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Five-Axis Machining
To really understand five-axis machining, the first thing we need to do is read up on what a five-axis machine tool is. Five-axis machine tools (5 Axis Machining), as the name suggests, refers to the X, Y, Z, three common linear axes plus two rotary axes. two rotary axes in the A, B, C axes have different modes of movement to meet the technical needs of various types of products. And in the mechanical design of 5-axis machining centers, machine tool manufacturers have always been committed to developing new modes of motion to meet a variety of requirements. Comprehensive the current market of various types of 5-axis machine tools, although its mechanical structure in a variety of forms, but there are mainly the following forms:
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The two rotating coordinates to directly control the direction of the tool axis (double pendulum head form)
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Two coordinate axes are at the top of the tool, but the rotation axis is not perpendicular to the linear axis (pitch pendulum head form)
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Two rotation coordinates directly control the rotation of the space (double rotary table form)
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Two coordinate axes are on the table, but the rotational axis is not perpendicular to the linear axis (pitch-type table form)
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Two rotational coordinates, one acting on the tool and one on the workpiece (one pendulum, one rotary form)
*Terminology: if the rotational axis is not perpendicular to the linear axis, it is considered to be a term: if the axis of rotation is not perpendicular to the linear axis, it is considered a "pendulous" axis.
Having looked at these structures of 5-axis machine tools, I believe we should understand what is moving and how 5-axis machine tools move. But what characteristics can such a diverse machine structure actually show when machining? Compared with the traditional three-axis machine tools, and what are the advantages? Next let's take a look at the five-axis machine tools have what luminous point.
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Features of 5-axis machine tools
When it comes to the features of 5-axis machine tools, it is important to compare them with traditional 3-axis equipment. Production of three-axis machining equipment is more common, there are several forms of vertical, horizontal and gantry. Common processing methods are end mill end edge processing, side edge processing. Ball tool profiling and so on. But no matter which form and method have a *** with the same characteristics, that is, in the process of machining the direction of the tool axis always remain unchanged, the machine tool can only be through the X, Y, Z three linear axes of interpolation to realize the movement of the tool in the spatial Cartesian coordinate system. Therefore, in the face of the following products, three-axis machine tools, low efficiency, poor surface quality or even impossible to process the disadvantages exposed.
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And compared with the three-axis CNC machining equipment, the five linkage CNC machine tools have the following advantages:
1. Maintaining the optimal cutting state of the tool and improving the cutting conditions
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As in the above figure, in the left figure of three-axis cutting mode, the cutting condition gradually deteriorates when the cutting tool moves toward the top or the edge of the workpiece. To maintain the optimum cutting condition even here, it is necessary to rotate the table. If we want to machine an irregular plane completely, we have to rotate the table several times in different directions. As you can see, the 5-axis machine also avoids the situation where the center point of the ball end milling cutter has a linear velocity of 0, and obtains a better surface quality.
2. Effective avoidance of tool interference
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As shown in the figure above, for parts such as impellers, blades, and integral blisks used within the aerospace sector, three-axis equipment cannot meet the process requirements due to interference. A 5-axis machine can do the job. At the same time, five-axis machines can also use shorter tools for machining, improve system rigidity, reduce the number of tools, avoiding the generation of specialized tools. For our business owners, means that in terms of tooling costs, five-axis machine tools will save you money!
3. Reduce the number of clamping, a clamping to complete the five-sided machining
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As can be seen in the above figure five-axis machining centers can also reduce the datum conversion, improve machining accuracy. In actual machining, only one clamping is required, and machining accuracy is more easily ensured. At the same time, 5-axis machining centers reduce the number of fixtures, shop floor space and equipment maintenance costs due to the shorter process chain and reduced number of machines. This means that you can accomplish more efficient and higher quality machining with fewer fixtures, less shop floor space, and less maintenance!
4. Improved machining quality and efficiency
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As shown in the diagram, 5-axis machines can use tool side edge cutting for more efficient machining.
5. Shorten the production process chain, simplify production management
The complete machining of five-axis CNC machine tools greatly shortens the production process chain, which can make production management and planning and scheduling simplified. The more complex the workpiece, it is relative to the traditional process of decentralized production methods, the more obvious the advantages.
6. Shorten the cycle of new product development
For aerospace, automotive and other fields of enterprises, some new product parts and molding die shape is very complex, the precision requirements are also very high, so with high flexibility, high precision, high integration and complete machining capabilities of the five-axis CNC machining centers can be a good solution to the development of new products in the process of processing of complex parts of the accuracy and cycle time problems. Greatly shorten the development cycle and improve the success rate of new products.
And so on...
In summary, five-axis machine tools have too many advantages, but five-axis machine tool attitude control, CNC, CAM programming and post-processing are much more complex than three-axis machine tools! At the same time, we talk about five-axis machine tools, we have to talk about the problem of real and fake five-axis, we all know that the biggest difference between real and fake five-axis lies in the RTCP function, however, what is RTCP, and how it is generated and how it should be applied? Below we will combine the machine structure and programming after processing to understand the specific RTCP, to understand his true face.
RTCP, in the CNC GNC61 high-grade five-axis CNC system, RTCP that is Rotated Tool Center Point, that is, we often say that the tip of the point to follow the function. In five-axis machining, the pursuit of the tip point trajectory and the attitude between the tool and the workpiece, due to the rotary movement, resulting in additional movement of the tip point. CNC system control points often do not coincide with the tip point, so the CNC system to automatically correct the control point to ensure that the tip point according to the instruction of the established trajectory movement. The industry has also referred to this technology as TCPM, TCPC or RPCP and other functions. In fact, the definition of the function of these designations are similar to RTCP, strictly speaking, RTCP function is used in the double pendulum head structure, is the application of the pendulum head rotation center point to compensate. And similar to the RPCP function is mainly used in the form of double rotary table machine tools, compensation is due to the workpiece rotation caused by the linear axis coordinate changes. In fact, these functions are the same way, are to maintain the center of the tool and the tool and the workpiece surface of the actual contact point unchanged. So in order to express the convenience of this paper to unify such technology for RTCP technology.
So how did the RTCP function come about? Years ago, when 5-axis machine tools were first introduced to the market, the RTCP concept was highly publicized by machine tool manufacturers. At that time, the RTCP function was more of a gimmick for the sake of technology, and more people were enthusiastic and hype about the technology itself. In fact, RTCP function 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. With RTCP technology machine tools (that is, the domestic said real five-axis machine tools), the operator does not have to put the workpiece and the rotary table axis line alignment, random clamping, the machine automatically compensates for the offset, greatly reducing the auxiliary time, while improving the machining accuracy. At the same time, the post-processing production is simple, as long as the output of the tool tip point coordinates and vectors on the line. Like we said before, in terms of mechanical structure, five-axis CNC machine tools mainly have double pendulum head, double rotary table, a pendulum and a rotary structure. Below we will take the double rotary table five-axis machine tools, CNC GNC61 high-grade five-axis CNC system as an example, a detailed introduction to the RTCP function.
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Define the concepts of the fourth and fifth axes in a five-axis machine tool: the rotation of the fourth axis in a double rotary table structure affects the attitude of the fifth axis, and the rotation of the fifth axis can not affect the attitude of the fourth axis. The fifth axis is the rotary coordinate on the fourth axis.
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Well, after reading the definition note let's explain. As shown above, the 4th axis of the machine is the A-axis and the 5th axis is the C-axis. The workpiece is placed on the C-axis rotary table. When the 4th axis A axis rotation, because the C axis is mounted on the A axis, so the C axis attitude will also be affected. Similarly, for the workpiece that we place on top of the rotary table, if we program the tool center cutting, the change in the rotation coordinates will inevitably lead to a change in the X, Y, and Z coordinates of the linear axes, creating a relative displacement. In order to eliminate this displacement, the machine must be compensated, RTCP is to eliminate this compensation and the function.
So how does the machine compensate for this offset? Next we will analyze how this offset is generated.
According to the previous section, we all know that it is the change in the rotary coordinates that causes the offset of the linear axis coordinates. Then it is particularly important to analyze the center of rotation of the rotary axis. For the double rotary table structure machine tool, the C-axis, which is the control point of the 5th axis is usually in the rotary center of the machine table. And the 4th axis is usually selected as the control point of the midpoint of the fourth axis axis.
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The CNC system, in order to realize the five-axis control, needs to know the relationship between the 5th-axis control point and the 4th-axis control point. That is, the initial state (machine tool A, C-axis 0 position), the fourth axis control point as the origin of the fourth axis rotary coordinate system, the fifth axis control point of the position vector [U,V,W]. It is also necessary to know the distance between the A and C axis axes. For a double rotary table machine, an example is shown below.
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Speaking of this, you can see that for machines with RTCP function, the control system is to keep the tool center always in the position being programmed. In this case, the programming is independent, programming that is not related to the motion of the machine. When you use programming on a machine, you don't have to worry about machine motion or tool length, all you have to think about is the relative motion between the tool and the workpiece. The control system does the rest for you. As an example:
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As shown above, without the G203 RTCP function off, the control system does not consider tool length. The tool rotates around the center of the axis. The tool tip will move out of its location and will no longer be fixed.
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As in the figure above, with the G203 RTCP function ON, the control system only changes the tool direction and the tip position remains fixed. the necessary compensating movements in the X, Y and Z axes are automatically accounted for.
G203 is the RTCP on command in the CNC control, which is usually already called in the CNC program of the CAM system. The CNC program contains only the X/Y/Z points to be approached and the direction vectors A, B, and C describing the tool direction; in other words, the CNC program contains only the geometry and tool direction data.
And for the 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 belong to the false five-axis, the so-called false five-axis, in fact, refers to the machine tool without RTCP function. True or false five-axis, neither to look at the appearance of the five axes is not to see whether the linkage, to know the false five-axis can also do five-axis linkage. The difference of fake 5-axis is mainly because it does not have the real 5-axis RTCP algorithm, which means that the programming of fake 5-axis needs to take into account the swing length of the spindle and the position of the rotary table. This means that when programming with a dummy 5-axis CNC and machine tool, you must rely on CAM programming and post-processing techniques to plan the toolpath in advance. If the machine is changed or the tool is changed, the same part must be re-programmed with CAM programming and post-processing. And fake 5-axis machine tools need to ensure that the workpiece is in its table rotary center position when clamping the workpiece, which means a lot of clamping time for the operator to find the right position, and the accuracy is not guaranteed. Even for indexing, the fake 5-axis is a lot of trouble. While the real five-axis only need to set a coordinate system, only one time to align the tool, you can complete the processing. The following figure illustrates the coordinate transformation of the false five-axis with the NX post-processing editor settings as an example.
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As shown above, the fake five-axis relies on post-processing technology to compensate for the displacement of the rotary axis to the linear axis coordinates by indicating the relationship between the center positions of the fourth and fifth axes of the machine. Its generated CNC program X, Y, Z is not only programming convergence points, but also contains the necessary compensation on the X, Y, Z axis. The result of such processing will not only lead to insufficient machining accuracy and low efficiency, the generated program is not universal and the required labor cost is also very high. At the same time, due to the different rotary parameters of each machine tool, there are corresponding post-processing files, which will also cause great inconvenience to production. Moreover, the generated program of the fake 5-axis cannot be changed, so it is basically impossible to realize manual 5-axis programming. At the same time, because there is no RTCP function, its derivatives of the many five-axis advanced features can not be used, such as five-axis tool filler function. In fact, for 5-axis machine tools, it is just a tool for us to realize the machining results, and there is no difference between true and false. The important thing is that our process determines the choice of what way of processing, relatively speaking, the real five-axis machine tool is more cost-effective. As for the CNC GNC61 CNC system, it not only has RTCP function, but also supports 3D tool interpolation, C spline interpolation, NURBS spline interpolation, large arc interpolation, conic interpolation, and many other high-end interpolation functions, so as to realize a more efficient and concise, high-quality processing.
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Five-axis machine tool machining S-type specimen
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Machine tool machining titanium alloy impeller