Description
A Scale can be used if a fullsize (or truesize) drawing would be too big to fit on the paper being used or where drawing at larger than fullsize is useful to show greater detail (e.g. when designing a circuit board). They can also be used to convert units of measurement (e.g. objects measured in inches drawn fullsize, but in millimetres on the drawing).
Drawing at fullsize is said to be drawn at a ratio of 1 : 1 (i.e. 1 unit in reality is drawn as 1 unit on paper). For a drawing to be halfsize 1 unit in reality would be equal to ½ size units on paper, so would have a ratio of 1 : 2, quartersize would be 1 : 4 and so on. The ratios are derived from a Representative Fraction which is given as:
Features and Applications
Plain Scale

 Start by deciding how many units long the scale needs to be (e.g. the longest dimension on the drawing).
 Draw a line to represent this length to scale (e.g. half actual size)
 From one end, a second line at an angle to it.
 Step off along this second line equal divisions of any convenient size up to the number of units required.
 Join the end of the last division to the end of the first line and draw lines parallel to it through all other divisions as shown.
 Subdivide the first scale division as required using a similar procedure and start numbering the scale from the end of this.


Diagonal Scale

 It is not practical to create very small subdivisions on Plain Scale, hence the need for a Diagonal Scale.
 Begin by creating a Plain Scale as above but draw a rectangle equal to the length of scale and any convenient height.
 Create the major subdivisions as before (e.g. ¼, ½, ¾, or cm, etc.) and extend the divisions vertically.
 Divide the height of the rectangle into as many minor subdivisions as are needed (e.g. ⅛,⅜,⅝,⅞ or mm, etc.) and draw horizontal lines through the points.
 Draw diagonals to each of the vertical rectangles created by extending the subdivisions.
 Half way up, a diagonal would cross the rectangle at half its width and so on, thus providing the minor subdivisions (see the example measurement shown of 3.450 metres).


Note:
There are occasions when drawing accurately to scale can provide more than a good representation, or model, of the finished product. Scale drawings can be measured from and used to transfer sizes of a complex shape on to a large sheet of material for example, but in general and with the advent of CAD, measuring off an Engineering Drawing is not as common as it once was  and has never been satisfactory for really accurate work.