Drawing the set
work size of the component’ should avoid the possibility
of error.
Definitions
It may be helpful at this point to summarise some of the
terms referred to in a list of definitions and to add to
them others in common use:
Coordinating plane: Line representing the hypothetical
boundary between two adjoining building elements.
Coordinating dimension: The distance between two
coordinating planes.
Controlling dimension: The key dimension—normally
between coordinating planes—which is the crucial
determining dimension in an assembly and which must
remain sacrosanct while intermediate dimensions may
be permitted some tolerance.
Work size: The actual finished size of a component.
Permitted deviation (sometimes known as
manufacturer’s tolerance): The amount (plus or minus)
by which the finished size of a component may vary
from its stated work size and still be acceptable.
Dimension line: The line drawn between two planes with
a view to showing the dimension between them.
Extension line: The line drawn from a plane which is to
be dimensioned and intersecting the dimension line.
Leader line: The line joining a note with the object which
is the subject of that note.
Some examples
Appendix 5, dealing with non-active lines, gives
examples of the types of dimension line recommended
for different purposes.
The following comments may be helpful in establishing
the correct approach to dimensioning such diverse
drawings as the site plan, primary element general
arrangement plans, general arrangement and assembly
sections, and component and sub-component details.To set out a building it is necessary to establish a datum
parallel to one of the building’s axes. The criteria by
which this datum is selected will vary. Where there is an
improvement line required for the site, or an established
building line, these will obviously be important starting
points. If the site is relatively uncluttered then existing
physical features—boundary fencing, adjoining buildings,
etc.—will be used. In certain specialised structures
orientation may well be the overriding factor.The important thing is that the chosen starting
points should be unambiguous and clearly recognisable
on site.It is better to establish the datum some distance from
the perimeter of the new building so that it may be
pegged in as a permanent record during construction.
The building may then be set out from it by offsets. A
datum which coincides with one of the new building’s
faces will be obliterated as soon as excavation starts.Where the construction is load-bearing the setting-out
dimensions from the datum should be given to the
outside face of the wall. Where the structure is framed
this dimension should be to grid centre-lines.The dimensioning of the plan shown in 4.15is largely
self-explanatory. Note the three strips of dimensions
along the external walls, the string picking up the grid
being the outermost line of the three. Overall dimensions
are included, partly as an arithmetical check for the
dimensioner, partly to aid the estimator.Generally speaking, internal setting out is effected by
judiciously selected strings of dimensions. Where the