Engineering Drawing — Orthographic, Isometric & Sectional Views
What Is an Engineering Drawing?
A written description of a component leaves too much to interpretation. An engineering drawing communicates shape, size, and detail in a form that any trained engineer anywhere can read without ambiguity. The conventions used in engineering drawings are standardised in Britain under BS 8888 — the British Standard for technical product documentation. Knowing these conventions is not just useful background; the exam tests your ability to interpret and produce drawings to these standards.
Third-Angle Orthographic Projection
Third-angle orthographic projection is the system this course requires you to use. British Standard BS 8888 permits both first-angle and third-angle projection, so a drawing must always carry the symbol that says which one it uses. It shows an object as three flat views — front, plan (top) and end (side) — laid out on paper according to a fixed rule: the view of the top sits above the front view, and the view of the right-hand side sits to the right of the front view. Think of the object being placed inside a transparent glass box; the views are projected onto the glass walls nearest to each face, then the walls are unfolded flat.
Third-angle projection uses a specific symbol: a truncated cone drawn in side view, with the smaller circular end of the cone on the right. You will often see this symbol in the title block of a drawing. Always check for it when reading a drawing — if it is absent or shows first-angle projection (where the views are arranged on the opposite sides), the layout will be different and you risk misreading dimensions.
In third-angle projection (the system this course uses): the top view (plan) sits above the front view, and the right-side view sits to the right of the front view. The symbol is a truncated cone with the small circle on the right. Do not confuse this with first-angle projection, which places views on the opposite sides.
The Three Views in Orthographic Projection
A typical orthographic drawing uses three standard views:
Front view (elevation): The most informative face of the object. It is usually chosen to show the greatest number of features clearly. This view is drawn first and the others are projected from it.
Plan view (top view): What the object looks like from directly above. In third-angle projection this view is positioned above the front view.
End view (side view): What the object looks like from one side. In third-angle projection the right-side view is placed to the right of the front view.
All three views share the same scale and are drawn in alignment — features that appear in one view project directly across to the same position in the adjacent view. This alignment allows you to locate any feature precisely in three dimensions from a flat drawing.
Standard Line Types (BS 8888)
Different lines carry different meanings in engineering drawings. Using the wrong line type reads as a misunderstanding of the drawing convention — so accuracy here matters.
Continuous thick line — Visible edges and outlines of the object as seen from the viewing direction. These are the most prominent lines on the drawing.
Continuous thin line — Dimension lines, leader lines, projection lines and hatching lines. Thin because they should not compete visually with the outline.
Thin dashed line — Hidden detail: edges or features that exist on the object but are not visible from the current viewing direction (for example, a drilled hole seen from above, or an internal pocket seen from the front).
Thin chain line (long-dash short-dash) — Centre lines: the axes of holes, cylinders, arcs and lines of symmetry. These run through the centre of any circular or symmetrical feature.
Thick chain line — Cutting plane: shows where an imaginary cut has been made through the object to create a sectional view.
Five BS 8888 line types: continuous thick (visible edges), continuous thin (dimensions/leaders), thin dashed (hidden edges), thin chain (centre lines), thick chain (cutting plane). Each carries a specific meaning — using the wrong line for a feature is an error.
Scenario: A front view of a metal bracket shows a rectangular outline, a dashed rectangular shape inside the outline, thin lines with arrows and numbers running between the outline and the edge, and a long-dash short-dash line running vertically through the centre.
Interpreting the lines:
• The rectangular outline — continuous thick line: the visible edges of the bracket as seen from the front.
• The dashed rectangle inside — thin dashed line: a rectangular slot or recess cut into the back face of the bracket, not visible from the front (hidden detail).
• The thin lines with numbers — continuous thin dimension lines with arrowheads showing the width and height of the bracket in mm.
• The long-dash short-dash line — thin chain centre line: the bracket is symmetrical about this vertical axis.
Being able to identify these four distinct line types from a description is the kind of task the exam may set. Practise naming the line type and explaining its meaning in one sentence each.
Isometric Projection — A 3D Picture
Isometric projection is used alongside orthographic views to give a clear three-dimensional impression of an object. Three axes are used: one vertical and two running at 30° either side of the horizontal. Lengths are drawn true to scale along these three axes only — diagonal and curved features must be constructed indirectly (by plotting points along the axes and connecting them). Circles in isometric projection become ellipses.
Isometric projection is not a view from a specific angle in the way orthographic projection is. It is a convention that produces consistent, proportionally correct pictures. It is particularly useful for assembly drawings and instructions because the shape of the part is immediately clear to anyone, including non-specialists.
Sectional Views — Cutting to See Inside
Some internal features — such as bores, internal threads, or recesses — are difficult to describe with dashed lines alone. A sectional view imagines the object sliced along a flat plane. The cut is shown in the other views using the thick chain cutting plane line, with arrows indicating the direction of view. The cut surfaces exposed by the imaginary saw cut are shown with hatching — evenly spaced diagonal lines drawn at 45°. Thin, consistent hatching lines distinguish material from empty space and are drawn across the entire cut face.
The section label is typically written as “Section A–A” (or B–B, C–C for multiple sections on the same drawing). The letters appear at each end of the cutting plane line to identify which section view corresponds to which cut.
Scenario: A drawing of a cylindrical housing shows a front orthographic view and a side view. On the front view, a thick chain line (cutting plane) runs horizontally across the centre, labelled “A–A” with arrows pointing downward. A separate view labelled “Section A–A” shows the inside of the housing with hatching at 45° covering the top and bottom walls, and a clear circular bore in the centre with no hatching (empty space).
Reading the section: The hatched areas represent the solid metal walls of the housing that have been cut through. The un-hatched circle in the middle is the hollow bore — empty space through which a shaft or pipe would pass. Without the sectional view, this internal bore would only be visible as a dashed circle in the front view, which is less clear.
When drawing or interpreting sectional views, remember: hatching covers only the material that the cutting plane has physically sliced through. Features beyond the cut plane (visible in the background) are drawn as normal outlines, not hatched.
Datums and Dimensions on Drawings
Just as dimensions in a workshop are measured from a datum point or line, engineering drawings record dimensions from a datum. A drawing datum prevents cumulative measurement error in the workshop. All dimensions are written in millimetres by default on UK engineering drawings unless another unit is stated. Dimension lines are continuous thin lines with arrowheads touching the projection lines that frame the feature being dimensioned. The number sits above or in a gap in the dimension line.