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Distortion of the shape of the component parts leads to a decrease in the operational properties of these parts Thus, in movable joints the deviation of the component parts from the correct cylindrical shape leads to an inadequacy of its movements, rapid wear due to contact over a limited surface. In fixed joints, distortion of the shape leads to uneven tension in the joints, because of this, the strength of the joint, the tightness and the accuracy of the centering decrease. Distortion form also affects labor and assembly accuracy, increases the amount of trimming work affects the accuracy of basing of parts in the manufacture and control. Deviations in the location of surfaces from their nominal values are extremely detrimental to the reliability of the machines, causing additional static and dynamic loads in separate parts and joints, which leads to rapid wear and fatigue failure of the parts. Tolerances location or shape to be set for shafts or openings may be independent and dependent. Dependent location tolerances are set for the items, which are interfaced with the object at the same time on two or more surfaces, and for which the interchangeability requirements are reduced to ensure the collection, ie, connectivity details on all mating surfaces. Dependent tolerance indicated in the drawing or other technical documents value which may be greater than the value of which depends on the deviation of the actual size of the element and / or the base material of the maximum limit. On the example of the shaft of the unified reducer of the MOSSELMASH plant, under the designation H090.040, three variants of the assignment of the dependent tolerance of alignment are considered, which, on the one hand, allows to expand the capabilities of the technological equipment and reduce the cost of processing, and on the other hand, ensure the assembly of connections without fitting. For our example, the use of the third variant of the notation extends the alignment tolerance of the two shaft surfaces from 0.010 to 0.048 mm, which is not only economically feasible, but also allows the use of full interchangeability to be competently used.

The unified system of tolerances and fits is a very important system that provides internationally the interchangeability of products, and is widely used in machine-building and instrument-making industries, in repair and maintenance of machinery and in other fields. The new standard 286-1:2010 of International Organization for Standardization provides an improved international system of tolerances on linear dimensions containing series of tolerances and deviations. It contains the clear indication that the system refers to the two types of dimensional features, namely a cylinder and two parallel planes. In the older version of the standard 286-1:88, to interpret the part feature dimension the rule of outer limits was applied a priori, however, in the international standard 14405-1:2010, the interpretation of dimension on the double-point measurement was established. It means that the accuracy of form deviation no longer depends on the dimension accuracy and the requirements to the form should be established separately. Now, in many cases, normalization only of the diameter tolerance is not enough to control the design and performance characteristics of the joint. Theoretically, as provided by geometrical product specifications, it is necessary to establish the requirements and determine the outer limit; as well as in some cases, it is necessary to normalize the requirements for tolerances of form, waviness and surface roughness. The article describes the changes in the terminology from the perspective of the theory of precision and interchangeability. It is concluded that all minor changes in the terms, concepts and regulations do not affect the overall construction of the unified system of tolerances and fits, but clarification of a number of old terms and introduction of new concepts take place. Most significant is the arising of new link between the standards on geometric characteristics of products based on the principle of the general construction of models of precision and common terminology. Of particular note is a bad translation of English version into Russian language. The scientists working in the theory of precision and interchangeability were not involved in the translation. Most of the newly introduced terms could be interpreted more technically-oriented and more understandable.

The article considers the factors influencing on the quality of manufacture and repair of agricultural machinery from the perspective of application of one of the newest methods of the quality analysis known as Principle of 5 M - men, methods, materials, machines, measures. It is defined that in all five indices the domestic agricultural machinery considerably lags behind the foreign one. The main reasons are the use of cheap raw materials, the lack of modern processing equipment with high precision, the lack of highly skilled and highly paid workers, the designing of machines by means of archaic methods, the small degree of information support on modern technologies and specifics of branch machinery, especially internationally. The role of control in formation of quality of manufacture and repair of agricultural machinery is highlighted. The purchasing manager of a large repair enterprise has to provide in the contract for delivery of incoming production the conditions on the presence of a defined limit of defects with selective check of incoming consignment. For a small enterprise, there is the simple recommendation to take a technical documentation and instruments of control along when going on a warehouse for spares, and to buy only good products of manufacturer or well-established similar products. The main objectives of metrological service in repair enterprises are defined. Modern metrological provision during machine repair is the system ensuring the quality of control that includes: the strict ensuring unity of measurements; the choice of critical control points, which are subject to rigid control; the feasibility study of choice of measuring instruments by criteria of minimization of costs and losses for controlled products; the regular actions on adjustment of measuring instruments, the tests and control during maintenance and repair of agricultural machinery.