Contents

Classification of Tolerance

Tolerance can be classified under the following categories:

1. Unilateral toleranceย

2. Bilateral toleranceย

3. Compound tolerance

4. Geometric tolerance

Unilateral Tolerance

When the tolerance distribution is only on one side of the basic size, it is known as unilateralย tolerance. In other words, tolerance limits lie wholly on one side of the basic size, either aboveย or below it. This is illustrated in Fig. (a). Unilateral tolerance is employed when precisionย fits are required during assembly.ย

This type of tolerance is usually indicated when the matingย parts are also machined by the same operator. In this system, the total tolerance as related toย the basic size is in one direction only. Unilateral tolerance is employed in the drilling processย wherein dimensions of the hole are most likely to deviate in one direction only, that is, the holeย is always oversized rather than undersized. This system is preferred because the basic sizeย is used for the GO limit gauge. This helps in standardization of the GO gauge, as holes andย shafts of different grades will have the same lower and upper limits, respectively. Changes inย the magnitude of the tolerance affect only the size of the other gauge dimension, the NOT GOย gauge size.

Bilateral Tolerance

When the tolerance distribution lies on either side of the basic size, it is known as bilateralย tolerance. In other words, the dimension of the part is allowed to vary on both sides of the basicย size but may not be necessarily equally disposed about it. The operator can take full advantageย of the limit system, especially in positioning a hole. This system is generally preferred in massย production where the machine is set for the basic size.ย

 Tolerances (a) Unilateral (b) Bilateral

This is depicted in Fig.(b). In caseย unilateral tolerance is specified in mass production, the basic size should be modified to suitย bilateral tolerance.

Compound Tolerance

When tolerance is determined by established tolerances on more than one dimension, it isย known as compound tolerance For example, tolerance for the dimension R is determined byย the combined effects of tolerance on 40 mm dimension, on 60ยบ, and on 20 mm dimension. Theย tolerance obtained for dimension R is known as compound tolerance (Fig.). In practice,ย compound tolerance should be avoided as far as possible.

 Compound tolerance

Geometric Tolerance

Normally, tolerances are specified to indicate the actual size or dimension of a feature suchย as a hole or a shaft. In order to manufacture components more accurately or with minimumย dimensional variations, the manufacturing facilities and the labour required become moreย cost intensive. Hence, it is essential for the manufacturer to have an in-depth knowledge ofย tolerances, to manufacture quality and reliable components economically. In fact, dependingย on the application of the end product, precision is engineered selectively. Therefore, apart fromย considering the actual size, other geometric dimensions such as roundness and straightness of aย shaft have to be considered while manufacturing components. The tolerances specified shouldย also encompass such variations. However, it is difficult toย combine all errors of roundness, straightness, and diameterย within a single tolerance on diameter.

Geometric tolerance

Geometric toleranceย is defined as the total amount that the dimension of aย manufactured part can vary. Geometric tolerance underlinesย the importance of the shape of a feature as against its size.ย

Geometric dimensioning and tolerancing is a method ofย defining parts based on how they function, using standardย symbols. This method is frequently used in industries.

Depending on the functional requirements, tolerance on diameter, straightness, and roundness may beย specified separately. Geometric tolerance canย be classified as follows:

Form tolerancesย

Form tolerances are aย group of geometric tolerances applied toย individual features. They limit the amountย of error in the shape of a feature and areย independent tolerances. Form tolerances asย such do not require locating dimensions. Theseย include straightness, circularity, flatness, andย cylindricity.

Orientation tolerancesย

Orientation tolerancesย are a type of geometric tolerances usedย to limit the direction or orientation of a featureย in relation to other features. These are relatedย tolerances. Perpendicularity, parallelism, andย angularity fall into this category.

Positional tolerancesย

Positional tolerancesย are a group of geometric tolerances thatย controls the extent of deviation of the locationย of a feature from its true position. This isย a three-dimensional geometric toleranceย comprising position, symmetry, and concentricity.

Geometric tolerances are used to indicate the relationship of one part of an object withย another. Consider the example shown in Fig. Both the smaller- and the larger-diameterย cylinders need be concentric with each other. In order to obtain a proper fit between the twoย cylinders, both the centres have to be in line with each other. Further, perhaps both the cylindersย 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 centresย of these two cylinders. This information can be represented in the feature control frame thatย comprises three boxes. The first box on the left indicates the feature to be controlled, which isย represented symbolically. In this example, it is concentricity. The box at the centre indicatesย the distance between the two cylinders that can be tolerated, that is, these two centres cannotย be apart by more than 0.01 mm. The third box indicates that the datum is with X. The differentย types of geometric tolerances and their symbolic representation are given in Table

 Accumulation of tolerances

 Progressive dimensioning

It is essential to avoid or minimize theย cumulative effect of tolerance build-up, as it leadsย to a high tolerance on overall length, which isย undesirable. If progressive dimensioning from aย common reference line or a baseline dimensioningย is adopted, then tolerance accumulation effect canย be minimized. This is clearly illustrated in Fig