Engineering Drawing and Graphical Representation
Engineering Drawing and Graphical Representation
The exam tests projection conventions, line types, dimensioning practice, scales, sectioning and isometric drawing.
Conventions (BIS SP:46)
- First-angle projection: the object lies between the observer and the plane, so the view from above (plan) is placed below the elevation, and the view from the left is placed on the right.
- Lines: continuous thick = visible outline; dashed medium = hidden edge; chain thin = centre line and pitch circle; continuous thin = dimension, extension and leader lines; hatching thin at 45°.
- Dimensioning: numerals above the dimension line, readable from the bottom or right; the extension line starts about 1 mm clear of the object; never dimension a hidden line; use R for radius, Ø for diameter, SØ for a spherical diameter.
- Scales: representative fraction = drawing length / actual length; plain scale reads two units, diagonal scale three.
- Isometric: the three axes are at 30° to the horizontal and every true length is reduced by the isometric scale factor 0.816.
Worked example
Draw a plain scale of RF 1:40 reading metres and decimetres, long enough for 4 m. Scale length = 4000 / 40 = 100 mm. Divide it into 4 primary parts of 25 mm, each representing 1 m. Subdivide the first part into 10 equal parts of 2.5 mm, each representing 0.1 m. Place 0 at the end of the first primary division, metres to the right, decimetres to the left.
Traps
- Plotting isometric views with true lengths - the drawing comes out about 22 % oversize.
- Putting the plan above the elevation, which is third-angle.
- Drawing hidden edges thick.
- Dimensioning to a hidden line instead of adding a section.
- Hatching adjacent parts in the same direction, so the parts cannot be told apart.