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Chapter 3
DIMENSIONING
Dimensioning refers to the act of giving dimensions. BIS (SP 46: 2003) defines dimension as a numerical value expressed in appropriate units of measurement and indicated graphically on technical drawings with lines, symbols and notes.
The important aspects of dimensioning are as follows:
Units of Measurement The most convenient unit for length is millimetre. In civil engineering and architectural drawing, inch or foot is often used as a unit of length. Angles are shown in degrees.
Symbols are incorporated to indicate specific geometry wherever necessary.
Notes are provided to give specification of a particular feature or to give specific information necessary during the manufacturing of the object.
ELEMENTS OF DIMENSIONING
A line on the drawing whose length is to be shown is called an object line. The object line is essentially an outline representing the feature(s) of the object. While showing an angle, the two lines forming the angle will be the object lines.
Dimensioning is often done by a set of elements, which includes extension lines, dimension lines, leader lines, arrowheads and dimensions. These are shown in Fig. 3.1.
RULES OF DIMENSIONING
- Between any two extension lines, there must be one and only one dimension line bearing one dimension.
- As far as possible, all the dimensions should be placed outside the views. Inside dimensions are preferred only if they are clearer and more easily readable.
- All the dimensions on a drawing must be shown using either Aligned System or Unidirectional System. In no case should, the two systems be mixed on the same drawing.
- The same unit of length should be used for all the dimensions on a drawing. The unit should not be written after each dimension, but a note mentioning the unit should be placed below the drawing.
- Dimension lines should not cross each other. Dimension lines should also not cross any other lines of the object.
- All dimensions must be given.
- Each dimension should be given only once. No dimension should be redundant.
- Smaller dimensions should always be placed nearer the view. The next smaller dimension should be placed next and so on.
- All notes should be written horizontally.
Dimensioning of Circular Features
- A circle should be dimensioned by giving its diameter instead of radius. The dimension indicating a diameter should always be preceded by the symbol ø, Fig. 3.14.
Dimensioning of Spherical Features
Spherical features may be dimensioned by giving either the radius or diameter of a sphere. The symbols SR or S ø must precede the dimension for radius or diameter respectively, Fig. 3.19.
Dimensioning of Square Features
Square features (e.g., a rod of square cross-section) are dimensioned using symbol or SQ as shown in (i) or (ii), Fig. 3.22(a).
USE OF NOTES
Notes are used in technical drawings to give specifications of particular features or some specific information. A note may be a general sentence applied to the entire or some part of the drawing, or a note may be a specific sentence applied to a particular feature. The use of notes in dimensioning of some specific feature is explained below.
- Circular hole Fig. 3.27(a): A hole of diameter 16, drilled to the depth of
- Spot face Fig. 3.27(b): A spot face of diameter 22 on a hole of diameter
- Counterbore Fig. 3.27(c): A counterbore of root diameter 10, top diameter 20 and depth 10.
- Keyway Fig. 3.27(d).
KEYWAY 4 WIDE x 3 DEEP
