Dimensional accuracy

Technical requirements for castings are divided into general (regulating dimensional and mass accuracy, surface roughness, mechanical properties, quality of cast metal) and special (assessing tightness, corrosion resistance, wear resistance and other properties). When assessing technical requirements for castings, the relevant GOSTs and OSTs are used.

In addition, a number of special requirements can be reflected in the documentation (for example, in drawings) for the manufacture of cast parts. If it is technically difficult or economically unprofitable to ensure the required dimensional accuracy, surface roughness of the part or other requirements during casting, this is achieved by subsequent machining.

In general, the accuracy of a casting is assessed according to GOST R53464-2009, by its dimensional accuracy classes (DAC) and mass accuracy (MAC), as well as warpage degrees (WD) and surface accuracy (SPA). Mandatory requirements are to ensure that the casting meets the specified dimensional accuracy and mass accuracy classes. The dimensional accuracy of a casting is the degree of compliance between the actual dimensions and those specified in the drawing (or technical specifications). GOST R53464-2009 provides for 22 dimensional accuracy classes — from 1 to 16 (including classes 3t, 5t, 7t, 9t, 11t, 13t). Larger tolerances on the casting dimensions correspond to a higher numerical value of the dimensional accuracy class. If a casting element is formed by two half-molds and is perpendicular to the parting plane, then the tolerance on its size is determined by the dimensional accuracy class of the casting. The same casting elements that are formed by only one part of the mold (or one rod) have dimensional tolerances that are 1-2 classes more accurate than the casting itself. For machined surfaces of castings, symmetrical arrangement of tolerance fields is established, and for unmachined surfaces, both symmetrical and asymmetrical arrangement is allowed. The accuracy of the dimensions of a casting depends on its overall dimensions and complexity of the configuration, the casting method, and the chemical composition of the alloy. The more complex the shape and the larger the overall dimensions of the casting, the greater the error in the manufacture of the casting mold and pattern equipment and, as a consequence, the lower the accuracy of its dimensions. In addition, the enlargement and complexity of the casting configuration contribute to a decrease in the stability of temperature conditions during its crystallization and cooling. Each casting method is characterized by a number of factors that determine the accuracy of the castings. For example, when casting in metal molds (in a chill mold, under pressure, etc.), the accuracy of the castings significantly depends on the quality of the mold, the number of joints in it and the accuracy of the connection of its individual parts, as well as on the temperature of the mold during pouring. The accuracy of the dimensions of castings obtained by the method of casting on lost-foam patterns is determined by the quality of the mold material and the accuracy of its manufacture, the selected pattern composition, and the molding method.

Special requirements for castings arise from their functional tasks and operating conditions. These include ensuring: tightness under low and ultra-high vacuum conditions, as well as in a sufficiently wide range of increased gas or liquid pressure; corrosion resistance in aggressive environments (both liquid and gaseous) at room and elevated (up to 300 °C) temperatures; heat resistance – the ability to withstand cyclic loads caused by repeated heating and cooling; wear resistance under rolling or sliding friction with or without lubrication; dimensional stability under alternating loading or elevated temperatures; decorativeness – the possibility of applying various functional protective coatings to the surface of the casting to improve its marketable appearance and a range of operational properties (corrosion resistance, wear resistance).

The implementation of the above special requirements for castings is achieved by selecting the required composition of the casting alloy, the optimal casting method, mechanical and thermal treatments, as well as the formation of functional protective and decorative coatings on the surface of the casting.