This is highly essential in graphical language as it affords one knowing the actual size of an object. Also it affords one of the opportunity of easy construction or fabrication of the object. Dimension falls into two part; namely.
I The Technique of dimensioning. II The Selection of dimensions.
I. THE TECHNIQUE OF DIMENSIONING: It is use for correct drawing, lettering and positioning of dimension on a drawing.
II. THE SELECTION OF DIMENSIONS: It is use to ensure the corect functioning of the part as well as enabling the workman to make it without having to calculate any sizes. In other words, the former is called DATUM dimensioning while the latter is known as CHAIN dimentioning.
Dimensions are placed so that they can be read from the bottom and right hand side of the drawing and above dimension lines. Except in very few instances (such as diameter and radii) dimension should be made outside the drawing and smallest dimension should be placed nearest the outline of the drawing.
RULES IN DIMENSIONING
(a) Dimensions must be placed outside the outline of the view wherever possible. (b) There should preferably be a small gap between the outline and the start of the projection line.
(c) Dimension line must be thin and not thick.
(d) Centre lines or their extension must not be used as dimension lines. (e) Dimension figures should be placed normal to the dimension lines. (f) Dimension lines should be placed on the view which shows the features to which they refer clearly
(g) Dimensions must never be cramped.
(h) The smallest dimension should be placed nearest to the outline to avoid dimension and projection lines crossing.
(i) It is note worthy that the dimension be placed centrally or above the extension of one arrow heads.
(j) Dimension less than unity should be preceeded by a zero.
Angles are dimensioned by drawing a circular arc with its centre on the point of the angle.
Linear dimensions should be placed as shown above depending upon the space available. Dimensions should not be crowded together.
Sloping dimension lines should have the figure placed. Dimension should not be placed inside 30′ shaded sector.
Circles are dimensioned by their diameter. The symbol ( ) should be placed before the figure.
Arcs and incomplete circles are dimensioned by their radii, the letter R should be placed before the figure.
Squares are dimensioned by placing the symbol (¤) infront of the figure.