Engineering Drawing and Orthographic Projection

Introduction to Engineering Drawing

Engineering drawing (also called technical drawing) is a precise and standardised way of representing the shape, size, and details of an object on paper using lines, symbols, and dimensions, so that the object can be understood and produced accurately by anyone who reads the drawing, anywhere in the world. Unlike freehand sketches or artistic drawings, engineering drawings follow strict rules and conventions so that they can be interpreted the same way by different people, such as designers, engineers, and factory workers. Engineering drawing is a compulsory skill for architects, engineers, technicians, and craftsmen because it is the universal language of design and construction.

Importance of Engineering Drawing

Engineering drawing is important because it allows designers to communicate their ideas clearly and accurately without ambiguity. It provides exact measurements and specifications needed to manufacture or construct an object correctly. It saves time and materials by allowing problems to be identified on paper before production begins. It serves as a permanent record that can be used for future reference, repairs, or modifications. Without engineering drawing, it would be extremely difficult to mass-produce identical parts or construct complex buildings and machines accurately.

Drawing Instruments and Their Uses

Several instruments are used in engineering drawing to achieve accuracy and neatness. The drawing board is a flat, smooth surface on which drawing paper is fixed for drawing. The T-square is used together with the drawing board to draw horizontal lines and to guide set squares for vertical and inclined lines. Set squares (45° and 30°/60° triangles) are used to draw vertical lines and lines at specific angles. The protractor is used to measure and draw angles accurately. Compasses are used to draw circles and arcs. A scale rule is used to measure lengths and to draw objects to a reduced or enlarged size while keeping accurate proportions. Pencils of different grades (such as H for hard, light lines and HB or B for darker lines) are used for different parts of the drawing, while an eraser is used to correct mistakes.

Types of Lines Used in Engineering Drawing

Different types of lines have specific meanings in engineering drawing. Outlines (visible lines) are thick, continuous lines used to show the visible edges and outline of an object. Hidden lines are thin, dashed lines used to show edges that are not visible from the current viewing direction. Centre lines are thin lines made of alternating long and short dashes, used to indicate the centre of circles, arcs, and symmetrical objects. Dimension lines are thin lines with arrowheads at both ends, used to show the measurement of a particular part of the drawing. Extension lines extend from the object to allow dimension lines to be placed slightly away from the drawing itself. Cutting plane lines show where an imaginary cut has been made through an object to reveal its internal features.

What is Orthographic Projection?

Orthographic projection is a method of representing a three-dimensional object using two-dimensional views, where each view shows the object as seen from a specific direction, with all lines of sight perpendicular (at right angles) to the plane of projection. Because a single view cannot fully describe a three-dimensional object, orthographic projection typically uses multiple views — commonly the front view, top view (plan), and side view (end view) — to give a complete and accurate description of an object's shape and size.

The Three Principal Views

The front view (front elevation) shows what the object looks like when viewed directly from the front. The top view (plan) shows what the object looks like when viewed directly from above. The side view (end elevation) shows what the object looks like when viewed directly from the side, usually the left or right. Together, these three views allow a person to fully understand the length, width, and height of an object without needing to see it in three dimensions.

First Angle and Third Angle Projection

There are two main systems of orthographic projection used around the world. In First Angle Projection (commonly used in Nigeria, the UK, and most of Africa and Europe), the object is imagined to be placed between the observer and the plane of projection; the top view is placed below the front view, and the side view (from the left) is placed on the right of the front view. In Third Angle Projection (commonly used in the United States and Canada), the plane of projection is imagined to be between the observer and the object; the top view is placed above the front view. Nigerian students are generally taught First Angle Projection, so it is important to know and practise its correct arrangement of views.

Principles of First Angle Projection

In First Angle Projection, the object is placed in the first quadrant, and each view is projected onto the plane behind it, then that plane is folded down or sideways to lie flat on the paper. This results in the standard Nigerian arrangement: the front view in the centre, the top view directly below the front view, and the side view to the right of the front view (when viewed from the left side of the object).

Dimensioning in Engineering Drawing

Dimensioning is the process of indicating the exact size and measurements of an object on a drawing, using dimension lines, extension lines, and figures (numbers) that state the actual measurement, usually in millimetres. Correct dimensioning ensures that anyone reading the drawing knows precisely how large each part of the object should be, without needing to physically measure the drawing itself. Dimension figures should be written clearly, placed so they do not touch the outline of the object, and should never be repeated unnecessarily.

Scale in Engineering Drawing

A scale is the ratio between the size of a drawing and the actual size of the real object. A full scale (1:1) means the drawing is the same size as the object. A reducing scale (for example, 1:2, 1:10, 1:100) is used when the actual object is too large to fit on the paper at full size, such as buildings or large machines. An enlarging scale (for example, 2:1, 10:1) is used when the actual object is too small to be drawn clearly at full size, such as tiny mechanical parts.

Freehand Sketching

Before making a formal, instrument-drawn engineering drawing, technicians and engineers often make a quick freehand sketch without the use of drawing instruments, to plan the layout and record initial ideas. Freehand sketches are not as precise as formal drawings but are useful for quickly communicating an idea before detailed measurements and instruments are applied.

Applications of Engineering Drawing

Engineering drawing is applied in many fields: architects use it to design buildings and houses; mechanical engineers use it to design machine parts and engines; electrical engineers use it to design circuits and wiring layouts; and furniture makers use it to plan chairs, tables, and cabinets. Every manufactured product you use, from a mobile phone to a car, was first designed using the principles of engineering or technical drawing.

Good Practices in Engineering Drawing

To produce a good engineering drawing, always work on a clean, properly fixed sheet of paper; use sharp pencils and keep instruments clean; use the correct type and thickness of line for each purpose; ensure all dimensions are accurate and clearly written; and always keep the drawing neat, since a messy drawing can easily be misread and lead to costly mistakes during construction or manufacturing.

Summary

Engineering drawing is a standardised method of representing objects accurately using lines, symbols, and dimensions, essential for communication in design and construction. Common instruments include the drawing board, T-square, set squares, protractor, compasses, and scale rule. Orthographic projection represents a three-dimensional object using multiple two-dimensional views (front, top, and side views); Nigeria uses First Angle Projection, where the top view is placed below the front view. Correct dimensioning and appropriate scale ensure that drawings communicate accurate size information, making engineering drawing an essential skill for architects, engineers, and craftsmen.