Grades of Fit

Grades of Fit


(in machine building), characteristics of the precision of fabrication of parts, assemblies, machines, or instruments; determined by the values of tolerances given in the standards. Grades of fit may be defined for separate geometric parameters of articles (for example, angles, linear dimensions, and gear parameters), as well as for entire articles (for example, for metalcutting machine tools, roller bearings, and so on). Grades of fit of the separate geometric parameters are component parts of the systems of tolerances and fit for standard joints in machine building—such as smooth, threaded, taper, and splined joints—and usually extend to the tolerances of a given geometric parameter in any article.

Grades of fit in most standard systems are determined on the basis of the tolerance unit i, which depends on the nominal size. For example, the unit of tolerance (in microns) for smooth cylindrical joints with dimensions of 1–500 mm is Grades of fit in most standard systems are determined (in the all-Union OST system) and Grades of fit in most standard systems are determined + 0.001d (in the ISO system), where d is the average value of the nominal dimensions (in millimeters) for the given interval, within the limits of which the tolerance is assumed to be constant. In calculating tolerances, each grade of fit corresponds to the selection of a certain number of tolerance units, which make up a geometric progression, usually with the denominator Grades of fit in most standard systems are determined or, less frequently, 1.25 or 2.

Each system of tolerances consists of several grades of fit, which are designated by sequential numbers. Usually the numbers of grades of fit increase with increasing tolerance. For example, the OST tolerance system for cylindrical couplings with dimensions of 1 to 500 mm contains the grades of fit 1, 2, 2a, 3, 3a, 4, and 5, which are usually used for the dimensions of the joint, and 7, 8, 9, and 10, which are used for the noncritical, or “free,” dimensions. Classes of greater precision than 1 have been also established. They are 09, 08, 07, 06, 05, 04, 03, and 02 (in order of decreasing tolerance) and are designed for measuring devices (calipers and gauges) and for parts of high-precision joints (precision bearing fits). The ISO system contains 18 basic grades of fit (sometimes called qualities), which are designated by the numbers 01, 0, 1, 2, . . . , 16.

Grades of fit have been established for some articles as a whole. In this case, standards are developed for the machine, instrument, or assembly that define tolerances of the main performance indicators and of other properties of the article that affect the precision of its performance. For example, the grades of fit of metalcutting machine tools determine the deviations in the dimensions and geometric shape of the surfaces of parts machined on a particular lathe, as well as the limiting errors of the base surfaces of the lathe and the limiting errors in the mutual displacement of the working elements of the lathe. The grades of fit of roller bearings determine the limiting errors in the rotation of the bearings, as well as the finish of their mounting surfaces.

Grades of fit are important engineering and economic characteristics of articles; they define the degree to which the parameters of the article approach their design values. Degrees of fit determine the assembly precision, labor requirements, and production costs, as well as selection of the equipment for production and monitoring. They may also influence the selection of material for the article and its design and other properties.