XI Международная студенческая научная конференция Студенческий научный форум - 2019

The problem of technological quality assurance of machine parts is solved on the basis of the development of standard technological processes. Since there are countless different details, it is not possible to analyze the quality control methods for each of them. All items are classified by breaking them down by type. This approach turned out to be legitimate and useful, since it is possible to work out the unity of the technological solution for parts of each type outside of their connection with a specific industry. There is a notion of typical details. For example, a gear wheel is found in engineering technology and in instrument making. Nevertheless, despite the huge difference in size, the gear wheel is a typical part, and we can speak of uniform technological methods and features for the preparation of such parts. Therefore, a typical part brings to life a typical technological process.

Typical technological process / standard technology / designed for the most common design solutions of parts, consistently repetitive elements. So, for parts such as shafts, a stepped form is characteristic, which determines the ratio of length to diameter, etc. Therefore, the most convenient is the typical machining in centers, the choice of a certain type of tooling and machine tools. Typical technology is the basis of the surface quality of parts, on which various processing methods can be implemented, taking into account the operational features of the parts. Torsion shafts and shafts operating under flexural-alternating load conditions may have the same technical processing. At the same time, an account should be taken of both hereditary phenomena and the specifics of carrying out finishing operations, which can differ significantly from each other in both cases. So, bending shafts should have a specific surface roughness and subjected to special heat treatment, which in the case of shafts, working on torsion can not provide.

The task of improving the quality of machines should be addressed by improving the quality of all parts, but this requirement cannot be extended to all parts equally. There is a circle of details that most determine the quality of the whole machine. For such parts achieved a very high rates of geometric accuracy. This is achieved by using rigid and precise machine tools using specific processing methods and high-precision measuring devices.

A large group consists of parts such as rings, sleeves and sleeves. The achievement of high quality indicators under production conditions can be considered as a peculiar technological superstructure above the base in the form of a typical process of machining parts.

Body parts have two groups of critical surfaces that determine quality indicators: bearing holes and flat guide surfaces.

These types of parts are the basis for the creation of machines. Details in the form of the above bodies of rotation in the total number of engineering parts 35%, their production accounts for 27% of the total cost of manufacture of all parts; 15% of all parts are body parts, but their production accounts for 53% of the total cost. Thus, the manufacture of the remaining 50% of the parts consumes only 20% of the funds.

For parts such as slabs, geometric quality indicators are crucially dependent on their dimensions. So, for plates-tables of 1120x630 mm, the deviation from flatness does not exceed 6 microns on average, and the deviation from parallelism of the guides and the main plane of the table is within 5 microns.

Basic details in the form of columns. Racks can have exact guides. The quality indicators in the form of geometric characteristics in this case correspond to the deviations for the surfaces of the body parts of the plates and are within 3-5 μm.

For other parts that are less common in mechanical engineering, there are also corresponding quality indicators. The values ​​given do not represent the maximum permissible accuracy of shape and size; they may be higher. However, they show a high level of quality characteristics that is consistently achieved in the mechanical assembly production. In all cases where it is possible to reduce the requirements for geometric accuracy, this should be done for economic reasons. The main technological difficulty in achieving high quality indicators is due to the fact that each element of the technological system, with its operation, introduces its own errors in the overall value of the quality indicator.

One of the methods for assessing the technological impact on the quality indicator is the use of the principles of probability theory. The establishment of correlation dependences allows us to estimate the influence of each of the elements on their total result. However, for such an assessment requires a kind of information obtained as a result of measurements of already produced products. In this case, the effect of man on the technological process for its improvement is significantly weakened.

The calculation and analytical method for determining quality indicators is based on the assessment of the actions of each of the elements of the technological system. In the first approximation, the value of the six elements of the system is estimated even before its operation begins or even before the creation of such a system in the metal.

With the help of calculations and experimental data, the error of installation of workpieces on machine tools, the influence on the geometrical accuracy of a part of elastic displacements of the system, its thermal deformations, wear of cutting tools, the error of their adjustment and the geometrical accuracy of machine tools are estimated. Since each of the named errors represents a vector in space, the addition of errors as vector quantities for technological solutions represents known inconveniences. If we consider the errors as random, and some of them are systematic constants, and take into account the laws of their distribution, then the summation of errors is greatly simplified. The total value of the expected error must be less than or equal to the tolerance on the parameter set by the designer. If the error is calculated by several micrometers, then its components turn out to be significantly less and ensuring them in practice is associated with overcoming significant technological difficulties. Consideration of ways to overcome them is of fundamental interest.

Technological support of quality indicators of parts begins at the design stage. Since the technological inheritance of constructive forms, the designer must imagine a picture of the deformed state of the shaft during processing. For example, hollow shafts with a conical hole are machined “from the hole” i.e. at its base. At the same time, a conical stopper is installed in the shaft hole and further processing is carried out in the centers. Deformation as a component of the total error can be determined by calculation and taken into account when installing blanks on the machine. With a complex shape of the outer surface of the shaft, such a calculation is somewhat difficult and an experiment organized in factory laboratories must come to the rescue. The designer must take into account the specified errors along with machining the part for manufacturability.

The integrity of the responsible surfaces of the shaft is directly related to the choice of material and the conduct of heat treatment. The most appropriate solution for such shafts is the use of steels produced in vacuum, although the disadvantages of the microstructure of the metal, not vacuum melting, caused by poor heat treatment, can be eliminated by heating the high frequency currents of the shaft shafts with air cooling. At the same time, nonmetallic inclusions remain and can be detected as defects on the surface of small roughness. Such defects can be presented in the form of characteristic holes. The opinion that these defects do not affect the operation of kinematic pairs, if the latter have small deviations of the shape, is erroneous. Obviously, in general, the quality of the shaft-bush pair is decreasing.

Great attention should be paid to the choice of blanks and the formation of requirements for them. Even for a typical technology, it is necessary to take into account that the spatial deviations of the shafts after the roughing pass amount to 0.06 from the deviations of the workpiece, and after the finishing pass - to 0.04 deviations arising after the roughing cut.

These data, of course, can vary depending on the rigidity of technological systems, but when ensuring the quality of the shafts should be taken into account. It is impossible to correct spatial errors solely on finishing operations. Moreover, with multi-pass grinding of shafts with a constant feed, the initial error remaining after processing with a blade tool increases constantly, as the difference between the set and actual cutting depths constantly increases.

In order to continuously reduce errors, it is necessary to reduce the feed and depth with each subsequent pass.

In case of centerless grinding, it is often necessary to correct the shape deviation in the form of hereditary three- and five-sided, which is ensured by the rational adjustment of the machines. Therefore, to ensure high requirements for shape deviations, it is not possible to grind workpieces with the same machine setup, for example, with an oval initial error and workpieces with original pentahedrons in cross section (shape deviations are set using round meters). Analysis of machine setups is very convenient to carry out using Fourier series.

Processing shaft, as a rule, carried out in the centers. The resulting hereditary error is very stable. Measures to combat such an error are the use of holes with curvilinear generators, ensuring the necessary ratio of the angles of the center holes and centers, increasing the accuracy of the shape of the center holes. Good results were achieved when grinding center holes, as well as when editing faceted carbide centers with the number of faces 3 or 5.

If to reduce the deviation of the form to an even greater degree, then comes a kind of limit, and the technological system, being conservative, does not provide such a reduction. To further improve the quality of the shafts on this parameter, special methods should be applied.

So it is possible, according to a certain law, to change the circular flow of the grinding of shafts. Another method is the creation of special oscillating systems installed on the table grinding machines, in order to blur the hereditary errors.

The problem of reducing shape deviations turns out to be very difficult, and it is wrong to think that such technological methods as super-finishing can always reduce errors. The harmonic analysis helps to solve the problem of reducing errors.

The industry has accumulated rich experience in ensuring a given roughness as a quality parameter.

However, it is not yet possible to suggest rigorous mathematical dependencies of roughness on many production factors and it is necessary to use empirical formulas. If the geometrical dimensions of the part, its material, the type of lathe, the type of tool and the depth of cut are known, then it is possible to designate the optimum machining conditions to ensure a given roughness. Successfully solved similar problems in the selection of optimal methods for processing blanks according to given parameters of their surface. The use of computers greatly simplifies this work.

Typical technological processes for the manufacture of rings, sleeves, and sleeves are similar to each other. The main technological difficulties of manufacturing these parts is to ensure the requirements for small deviations of the shape of the outer and inner surfaces, small deviations from cylindricity, beating surfaces. Overcoming these difficulties on the background of typical technology is the basis for improving the quality of parts.

Structural elements of parts in the form of holes, grooves generate shape deviations on the responsible surfaces. Such deviations should be overcome on the basis of the calculation of the arising elastic displacements under the action of cutting forces. The latter are chosen on the basis of the consideration that the movements must be less than the tolerance for the deviation of the form.

In the details of the specified type, made on unchanged technological routes, of the same chemical composition, but from blanks produced by different methods, the result is a different level of residual stresses. Heat treatment changes the level of stress, even their sign changes, but the overall conclusion remains the same and must be taken into account in process quality assurance.

The effect of technological inheritance especially should be considered when making the type of rings. Ring blanks made on horizontal forging machines invariably receive the deviation of the shape of the outer surface in the form of an oval. This error is extremely stable, it decreases on all operations of the technological process. Setting the task of improving the quality, you can not ignore the shape of the workpiece. For high-quality rings, it is necessary to limit the deviation of the shape of the blanks. The second condition for improving the quality should be considered the use of clamping devices with fixing blanks on the ends. These activities are quite possible to prevent the transfer of harmful hereditary properties.

The problem of ensuring the quality of parts such as rings, sleeves and sleeves is directly related to the peculiarities of fixing them during machining. Even when securing blanks with distributed loads, the transfer of errors from the outer surface to the inner one is noticeable. Therefore, it is extremely important to ensure small deviations in the shape of the mounting surfaces.

These parts often work under wear conditions, and therefore, compressive stress is preferable in the surface layers. However, due to the variety of processing methods, various combinations of power and thermal factors of the tool impact on the surface being processed, residual tangential stresses, different in size and in sign, appear, which should be taken into account in the technological formation of such a quality indicator as wear resistance.

The question of stresses is directly related to deviations of the shape of the surfaces of rings, sleeves, sleeves. Real surfaces always have a waviness (facet). After turning the workpieces with a diameter of 50-80 mm, a layer with a structure different from that of the base material appears below such a surface. The depth of this layer is 25-50 microns. After heat treatment at the grinding operation, very small deviations of the shape can be achieved. However, it was found that a belt of austenitic grains is located at a depth of 10–12 µm from the surface of the ground sample. The thickness of this belt is different and periodically repeated. Over time, the austenite layer unstable in structure turns into martensite. At the same time, naturally, the volume of material changes (increases). In those places where the austenite layer was wider, there is a greater change in volume (increase), and vice versa. Therefore, the part that had very small deviations of the shape after grinding, gets hereditary waviness. To reduce the shape deviations, it is necessary to treat the surface in question additionally using methods that create compressive stresses, since they slow down the process of austenite to martensite.

One of such methods is diamond smoothing. As a result of this treatment, the deviation of the form for the same period of time is almost 3 times less than after grinding with elboron.

Structural forms of body parts directly affect the heat sink when boring the main holes. The consequence of it is a deviation from alignment. When sequentially boring quality indicators are lower than at the same time. The best results were obtained while simultaneously boring symmetrical parts of the hulls.

Particularly noteworthy is the danger of distorting the shape of the main holes of the body parts when they are mounted on machine tools. For technological support of the quality of body parts in connection with the use of technological equipment, experimental testing of the fixing scheme in the factory laboratories with indication of the fixing forces and coordinates of their application is necessary. The highest accuracy is provided by the fixing scheme, which corresponds to the fixing scheme of the body after its assembly in the finished machine. For parts of other types there are their own methods of quality improvement, and the issue is solved in the same way as it is solved in relation to the details discussed above. In various branches of engineering there is an increased interest in flexible production, including automated, the use of machine tools with program control. In this regard, it is sometimes poured that the issues of technical quality assurance of product quality can be solved only through this so-called new technique. Such a point of view is certainly erroneous. First, these technological systems have almost the same drawbacks as conventional systems, secondly, the scale of their application is small and does not play a tangible role in the total mass of machine parts, thirdly, their reliability is not at that level so that we can talk about sustainable technological processes. However, the trend of development and improvement of such technological systems is obvious. The problem of technical quality assurance of machine parts should be solved with the use of any technological systems in the first place - automatic. With increasing accuracy in the assembly requires a special approach to the assessment of bases as geometric images. Production errors and deformations on the assembly cause significant deviations from flatness, cylindricalness, taper, perpendicularity, etc. Therefore, we should take into account the actual shapes of the base surfaces.

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