Classification of Tolerance
Tolerance can be classified under the following categories:
1. Unilateral tolerance
2. Bilateral tolerance
3. Compound tolerance
4. Geometric tolerance
When the tolerance distribution is only on one side of the basic size, it is known as unilateral tolerance. In other words, tolerance limits lie wholly on one side of the basic size, either above or below it. This is illustrated in Fig. (a). Unilateral tolerance is employed when precision fits are required during assembly.
This type of tolerance is usually indicated when the mating parts are also machined by the same operator. In this system, the total tolerance as related to the basic size is in one direction only. Unilateral tolerance is employed in the drilling process wherein dimensions of the hole are most likely to deviate in one direction only, that is, the hole is always oversized rather than undersized. This system is preferred because the basic size is used for the GO limit gauge. This helps in standardization of the GO gauge, as holes and shafts of different grades will have the same lower and upper limits, respectively. Changes in the magnitude of the tolerance affect only the size of the other gauge dimension, the NOT GO gauge size.
When the tolerance distribution lies on either side of the basic size, it is known as bilateral tolerance. In other words, the dimension of the part is allowed to vary on both sides of the basic size but may not be necessarily equally disposed about it. The operator can take full advantage of the limit system, especially in positioning a hole. This system is generally preferred in mass production where the machine is set for the basic size.
|Tolerances (a) Unilateral (b) Bilateral
This is depicted in Fig.(b). In case unilateral tolerance is specified in mass production, the basic size should be modified to suit bilateral tolerance.
When tolerance is determined by established tolerances on more than one dimension, it is known as compound tolerance For example, tolerance for the dimension R is determined by the combined effects of tolerance on 40 mm dimension, on 60º, and on 20 mm dimension. The tolerance obtained for dimension R is known as compound tolerance (Fig.). In practice, compound tolerance should be avoided as far as possible.
Normally, tolerances are specified to indicate the actual size or dimension of a feature such as a hole or a shaft. In order to manufacture components more accurately or with minimum dimensional variations, the manufacturing facilities and the labour required become more cost intensive. Hence, it is essential for the manufacturer to have an in-depth knowledge of tolerances, to manufacture quality and reliable components economically. In fact, depending on the application of the end product, precision is engineered selectively. Therefore, apart from considering the actual size, other geometric dimensions such as roundness and straightness of a shaft have to be considered while manufacturing components. The tolerances specified should also encompass such variations. However, it is difficult to combine all errors of roundness, straightness, and diameter within a single tolerance on diameter.
Geometric tolerance is defined as the total amount that the dimension of a manufactured part can vary. Geometric tolerance underlines the importance of the shape of a feature as against its size.
Geometric dimensioning and tolerancing is a method of defining parts based on how they function, using standard symbols. This method is frequently used in industries.
Depending on the functional requirements, tolerance on diameter, straightness, and roundness may be specified separately. Geometric tolerance can be classified as follows:
Form tolerances are a group of geometric tolerances applied to individual features. They limit the amount of error in the shape of a feature and are independent tolerances. Form tolerances as such do not require locating dimensions. These include straightness, circularity, flatness, and cylindricity.
Orientation tolerances are a type of geometric tolerances used to limit the direction or orientation of a feature in relation to other features. These are related tolerances. Perpendicularity, parallelism, and angularity fall into this category.
Positional tolerances are a group of geometric tolerances that controls the extent of deviation of the location of a feature from its true position. This is a three-dimensional geometric tolerance comprising position, symmetry, and concentricity.
Geometric tolerances are used to indicate the relationship of one part of an object with another. Consider the example shown in Fig. Both the smaller- and the larger-diameter cylinders need be concentric with each other. In order to obtain a proper fit between the two cylinders, both the centres have to be in line with each other. Further, perhaps both the cylinders are manufactured at different locations and need to be assembled on an interchangeable basis.
It becomes imperative to indicate how much distance can be tolerated between the centres of these two cylinders. This information can be represented in the feature control frame that comprises three boxes. The first box on the left indicates the feature to be controlled, which is represented symbolically. In this example, it is concentricity. The box at the centre indicates the distance between the two cylinders that can be tolerated, that is, these two centres cannot be apart by more than 0.01 mm. The third box indicates that the datum is with X. The different types of geometric tolerances and their symbolic representation are given in Table
|Accumulation of tolerances
It is essential to avoid or minimize the cumulative effect of tolerance build-up, as it leads to a high tolerance on overall length, which is undesirable. If progressive dimensioning from a common reference line or a baseline dimensioning is adopted, then tolerance accumulation effect can be minimized. This is clearly illustrated in Fig