First angle and third angle projection are the two standard methods used in engineering drawings to represent 3D objects in 2D views. Understanding the difference between these projection systems is essential for interpreting technical drawings correctly.
This guide explains how first and third angle projection work, how views are arranged, and how to identify each method using standard symbols.
In first angle projection, the object is placed between the observer and the projection plane. Views are arranged opposite to the direction of sight.
In third angle projection, the projection plane is placed between the observer and the object. Views are arranged in the same direction as the observer’s line of sight.
Engineering drawings include a standard symbol to indicate whether first angle or third angle projection is used. This symbol is usually placed in the title block of the drawing to prevent misinterpretation.
The symbol identifying the type of projection used should be placed in a space provided in the drawing block
Views Using Reference Arrows.
The principal view on which all of the other views are based should be selected as being the most informative. This may be the view which is most recognisable during manufacture or use. i.e. the front view of a house or the side view of a car.
| Line | Description.......... | Application |
| Continuous Thick | Outline Edge |
| Continuous Thin | Intersection.. Dimension.. Projection Leader.. Hatching |
| Continuous Freehand | Limits of Partial or interrupted view |
| Continuous Thin With Zigzags | Shortened Sections |
| Dashed Thick | Hidden Outlines Hidden Edges |
| Dashed Thin | Hidden Outlines Hidden Edges |
| Chain Thin | Centre lines Lines of Symmetry |
| Chain Thick | Special Surfaces |
| Chain Thin Thick ends | Cutting Planes |
| Chain Thin double-dashed | Centroidal Lines Initial Outlines Prior to Forming /Machining |
Two thicknesses of line should be used on a drawing. The "thick" line should be at least twice as thick as the "thin" line. The thickness of the line should be based on the sizes... 0,18mm 0,25mm 0,35mm 0,5mm 0,7mm 1,0mm 1.4mm and 2mm
The minimum space between parallel lines should always be greater than twice the thickness of the heaviest line.
Leader Lines terminate
|
Lettering should all be in capitals and underlining should be avoided.
The lettering size should be at least 3,5mm on the finished drawing/ plot(A0 ,A1). For larger
sized lettering such as the drawing number and title in the title block 5mm is used.
Details of the lettering is found on webpage Lettering . Smaller lettering sizes
are acceptable on the smaller drawing sheet sizes A2, A3, A4 (2,5mm and 3,5mm respectively)
With the normal paper in landscape orientation and the title block on the bottom right and side,
notes to be parallel with the bottom edge.
General notes shall be normally grouped together. Notes related to specific views
may be located adjacent to the view.
The minimum letter sizes are specified to meet the requirements of reading
microfilm copies of
drawings. This requirement is specified in BS EN ISO 6428 .
Hatching is used to show areas of sections.
In case of large areas the hatching may be limited to a zone following the contour of the hatched area
Orthographic projection uses multiple views to describe a component. These typically include front, top, and side views arranged according to the projection method being used.
Reference arrows may be used to indicate viewing direction for additional views, ensuring clarity in complex drawings.