Ensuring interchangeability in aircraft production using cutting stands

. The paper discusses the basic principles and modern views on methods for ensuring interchangeability when cutting joints and docking airframe units, describes the purpose, provides a layout diagram and describes the design of the aircraft fin cutting stand, the equipment used and tools used to ensure the interchangeability of aircraft airframe units at joints.


Introduction
The creation of highly efficient, reliable and competitive aviation equipment places exceptional demands on its quality, as the most important and general indicator of design excellence.The quality of aviation equipment is established at the stage of its development, formulated during production and testing, and implemented during operation.
The production of individual units and assembly of aircraft (helicopters) can be carried out at different factories and even in different countries, but regardless of this, all individual units (wing, tail elements, fuselage sections, takeoff and landing and some other devices) must be interchangeable.Therefore, the interchangeability of units is one of the most important indicators in the formation of quality in aircraft production and, at the same time, the most important technical problem, the solution of which allows not only to better organize production, but also to reduce its time and to some extent improve repairs in the process operation [1][2][3].
The correspondence of mass-produced units in geometry, aerodynamic shape, structural rigidity and weight allows the flight characteristics obtained on the prototype to be retained on production aircraft.
The interchangeability of aircraft (helicopter) structural elements usually includes requirements for identical dimensions and shape, weight, alignment, strength and rigidity, and connection of aircraft structural elements without additional processing.Interchangeability requirements must be taken into account in all aircraft production processes, starting with the manufacture of parts, assembly of components, compartments and assemblies, and installation of systems in compartments and assemblies.
If the dimensions and shape of rigid parts and joint assemblies do not change under the influence of their own weight or joining processes, then the main condition for ensuring interchangeability is the rational choice of tolerances and fits for the connected elements of the units.Processing and assembly of such elements of connectors and joints are carried out independently of each other.
When manufacturing insufficiently rigid assemblies, connectors and joints, it is not possible to apply to them the tolerances, methods and measuring instruments used in general mechanical engineering.This is explained by the fact that rigid units retain their dimensions and shape unchanged regardless of their position in space, while the dimensions and shape of elements of connectors and joints of low rigidity, depending on their position in space and connections with other parts of the unit, can change [4,5].
In aircraft construction, as a rule, a coherent system of linking shapes and sizes is used in the manufacture of an aircraft airframe, which is assembled from a fairly large number of long and low-rigid parts, which, under the influence of their own mass, distort the required shape and location of individual surfaces.The independent system is common in the manufacture of relatively rigid and small-sized structures -engines, chassis and the like, the parts of which do not lose their shape under the influence of their own gravity.Coordination of the shapes and sizes of parts of components and assemblies is carried out using various carriers of shapes and sizes, assemblies and joints.These can be hard media in the form of flat and spatial templates and standards, or digital models.

Materials and methods
When assembling units and mutual assembly-docking, it is possible to fully or incompletely ensure interchangeability.With complete interchangeability of connectors and joints, the units must be connected without modification or adjustment.This is achieved by the fact that the tolerances on the links of the dimensional chain are calculated using the maximum and minimum, or by the probabilistic method.However, in the aviation industry, for the assembly of large-sized units, the method of incomplete interchangeability is widely used, in which, during the assembly-joining process, mechanical modification of the mating surfaces of connectors and joints is carried out.This method of producing units is dictated by economic considerations, since the cost of manufacturing units as a whole is reduced due to the expansion of the tolerance range on their main dimensions.
In the production of units with incomplete interchangeability, the method of adjusting the closing link is often used, which is implemented in several ways.A method of mechanical modification of the closing link, when the mating surfaces of the parts of the units are manufactured with wide tolerances and subsequently modified by machining.For airframe units this is done in cutting shops.Another method involves adjusting the closing link in connector and joint assemblies using various types of compensators in the form of structural elements (corners, overlays, profiles), eccentrics, cardans, floating anchor nuts, gaskets, and the like.Compensators allow you to eliminate errors in linear and angular dimensions, as well as various deviations of the axes of the connected units.
Issues related to interchangeability are resolved during the design of the aircraft and its manufacture at the aircraft manufacturing plant.At the same time, in order to meet production and operational requirements, the airframe is divided into units, compartments, and units; provide connectors for dividing systems and cutouts for access to hard-to-reach places.As a result of the division of the structure, a scheme for ensuring interchangeability is drawn up, reflecting the connection between individual units, compartments and systems of the aircraft.
The development of technological processes for assembling units and manufacturing assembly equipment is carried out in accordance with the scheme for ensuring interchangeability and requirements for the relative position of units and the accuracy of their contours.Mutual linking of joints and connectors of engines, instruments and equipment with joints and connectors on the airframe is also carried out according to the scheme for ensuring interchangeability [6,7].
The implementation of a scheme for ensuring interchangeability makes it possible to reduce labor intensity and shorten the cycle of assembly work, creates conditions for the cooperative production of aircraft and the possibility of quick replacement during operation of damaged or worn-out units.
To ensure the interchangeability of aircraft airframe compartments and assemblies, after their final assembly, mechanical modification of connectors and joints is carried out in special cutting stands.These stands eliminate various types of manufacturing errors in the actual dimensions of assembled units in butt joints and holes for butt bolts, the occurrence of which is associated both with deformations of the assembled product itself and with deformations of some elements of the assembly fixture.These errors are one of the main reasons for the violation of the interchangeability of compartments and units at connectors and joints.
When forming the most common fork and flange joints, two options for joining compartments can be distinguished.In one of the options, holes for butt bolts (OSB) and butt planes located on individual parts are processed to the final size during their manufacture using a jig plate (KP), then these joint parts are sent for assembly into a fixture and then the assembled compartments are sent for joining .
In the second case, the OSB holes and joint planes are processed on the jig plate only preliminary, that is, not to the final size, but taking into account the allowances of the parts for further processing of the joint, and then they are delivered to the assembly slip.After assembly, the compartment enters the cutting bench for processing of OSB and butt joints, and only then the compartments are joined.
Assembly without cutting is used for rigid monolithic joints or installation of butt bolts with a large gap.In the case where the joint has large dimensions (diameter) and consists of several sections or butt assemblies, cutting the joint in a cutting stand is a mandatory operation [8][9][10][11].
In the production of airplanes and helicopters, special and universal cutting stands are used.A special stand is designed for processing one specific standard size of compartment or unit, a universal stand is designed for processing similar groups of compartments and units.When switching from processing one standard size compartment (unit) to another, the stand is not completely dismantled, but only reconfigured.
The processing of holes and grooves in connectors and joints is carried out in several transitions, the number of which depends on the grade of material from which the joints are made, and the required accuracy in the formation of holes and grooves.The compartments and units processed in the stands are interchangeable and the joining of such units is reduced only to operations related to the installation of butt bolts, connection of system communications and adjustment of control rods passing in the joint or connector area.
The most common operations performed at the cutting machine are milling joint planes, milling grooves for butt bolts, cutting holes, counterbore sockets for bolt heads and nuts, and others [12].
A cutting stand in its general form is a spatial structure that consists of base elements for installation, basing, fixing and securing the product, conductors and metalworking special and universal machine heads.Specific parts of the cutting stand are holders, jig heads, and a fixing ring.These parts are manufactured for each standard size of the assembled compartment or unit.The remaining parts and elements of the stand are not related to the design of the product being processed; they are standardized or manufactured according to drawings using mechanical engineering methods.The process of manufacturing the base elements of the stand depends on the method of installation of the stand and the principle of linking technological equipment during the manufacture of parts and assembly of the compartment.
When using the standard-template method for installing the assembly fixture for the compartment, the cutting stand is assembled according to the installation standard.The contours of switches and supports are obtained according to the surface standard (ETS).Switches and holders with pre-processed (cast) contours and drilled mounting holes (MO) are delivered to the ETP.Using the ETP method, the contours of switches and supports are obtained using the cast method, and according to the installation standard, holes are drilled into them for installation base holes (UBO).
According to the installation standard, OSB holes are formed in the jig head blank and the assembly fixture joint blanks.After processing, the switches and joint assemblies are sent for installation of the assembly fixture (JF), and the holders are sent for installation of the cutting stand.Next, the assembly fixture is installed, and then the compartment is assembled in it.The assembled compartment with pre-drilled OSB, pre-processed joints and drilled UBO holes enters the cutting bench.
When using optical-laser measuring instruments, the bases are the mounting holes in the forks of the beams into which target signs are inserted.Switches, clamps, a cutting stand plate and mechanisms for moving the clamps are installed on the beams.
As an example, consider a cutting stand for joining aircraft fin assemblies (Fig. 1).The basic axes of this cutting stand are: the axis of the rudder (RP), the axes of the fuselage frame, the axis of the side rib, as well as the axes of the front and rear stringers of the unit.Tolerances for non-parallelism of the axes of the machine and stand are about 0.02 mm per 1000 mm of length.
At the stand, cutting operations are carried out -processing the end surface, that is, removing the allowance and cutting holes for butt bolts.They process the joint with an end mill, installing it in the machine spindle, and also countersink and deploy the OSB.
First, the cutting stand is cleaned of chips and the keel is installed and secured into it using clamping mechanisms.Check the serviceability of the conductor units.The clamps should move smoothly without jamming [13][14][15][16].
The processing is carried out through replaceable jig bushings, and the jig bushings, cutting and control tools are changed in accordance with the accepted processing process.To link the dimensions and shape of the assembled product with the elements of the cutting stand, it is necessary to include in the equipment linkage diagram a chain of dimensions associated with the manufacture of the cutting stand slab [17].The requirements for connectors and joints in terms of accuracy and quality of surface treatment depend on the accuracy and nature of the fit of the bolts in the joint holes.The most common installation of bolts in holes is H7/f7, H8/h8.

Research and results
In table 1 shows the requirements for joints and connectors when fitting H8/H8 bolts.It indicates in letter designations the dimensions of the geometric elements of the joints or connectors, in particular: D -the internal diameter of the holes, K and I -the width and length of the groove, F -the width of the plug groove, f -the width of the eye, L -the outer size of the plug.The tolerance field of the made round holes corresponds to H8, the grooves -H11, the dimensions of the mating surfaces of the ear-plug connections -d11 and H11, and the outer size of the plug -h12.
The processing of holes and grooves in connector and joint assemblies is carried out in several transitions, the number of which depends on the grade of material from which the joint is made and the required accuracy when processing grooves and holes [18].In table Figure 2 shows as an example some diameters of drills, countersinks and reamers used along transitions when processing components made of aluminum alloys and steel 30KhGSA.Note -the surface roughness of the Ra2.5 and Rz20 holes is achieved when they are cut with reamers, and Rz80 -when drilling.
Cutting is carried out in the following sequence.Install and fix the unit in the cutting stand.After performing all the necessary control operations, the holes are cut through the conductor bushings located on the conductors of the holes of the RP hinge and the holes for attaching the keel to the fuselage at the front and rear points.Through the bushings, countersinking (with two countersinks) and deployment (with two reams) are sequentially performed.In conclusion, produce control of the position and dimensions of the holes made, for which mandrels with pins of the corresponding diameters, made with an accuracy of H8, are inserted into the holes.
The described sequence of operations is observed in the processing of each of the holes of the OSB.The design of the conductors and working tools used depends on the design of the butt assembly being processed.The structural diagram of the conductor for cutting the hole in the keel assembly at the rear point of its attachment to the fuselage is shown in Fig. 2.  The design of the conductor shown in Fig. 2 makes it possible to ensure the required dimensional accuracy and surface roughness of the holes, and to reduce the effort during their deployment.For greater processing accuracy and reduced effort during deployment, guide bushings are used in the jig; holes are machined with countersinks and reamers with special floating grips.
With such grips, the accuracy of hole processing depends on the accuracy of the installation and direction of the tools in the jig, and not on the accuracy of the installation of the axis of the power head relative to the axis of the hole.The accuracy of processing the diameter of the hole and the accuracy of the size between the centers of the holes depends on the magnitude of manufacturing errors that arise when processing the hole.

Conclusion
Thus, the errors consist of errors in processing the holes and jig bushings of the cutting stand, errors caused by the presence of gaps between the jig bushing and the diameter of the tool, as well as the misalignment of the tool in the jig bushing.
The final value of the error in the location of the centers of the holes in the unit depends on the design of the joint plate, jig heads and the method of basing the compartment and unit when installed in a cutting stand.
In cases where it is necessary to fit butt bolts into both OSB holes of the joined compartments with an interference fit, it is necessary to carry out joint cutting of the OSB when joining the compartments.

Fig. 2 .
Fig. 2. Conductor for cutting holes in the keel assembly, rear attachment point to the fuselageThe scheme and the main characteristics of the used drilling-milling head used to perform the operations of drilling and countersinking holes, milling planes are shown in Fig.3.

Table 1 .
Requirements for connectors and joints in terms of accuracy and quality of processing

Table 2 .
Diameters of the tool for processing bolt holes