Page 272 - Aeronautical Engineer Data Book

218 Aeronautical Engineer’s Data Book
15 35

25

10

B
A
Note how the datum servics, A, B are shown
Fig. 12.3 Datum surfaces

itself rather than an ‘imaginary’ surface. This
means that the datum surface normally plays
some important part in the operation of the
elements – it is usually machined and may be a
mating surface or a locating face between
elements, or similar (see Figure 12.3). Simple
machine mechanisms do not always need
datums; it depends on what the elements do and
how complicated the mechanism assembly is.
A tolerance is the allowable variation of a
linear or angular dimension about its ‘perfect’
value. British Standard BS 308: 1994 contains
accepted methods and symbols (see Figure 12.4).

12.4 Toleranced dimensions
In designing any engineering component it is
necessary to decide which dimensions will be
toleranced. This is predominantly an exercise
in necessity – only those dimensions that must
be tightly controlled, to preserve the function­
ality of the component, should be toleranced.
Too many toleranced dimensions will increase
significantly the manufacturing costs and may
result in ‘tolerance clash’, where a dimension
derived from other toleranced dimensions