GENERAL TOLERANCING AND DIMENSIONING
There are two types of tolerances, namely: General and geometric tolerances. General tolerance is used to control the size of linear and angular features. General linear tolerances are about two to three orders of magnitudes smaller than the linear dimensions. They are established from basic sizes and tolerance grades. Linear sizes include length, width, breadth, height, depth, thickness, arc length, diameter, and radius. Angular tolerance is used to control angular dimension. General tolerances are specified based on the functional requirements for manufactured items. They are usually selected from national and or international standards. The commonly used international standard is the International Tolerance (IT) Grade, defined in ISO 286. This grade identifies 18 tolerance grades of relative accuracy that manufacturing processes can produce for a given dimension. Grades are designated as IT01 to IT16, with smaller numbers representing tighter tolerances. Tolerances are measured in micrometers [µm] in metric units and microinches [µin] in English units. For measuring tools, grades 01, 0, and 1 to 8 are recommended. For components made from metals, grades 7 to 15 are recommended. For large manufacturing tolerances, grades 11 to 16 may be used. Size limits of items depend on both the tolerance grade and the fundamental deviation. The fundamental deviation class defines the relative position of the upper- or lower-limit size of an item from a theoretical reference size called the basic or preferably design size. The classes are designated by letters and symbols. The fundamental deviation classes for holes are A, B, C, D, E, F, G, H, JS, J, K, L, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, and ZC. The rest of the letters, that is, I, L, O, Q, and W, are not used. For shafts, the same symbols are used, but in lower-case letters. For holes classes A to H, the lower deviation is above the basic size, with the lower deviation for H class being zero. For holes having symbols J to ZC, the fundamental deviation is below the basic size. Practically, the lower deviation for holes A to H is the fundamental deviation, and for holes J to ZC, the fundamental deviation is the upper deviation. The fundamental deviations for shafts are opposite to those of holes. That is, for shafts a to h, the upper deviation is below the basic size and having the upper deviation being zero for shaft h. Then, for shafts having symbol in between j and zc, it is above the basic size. The fundamental deviation for shafts a to h is the upper deviation, and for shafts j to zc, it is lower deviation. Please refer to ANSI B4.2 and B4.4M for details. Tables are available in these documents with tolerances of size limits based on tolerance grades and preferred size ranges. There are two basic ways that general tolerance may be specified and these are with symbols or values.
A2.1 SYMBOLIC SPECIFICATION
In symbolic specification, the design size and tolerance grade are indicated. For instance, 40H8 is a symbolic specification of tolerance on a size. In this example, 40 mm is the design or functional size, H identifies the fundamental deviation class, and 8 is the international tolerance grade. The end user of this specification would have to determine the size limits for the design size of 40 mm. In this example, the limit format for 40H8 is 40.039/40.000, where 40.039 mm is the maximum size (upper limit) and 40.000 is the minimum size (lower limit) permitted by H8 tolerance specification.
A2.2 VALUE SPECIFICATION
In value specification, the size range or limits are indicated. The size range specification gives the functional size and the deviations around it. Two formats for size range specification are in use, namely: Unilateral and bilateral. In unilateral specification, the total value of the tolerance is applied to one side of the design size, as shown in Figure A2.1. Bilateral tolerance could be of equal or unequal deviations relative to the design size. Figure A2.2 shows an example of equal bilateral specifications. The limit specification format gives the upper- and lower-limit values of size, as shown in Figure A2.3. The usual practice is to indicate value specification on working drawings and symbols on assembly drawings. The limits format of value specification is more directly related to measurement and inspection and should be preferred.
Figure A2.1. Unilateral tolerance specification.
Figure A2.2. Bilateral tolerance specification.
Figure A2.3. Limits specification.
A2.3 HOLE-BASIS OR SHAFT-BASIS FIT SYSTEMS
An assembly fit is created by combining the tolerances of mating parts and determines the relative clearance or interference between the parts. A fit must be chosen very carefully in order to ensure functionality of an assembly. The type of device and its applications are important factors in determining a fit. Now, several combinations of shaft and hole sizes are possible in defining a fit. So, standardization is an economic advantage because it reduces variety and simplifies design choices. National (ANSI/ ASME) and international standard (ISO) fits have been developed based on hole- and shaft-basis. It is better to select hole-basis fit because it is easier to produce shafts to the required size on this basis. A hole-basis system uses the design size of hole as a reference for tolerance disposition, while a shaft-basis system uses the design size of shaft as a reference. Hole-basis is used when a shaft component has variable cross-sectional sizes along its length or hole component has a constant cross-section along its length. However, the shaft-basis system is very good for manufacturing bright drawn bars. Shaft-basis is used when a shaft component has constant cross-sectional size along its length or hole component has variable cross-sectional sizes along its length. Tables of preferred tolerances and fits are available, so designers need not calculate tolerances and fits from scratch. Table A2.1 shows the recommended metric preferred fits for general applications.
Table A2.1. Preferred fits (ANSI B4.2)