machine tool can also use a shorter tool for processing, improve the rigidity of the system, reduce the number of tools, and avoid the generation of special tools. For our business owners, it means that in terms of tool costs, five-axis machine tools will save you money!

 

3. Reduce the number of clamping, one clamping to complete five-sided processing

 

 

 

As can be seen in the figure above, the five-axis machining center can also reduce the benchmark conversion and improve the 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, the number of fixtures, workshop floor space and equipment maintenance costs are also reduced. This means that you can use fewer fixtures, less plant area and maintenance costs to complete more efficient and higher quality processing!

 

As shown in the figure, the five-axis machine tool can use the tool side edge cutting, and the processing efficiency is higher.

 

 

5. Shorten the production process chain and simplify production management

 

The complete machining of the five-axis CNC machine tool 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 decentralized production methods.

 

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, the five-axis CNC machining center with high flexibility, high precision, high integration and complete processing capability can well solve the problem of precision and cycle 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 is too many advantages, but the five-axis machine tool attitude control, CNC system, CAM programming and post-processing are more complex than the three-axis machine tool! At the same time, when we talk about five-axis machine tools, we have to say true and false five-axis problem, we all know that the biggest difference between true and false five-axis RTCP function, but what is RTCP, how it is produced and how to apply it.

 

03 About RTCP

 

RTCP, in the high-end five-axis numerical control system, RTCP is considered to be the Rotated Tool Center Point, which is what we often call the knife point following function. In five-axis machining, when pursuing the trajectory of the tool point and the attitude between the tool and the workpiece, the additional motion of the tool point is generated due to the rotary motion. The control point of the numerical control system is often not coincident with the knife point, so the numerical control system should automatically correct the control point to ensure that the knife point moves according to the instruction set trajectory. This technology is also known in the industry as TCPM, TCPC or RPCP and other functions. In fact, the functional definitions of these names are similar to RTCP. Strictly speaking, RTCP function is used in the double swing head structure and is compensated by the rotation center point of the swing head. The function similar to RPCP is mainly applied to the machine tool in the form of double turntable to compensate for the change of linear axis coordinates caused by the rotation of the workpiece. In fact, these functions are the same, in order 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 paper unifies this kind of technology as RTCP technology.

 

 

 

Many years ago, when five-axis machine tools were just popularized in the market, the concept of RTCP was hyped by machine tool manufacturers. At that time, the RTCP function was more like a gimmick for technology, and more people were enthusiastic and hyped about its 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 for customers. For machine tools with RTCP technology (also known as true five-axis machine tools in China), the operator does not have to align the workpiece accurately with the axis of the turntable and clamp it casually. The machine tool automatically compensates for offset, greatly reducing auxiliary time and improving machining accuracy. At the same time, post-processing production is simple, as long as the output tool point coordinates and vectors on the line. 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.

 

Below we will take the dual turntable high-end five-axis CNC system as an example to introduce the RTCP function in detail.

 

The concept of the fourth axis and the fifth axis is defined in the five-axis machine tool: in the structure of the double rotary table, 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 a rotational coordinate on the fourth axis.

 

 

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

 

So how does the machine tool compensate for this offset? Next, we will analyze how this offset is generated.

 

 

According to the foregoing, we all know that the shift 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 5th axis, is usually at the rotation center of the machine tool table. The fourth axis usually selects the midpoint of the fourth axis axis as the control point.

 

 

In order to realize the 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, in the initial state (machine tool A, C axis 0 position), the position vector [U,V,W] of the fifth axis control point in the fourth axis rotating coordinate system with the fourth axis control point as the origin. At the same time, you also need to know the distance between the axis of A and C. For a double turntable machine, an example is shown in the figure below.

 

At this point, we can see that for the machine tool with RTCP function, the control system keeps the tool center always in the programmed position. In this case, the programming is independent and independent of the machine movement. When you program on a machine tool, you don't have to worry about machine tool motion and tool length, all you need to consider is the relative motion between the tool and the workpiece. The rest of the work control system will do for you. For example:

 

 

 

As shown above, the control system does not consider the tool length without RTCP function off. The tool rotates around the center of the shaft. The tip will move out of its position and will no longer be fixed.

 

 

As shown in the above figure, with RTCP function 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.

For five-axis machine tools and CNC systems that do not have RTCP, how to solve the problem of linear axis coordinate offset? We know that many domestic five-axis CNC machine tools and systems are false five-axis, the so-called false five-axis, in fact, refers to the machine tool without RTCP function. The true and false five-axis is neither to look at the appearance nor to see whether the five-axis is linked. It is necessary to know that the false five-axis can also be linked to the five-axis. The difference between the false five-axis is that it does not have a true five-axis RTCP algorithm, which means that the false five-axis programming needs to consider the length of the spindle and the position of the rotating table. This means that when programming with false five-axis CNC systems and machine tools, 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 carried out again. In addition, the fake five-axis machine tool needs to ensure that the workpiece is located at the rotation center of its worktable when clamping the workpiece. For the operator, this means that a large amount of clamping time is required and the accuracy is not guaranteed. Even do indexing processing, false five axis is also a lot of trouble. However, 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 axes:

 

As shown in the above figure, the false five-axis is to rely on post-processing technology to show the position relationship between the center of 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 generated CNC program X, Y, Z is not only the programming point, but also contains the necessary compensation on the X, Y, Z axis. The result of this treatment will not only lead to insufficient machining accuracy, low efficiency, the generated program is not universal, and the labor cost is also high. At the same time, due to the different rotation parameters of each machine tool, there must be a corresponding post-processing file, which will cause great inconvenience to production. Moreover, the false five-axis generation program 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 five-axis advanced functions derived from it cannot be used, such as the five-axis tool compensation function. In fact, for a five-axis machine tool, it is just a tool for us to achieve the processing results, and there is no difference between true and false. The important thing is that our process determines the choice of processing methods, relatively speaking, the true five-axis machine tool is more cost-effective.