Dimensional Variations I

It is not possible and not necessary to adhere strictly to the dimensions specified in the manufacture of workpieces.

The accuracy of the dimension to be observed in the manufacture is clearly given with the dimension figure. Each dimension figure includes an information about the tolerance (Fig. 4.65.).

Fig. 4.65. 1 deviation

2 tolerance

Tolerance is the allowable deviation from the specified dimension (nominal size)! It is the difference

between the upper and lower limits between which a size must be held; hence, upper and lower deviations. The tolerance is not arbitrary. It is governed by the function of the object and specified by the designer accordingly. He has to take his decision according to the economical principle:

Select the deviations as coarse as possible and as fine as necessary!

Depending on the requirements, the tolerance can be specified according to three possibilities: − Dimensions without specification of a tolerance − for purposes of insignificant requirements (the allowable deviations are given in Tables)

− Dimensions where deviations are given in numerals − for purposes of medium requirements − Dimensions with a coded indication of the deviations (letter−numeral−system) − for

purposes of higher requirements, e.g. for fits (this field will be dealt with in Chapter 7).

4.4.1. Dimensions without Specification of a Tolerance

Dimensions which are not subject to special functional requirements are not provided with any statement for allowed deviations in the drawing. This is a simplification! The deviations applicable to this case are given in Tables. Every designer must know, for example, that, when giving the dimension of 50, he specifies that this dimension has to be observed between the values of 49.7 and 50.3. Every user of the engineering drawing in question must also know this. This way of statement of tolerances must not be used for dimensions from which other dimensions are dependent (assembly dimensions, dimensions of assembled parts).

When using dimensions without the specification of tolerances, an indication is given in the title block of the drawing (Fig. 4.66.).

Fig. 4.66. Permissible dimensional variations for dimensions without tolerance specifications X

bis 6 bis 30 bis 120 bis 315 bis 1000 bis 2000 bis 4000

±0,1 ±0,2 ±0,3 ±0,5 ±0,8 ±1,2 ±2 ±3

x Nominal size range Repetition

1. What is tolerance when manufacturing workpieces? 2. For what reasons, tolerance specifications are required?

3. Considering Fig. 4.6.7., as certain the dimensions which are the limits, the upper and the lower limits, which have to be observed in the manufacture of this object?

Fig. 4.67. x 3 thick 1 Design dimension 2 Upper deviation 3 Lower deviation 1 2 3 3 3,1 8 7,8

4.4.2. Dimensions where Deviations are Given in Numerals

In cases where objects or shapes or their parts in conjunction with other parts have to fulfil certain functions, the dimensional variations are given directly in the form of numerals.

The lower deviation is written behind the dimension figure deep (Fig. 4.68.).

Fig. 4.68. 1 Nominal size

2 Upper deviation 3 Lower deviation

The dimensional variations have a height of 0.7 dimension figure height. But they are not smaller than 2 mm. The complete representation of the basic terms of tolerance are given in Fig. 4.69.

Fig. 4.69. Basic terms of tolerance specification 1 Reference line, 2 Zero line, 3

Tolerance zone 4 Nominal size N 5 upper deviation UD = MS − N 6 lower deviation LD = SS − N 7 smallest size SS = N + LD 8 maximum size MS = N + UD 9 tolerance T = MS − SS 9 T = UD − LD

− The tolerance zone is the field which illustrates the difference between maximum limit and minimum limit. It shows the magnitude of the tolerance and its position with respect to the zero line.

There are five possibilities of the position of the tolerance zone with respect to the zero line which are given in Fig. 4.70.

Fig. 4.70. Possibilities of the location of the tolerance zone with respect to the zero line

The extreme positions or plus−plus and minus−minus can only be understood in connection with the solution of the problem of fits (see Chapter 7).

When manufacture is the main point of view for entering the dimensional variations (the requirements regarding the function are significant), then the following rule bolds:

− Give the tolerances in manufacturing direction! That is to say:

external dimensions which become smaller during machining are provided with a negative deviation,

external dimensions which become larger during the working process are provided with a positive deviation (see Fig. 4.71.).

If, in manufacture, the basic size is to be achieved − it is the next limit −, then the tolerance zone remains to be the reserve for the permissible “inaccuracy”.

Fig. 4.72. shows two examples for the way of entering of dimensional variations in drawings according to points of view of manufacture.

3 Internal dimension becomes larger during machining

Fig. 4.72. Part to be milled and part to be turned in a lathe

The same rule is applicable to the dimensioning of angles between two surfaces (see Fig. 4,73.). Distances are indicated by the ± tolerance (Fig. 4.74.).

Fig. 4.73.

Fig. 4.74.

Fig. 4.75.

When the function is the main point of view of entering dimensional variations in a drawing, then the reference plane is decisive for the kind of dimensional variations (Fig. 4.76.).

Fig. 4.76. Dependence of the statement of dimensional variations on the function (plane of reference) a Example 1, b Example 2, 1 Plane of reference, 2 Internal dimension, 3 External dimension Repetition

1. Determine the six basic sizes of the tolerance specification according to Fig. 4.67. for the dimension 24.3 2. Determine the six basic sizes for any four dimensions shown in Fig. 4.72. and in Fig. 4.75.

3. Give reasons for the signs of the dimensional variations shown at the part to be turned in a lathe in Fig. 4.72.!