How to design fixtures for CNC machining of thin-walled parts?

With the rapid development of the modern manufacturing industry, thin-walled parts are widely used in aerospace, automotive, electronics, and other industries due to their lightweight, high strength, and other advantages.

However, thin-walled parts with thin wall thickness, low stiffness, etc., are very prone to deformation during CNC machining, which seriously affects machining accuracy and efficiency. Fixture as a machine tool auxiliary equipment, in thin-walled parts processing plays an important role. Scientific and reasonable fixture design can not only effectively prevent deformation of the workpiece, and improve machining accuracy, but also shorten the clamping time and improve productivity.

Therefore, an in-depth study of thin-walled parts CNC machining fixture structure design has important theoretical significance and practical application value. This paper will focus on the fixture structure design in the CNC machining of thin-walled parts, aiming to provide theoretical guidance and technical support for the high-precision and high-efficiency machining of thin-walled parts.

Fixture structure design needs analysis

Thin-walled parts are parts whose wall thickness is relatively small compared with other dimensions, usually with a wall thickness of less than 2 mm. These parts are widely used in aerospace, automotive, electronics, and other industries, such as aircraft skins, car bodies, electronic product shells, etc.

Thin-walled parts with lightweight, high strength, low stiffness, and other characteristics, for its CNC machining have brought great challenges. Fixture is an indispensable auxiliary equipment in CNC machining, especially important for the processing of thin-walled parts. The main role of the fixture includes positioning the workpiece, supporting the workpiece, fixing the workpiece, and guiding the tool.

Demand analysis of fixture structure design is the first step of fixture design, which directly determines the function and performance of the fixture. For the machining characteristics of thin-walled parts, fixture structure design needs to meet the needs of various aspects.

First, the support structure design is a key link in fixture design. Thin-walled parts are very easy to be deformed under the action of machining force, so it is necessary to carry out a reasonable support structure design of the fixture to maximize the overall rigidity of the workpiece and reduce the amount of deformation.

The support structure needs to be optimized according to the shape of the workpiece, size, machining parts, and other factors to optimize the layout, not only to provide sufficient support but also to ensure that the machining space of the parts. Second, the design of positioning elements is an important part of fixture design.

Fixtures must be able to reliably locate thin-walled parts to ensure that their position and attitude remain stable during machining. Positioning elements such as positioning pins, positioning blocks, etc., need to be accurately matched with the positioning reference on the workpiece because positioning accuracy directly affects machining accuracy.

Third, the design of the clamping mechanism is the key to ensuring the stability of the workpiece.

The clamping mechanism needs to meet the requirements of the clamping force at the same time, to avoid deformation caused by local stress concentration. Fast, flexible clamping methods such as pneumatic clamping, hydraulic clamping, etc., can significantly improve the clamping efficiency and shorten the non-cutting time.

Fourth, the design of guiding and alignment elements should not be ignored. Guiding elements such as guide bushings, guide plates, etc., need to accurately control the trajectory of the tool to ensure the machining surface quality.

Clamp structure design

1. Support structure design

The designed fixture support structure for thin-walled parts adopts a combination of multi-point support and local support.

First, several support points are arranged at the back of the thin-walled part, in the number and position of the support points are optimized according to the size and shape of the part as well as the distribution of the machining force.

Second, through finite element analysis, the optimal layout of the nine support points is determined to minimize the occupation of the machining space of the part while meeting the support stiffness requirements.

Finally, a carbide ball with a diameter of 10mm is used to contact the back of the part to form a point contact support. The spherical support can effectively avoid the stress concentration problem caused by the unevenness of the support surface while facilitating the uniform transmission of the support force.

The support balls are loaded by springs, and the applied preload can be adjusted according to the stiffness of the part and the amount of machining force.

In addition to multi-point support, a specialized support structure is also provided in the locally weak areas of thin-walled parts. For the U-shaped groove part of the machined part, a bridge-type support structure is designed.

A three-point support is used, with one support point on each side wall and at the bottom of the U-shaped groove, forming a “zigzag” support. The bridge-shaped support consists of two height-adjustable support blocks and a connecting beam. By adjusting the height of the support blocks, it can be adapted to different sizes of U-shaped grooves.

The support blocks are in the form of polycrystalline diamond inlaid corundum blocks. The corundum blocks are arranged on the top surface of the support block and are in direct contact with the wall surface of the U-shaped groove, which can minimize the abrasion on the side wall of the U-shaped groove while providing the support force.

The combination of multi-point support and local support gives full consideration to the support needs of different parts of the thin-walled parts, which can effectively inhibit the overall deformation, but also strengthen the local weak areas in a targeted manner to ensure the stability of the parts in the machining process.

2. Design of Positioning Elements

The design of thin-walled parts fixture positioning elements using a combination of reference positioning and local positioning. In the fixture base set up two V-shaped positioning blocks, and with the workpiece bottom surface of the two positioning holes with the realization of the workpiece in the horizontal plane of the positioning. v-shaped positioning block made of integral tungsten carbide material, the top angle of 90 °, and positioning holes with the self-centering effect can be achieved, thereby improving positioning accuracy.

In addition to datum positioning, to set up local positioning elements in the characteristic parts of thin-walled parts. For a rectangular groove on the workpiece, a T-shaped positioning block is designed to cooperate with the two side walls and the bottom surface of the groove, to realize the positioning of the workpiece in the vertical direction. The T-shaped positioning block is made of high manganese steel, and the clearance between the transverse width and the width of the groove is 0.02mm, and the clearance between the longitudinal height and the depth of the groove is 0.01mm. the setting of the clearance is based on the manufacturing accuracy and positioning precision requirements of the workpiece and the positioning block.

The T-shaped positioning block is fixed to the fixture base by screws, and the position can be fine-tuned through the long holes to adapt to different sizes of grooves. Polycrystalline diamond inlaid corundum blocks are provided on both sidewalls of the T-shaped locating blocks, which are in direct contact with the groove sidewalls to minimize wear on the groove sidewalls while providing positioning constraints.

The combination of benchmark positioning and local positioning makes full use of the geometric features of the thin-walled parts, and while ensuring the overall positioning accuracy, it can further improve the positioning accuracy of the key feature parts, laying a solid foundation for the high-precision machining of thin-walled parts.

3. Clamping mechanism design

The designed thin-walled parts fixture adopts a two-way hydraulic clamping mechanism, which realizes reliable clamping of the workpiece by applying uniform clamping force on two opposite sides of the workpiece. The clamping mechanism mainly consists of two hydraulic cylinders, two clamping blocks, and connecting parts.

The hydraulic cylinder is a domestic SCJ-63×50 double-acting hydraulic cylinder with a rated working pressure of 16MPa and a maximum thrust of 5kN, and the clamping block is made of whole 45-gauge steel, which is tempered to a surface hardness of 40-45HRC to improve its abrasion resistance and deformation resistance.

The clamping surface of the clamping block is made of polycrystalline diamond inlaid corundum block, which is in direct contact with the surface of the workpiece and effectively reduces the wear on the surface of the workpiece while providing the clamping force.

At the same time, considering the unevenness of the surface of the thin-walled parts, a spherical pad is set between the clamping block and the hydraulic cylinder to realize the automatic centering of the clamping force and ensure the uniform distribution of the clamping force.

The clamping block and the hydraulic cylinder are connected by a two-way screw mechanism, and the position of the clamping block is quickly adjusted by rotating the nut to adapt to thin-walled parts of different sizes.

The bidirectional screw mechanism adopts Tr16×2 trapezoidal thread, with a maximum axial thrust of 8kN, which fully meets the requirements for the transmission of clamping force.

4. Design of guidance and alignment elements

The designed fixture for thin-walled parts adopts a combination of guide bushings and locating pins for tool guidance and alignment. A set of carbide guide bushings with a diameter of 20 mm is designed on the fixture for the machining of internal holes in the workpiece. The tool is precisely guided into the bore of the workpiece through a direct fit with the tool.

The guide bush is made of polycarbonate material, the inner hole is precision ground with a surface roughness of 0.4μm, and the clearance between the guide bush and the tool is controlled at 0.01-0.02mm, which not only ensures the high precision of the tool movement but also effectively prevents the tool and the guide bush from excessive wear. The guide sleeve is mounted on the fixture base through precision fit, and the coaxiality error is controlled within 0.005mm.

At the same time, a conical chamfer is designed on the end face of the guide sleeve, with a chamfer angle of 60° and a chamfer height of 2mm, to guide the tool into the guide sleeve smoothly.

For finishing features such as threaded holes and deep grooves on the workpiece, locating pins are designed to align the tool. The locating pin is made of hardened progressive die steel material with a nitrided surface and a hardness of more than 1000 HV.

The outer circle of the locating pin is ground with a surface roughness of 0.2μm, which can minimize the friction between the locating pin and the tool without affecting the tool alignment accuracy. The clearance between the locating pin and the tool is controlled at 0.005～0.008mm, and the length of the fit is 1.5 times the tool diameter, which can effectively improve the alignment accuracy.

When in use, the positioning pin and the end face of the tool are closely fitted first, and then the positioning pin is slowly inserted into the hole reserved for the finishing feature of the workpiece to complete the precise alignment of the tool to ensure the positional accuracy of the machined feature.

Experimental design and validation

1. Experimental program design and implementation

To fully verify the feasibility and effectiveness of the designed fixture structure design scheme, the actual machining experiments are carried out at a CNC machining center.

The experiment is carried out in a constant temperature and humidity workshop with a temperature of 20℃±2℃ and a relative humidity of 50%±5% to ensure that the influence of the experimental environmental parameters on the machining accuracy is within the controllable range.

The CNC machining center is selected from an imported vertical machining center, in which the positioning accuracy of the X, Y, and Z axes is better than 0.005mm, and the repeat positioning accuracy is better than 0.003mm, which has high motion accuracy and trajectory accuracy, and can meet the requirements of high-precision machining of thin-walled parts.

In addition, the tools used in the experimental process are all high-precision end mills of national brands, and the radial runout and axial runout of the tools are all controlled within 0.005mm after inspection.

The experimental program design selected an automotive engine head cover as the experimental object, the material is A380 aluminum alloy, with an overall size of 600mm × 400mm × 120mm, and the thinnest wall thickness of 2.5mm. experiments focus on examining the design of the fixture and the traditional fixture in different processing conditions on the machining quality of thin-walled parts.

To comprehensively assess the performance of the fixture, the experiment selected roughing, semi-finishing, and finishing three stages designed orthogonal experiments, spindle speed, feed rate, and depth of cut were selected as the factors to be examined, and three levels were selected for each factor, with a total of 27 groups of experiments implemented.

The experimental evaluation indexes mainly include workpiece deformation, key feature dimensional accuracy, surface roughness, and machining efficiency.

Among them: workpiece deformation is obtained by measuring the shape change of the workpiece before and after machining using a coordinate measuring machine; key feature size accuracy is obtained by measuring the difference between the actual value and the design value of 20 key dimensions on the workpiece; surface roughness is obtained by using a surface profiler; machining efficiency is obtained by recording the machining time of each experiment.

2. Analysis of experimental results

The experimental results are shown in Table 1. In 27 groups of orthogonal experiments: the maximum deformation of the workpiece using the designed fixture is controlled within 0.08mm, which is 52.9% lower than that of the traditional fixture of 0.17mm; the accuracy of the key feature dimensions is improved from 0.026mm to 0.014mm, which is 46.2% higher, and all 20 feature dimensions satisfy the tolerance requirements of the drawings; the surface roughness is reduced from 0.92μm to 0.63μm, a reduction of 31.4%; the average machining time is shortened from 68min to 55min, a reduction of 19.1%, which improves the productivity.

The experimental results show that, compared with the traditional fixture, the designed fixture has significant advantages in workpiece deformation control, machining accuracy guarantee, surface quality improvement, and machining efficiency enhancement, i.e., the designed fixture for thin-walled parts has substantial advantages in the machining of complex thin-walled parts. It has a good prospect for engineering applications.

Table 1 Comparison of machining performance of different fixture programs

Conclusion

For the thin-walled parts in the CNC machining of the workpiece deformation, low machining accuracy, and productivity are not high, so put forward an innovative fixture structure design program. The program uses a combination of multi-point support and local support, datum positioning and local positioning, two-way hydraulic clamping and guide bushings, and other technical means to solve the thin-walled parts clamping problems effectively.

Processing experimental results show that, compared with traditional fixtures, the design of fixtures in the workpiece deformation control, machining accuracy protection, surface quality improvement, and machining efficiency, etc. has obvious advantages, can significantly improve the machining quality of thin-walled parts and production efficiency.

With the continuous development of the manufacturing industry and the increasing requirements for the machining of thin-walled parts, there is an urgent need to develop a more intelligent, precise, and flexible fixture system, so the subsequent attempts to combine intelligent sensing, real-time monitoring, closed-loop control, online compensation and other technologies with fixture design, and to develop intelligent fixture optimization technology based on machine learning.