Dimensional tolerances define the permissible actual dimensions of a component relative to its nominal dimension. The ISO tolerance table can be used to find the limit dimensions and fundamental deviation values for a nominal dimension in accordance with the SFS-EN ISO 286 standard.
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ISO TOLERANCES
Dimensional tolerances define the actual sizes permitted for a part in relation to its nominal size. The need to define tolerances and fits for machined workpieces arises from the requirements for interchangeability of mass-produced parts, the inaccuracy of manufacturing methods, and the fact that an exact dimension is not necessary for every dimension of a workpiece. To ensure that the desired fit functions properly, the workpiece must be manufactured so that the toleranced dimensions fall between the two permitted limit values. A tolerance is the permitted amount of variation in a dimension that ensures the functional effect of the fit is achieved.
To achieve the required clearance or interference fit between mating workpieces, the nominal size requires either a positive or negative allowance. The internationally recognized designation system for tolerances of linear dimensions is defined in the ISO 286 standard. It provides a system of tolerances and deviations applicable to two types of dimensional elements: a “cylinder” and “two parallel opposing surfaces.” The primary purpose of the system is to achieve a functional fit.
The terms “hole,” “shaft,” and “diameter” are used for cylindrical dimensional elements, such as tolerancing the diameter of a hole or shaft. For simplicity, these terms are also used for two parallel opposing planes, such as tolerancing the thickness of a key or the width of a groove. Applying the ISO designation system for tolerances of linear dimensions to fits formed by these elements requires the nominal dimensions of the hole and shaft to be identical.
A fit can be defined in two different ways: empirically or by calculation, using permitted clearances and/or interferences derived from the functional requirements and manufacturing capabilities of the mating parts. The performance of a fit is affected by more properties than just the dimensions of the mating parts and their tolerances. Other effects must be taken into account in order to define the fit fully from a technical standpoint. These effects may include, for example, deviations in form, orientation, and location; surface characteristics; material density; operating temperature; heat treatment; and the materials of the mating parts. In addition to dimensional tolerances, form, orientation, and location tolerances may be required for the dimensional elements of mating parts in order to control the intended function of the fit.
When selecting a fit system, the first decision is whether to use the “hole-basis fit system” (hole H) or the “shaft-basis fit system” (shaft h). The choice of fit system is based on economic considerations, and the systems have no technical differences in terms of the functionality of the parts. The “hole-basis fit system” is the most commonly used option. This choice avoids the unnecessary need for a large number of tools, such as reamers, and measuring instruments. The “shaft-basis fit system” should be used only when it offers clear economic advantages—for example, when several parts with different deviations in their holes need to be installed on a drawn steel shaft without machining the shaft.
All elements of parts always have a size and a geometric form. The function of a part requires restrictions on dimensional deviations and deviations in geometric characteristics (form, orientation, and location); without such restrictions, the function is impaired. Tolerancing should be presented completely on drawings to ensure that the size and geometry of all elements are controlled and that nothing is left unclear or subject to interpretation during manufacturing or inspection. The use of general tolerances for dimensions and form provides a simple way to ensure that this requirement is met.
The purpose of ISO 2768 is to simplify drawing specifications. It defines general geometrical tolerances that can limit elements shown on drawings without separate tolerance indications. It defines three general tolerance classes for geometrical tolerances and is primarily used for parts manufactured by machining.
When selecting a tolerance class, the corresponding customary manufacturing accuracy must be taken into account. If tighter geometrical tolerances are required, or if larger geometrical tolerances are permitted and more economical for a particular element, such tolerances must be indicated directly in accordance with ISO 1101. General geometrical tolerances apply to all other geometrical tolerances except cylindricity, profile of a line, profile of a surface, angular location, coaxiality, and total run-out tolerances.
The Camcut ISO tolerance table can be used to look up the limit values and fundamental deviation values for a nominal dimension in accordance with the SFS-EN ISO 286 standard.
