Table of Contents

Beam parts are important bearing parts of the aircraft fuselage load, including edge, slot cavity, bars, mitigation holes, and other typical structural features, and most of them are thin-walled parts, there are problems such as poor machining process and easy to be deformed by machining. Milling beam parts often use assembly fixtures for clamping and fixing, the use of bolts and pressure plate clamping parts around, need a lot of time to adjust, find the right and fixed, the clamping process is cumbersome, labor-intensive, production preparation time is long, and at the same time, the milling force is not uniform, easy to produce deformation, which leads to a high rate of scrap of the workpiece. Especially in the finishing process, due to the double-sided beam parts without external support and their rigidity not enough, if the cutting force is large or resonance in the cutting process will cause vibration of the belly plate, resulting in quality problems in the belly plate.

The traditional platen clamping method of low productivity, and processing quality consistency is difficult to ensure and can not meet the process requirements, therefore, there is a need to explore a new CNC machining process to solve the above problems. To solve the defects of low clamping efficiency and inconsistent quality in the traditional machining and positioning methods for double-sided beam parts, this paper designs a fast positioning and clamping milling fixture based on vacuum adsorption technology for finish machining of work-type beam parts to solve the problems of low clamping efficiency and machining deformation.

Adopting the vacuum adsorption principle to design the fast clamping program, when finishing parts, using multiple vacuum adsorption live blocks for vacuum adsorption positioning, while providing support, so that the parts are subjected to uniform clamping force, supplemented by the pressure plate on this basis, can effectively inhibit cutting vibration due to the lack of rigidity in the process of machining, reduce the impact of uneven clamping force on the machining deformation, and improve the clamping efficiency and positioning accuracy. The clamping efficiency and positioning accuracy are improved.
Structure and process of beam parts

Beam parts of the aircraft fuselage are a key part of the modern aircraft, generally used die forging parts machining manufacturing, belong to the more difficult to process large parts. The shape of the beam is mostly a straight line along the longitudinal direction of the fuselage, the internal design of many cavity features, the cavity is generally a flat-bottomed groove with a closed boundary profile, which can ensure the overall mechanical properties of the component while removing excess material, to achieve the lightweight design requirements of the aircraft. To improve structural efficiency, the cross-section shapes of beam structures include U-shape (single-sided beams) and I-shape (double-sided beams), etc. The cross-section shapes of typical double-sided beam parts are shown in Fig 1.

Beam parts are aircraft stress thin-walled parts, generally using high specific strength and specific modulus of titanium alloy, aluminum alloy or alloy steel, and other materials. Due to its special structural characteristics and material properties, there are large differences resulting in a large coefficient of difficulty in processing, technical requirements, and assembly requirements.

Beam parts are generally larger external dimensions, double-sided beams on both sides of the distribution of several groove cavities, parts of the web, bar, edge of the thin thickness of the strip, the cross-section relative to the outer dimensions of the smaller, rigid, easy to cut vibration, machining processability is poor in the machining process is very easy to produce deformation, the size is not easy to ensure that affects the machining quality of the parts, the material removal rate of the large, especially in the finishing stage of the requirements of the process equipment Higher.

Fixture structure design

 

Fixture and workpiece have a strong correlation, according to statistics, 20% ~ 60% of the machining error is caused by the clamping, fixture accuracy (not only positioning accuracy), the fixture structure of the stiffness, and deformation of the machining accuracy of the greater impact. Therefore, the structural design and optimization of the fixture is very important. By reasonably arranging the clamping elements and suitable clamping force, the fixture can realize the positioning, constraint, and support of the workpiece during the machining process to ensure the accurate positioning and firm clamping of the workpiece. Since the vacuum adsorption fixture realizes the vacuum clamping and positioning by the sealing elements, the sealing structure is the focus and key part of the design of the vacuum adsorption fixture.

In addition, in the fixture design process, to ensure the strength and rigidity of the fixture, and according to the requirements of the CNC equipment, allowing the tool to reach as many machining surfaces as possible, or even all machining surfaces, to reduce the machine’s downtime as well as fixtures, tools, workpiece system adjustment time.

1. Working Principle

 

The working principle of a vacuum clamping fixture is: using a vacuum air pump to vacuum the closed cavity formed between the part and the fixture, generating negative atmospheric pressure, the part is tightly adsorbed on the fixture and subjected to a uniformly distributed clamping force. The vacuum clamping system does not damage the clamping surface, does not require adjustment of the workpiece, and significantly reduces the deformation of the part due to uneven clamping force, which can improve the machining accuracy and surface roughness of the part. The clamping of the fixture is realized by the vacuum air pump and several positioning blocks, but the clamping force is mainly provided by the vacuum air pump, by turning on and off the vacuum air pump to realize the clamping and loosening of the fixture on the part.

2. Design program

 

Work-type beam parts need to be double-sided milling, the machining process requires high precision. According to the use of precision requirements the parts, should minimize the deformation, and strive to achieve as many machining processes in a clamping state. The fixture adopts the vacuum adsorption type clamping program, the bottom of the positioning block is fixed on the positioning plate of the fixture body through bolts, and the top is in contact with the part, to realize clamping through vacuum adsorption. Multiple adsorption forces are uniformly distributed in the inner cavity plane of the part, so that the overall force of the part, reduces the deformation of the part.

Due to the increased contact area, under the condition of a certain pressure of the air pump, it can prevent the parts from vibration in the process of machining caused by the problem of low machining accuracy. In the use of fixtures, the separate design of the live block adapts to the different cavity shapes of the parts, thus maximizing the expansion of the use of fixtures. The composition of the milling fixture is shown in Fig 2.

The base of the fixture is a combination of the base plate, support plate, and reinforcement welded together, which mainly plays the role of support and fixation. A certain number of countersunk holes are evenly distributed on the base plate to fix the fixture on the machine table, and 4 holes are designed for lifting bolts to lift the fixture. Considering the overall lightweight requirements of the fixture, the support plate is designed with several oblate relief holes, and threaded holes are designed on the top surface to fix the locating plate as shown in Fig. 2.

The positioning plate is a flat plate designed according to the external dimensions and structural characteristics of the part and plays the role of supporting and fixing several vacuum positioning blocks and parts. Steps are designed according to the height of the part’s rim to avoid interference between the part and the positioning plate.

The plate also contains pin holes and threaded holes to accurately position the vacuum positioning blocks and threaded holes to secure the plate to the fixture base. At the same time, pads are designed to adjust the vacuum positioning blocks according to the height of the rim strips to ensure that all vacuum positioning blocks have the same top surface height.

3. Structural Design of Vacuum Positioning Block

 

The vacuum positioning block is a key component of the milling fixture and is based on the structural characteristics of the beam parts designed for the movable positioning unit, its number, and external dimensions by the number of parts of the slot cavity and the size of the internal shape to determine the structure of the two are generally similar to the dimensions of the external dimensions of the different (see Fig 3).

The upper surface of the vacuum positioning block is the effective area of the vacuum adsorption work, to ensure the adsorption effect, the vacuum positioning block of the machining accuracy and surface roughness requirements are high. Since pneumatic clamping requires high sealing, a sealing groove is designed on the upper surface of the vacuum positioning block, which can be embedded with rubber strips to seal the entire vacuum cavity to avoid air leakage during clamping and machining. The profile of the sealing groove is similar to the inner profile of the part’s slot cavity and is 3 to 5mm smaller than the inner profile of the part’s slot cavity to ensure that the part’s slot cavity is completely covered by the belly plate.

The vacuum positioning block is adapted to the contour of the groove cavity and has a clearance of 2 to 3mm from the groove cavity, and its typical structure is shown in Fig 4. In Fig. 4a, the vacuum positioning block is designed with vacuum extraction holes and bolt holes, both of which are through holes, playing the roles of vacuum extraction and fixing of parts, respectively, and there are vacuum extraction slots and sealing grooves on the top surface of the positioning block. The vacuum pumping groove uniform and vertical cross-distribution, and the adsorption of the slot cavity contour to adapt to the slot width of 1mm, the depth of the slot 0.5mm, vacuum pumping groove, and vacuum pumping holes through the formation of the vacuum channel.

The sealing groove is designed in a circular shape along the periphery, and the width of the groove is determined according to the specifications of the selected sealing strip. This milling jig selects a round sealing strip, diameter ϕ6mm, to ensure the reliability of sealing, sealing groove cross-section is designed as a drum, with a diameter of ϕ6mm drum milling cutter processing from the groove depth of 5mm, sealing groove centerline from the edge of 5.5mm.

There are two pin holes on the bottom surface of the vacuum positioning block, which play a role in positioning. According to the shape and size of the positioning block, the bottom of the vacuum positioning block is generally designed with 6 threaded blind holes (see Fig. 4b) and screwed into the threaded bushings (see Fig. 4c), and the vacuum positioning block is fixed to the positioning plate from the bottom with bolts, and its connection status is shown in Fig. 5.

positioning process and applications

 

End stops, side stops and platens are used to hold and assist in pressing the part as shown in Fig 6. The process of part clamping and positioning is realized as follows:

（1） Assemble the fixture components and place the fixture on the machine table using a lifting device. The datum surface and the two datum holes have been pre-set on the side and ends of the positioning plate, through the percentage table according to the two holes on one side of the positioning method to find the correct clamping, to ensure that the parallelism and perpendicularity with the machine tool table, through the bolts will be fixed on the machine tool table.

（2） Connect the vacuum system. Connect the vacuum holes of all vacuum positioning blocks to the vacuum system through the vacuum interface and connecting pipe.

（3）Place the parts. Embed ϕ6mm rubber strips into the ring sealing grooves on the upper surface of all vacuum positioning blocks, place the semi-finished parts on the fixture so that all the grooves on the first side correspond to the vacuum positioning blocks one by one, and make sure that the web surface of the parts is in full contact with the upper surface of the vacuum positioning blocks, as shown in Fig. 6.

（4）Turn on the vacuum pump to realize rapid positioning clamping and processing of the part. At this time, the workpiece is subjected to downward adsorption force to ensure that the web surface of the part and the upper surface of the vacuum positioning block are adhered to, to realize rapid positioning and clamping.

At the same time, the top surface and the part are fixed by bolts, and both sides are clamped by pressure plates to avoid the adsorption clamping and processing of the part.
Execute the numerical control program to complete the machining tasks such as fine-milling the height of the edge strip, fine-milling the shape of the part, fine-milling the inner shape of the groove cavity and the web, and making holes.

（5）Turn off the vacuum system. After the workpiece processing is completed, close the vacuum system, at this time the suction force is 0, remove the two sides of the auxiliary clamping pressure plate, lifting out the processed workpiece, for the next machining process to prepare or end of processing.

The fixture has been successfully applied to double-sided beam parts finishing, the actual operation is simple, with rapid clamping, reliable positioning of the workpiece, and high productivity. The processing deformation is small, the thickness is uniform, and the quality meets the requirements. Therefore, the application of vacuum adsorption fixtures solves the problems of low machining efficiency and finishing deformation of beam parts. Using vacuum adsorption, the parts are fixed to avoid interference in the machining process, thus completing multi-process machining.

Conclusion

 

This paper analyzes the structural characteristics and process requirements of aircraft beam parts, for the CNC machining of beam parts to design a double-sided beam parts finishing vacuum adsorption milling fixture structure, put forward a vacuum adsorption, fast clamping program.

The milling fixture adopts the vacuum adsorption system to fix the parts, is easy to operate, has reliable positioning, and no clamping stress deformation, in practical application effectively improves the parts in the milling process due to uneven clamping force caused by the processing of deformation problems, to achieve accurate positioning of the workpiece and fast clamping, improve the clamping efficiency, the machining accuracy of the parts and the surface roughness of the parts, the machining of thin-walled parts have reference significance.

In the fixture design process, the vacuum positioning block can change the shape design according to different machining needs, so that the milling fixture is both personalized and universal, and can be widely used in mass production of double-sided beam parts. The use of vacuum positioning blocks to support the web increases the rigidity of the processing system and uniformly distributes the clamping force, resulting in improved machining reliability.