Engineering Drawing

Selection of Drawing Scale

The 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 selected as follows:

1. Size of the object and size of the drawing sheet — The scale is chosen so that the whole object (or the required portion) fits within the available sheet with space left for title block, notes and margins. A large area needs a small scale; a small component needs a large scale.

2. Required accuracy and purpose of the drawing — Working/construction drawings need larger scales so that dimensions can be read and measured accurately, while location or index plans may use small scales.

3. Standard (recommended) scales — Only standard scales are used so that readings are simple and comparable. For engineering drawings these are:

   • Reducing scales: 1 cm = 10 m, 1 cm = 20 m, 1 cm = 50 m, 1 cm = 100 m, etc. (for maps and site plans)
   • Enlarging scales: 10 cm = 1 cm, 5 cm = 1 cm, etc. (for small details)
   • Full size: 1 cm = 1 cm (for details drawn actual size)

4. Representative Fraction (RF) — The scale is expressed as RF = length on drawing / actual length. The RF must be kept as a convenient fraction (e.g. 1/100, 1/500) so that the drawing can be measured directly with a scale rule.

5. Availability of space and number of views — If several views, sections and notes must be shown on one sheet, a smaller scale is adopted; if only one detail is shown, a larger scale is used.

6. Convenience of the user — The scale should allow the drawing to be easily read, carried and reproduced (photocopying/scanning) without loss of clarity.

Methods of Showing Dimensions in Drawing

Dimensions are shown by dimension lines with arrowheads, extension (projection) lines and the dimension figure. The main methods are:

1. Aligned system — The dimension line is placed parallel to the side being dimensioned, and the figures are written parallel to the dimension line, i.e. they are aligned with it. The figure is read from the bottom or from the right-hand side of the drawing. This is the method generally used in engineering drawing.

2. Unidirectional system — All dimension figures are written horizontal, irrespective of the direction of the dimension line, and are read from the bottom of the sheet only. This is preferred where dimensions are numerous and closely spaced.

3. Chain (continuous) dimensioning — Dimensions are placed end to end in a single line, each dimension being measured from the previous one. It is used where a series of small dimensions must be shown.

4. Parallel dimensioning — All dimensions are measured from one common base line (datum), the dimension lines being drawn parallel to one another. It avoids the accumulation of errors and is used for accurate work.

5. Combined dimensioning — A combination of chain and parallel dimensioning, used when both the overall size and the individual sizes are required.

6. Dimension by size and by coordinates — Sizes are given directly (e.g. diameter, radius, thickness), or the position of a point is fixed by coordinates (x, y) from a datum.

Common rules observed:

• Every dimension is given once only, on the view where the feature appears most clearly.
• Dimension lines are drawn as thin continuous lines, with arrowheads touching the extension lines.
• Extension lines are drawn perpendicular to the object line, with a small gap (about 1.5 mm) from the object.
• Figures are placed above the dimension line (aligned system) or in the middle of a broken dimension line (unidirectional system).
• Radius is marked with R, diameter with ⌀, and a centre line is used for circles and symmetrical parts.
• Dimensions are given in mm (or as stated) and the unit is written once in the note, not repeated with every figure.