With the rapid development of the modern manufacturing industry, complex curved parts are more and more widely used in aerospace, automotive, mold, and other industries.
Traditional 3-axis CNC machine tools have been unable to meet the processing needs of complex surface parts; 5-axis CNC machine tools, with their unique advantages, have become the preferred equipment for complex surface processing.
Compared to three-axis CNC machine tools, five-axis CNC machine tools add two rotary axes, which can be realized through the multi-directional movement of the workpiece or tool, thereby achieving high efficiency and precision machining of complex surfaces.
To fully utilize the machining potential of five-axis CNC machine tools and enhance the quality and efficiency of complex surface part processing, a thorough study is necessary.
This study focuses on the principles of five-axis CNC machine tools, the difficulties in complex surface machining, and the machining processes.
5-axis CNC machine tool working principle
Five-axis CNC machine tool is based on three-axis CNC machine tools, adding two rotary axes (usually A-axis and C-axis) to form a high-end CNC machining equipment.
Among them, the X-axis, Y-axis and Z-axis control the linear movement of the tool in three orthogonal directions, the A-axis rotates around the X-axis, and the C-axis rotates around the Z-axis.
Through the collaborative work of the five axes of motion, the five-axis CNC machine tool can realize the movement of the tool relative to the workpiece in any angle and direction, so as to meet the needs of complex surface machining.
In the actual machining process, the CNC system controls the servo motor to drive each axis of motion according to the pre-programmed CNC program.
This control makes the tool move along the predetermined trajectory and speed, allowing it to complete the cutting of the workpiece.
Compared with three-axis CNC machine tools, five-axis CNC machine tools provide a higher degree of axial freedom for the tool. They can achieve ±120° rotation on the A-axis and 360° rotation on the C-axis.
This capability allows the tool to approach the workpiece surface in the optimal attitude, reducing the risk of tool interference and collision. As a result, operators improve machining efficiency and surface quality.
Additionally, the five-axis linkage enables constant cutting force and heat by dynamically adjusting the tool attitude, thereby prolonging the tool’s service life and reducing the impact of tool wear on machining accuracy.
Complex surface machining difficulties
Complex surface machining is a significant challenge in the field of CNC machining, and its processing difficulty is primarily reflected in three key aspects.
First of all, complex surfaces usually have variable radius of curvature and irregular geometry, making it difficult to apply the traditional tool path planning methods.
Second, complex surface machining challenges operators in tool selection and optimization.
They must select different types of tools, such as ball-end, round-nose, and taper tools, based on the geometric features of the surfaces.
Additionally, they optimize the geometric parameters of the tool, including edge radius, helix angle, and the number of edges, to achieve the optimal cutting effect.
Finally, complex surface machining requires machine tools to have superior dynamic characteristics.
It also demands high control accuracy from the machine tools.
During surface machining, the relative motion between the tool and the workpiece changes rapidly in speed and acceleration.
These rapid changes can easily cause machine vibration and positioning errors. As a result, machining accuracy and surface quality may be affected.
Therefore, operators must use high-precision, high-rigidity CNC machine tools.
They also need to optimize the parameters of the servo control system to enhance the machine tool’s dynamic response and improve trajectory following accuracy.
Complex surface machining process based on 5-axis CNC machine tools
Workpiece clamping and positioning
In complex surface machining, workpiece clamping and positioning is the first section, directly affecting the accuracy and efficiency of subsequent processing.
Unlike traditional three-axis machining, five-axis machining constantly changes the spatial attitude of the workpiece as the tool moves.
This change places higher demands on the clamping method.
> First of all, ensuring Workpiece Stability Through Effective Clamping Methods
The clamping system must withstand multi-directional cutting forces and inertia forces.
It ensures that the workpiece remains stable during machining without deforming or shifting position.
Operators usually use vacuum adsorption, electromagnetic adsorption, chuck clamping, and other methods based on the material, size, and shape of the workpiece.
They select the appropriate clamping force to ensure stability. If the clamping force is too small, the workpiece may slip; if it is too large, it may cause deformation.
> Secondly, advanced Fixture Solutions for Complex Workpiece Machining
The clamping should minimize the shielding of the workpiece machining surface, leaving enough space for the tool to avoid interference between the tool and the fixture.
In actual processing, operators optimize the fixture layout based on the structural characteristics of the workpiece.
They utilize specialized jaw structures, including rotatable jaws and retractable support rods, to maximize exposure of the machining surface.
> Finally, Workpiece Coordinate System Establishment Using Touch Probes
Clamping needs to consider the positioning accuracy of the workpiece, specifically the position and orientation relationship between the workpiece coordinate system and the machine coordinate system.
Positioning accuracy directly determines the accuracy of the machining trajectory, which in turn affects the dimensional and geometric tolerances of the machined surface.
In five-axis machining, operators usually use Touch Probes and other specialized probes to measure the workpiece.
They perform multi-point sampling to fit the spatial position of the workpiece and establish the workpiece coordinate system.
The repeatable positioning accuracy of the probe can reach ±1 μm, and the angular positioning accuracy can reach ±0.5 °, which meets the requirements of most complex surface machining.
Tool path planning
Tool path planning is the core of complex surface machining, which directly determines the machining efficiency, surface quality, and tool life.
Compared with three-axis machining, five-axis machining tool path planning is more complex.
Operators need to consider the relative position of the tool’s attitude and the workpiece surface in three-dimensional space to avoid tool interference and collision.
At the same time, they must optimize cutting parameters and control factors such as residual height and tool spacing.
> Common Five-Axis Tool Path Planning Methods
Currently, scholars have proposed various five-axis tool path planning methods, including mesh-based methods, parametric surface-based methods, and space curve-based methods.
Among these methods, operators widely use the parametric surface-based method in complex surface machining.
This method generates a series of equidistant contour lines by performing equal-parameter profiling of the surface.
Then, the system generates tool position points along the contour lines and connects these points to form the tool path
This method can effectively control the residual height and tool spacing to achieve a more uniform distribution of cutting allowances.
> Addressing Residual Height and Curvature Variations
The relationship between residual height tool spacing, and tool radius can be expressed as follows:
Where: Rz is the residual height, that is, the maximum height difference between two adjacent tool paths;
ae is the tool travel distance, i.e., the distance between two adjoining tool paths; r is the tool radius.
Equation (1) shows that the residual height is proportional to the square of the tool travel distance and inversely proportional to the tool radius.
Therefore, to obtain a smaller residual height, it is necessary to either reduce the toolpath spacing or increase the tool radius.
However, for surfaces with significant variations in curvature, the equidistant contour line method may have the problem of tool overcutting or undercutting, which affects the surface quality.
To solve this problem, operators utilize the adaptive contour line method to adjust the pitch of the contour lines in response to changes in surface curvature.
They reduce the pitch in areas with larger curvature and increase it where the curvature is smaller. This approach achieves uniform control of the residual height.
Processing parameter setting
A reasonable setting of machining parameters is a key factor in achieving efficient and high-quality machining of complex surfaces.
Five-axis CNC machining mainly involves parameters such as spindle speed, feed rate, depth of cut, and cutting width.
When selecting these parameters, operators must consider factors like the workpiece material, tool material, tool geometry, and machining strategy.
> Influence of Spindle Speed, Feed Rate, Depth of Cut, and Cutting Width
Spindle speed and feed rate are the primary parameters that affect cutting speed and material removal rate.
Higher cutting speed is conducive to improving machining efficiency, but too high a cutting speed may lead to increased tool wear and increased cutting temperature, which in turn affect the surface quality.
Therefore, it is necessary to select a suitable range of cutting speeds based on the characteristics of the workpiece material and the tool material.
For materials with high hardness, such as titanium alloy and cemented carbide, the cutting speed is usually 30 to 50 m·min-1;
For soft materials such as aluminum alloy, the cutting speed can be selected from 200 to 500 m·min-1.
The depth of cut and cutting width determine the amount of material removed per unit of time, which has a significant impact on processing efficiency and tool life.
A depth of cut that is too large is prone to cause tool vibration and deformation, and a cutting width that is too large will increase the lateral load on the tool, resulting in tool deflection.
Typically, the ratio of the cut depth to the tool diameter is maintained at 0.2 to 0.5, and the ratio of the cutting width to the tool diameter is controlled between 0.5 and 0.8.
> Optimization of Machining Parameters Using Intelligent Algorithms
Engineers can introduce intelligent algorithms, such as genetic algorithms and particle swarm algorithms, to optimize machining parameters.
These algorithms establish a mathematical model that links machining parameters with machining quality and efficiency.
They then search for the optimal combination of parameters.
Post-processing
After five-axis CNC machining of complex surface parts, a series of post-processing operations are also required to improve the surface quality and accuracy further, meeting the use requirements of the parts.
> Firstly, eliminating Residual Stress to Improve Surface Integrity
It is necessary to eliminate residual stress on the machined surface and enhance its integrity.
During the cutting process, high-speed friction and plastic deformation occur between the tool and the workpiece.
These actions cause the machined surface to develop tensile residual stress, which decreases surface hardness and reduces fatigue strength.
Operators use surface strengthening techniques such as shot peening to eliminate residual stress.
This process forms a compressive stress layer on the surface through high-speed jets.
The compressive layer balances the original tensile stress, enhancing surface hardness and wear resistance.
> Secondly, Enhancing Assembly Accuracy by Refining Machined Surface Finish
It is necessary to optimize the microscopic morphology of the machined surface and reduce the roughness.
Although five-axis CNC machining achieves high dimensional accuracy, tool vibration and chip adhesion often cause microscopic defects on the machined surface.
These defects include walking traces and furrows. Such imperfections affect the assembly accuracy and sealing performance of the parts.
To eliminate these defects, operators employ surface finishing techniques, including polishing and grinding.
They repair and smooth the surface morphology by micro-cutting and plastic deformation using micro-fine abrasives.
> Finally, surface Strengthening Techniques for Enhanced Part Durability
operators can apply surface strengthening techniques such as laser surface melting and plasma spraying.
These methods deposit wear-resistant and corrosion-resistant coatings on the processed surface, further enhancing the part’s performance and extending its service life.
Experimental verification
Experimental design
To verify the advantages of complex surface machining using five-axis CNC machine tools, a set of comparative experiments has been designed.
The aero-engine blade is selected as a typical complex surface part, and Ti-6Al-4V titanium alloy material is used.
The experiment includes two groups: the experimental group and the control group.
> Machining Methods and Parameters
The experimental group utilizes five-axis CNC machine tools for machining and adheres to the five-axis machining process outlined in the article.
The team clamps the workpiece using a vacuum adsorption system.
They apply the adaptive isoparametric surface method for the tool path, which dynamically adjusts the pitch of contour lines based on changes in curvature.
The machining parameters are optimized by a genetic algorithm, and the spindle speed is set at 8,000 r·min-1, the feed rate is set at 2,000 mm·min-1, the depth of cut is set at 0.2 mm, and the cutting width is set at 3 mm;
The post-processing was shot peening and nano-grinding.
The control group utilizes a traditional three-axis CNC machine and adheres to the conventional three-axis machining method.
The team fixes the workpiece using a fixture and adopts an equidistant parallel tool path.
They keep the machining parameters fixed and perform only conventional polishing during post-processing.
> Experimental Procedure
The experimental process involves several key steps.
First, the team performs workpiece modeling and generates the CNC program. Next, they carry out workpiece clamping and machine debugging.
They then conduct rough machining three times, followed by semi-finish machining twice. After that, they perform finishing machining once.
Finally, they complete surface treatment, accuracy measurement, and surface quality measurement.
> Evaluation Indexes and Control of Variables
The evaluation indexes include shape accuracy, surface roughness, machining efficiency, surface integrity, and tool life.
Among them, shape accuracy is evaluated by measuring the deviation of key feature points using a CMM and applying the standard deviation method.
Surface roughness was measured using a white light interferometer to measure the Ra value; machining efficiency was assessed by recording the total machining time;
Surface integrity was measured using an X-ray diffractometer to measure residual stress, and tool life was assessed by recording the frequency of tool change.
The researchers strictly controlled the variables during the experimental process.
They ensured that, aside from the machining method, other factors—such as material batch and environmental conditions—remained consistent.
This approach guaranteed the reliability and reproducibility of the experimental results.
Analysis of experimental results
The machining results of the experimental group and the control group are presented in Table 1, indicating that five-axis machining outperforms traditional three-axis machining in all key evaluation indexes.
In terms of shape accuracy, the maximum deviation of 5-axis machining is only 0.015 mm, while 3-axis machining reaches 0.042 mm, which reflects the accuracy of 5-axis machining in dealing with complex surfaces.
Surface roughness measurement results indicate that the surface roughness (Ra) achieved by five-axis machining reaches 0.2 μm. In comparison, three-axis machining yields a roughness of 0.8 μm.
This significant improvement comes from the ability of five-axis machining to dynamically adjust the tool’s attitude and maintain optimal cutting conditions.
> Efficiency, Residual Stress, and Tool Wear Performance
In terms of machining efficiency, the three-axis machining process takes 300 minutes to complete the entire experimental process. In comparison, the five-axis machining process takes only 180 minutes, which is 40% more efficient.
The surface residual stress measurement results show that the residual stress on the workpiece surface after five-axis machining is -50 MPa.
This value indicates a state of compressive stress. Such stress helps improve the part’s fatigue strength and extends its service life.
On the other hand, the residual stress after 3-axis machining is +120 MPa, which represents a tensile stress state that may adversely affect the part’s long-term performance.
Additionally, the five-axis machining process results in less tool wear. On average, operators replace the tool after processing every five workpieces.
This practice not only reduces processing costs but also improves the stability and consistency of machining.
Conclusion
We conducted an in-depth study of complex surface machining using five-axis CNC machine tools.
This study systematically describes the working principles of five-axis machining, highlights the difficulties in complex surface machining, and explains the key process points.
Through experimental verification, five-axis machining in shape accuracy, surface quality, machining efficiency and tool life have shown significant advantages.
As technology continues to advance, five-axis CNC machining is playing an increasingly significant role in aerospace and automotive manufacturing.
It is also driving the manufacturing industry toward higher precision and greater efficiency