Tolerancing dimensions

There are essentially two methods of adding tolerances to dimensions: firstly universal tolerancing and secondly specific tolerancing. In the universal tolerance case, a note is added to the bottom of the drawing which says something like 'all tolerances to be ±0.1mm'. This means that all the features are to be produced to their nominal values and the variability allowed is plus or minus 0,1mm. However, such a blanket tolerance is unlikely to apply to each and every dimension on a drawing since some will be more important than others. Invariably, functional dimensions require a tighter (smaller) tolerance than non-functional dimensions.

A variation of universal tolerancing is where there are different classes of tolerance ranges applicable within a drawing. There are various ways of showing this on a drawing. One way is by the use of different numbers of zeros after the decimal marker. For example, a drawing may say:

'All tolerances to be as follows: XX (e.g. 20) means ±0,5mm,

XX,X (e.g. 20,0) means ±0,lmm XX,XX (e.g. 20,00) means ±0,05mm'

In this case, any dimension on a drawing can be related to one of the three ranges given by the number of zeros used in the dimension value after the decimal marker.

The other method of dimensioning is specific dimensioning in which every dimension has its own tolerance. This makes every dimension and the associated tolerance unique and not related to any other particular tolerance, as is the case with general toler-ancing. Figure 4.14 shows various ways of tolerancing dimensions. The first three are bi-lateral tolerances in that the tolerance is plus and minus about the nominal value whereas the last three are unilateral tolerances in that either the upper or the lower value of the tolerance is the same as the nominal dimension. The use of bilateral or uni-lateral tolerances will depend upon the tolerance situation and the functional performance. Note that, irrespective of whether bi-lateral or uni-lateral tolerancing is used, there are two general methods of writing the tolerances. The first is by putting the nominal value (e.g. 20) followed by the tolerance variability about that nominal dimension (e.g. +0,1 and -0,2). Alternatively, the maximum and minimum values of the dimension, including the tolerance can be given (e.g. 20,15 and 19,99). When dimensions are written down like this either as a tolerance about the nominal value or the upper and lower value method, the largest allowable dimension is placed at the top and the smallest allowable dimension at the bottom.

Normally, a mixture of general and specific tolerances is used on a drawing. The reason is that most dimensions are general and can be more than adequately covered by one or two tolerance ranges yet

Bi-lateral (a)

20,00

Uni-lateral (d) Uni-lateral (e) Uni-lateral (f)

Figure 4.14 The variety of ways that it is possible to add tolerances to a dimension

20,15

there will be several functional dimensions that need specific and carefully described tolerance values. A good example of this would be the pulley bush in Figure 4.1. The bearing internal diameter tolerance would need to be tightly controlled to prevent vibration during high rotational speeds yet the outside diameter and the length could be defined by general tolerances.

Exactly the same principles apply to the dimensioning and hence tolerancing of angles. Indeed, the example shown in Figure 4.14 could just as easily have been drawn using angles as examples rather than linear measures.

Figure 4.5 has shown the difference between parallel, running and chain dimensioning. The important thing about parallel and running dimensions is that they are both related to a datum surface whereas this is not the case with chain dimensioning. When tolerances are added to parallel or running dimensions, the final variability result is significantly different from when tolerances are added to a chain dimension (see Figure 4.15). In the case of chain dimensioning, where each of the individual dimensions is cumulative, if tolerances are added to these dimensions, they too will be cumulative. This is not the case with running dimensions in that when a tolerance is applied to each running dimension the overall tolerances are the same for each dimension. In Figure 4.15, the three steps of the component are dimensioned using chain tolerancing (top) and running tolerancing (bottom). The shaded zones on the right-hand drawings show the tolerance ranges permitted by

Effect of Chain Tolerancing

Effect of Running Tolerancing

Figure 4.15 The effect of different methods of tolerancing on the build-up of variability that particular method of dimensioning. In each case the tolerance on each dimension is ± 1mm which is very large and only used for convenience of demonstration. Thus, with chain tolerancing, the final tolerance value at the end of the third step will be ±3mms whereas with running tolerances it will only be ± 1mm.

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