DRAWING SCALE

Drawing Scale - The ratio of the linear dimension of an object's element as represented in the drawing to the real linear dimension of the same object's element is known as scale. Full-size drawings, if possible, should be used to show true forms and sizes. If this isn't achievable, the greatest size possible should be utilized instead.

The complete designation of a scale should consist of the word Scale, followed by the indication of its ratio as:

SCALE 1 : 1 for full size

SCALE × : 1 for enlarged scales

SCALE 1 : × for reduced scales

The scale should be specified in or near the title block if all drawings are drawn to the same scale. When more than one scale is needed on a design, only the main scale should be shown in the title block, and all additional scales should be displayed next to the part's item reference number or near the drawings. 

Small items are sometimes drawn larger than they are, while huge components and assemblies are drawn to a smaller scale due to necessity. The scale used in a drawing should always be stated. On a full-size drawing, the scale will be stated as 'ORIGINAL SCALE 1:1'.

When used for manufacturing, drawings should not be scaled, and cautions against the practice are frequently printed on standard drawing sheets, such as 'DO NOT SCALE' and 'IF IN DOUBT, ASK.' A drawing must be properly dimensioned or referenced to ensure that all required sizes are available.

The recommended multipliers for scale drawings are 2, 5, and 10.

1:1 denotes a drawing drawn full-size.

2:1 denotes a drawing drawn twice full-size.

5:1 denotes a drawing drawn five times full size.

Other common scales are 10 :1, 20 :1, 50 :1, 100 :1, 200:1, 500 :1, and 1000 :1.

A scale drawing can be misleading; a component drawn twice full-size will span four times the space of drawing paper as a component drawn full-size, and its actual size may be difficult to understand. It is usual practice to provide a full-size visual view to aid in appreciation, assuming that the drawing is meant to be copied to the same scale and not replicated and reduced by microfilming.

The recommended divisors for scale drawings are also 2, 5, and 10.

1:1 denotes a drawing drawn full-size.

1:2 denotes a drawing drawn half full-size.

1:5 denotes a drawing drawn a fifth full-size.

Other common scales used are 1:10, 1:20, 1:50, 1:100, 1:200, 1:500, and 1:1000.

On a standard drawing sheet, the draughtsman will choose an appropriate scale based on the size of the object to be drawn. Keep in mind that the drawing must clearly display all relevant information and details. A local enlargement of a small area may be useful.

Division Of Lines

The process of dividing a given line AB, which is 89 mm long, into a number of segments is shown in the figure above (say 7). Draw a line AC and divide it into 7 equal sections. To make the appropriate divisions on AB, draw line B7 and, using the tee-square and set-square, draw lines parallel to line B7 across points 1 to 6.

Figure below shows an alternative method.

1. Draw vertical lines from A and B.
2. Place the scale rule across the vertical lines so that seven equal divisions are obtained and marked.
3. Draw vertical lines up from points 2 to 7 to intersect AB.

Diagonal Scale

The method for designing a diagonal scale of 40 mm to 1000 that can be read by 10 mm to 4000 is shown in the figure below. Diagonal scales get their name from the fact that diagonals are drawn in the rectangular area of the scale at the left end. A series of similar triangles result from the diagonals.

1. Draw a line 160 mm long.
2. Divide the line into four equal parts.
3. Draw 10 vertical divisions as shown and to any reasonable scale (say 5 mm) and add diagonals.

Plain Scales

The method of drawing a plain scale is shown in Fig. below. The example is for a plain scale of 30 mm to 500 mm to read by 125 mm to 2500.

1. Draw a line 150 mm long and divide it into 5 equal parts.
2. Divide the first 30 mm length into four equal parts and note the zero position or, the solution.

This method of calibration is widely used in industry, and scales for a wide range of scale ratios can be obtained.