Abstract: This article delves deeply into the precision design principles of the cutting fixture in CNC machining for ensuring micron-level precision. It conducts a detailed analysis from multiple dimensions, such as the precise determination of positioning references, the rational control of clamping forces, the optimized design of fixture structures, and the establishment of error compensation mechanisms. Combined with practical cases, it elaborates on the application of these principles in engineering practice, aiming to provide in-depth technical references for professionals in the CNC machining field and help improve the design level of cutting fixtures and the machining precision.

1. Introduction

In modern manufacturing, CNC machining technology, with its advantages of high precision, high efficiency, and high automation, is widely used in numerous fields such as aerospace, automobile manufacturing, and precision machinery. As an indispensable and important part of the CNC machining process, the cutting fixture directly affects the machining precision and quality of workpieces. Especially in precision machining scenarios where the precision requirement reaches the micron level or even higher, the precision design of the cutting fixture is particularly crucial. It not only needs to ensure the accurate position and stable posture of the workpiece during the machining process but also effectively resist various interfering factors such as cutting forces and vibrations, so as to guarantee that the machined parts meet strict dimensional tolerance and geometric tolerance requirements.

2. Precise Determination of Positioning References

2.1 Principles for Selecting References

• The primary principle is to select the surfaces of the workpiece that have been machined and possess relatively high precision and stability as positioning references. For example, for shaft-like parts, the center holes are usually preferentially selected as positioning references because the center holes are precision-machined, and their roundness and cylindricity can provide a high-precision reference for the rotational positioning of the shaft. For box-like parts, the bottom surface and the machined side surfaces are often chosen as positioning references to ensure the positional accuracy of the parts in space.
• The principle of unified references should be followed, that is, the same set of positioning references should be used as much as possible to machine various surfaces throughout the machining process. This can reduce the accumulation of errors caused by reference conversion and improve the positional accuracy among the machined surfaces. For example, when machining an engine block, the bottom surface and two locating pin holes are used as unified references, and all processes from rough machining to finish machining are carried out with this as the reference for clamping and machining, effectively ensuring the positional accuracy among the hole systems of the block and the flatness requirement of the joint surface between the block and the cylinder head.

2.2 Design of Positioning Elements

• For different shapes and precision requirements of workpieces, it is crucial to design appropriate positioning elements. For plane positioning, high-precision support plates or support pins are commonly used. The working surface of the support plate needs to be precision-ground, with a flatness reaching the micron level, which can provide stable plane support for the workpiece. Support pins can be divided into fixed and adjustable types according to needs and can be used to position the surfaces of workpieces with different heights or shapes.
• For positioning round holes, commonly used positioning elements include cylindrical pins, tapered pins, and mandrels. The cylindrical pin cooperates with the round hole of the workpiece, and its diameter tolerance and cylindricity need to be strictly controlled. Generally, the cylindricity can be controlled within the range of ±1 - 2μm to ensure the radial positioning accuracy of the workpiece. Tapered pins are suitable for occasions with high positioning accuracy requirements and the need for automatic centering. The taper of the tapered pin matches that of the round hole of the workpiece, and high-precision centering is achieved through the close fit of the tapered surfaces. Mandrels are mainly used for machining sleeve-like parts and can be divided into clearance fit mandrels and interference fit mandrels. Clearance fit mandrels are convenient for workpiece loading and unloading, but their positioning accuracy is relatively low; interference fit mandrels can provide higher positioning accuracy, but it is more difficult to load and unload workpieces, and appropriate expansion and contraction mechanisms or thermal assembly processes need to be adopted.
• In terms of the layout of positioning elements, the six-point positioning principle should be followed, and the positioning points should be reasonably distributed to limit the six degrees of freedom of the workpiece. For example, for a cuboid workpiece, three support points can be arranged on the bottom surface to limit three translational degrees of freedom, two support points can be arranged on the side surface to limit two rotational degrees of freedom, and one support point can be arranged on the end face to limit the last rotational degree of freedom, thus achieving the complete positioning of the workpiece. Meanwhile, the positional accuracy among positioning elements also needs to be strictly controlled. For example, the center distance tolerance between positioning pins is generally controlled within ±5 - 10μm to ensure the accuracy of workpiece positioning.

3. Rational Control of Clamping Forces

3.1 Calculation of Clamping Force Magnitude

• The magnitude of the clamping force must be accurately calculated. It should ensure that the workpiece will not be displaced or vibrated due to external forces such as cutting forces and inertial forces during the machining process, and it should not be too large to cause deformation or damage to the workpiece. When calculating the clamping force, various factors such as the material characteristics, shape structure, machining process, and cutting parameters of the workpiece need to be comprehensively considered. For example, for rough machining processes with large cutting forces, the clamping force should be increased accordingly, but the reasonable range needs to be determined through mechanical analysis and finite element simulation. Taking milling as an example, according to the milling force calculation formula  (where  is the milling force,  is the milling force coefficient,  is the cutting depth,  is the feed per tooth,  is the diameter of the milling cutter, and  is the number of teeth of the milling cutter), combined with parameters such as the friction coefficient between the workpiece and the fixture, the minimum required clamping force is calculated. In practical applications, a safety factor (usually taken as 1.5 - 2.5) is usually multiplied on the basis of the calculated value to ensure the reliability of clamping.

3.2 Determination of Clamping Force Direction

• The direction of the clamping force should face the main positioning reference and be as consistent as possible with the direction of the cutting force. This can reduce the displacement tendency of the workpiece and the magnitude of the clamping force. For example, when turning shaft-like parts, the clamping force should be directed along the axial direction towards the positioning end face of the chuck and be in the same direction as the axial component of the cutting force. This can effectively prevent the workpiece from axial displacement during the cutting process and reduce the strength requirement for the clamping device. For some workpieces with complex shapes, there may be multiple directions of cutting forces. In this case, vector analysis is needed to determine the optimal direction of the clamping force, so that the clamping force can effectively resist the action of the cutting force in all directions.

3.3 Selection of Clamping Force Application Points

• The application points of the clamping force should be selected at the parts of the workpiece with relatively high stiffness and that are less likely to cause deformation. For thin-walled parts, direct application of clamping force at the thin-walled parts should be avoided. Instead, distributed multi-point clamping or auxiliary supports can be used to evenly distribute the clamping force on the workpiece. For example, when machining a thin-walled aluminum alloy shell, an inflatable sleeve clamping device can be set inside the shell. By inflating the sleeve to make it expand evenly outward, the clamping force is applied to the shell from the inside, effectively avoiding the deformation of the shell caused by external clamping. Meanwhile, the application points of the clamping force should be as close as possible to the machining area to reduce the moment generated by the cutting force on the workpiece and prevent the workpiece from rotating or tilting.

4. Optimized Design of Fixture Structures

4.1 Enhancement of Overall Stiffness

• In order to reduce the deformation of the fixture under the action of cutting forces and improve the machining precision, the overall stiffness of the fixture must be fully guaranteed. In terms of structural design, adopting a reasonable cross-sectional shape and size is the key. For example, for the crossbeam of a fixture that bears a large bending moment, a rectangular or I-shaped cross-section can be adopted, and its thickness and width are optimized according to the magnitude of the force. Through finite element analysis, the minimum cross-sectional size that meets the strength and stiffness requirements is determined to reduce the weight of the fixture. Meanwhile, adding stiffeners is also an effective means to improve the stiffness of the fixture. Reasonably arranging stiffeners on parts such as the base and side plates of the fixture can significantly improve its ability to resist deformation. For example, on the base of a large fixture used in a machining center, a grid-like stiffener structure with crisscrossing patterns is adopted, which can increase the stiffness of the base by 30% - 50% and effectively reduce the vibration and deformation during the machining process.

4.2 Dynamic Balance Design

• In high-speed cutting machining, the dynamic balance performance of the fixture has an important impact on the machining precision and the service life of the machine tool spindle. If the fixture has unbalanced mass, centrifugal force will be generated during high-speed rotation, resulting in intensified vibration of the machine tool and a decline in machining precision. Therefore, for rotary fixtures such as lathe fixtures and grinder fixtures, dynamic balance design must be carried out. Firstly, in the fixture design stage, the mass of parts is reasonably distributed to make the center of gravity of the fixture as close as possible to the rotation axis. For example, when designing a chuck-type lathe fixture, the heavier clamping mechanisms are symmetrically arranged on both sides of the chuck to reduce the unbalanced moment. Secondly, for fixtures that cannot completely eliminate unbalance through structural design, dynamic balance tests and corrections are required. The unbalance amount and phase of the fixture are measured by a dynamic balance testing machine, and then corrections are made by adding or removing counterweight blocks at specific positions, so that the unbalance amount of the fixture is controlled within the allowable range (generally, the unbalance amount of high-speed fixtures needs to be controlled within ).

4.3 Modular and Adjustable Design

• In order to meet the machining needs of workpieces with different sizes and shapes and improve the universality and flexibility of fixtures, adopting the design concepts of modularization and adjustability is the development trend of modern cutting fixture design. The fixture is designed as a structure composed of multiple standard modules, such as positioning modules, clamping modules, and support modules. Through the combination and adjustment of different modules, fixtures suitable for different workpieces can be quickly constructed. For example, in aerospace manufacturing, for the machining of wing spars of different models of aircraft, the same basic fixture frame can be used, and by replacing positioning and clamping modules of different specifications, it can adapt to the size and shape changes of various wing spars. Meanwhile, adjustable mechanisms are set at some key parts of the fixture, such as adjustable positioning pins and clamping force adjustment devices, to further improve the adaptability of the fixture. For example, when machining shaft-like parts with different hole diameters, an adjustable-diameter mandrel can be used. By adjusting the taper sleeve or threaded structure on the mandrel, a tight fit with workpieces with different hole diameters can be achieved.

5. Establishment of Error Compensation Mechanisms

5.1 Compensation for Thermal Deformation Errors

• During the CNC machining process, due to the influence of factors such as cutting heat, friction heat of the moving parts of the machine tool, and environmental temperature changes, both the fixture and the workpiece will undergo thermal deformation, resulting in machining errors. In order to compensate for thermal deformation errors, it is first necessary to conduct in-depth research on the thermal characteristics of the fixture and the workpiece. Through thermal analysis software, the thermal models of the fixture and the workpiece are established to predict their temperature distribution and thermal deformation laws under different machining conditions. For example, for a fixture that undergoes continuous cutting for a long time, analyze the temperature rise curve of each part under the action of cutting heat to determine the region and direction with the largest thermal deformation. Then, corresponding compensation measures are taken in the fixture design according to the thermal deformation laws. For example, thermal expansion compensation mechanisms are set in the positioning elements or clamping devices of the fixture. For example, special materials with a thermal expansion coefficient matching that of the workpiece material are used to make compensation blocks. When the temperature rises, the expansion amount of the compensation blocks can offset a part of the thermal deformation of the workpiece or the fixture, thereby reducing machining errors. In addition, by controlling the temperature of the machining environment, such as using a constant temperature workshop or setting temperature adjustment devices around the fixture and the machine tool, the impact of thermal deformation on machining precision can also be effectively reduced.

5.2 Compensation for Wear Errors

• With the use of the fixture, the positioning elements and clamping elements will gradually wear out, resulting in changes in the positioning accuracy and clamping force of the workpiece and thus generating machining errors. In order to compensate for wear errors, a regular inspection and adjustment mechanism needs to be established. High-precision measuring instruments such as coordinate measuring machines are used to regularly detect the positioning accuracy of the fixture and the wear condition of the clamping elements. For example, for a cylindrical positioning pin, measure its diameter wear amount. When the wear amount exceeds a certain threshold (such as ±5μm), replace the positioning pin in time or restore its accuracy by methods such as sleeve insertion repair. Meanwhile, in the fixture design, the structure of replaceable positioning and clamping elements is considered to facilitate quick replacement after wear and reduce the downtime caused by maintenance. In addition, by setting fine-tuning mechanisms on the fixture, such as threaded adjustment devices or eccentric wheel adjustment devices, the positioning errors caused by wear can be compensated to a certain extent, enabling the fixture to maintain a high level of precision for a long time.

6. Case Analysis

Taking the CNC machining of an aero-engine blade as an example, the blade is made of titanium alloy, has a complex curved surface shape, and has extremely high precision requirements. The profile tolerance of the blade surface is required to be controlled within ±10μm.

6.1 Positioning References

• The bottom surface and two side surfaces of the blade tenon are selected as positioning references, and high-precision positioning blocks and positioning pins are used for positioning. The surface roughness of the positioning blocks and positioning pins is controlled within , the diameter tolerance of the positioning pins is controlled within ±3μm, and the center distance tolerance is controlled within ±8μm, ensuring the precise position of the blade in the fixture.

6.2 Clamping Force Control

• According to the calculation of the cutting force during blade machining, the clamping force magnitude is set to . A hydraulic clamping device is adopted, and the direction of the clamping force is perpendicular to the bottom surface of the tenon and consistent with the main direction of the cutting force. The application points of the clamping force are distributed at the tenon part of the blade, and the clamping force is evenly applied through multiple clamping points, avoiding the deformation of the blade during the clamping process.

6.3 Fixture Structure Optimization

• The fixture adopts an integral structure, and the main body is made of high-strength alloy steel. Stiffeners are set at key parts to improve the overall stiffness of the fixture. In order to adapt to the machining of different models of blades, the fixture is designed in a modular structure, and the positioning and clamping modules can be adjusted according to the size and shape of the blades. Meanwhile, since blade machining belongs to high-speed milling, the fixture has undergone dynamic balance design, and the unbalance amount is controlled within , effectively reducing the vibration during the machining process.

6.4 Error Compensation

• Aiming at the thermal deformation during titanium alloy machining, cooling channels are set inside the fixture, and the heat is taken away by the circulating cooling liquid to reduce the temperature rise range of the fixture and the blade. Meanwhile, thermal expansion compensation devices are adopted in the positioning elements, and ceramic materials with a thermal expansion coefficient similar to that of titanium alloy are used to make compensation blocks, effectively compensating for the thermal deformation errors. For the wear errors of the fixture, the positioning pins and clamping elements are regularly inspected and replaced to ensure the long-term precision stability of the fixture. Through the precise design and application of the above cutting fixture, high-precision machining of the aero-engine blade has been successfully achieved, and the profile error of the blade surface is stably controlled within ±8μm, meeting the performance requirements of the aero-engine.

7. Conclusion

In CNC machining, the precision design of the cutting fixture plays a decisive role in ensuring micron-level precision. Through various design principles and technical means such as precisely determining positioning references, rationally controlling clamping forces, optimizing fixture structures, and establishing error compensation mechanisms, the precision and reliability of the cutting fixture can be effectively improved, thus providing a powerful guarantee for machining high-quality and high-precision parts. With the continuous development of the manufacturing industry, the requirements for machining precision will become higher and higher, and the precision design of the cutting fixture will also face more challenges and opportunities. In the future, it is necessary to further conduct in-depth research on the application of new materials and new processes in the design of cutting fixtures, combined with advanced digital design and manufacturing technologies such as virtual simulation and artificial intelligence, and continuously promote the innovation and development of the precision design technology of cutting fixtures to meet the needs of modern manufacturing for high-precision and high-efficiency machining.