With the increasing maturity of CNC technology, 5-axis CNC machining centers have been widely used in various fields in recent years.
In practical applications, whenever people encounter a problem of high-efficiency and high-quality processing of special-shaped complex parts, 5-axis linkage technology is undoubtedly an important means to solve such problems.
More and more manufacturers tend to look for 5-axis equipment to meet high-efficiency and high-quality processing.
But, do you really know enough about 5-axis machining?
To truly understand 5-axis machining, the first thing we have to do is to understand what a 5-axis machine tool is.
5-axis machining, as the name implies, refers to the addition of two rotary axis to the three common linear axis of X, Y, and Z.
The two rotating shafts of the 3 axis ( A, B, and C axis) have different motion modes to meet the technical requirements of various products.
As for the mechanical design of 5-axis machining tools, machine tool manufacturers have always been unremittingly committed to developing new motion modes to meet various requirements.
In summary, there are various types of 5-axis machine tools currently on the market.
Although their mechanical structures are in various forms, there are mainly the following forms:
Two rotating coordinates directly control the direction of the tool axis (Double pendulum head form)
The two coordinate axis are at the top of the tool, but the rotation axis is not perpendicular to the linear axis (Nutate swing head form)
Two rotating coordinates directly control the rotation of the space (Double turntable form)
The two coordinate axis are on the worktable, but the rotation axis is not perpendicular to the linear axis (Nutate workbench form)
Two rotating coordinates, one acting on the tool and the other acting on the workpiece (one swing and one rotation form)
*Terms: If the axis of rotation is not perpendicular to the linear axis, it is considered a “nutate form” axis.
Having seen these 5-axis machine tools, I believe we should understand what and how the 5-axis machine tool is moving.
However, with such a diversified machine tool structure, what characteristics can it exhibit during machining?
Compared with traditional 3-axis machine tools, what are the advantages?
Next, let us take a look at the luminous points of the 5-axis machine tool.
Features of 5-axis machine tools
Speaking of the characteristics of 5-axis machine tools, it is necessary to compare with traditional 3-axis machines.
3-axis processing equipment is more common in production, and there are several forms such as vertical, horizontal and gantry.
The common processing methods include end cutting & side cutting of end milling cutter , profiling processing of ball nose cutters etc.
But no matter which form and method 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 movement of the tool in the spatial rectangular coordinate system through the interpolation of the three linear axis of X, Y, and Z.
Therefore, when faced with the following products, the disadvantages of the low efficiency, poor surface quality and even the inability to be processed of the 3-axis machine tool are exposed.
Compared with 3-axis CNC machining equipment, 5-axis linkage machine tools have the following advantages:
- Maintain the best cutting state of the tool and improve the cutting conditions
As shown in the figure above, in the 3-axis cutting mode in the left figure, when the cutting tool moves to the tip or the edge of the workpiece, the cutting state gradually deteriorates.
And to maintain the best cutting conditions here, it needs to rotate the table.
If we want to completely process an irregular plane, we must rotate the worktable several times in different directions.
It can be seen that the five-axis machine tool can also avoid the situation where the linear velocity of the center point of the ball end mill is 0 to obtain a better surface quality.
- Effectively avoid tool interference
As shown in the figure above, for the impellers, blades and blisks used in the aerospace field, the 3-axis equipment cannot meet the process requirements due to interference.
The 5-axis machine tool can satisfy this condition.
At the same time, the 5-axis machine tool can also use shorter tools for processing, which improves the rigidity of the system, reduces the number of tools, and avoids the production of special tools.
For our business owners, it means that the 5-axis machine tool will save money in terms of tool costs!
- Reduce the number of clamping and complete five-sided processing in one clamping
As can be seen from the above figure, the 5-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 in the number of the equipment for the five-axis machining center, the number of fixtures, workshop area and equipment maintenance costs have also been reduced.
This means that you can use fewer fixtures, less plant area and maintenance costs to complete more efficient and higher-quality processing!
- Improve processing quality and efficiency
As shown in the figure, the 5-axis machine tool can be cut by the side edge of the tool, which has higher processing efficiency.
- 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 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.
- Shorten the new product development cycle
For companies in the aerospace, automotive and other fields, some new product parts and molding molds have complex shapes and high precision requirements.
Therefore, a 5-axis CNC machining center with high flexibility, high precision, high integration and complete processing capabilities can well solve the accuracy and cycle problems of complex parts processing in the development of new products, greatly shorten the development cycle and improve the success of new products rate.
In summary, 5-axis machines have many advantages, but 5-axis machines are much more complex than 3-axis machines in terms of tool attitude control, CNC, CAM programming and post-processing!
At the same time, when talking about 5-axis machine tools, we have to talk about the true and false 5-axis issues.
We all know that the biggest difference between true and false 5-axis is the RTCP function.
But what is RTCP, how is it produced and how should it be applied?
Below we will combine the machine tool structure and programming post-processing to learn more about RTCP and understand it.
RTCP, in high-grade 5-axis CNC systems, is considered to be Rotated Tool Center Point, which is often referred to as the tooltip follow function.
In 5-axis machining, when pursuitng tool cusp locus and the attitude between the tool and the workpiece, due to the rotary motion, it produces additional movements of the tooltip.
CNC system control points often do not coincide with the tooltip, so the CNC system needs to automatically correct the control point to ensure that the tooltip move according to the prescribed trajectory.
The industry also calls this technology as TCPM, TCPC or RPCP and other functions.
In fact, the functional definition of these names is similar to RTCP.
Strictly speaking, the RTCP function is used in the double pendulum head structure, which is the application of the pendulum head rotation center point to compensate.
RPCP-like functions are mainly used on double rotary table machines to compensate for the change in linear axis coordinates caused by the rotation of the workpiece.
In fact, these functions are the same 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 article unifies such techniques for RTCP technology.
So how did the RTCP function come about?
Many years ago, when the five-axis machine tool was just popularized in the market, the RTCP concept was hyped by machine tool manufacturers.
At that time, the RTCP function was more of a technology for technology’s sake and more of hype for the technology itself.
In fact, the 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 to customers.
With a machine tool with RTCP technology (also known as a true 5-axis machine tool), the operator does not need to precisely align the workpiece with the axis of the turntable, and simply clamp it.
The machine tool automatically compensates for the offset, which greatly reduces the auxiliary time and improves the machining accuracy.
At the same time, the post-processing is easy to make, as long as the tooltip coordinates and vectors 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 structure.
In the following, we will take a double-turntable high-end 5-axis CNC system as an example to introduce the RTCP function in detail.
Define the concept of the 4th axis and the 5th axis in a 5-axis machine tool:
In the double-rotating 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.
Ok, let’s explain after reading the definition.
As shown in the figure above, the fourth axis of the machine tool is the A axis and the fifth 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 C axis attitude will also be affected.
Similarly, 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 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, we will analyze how this offset is generated.
According to the previous article, we all know that the linear axis coordinate shift is caused by the change of the rotating coordinate.
Then it is particularly important to analyze the center of rotation 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.
The fourth axis usually selects the midpoint of the fourth axis as the control point.
In order to realize five-axis control, the CNC system needs to know the relationship between the control points of the fifth axis and the control points of the fourth axis.
In the initial state (machine A and C axis 0 positions), the fourth axis control point is the origin in the fourth axis rotation coordinate system, and the fifth axis control point is the position vector [U,V,W].
It also needs to know the distance between the A and C axis.
For a double turntable machine tool, an example is shown in the figure below.
At this point, we can see that for RTCP-capable machines, the control system is designed to keep the tool center always at the position being programmed.
In this case, the programming is independent of the machine’s motion.
When you program on the machine, you don’t have to worry about machine movement or tool length, all you have to think about is the relative movement between the tool and the workpiece.
The rest will be done for you by the job control system.
As shown in the figure above, without the RTCP function, the control system does not consider the tool length.
The tool rotates around the center of the shaft.
The tip of the tool will move out of its position and will no longer be fixed.
As shown in the figure above, when the RTCP function is enabled, the control system only changes the direction of the tool, and the position of the tool tip remains unchanged.
The necessary compensation movements on the X, Y, and Z axis have been automatically calculated.
How to solve the problem of linear axis coordinate offset for 5-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 5-axis.
The so-called fake 5-axis actually refers to machine tools without RTCP function.
True and false 5-axis, neither look at the appearance nor whether the 5-axis are linked, because the false five-axis can also be used for 5-axis linkage.
The main difference between the fake 5-axis is that there is no real 5-axis RTCP algorithm, which means that the fake 5-axis programming needs to consider the spindle’s swing length and the position of the rotating table.
This means that when using fake five-axis CNC systems and machine tool programming, it 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.
And the fake 5-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 a lot of time for clamping and alignment, and accuracy cannot be guaranteed.
Even for index processing, fake 5-axis is a lot of trouble.
However, the true 5-axis only needs to set up one coordinate system, and only needs one tool calibration to complete the machining.
The following figure uses the NX post-processing editor settings as an example to illustrate the coordinate transformation of the fake 5-axis.
As shown in the figure above, the fake 5-axis relies on post-processing technology to show the center position relationship between the fourth axis and the fifth axis of the machine tool to compensate for the displacement of the rotary axis to the linear axis coordinate.
The CNC programs X, Y, and Z generated by it are not only programming approach points, but also include the necessary compensation on the X, Y, and Z axis.
The result of such processing will not only lead to insufficient processing accuracy and low efficiency, the generated program is not universal, and the required labor cost is also high.
At the same time, since the rotation parameters of each machine tool are different, there must be a corresponding post-processing file, which will also cause great inconvenience to production.
Furthermore, the generation program of fake five-axis cannot be changed, and it is basically impossible to realize manual 5-axis programming.
At the same time, because there is no RTCP function, many of its derivative 5-axis advanced functions cannot be used, such as the compensation function.
In fact, for a 5-axis machine tool, it is just a tool for us to achieve the processing results, and there is no difference between true and false 5-axis machine tool.
The important thing is that our process determines which method is used for processing,
Relatively speaking, true 5-axis machine tools are a more cost-effective way.
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