With the advancement of CNC technology, 5-axis CNC machining centers have become increasingly popular in various industries in recent years. In practical applications, when faced with the challenge of efficiently and effectively processing complex and irregular shaped parts, 5-axis linkage technology is often seen as a key solution.
An increasing number of manufacturers are seeking out 5-axis equipment to meet their demands for high efficiency and high-quality processing.
But, do you have a thorough understanding of 5-axis machining?
To truly understand 5-axis machining, we first need to understand what a 5-axis machine tool is. 5-axis machining, as the name suggests, involves the addition of two rotary axes to the three common linear axes of X, Y, and Z. The two rotating axes (A, B, and C axis) have different motion modes to meet the technical requirements of various products.
Machine tool manufacturers continuously strive to develop new motion modes to meet various requirements in the mechanical design of 5-axis machining tools.
In conclusion, there are various types of 5-axis machine tools currently available in the market. Although their mechanical structures vary, the main forms include:
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 understood the 5-axis machine tools, we should now delve into their movements.
However, with such a diverse array of machine tool structures, what unique properties can they display during machining?
In comparison to traditional 3-axis machine tools, what are the benefits?
Let us now examine the highlights of the 5-axis machine tool.
Features of 5-axis machine tools
Speaking of the characteristics of 5-axis machine tools, it’s important to compare them 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 and side cutting with an end milling cutter, and profiling processing with a ball nose cutter, among others.
However, no matter the form or method, one common feature is that the direction of the tool axis remains unchanged during the machining process.
The machine tool can only realize movement of the tool in the spatial rectangular coordinate system through interpolation of the three linear axes X, Y, and Z.
Therefore, when faced with certain products, the disadvantages of the 3-axis machine tool are exposed, such as low efficiency, poor surface quality, and even an inability to process the product.
Compared with 3-axis CNC machining equipment, 5-axis machining centers offer the following benefits:
- Maintain the best cutting state of the tool and improve the cutting conditions
As shown in the figure, in the 3-axis cutting mode on the left, when the cutting tool moves to the tip or edge of the workpiece, the cutting conditions gradually deteriorate. To maintain the best cutting state, the table must be rotated.
To fully process an irregular plane, the worktable must be rotated multiple times in different directions. It can be seen that the five-axis machine tool can also prevent the situation where the linear velocity of the center point of the ball end mill is zero, resulting in a better surface quality.
- Effectively avoid tool interference
As shown in the figure above, for the aerospace field components such as impellers, blades, and blisks, the 3-axis equipment fails to fulfill the processing requirements due to interference. The 5-axis machining tool can meet this requirement.
Additionally, the 5-axis machine tool can also employ shorter tools for processing, which enhances the rigidity of the system, reduces the number of tools required, and eliminates the need for special tools.
For business owners, this translates to cost savings in terms of tool expenses with the use of 5-axis machine tools.
- 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 bench conversion and improve machining accuracy. In actual processing, only one clamping is required, making it easier to guarantee the accuracy.
Moreover, due to the shortening of the processing chain and the reduction in the number of equipment for the 5-axis machining center, the number of fixtures, workshop area, and maintenance costs have also been reduced. This means you can use fewer fixtures, less workshop space, and incur lower maintenance costs to achieve more efficient and higher-quality processing!
- Improve processing quality and efficiency
As demonstrated in the figure, the 5-axis machine tool can perform cutting through the side edge of the tool, resulting in improved processing efficiency.
- Shorten the production process chain and simplify production management
The complete machining capability of the 5-axis CNC machine tool significantly shortens the production process and streamlines production management and planning. Its advantages become increasingly apparent for more complex workpieces compared to traditional methods with dispersed processes.
- Shorten the new product development cycle
For companies in the aerospace and automotive industries, the development of new products often involves complex shapes and high precision requirements. In these cases, the use of a 5-axis CNC machining center, with its high flexibility, precision, and complete processing capabilities, can effectively address the accuracy and cycle problems in the processing of complex parts. This, in turn, significantly reduces the development cycle and improves the success rate of new product development.
It’s important to note, however, that 5-axis machines are more complex than their 3-axis counterparts, with regards to tool attitude control, CNC, CAM programming, and post-processing.
Additionally, there are true and false 5-axis issues to consider. The distinction between true and false 5-axis lies in the presence or absence of the RTCP function.
To better understand RTCP and how it’s produced and applied, let’s dive into the machine tool structure and programming post-processing.
RTCP, which stands for Rotated Tool Center Point, is a crucial aspect of high-grade 5-axis CNC systems. It is also known as the tooltip follow function.
In 5-axis machining, the rotary motion of the tool produces additional movements of the tooltip, which affects the cusp locus and the attitude between the tool and the workpiece. To ensure the tooltip follows the prescribed trajectory, the CNC system must automatically correct the control point, which often does not coincide with the tooltip.
The same technology may be referred to as TCPM, TCPC, or RPCP. These names are similar in meaning to RTCP, with the main difference being in the way the technology is applied.
RTCP specifically refers to the application of the pendulum head rotation center point to compensate in the double pendulum head structure. On the other hand, functions like RPCP 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 essence, these functions aim to keep the center point of the tool and the actual contact point between the tool and the workpiece surface unchanged. For the purpose of this article, such techniques will be referred to collectively as RTCP technology.
The Origin of the RTCP Function
Years ago, when five-axis machine tools were first becoming popular in the market, the RTCP concept was heavily hyped by machine tool manufacturers. At that time, the RTCP function was more of a technology for technology’s sake and more of a marketing tool.
However, in reality, the RTCP function is not only a good technology, but also a valuable tool that can bring benefits and create value for customers. With a machine tool equipped with RTCP technology (also known as a true 5-axis machine tool), operators do not have to carefully align the workpiece with the turntable axis. Instead, they can simply clamp it and the machine tool will automatically compensate for the offset, which significantly reduces preparation time and improves machining accuracy. Additionally, post-processing is easier because the tooltip coordinates and vectors are easily output.
As mentioned before, five-axis CNC machine tools mainly come in the form of double swing heads, double turntables, or one swing and one rotation structures. In the following section, we will use a double turntable high-end 5-axis CNC system as an example to provide a detailed explanation of the RTCP function.
Defining the Fourth and Fifth Axes 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 fifth axis is the rotary coordinate on the fourth axis. However, the rotation of the fifth axis does not affect the attitude of the fourth axis.
Ok, let’s explain after understanding the definition.
As depicted in the figure, the fourth axis of the machine tool is labeled as the A-axis and the fifth axis is the C-axis.
The workpiece is positioned on the C-axis turntable. When the 4th axis, the A-axis, rotates, the attitude of the C-axis will be impacted as it is installed on the A-axis.
When programming the tool center cutting for the workpiece placed on the turntable, any change in the rotation coordinate will result in a change in the X, Y, and Z coordinates of the linear axis, leading to a relative displacement.
To address this displacement, the machine tool must perform compensation, which is where the RTCP function comes into play.
So, how does the machine tool compensate for the offset?
To answer that, we need to first analyze the source of the offset. As previously discussed, the linear axis coordinate shift is caused by the change in the rotating coordinate. Hence, it is crucial to analyze the center of rotation of the rotating axis.
In a machine tool with a double turntable structure, the control point of the C-axis, or the fifth axis, is typically located at the center of rotation of the machine table. The fourth axis usually chooses the midpoint of the fourth axis as its control point.
In order to achieve five-axis control, the CNC system must have knowledge of the relationship between the control points of the fourth and fifth axes. In the initial state, when the A and C axes are at position 0, the fourth axis control point is the origin in the fourth axis rotation coordinate system and the fifth axis control point is represented by the position vector [U, V, W]. The CNC system also needs to be aware of the distance between the A and C axis.
For double turntable machine tools, an example can be seen in the accompanying figure. It can be seen that for machines with RTCP capability, the control system is designed to keep the tool center always at the position specified in the programming. This means that programming is not affected by the machine’s movement.
When programming on the machine, you won’t need to consider machine movement or tool length. Simply focus on the relative movement between the tool and the workpiece. The job control system will handle the rest for you.
As illustrated in the figure, when the RTCP function is absent, the control system disregards the tool length. As a result, the tool rotates around the center of its shaft, causing the tip to deviate from its position and become unfixed.
As demonstrated in the figure, when the RTCP function is activated, the control system only adjusts the direction of the tool, while the position of the tool tip remains constant. The necessary compensations along the X, Y, and Z axes have been calculated automatically.
Regarding the issue of linear axis coordinate offset in 5-axis machine tools and CNC systems that lack RTCP, it is worth noting that many five-axis CNC machine tools and systems in China are considered “fake 5-axis”. This term refers to machine tools without the RTCP function. It is not determined by appearance or whether the 5 axes are linked, as false five-axis can still be used for 5-axis linkage.
The main distinction between fake 5-axis is the absence of a real 5-axis RTCP algorithm, meaning that programming for fake 5-axis must account for the spindle’s swing length and the position of the rotating table.
This implies that when using fake five-axis CNC systems and machine tools in programming, it’s necessary to utilize CAM programming and post-processing technology to pre-plan the tool path.
If the machine tool or tool is altered for the same part, CAM programming and post-processing must be performed once again.
The fake 5-axis machine tool must also ensure that the workpiece is positioned at the center of rotation of the worktable when clamping.
This results in a considerable amount of time spent on clamping and aligning for the operator, and accuracy cannot be guaranteed.
Even for index processing, the fake 5-axis is problematic.
On the other hand, the true 5-axis only requires setting up one coordinate system and only one tool calibration to complete the machining process.
The following figure uses the NX post-processing editor settings as an illustration to demonstrate the coordinate transformation of the fake 5-axis.
As depicted in the figure, the fake 5-axis relies on post-processing technology to compensate for the displacement of the rotary axis to the linear axis coordinate by showing the center position relationship between the fourth and fifth axes of the machine tool.
The CNC programs generated for X, Y, and Z axis not only include approach points but also the necessary compensation on these axes. This method leads to reduced processing accuracy, low efficiency, non-universal programs, and high labor costs.
Moreover, each machine tool has different rotation parameters, requiring a separate post-processing file, causing production inconvenience. Fake five-axis programming cannot be altered and manual 5-axis programming is almost impossible. The lack of RTCP function also limits its ability to use advanced derivative 5-axis functions, such as compensation.
In conclusion, the choice of 5-axis machine tool is not about true or false, but about the method used to achieve processing results. In terms of cost-effectiveness, true 5-axis machine tools are a more viable option.