Engineering Fundamentals and Drawing

Selection of Drawing Scale

Drawing scale is the fixed ratio between the length of an object as drawn on paper and its actual length in the field. It is written as, e.g., 1 cm = 5 m, or as a representative fraction (RF) such as 1:500.

Scale = (Length on drawing) / (Actual length in the field)

The scale is selected on the following considerations:

1. Size of the object/area to be represented — Large areas (a whole town, a road alignment of several km) need a small scale (e.g., 1:2500, 1:5000); small objects (a building, a bridge component) need a large scale (e.g., 1:50, 1:100).
2. Size of the drawing sheet — The scale must be such that the whole object fits within the available sheet, leaving room for the title block, legend and margin.
3. Purpose / stage of the work — Reconnaissance and location surveys use small scales; detailed design, working and as-built drawings use large scales. For example: location survey 1:2500, cross-section 1:100 or 1:50, structural details 1:20 or 1:10.
4. Degree of accuracy and detail required — More detail and precision demand a larger scale, since at a small scale fine features cannot be shown or measured.
5. Standard practice / departmental norms — Standard scales prescribed by the concerned department (e.g., cadastral survey 1:1000 or 1:2000, topographic map 1:1000 to 1:5000) should be adopted so that the drawing is comparable and legible.
6. Convenience of reading — A scale that gives a whole number relationship (1 cm = 1 m, 1 cm = 10 m) is preferred over an odd ratio, as measurements can be taken directly with a scale rule.

The chosen scale is always indicated on the drawing, either as a numerical statement or as a graphical scale bar, so that the drawing remains usable even if it is photographically enlarged or reduced.

Methods of Showing Dimensions in a Drawing

Dimensions are shown so that the size of every element is clear without measuring the drawing itself. The following methods are used:

1. Aligned system (aligned dimensioning) — The dimension line is placed parallel to the side being dimensioned, and the figures are written above the line, reading from the bottom or from the right-hand side of the sheet. This is the most common method in engineering drawing.

2. Unidirectional system — All dimension figures are written horizontally, i.e., readable from the bottom of the sheet only, irrespective of the direction of the dimension line. Preferred where dimensions are crowded or the drawing is to be read from one position.

3. Chain (continuous) dimensioning — Dimensions are given end to end in a single line, each dimension referring to the previous one. Used for a series of small distances such as window spacing along a wall.

4. Parallel dimensioning — Several dimension lines are drawn parallel to one another, all measured from a common base or datum line. Useful where many dimensions start from one reference edge.

5. Combined dimensioning — A combination of chain and parallel dimensioning, where overall dimensions are given along with intermediate (sub-divisional) dimensions. Most common in practice.

6. Dimensioning by coordinates — The position of each point is fixed by its distance from two reference axes (x and y). Used for irregular or curved work, and in setting out.

7. Dimensioning by notes / tabular dimensioning — Where the shape is standard or repeated (e.g., reinforcement bars, bolts), the sizes are given in a table or by a written note with a leader line, instead of dimensioning each item separately.

Elements of a dimension: each dimension consists of dimension lines (thin continuous lines with arrowheads at ends), extension or projection lines (drawn from the object to the dimension line), and the dimension figure placed above the line. Dimensions are placed outside the view where possible, are never duplicated, and are not shown on hidden or shaded lines.