ISO Tolerances

Dimensional tolerances define the permissible actual dimensions of a part in relation to its nominal dimensions. The determination of tolerances and fits for machined workpieces arises from the requirements for interchangeability in mass-produced parts, the inaccuracy of manufacturing methods, and the fact that precise measurements are not necessary for all dimensions of workpieces. To achieve the desired fit, the workpiece must be manufactured so that the tolerated dimensions are within two permissible limit values. Tolerance is the allowable amount of variation in dimensions so that the functional impact of the fits is achieved.

To obtain the required clearance or interference fit between the workpieces being assembled, either a positive or negative deviation from the nominal dimension is required. The internationally accepted system for indicating length dimension tolerances is the ISO 286 standard. It provides a system of tolerances and deviations suitable for two types of dimensional elements: "cylinder" and "two parallel opposite surfaces". The main purpose of the system is to achieve a functional fit.

The terms "hole," "shaft," and "diameter" are used for cylindrical dimensional elements (e.g., diameter tolerancing of a hole or shaft). For simplicity, these terms are also used for two parallel opposite planes (e.g., tolerancing of the thickness of a key or the width of a groove). Application of the ISO system of tolerance notation for length dimensions to fits formed by elements requires that the nominal dimensions of the hole and shaft are identical.

A fit can be defined in two different ways, either empirically or computationally, using the allowable clearances and/or interferences derived from the functional requirements of the parts to be assembled and manufacturing capabilities. More characteristics than the dimensions and their tolerances of the parts to be fitted together influence the fit operation. Other effects must be considered to fully define the fit technically. Such effects might include form, orientation, and location deviations, surface properties, material density, operating temperature, heat treatment, and the materials of the parts to be fitted together. Form, orientation, and location tolerances of the dimensional elements to be fitted together may be needed in addition to dimensional tolerances to control the intended function of the fit.

In selecting the fitting system, the first decision is whether to use the "hole basis system" (hole H) or the "shaft basis system" (shaft h). The choice of the fitting system is based on economic reasons and does not involve technical differences related to the functionality of the parts. The "hole basis system" is the most commonly used. This choice can avoid an unnecessarily large number of tools (e.g., reamers) and measuring instruments. The "shaft basis system" should only be used when it has undisputable economic advantages (e.g., when it is necessary to fit several parts with different deviation dimensions into a drawn steel shaft without machining it).

All elements of workpieces always have size and geometric shape. The functioning of the workpiece requires limitations on dimensional and geometric deviations (form, orientation, and location); in the absence of such limitations, this functioning compromises. Tolerancing should be fully presented in drawings to ensure the size and geometry of all elements are controlled and nothing is left ambiguous or up for interpretation during manufacturing or inspection. The use of general tolerances for size and form can ensure this requirement is met simply.

The purpose of standard ISO 2768 is to simplify drawing notations, and it defines general geometrical tolerances that can limit elements appearing in drawings without separate tolerance notations. It outlines three general tolerance classes for geometrical tolerances and is primarily used for parts manufactured by machining. When selecting a tolerance class, the corresponding usual manufacturing precision must be taken into account. If smaller geometrical tolerances are required or larger geometrical tolerances are acceptable and more economical for a given element, such tolerances should be indicated directly per standard ISO 1101. General geometrical tolerances apply to all other geometrical tolerances except for cylindricity, line profile, surface profile, angularity, coaxiality, and total runout.

The Camcut ISO tolerance table can be used to find limit and basic deviation values for nominal dimensions according to the SFS-EN ISO 286 standard.