2.1 Drawing and interpretation of orthographic elevations

Orthographic projection represents a three-dimensional object by a set of two-dimensional views, each drawn as seen looking straight at one face, with lines of sight parallel to each other and perpendicular to the plane of projection. Each view (an elevation or a plan) shows two of the object's three dimensions in true size, so together the views describe the object completely and can be measured. Orthographic drawings are the working drawings from which components are made, inspected and repaired. Example: the maintenance unit at PCEA Kikuyu Hospital works from the front elevation, end elevation and plan of a worn autoclave door hinge to have an exact copy machined.

The principal views

The front elevation is chosen to show the most characteristic shape of the object and the fewest hidden lines. The other views are projected from it, so the height in the front and end elevations lines up horizontally, and the length in the front elevation and plan lines up vertically.

First angle and third angle projection

• First angle projection: The object is imagined in the first quadrant, between the observer and the plane. The plan is drawn below the front elevation, the view from the left is drawn on the right, and the view from the right is drawn on the left. This is the method used in Kenyan TVET and KNEC examinations.
• Third angle projection: The plane is imagined between the observer and the object. The plan is drawn above the front elevation and the view from the left is drawn on the left. Many imported equipment manuals from North America use it.
• Projection symbol: The method is always shown by its standard symbol in the title block (ISO 5456-2), so the reader knows where each view belongs.

Lines used on orthographic drawings (ISO 128)

Procedure for drawing orthographic elevations

1. Study the object: Decide the front, the projection method and a scale that fits the sheet.
2. Plan the layout: Work out the space each view needs and leave room between views for dimensions.
3. Block in the views: Draw the overall rectangles of each view with faint construction lines, projecting across between views.
4. Add features: Draw holes, slots and steps in the view where they appear true shape first, then project them into the other views.
5. Use a 45 degree mitre line: Transfer depths between the plan and the end elevation through a line at 45 degrees.
6. Add hidden and centre lines: Draw hidden edges with dashed lines and every hole and cylinder with centre lines.
7. Line in and complete: Darken outlines, add dimensions, the title block, the projection symbol, the scale and the material.

Interpreting orthographic elevations

Reading a drawing is the reverse of drawing it: the reader builds a mental picture of the object from its views.

• Read the title block first: Note the projection method, scale, units (millimetres unless stated), material and drawing number.
• Identify each view: Use the projection symbol to know which is the plan and which is the end elevation.
• Match features between views: A circle in one view appears as a rectangle or hidden lines in another; follow projection lines to link them.
• Interpret lines: A line can be an edge, the limit of a curved surface or the edge view of a flat surface; check it in the other views.
• Note sections and auxiliary views: Hatched areas show material cut by a section plane; an auxiliary view shows an inclined face in true shape.
• Sketch to confirm: A quick pictorial sketch confirms the shape before anything is made.

Sectional and auxiliary views

• Sectional view: An imaginary cut through the object shows its interior; cut material is hatched with thin lines at 45 degrees, and the cutting plane is labelled (for example A-A).
• Half section: Used for symmetrical parts, showing half outside and half inside.
• Auxiliary view: Projected onto a plane parallel to an inclined face, so that the face appears in true shape and size.
• Use in biomedical work: Sections show internal passages in valves, pump housings and connectors that would otherwise appear only as confusing hidden lines.