Qco-ordinoting size of opening q permitted deviation in opening permitted deviation in component

4.8 The work size

We now have a situation which may be shown diagrammatically, as in 4.8.

It must be borne in mind, however, that neither the bricklayer nor the joiner is likely to achieve 100 per cent dimensional accuracy, and the best that can be done is to specify the degree of inaccuracy that will be regarded as acceptable.

In the present example two trades are involved and the degree of precision to be demanded must be realistically related to the nature of the materials in which they are working. It may be assumed that the bricklayer will set up a rough temporary timber framework as a simple template to which his brickwork may be built to form the opening. For the bricklayer, therefore, an opening which varies in size between x +15 mm and x -0 mm, with x being the co-ordinating dimension may be considered reasonable. In the timber component, however, it will be realistic to accept variation in size between y +5 mm and y -5 mm where y is the laid down work size of the component.

The final assembly of window and brickwork may therefore have two extreme dimensional situations, with a range of intermediate possibilities (4.9).

The selected method of sealing the gap between component and opening must take account of these variables if it is to work in all situations. The sizing of components and the establishment of their work sizes and permitted deviations is a whole field for study in its own right. An obvious instance is that of pre-cast concrete cladding panels with a compressible extruded plastic section providing the weathertight seal between them where, unless great care is taken, the maximum permitted gap between panels may be too great to hold the plastic section in compression, while the minimum permitted gap is too small for it to be inserted. The UK Building Research Establishment's paper on 'Tolerance and Fit' is important reading in this connection.

The above discussion is intended as the most basic of introductions to a complex subject. But it may be seen how the whole of dimensioning practice becomes simplified by the concept of the co-ordinating dimension. The assembly of window and wall will be dimensioned on the location plan as in 4.10, and if he is wise the architect will similarly designate the co-ordinating dimensions on his component drawing (4.11).

q co-ordinating dimension

q co-ordinating dimension

,maximum tolerance maximum opening - minimum component

,maximum tolerance maximum opening - minimum component q co-ordinating dimension y

/minimum tolerance

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