Definition & types of technical drawing (PDF download available)
This article defines technical drawing (drafting or projection) and uses different images to illustrate the meaning, and types of technical drawing widely taught in schools and practiced in industries. The eBook/technical drawing PDF document of this article is available to download for free. Although it contains all the information available in this article, it still has something extra on its last (17th) page: a link to hundreds of images of objects in 2 & 3 dimensions, and under different types of projections. Basically, both the article and eBook elaborate on the following:
- Definition of technical drawing.
- Types of technical drawing: parallel projection (orthographic—first angle, and third angle; oblique—cavalier, and cabinet; axonometric—isometric, dimetric, and trimetric), and perspective projection (1 point, 2 point, and 3 point).
- Objectives of technical drawing.
- Purpose of technical drawing.
- Application of technical drawing.
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1.0 Definition of technical drawing
Technical drawing can be defined as the graphic representation of an object, concept or idea using a universal language that consists of graphic symbols produced with the aid of drawing equipment/tools that can be used to measure straight and curved lines according to specified dimensions, scales, and codes of practice.
Technical drawing is used in many professions (engineering, architecture, manufacturing, construction, estate management, etc.) to draw or draft ideas and different views of physical objects like drainages, culverts, septic tanks, incinerators, houses, etc. Drawing—either artistic or technical—is one of the oldest forms of communication, and is believed to be older than verbal communication. Generally, there are two types of drawings: artistic drawing, and technical drawing:
Artistic drawing is the type of drawing that is abstract because its meaning is unique to the person/artist who creates it. In order to understand the meaning of an artistic drawing, one has to understand the artist’s point of view or motivation for producing a specific artistic drawing.
Sometimes, it is necessary to understand an artist in order to understand their artistic drawing because artists often take a unique/abstract approach when communicating through their drawings. This type of approach gives rise to various interpretations when their drawings are exposed to public view.
Regardless of how complex artistic drawings may appear, they express the clear feelings, beliefs, philosophies and ideas of the artists who create them. Artistic drawings are generally freehand drawings or drawings made without the use of drawing instruments/tools.
Technical drawing is the type of drawing that is not abstract because it doesn’t require an understanding of what its creator has in mind; rather, it requires an understanding that can only be gained by studying and using universally accepted tools, codes and conventions applicable to technical drawing.
In addition to the previously stated definition on technical drawing, we can say that technical drawing clearly, precisely and concisely communicates all important information conveyed by an idea produced in graphic form by the use of universally accepted codes of practice, tools, dimensions, notes, symbols, and specifications.
Technical drawing can be done manually, or with the use of computers. When any idea or object is drawn on computer, it is said to be drafted by computer aided design (CAD). One major advantage of using CAD is that revisions can be easily and speedily carried out on any draft.
Any student, architect, engineer, etc., must understand the theory behind projections, dimensioning, and conventions if they wish to become proficient in drafting and interpreting drafts. It is very important for people to understand manual (traditional) drawing/drafting before exposing themselves to CAD softwares. Why? Because an understanding of manual drawings would make it easier to use CAD.
2.0 Types of technical drawing
Technical drawings are constructed on the basis of fundamental principles of projection. There are two main types of technical drawing or projection: parallel projection, and perspective projection. (Note that each projection has various categories that will be illustrated further below.)
A projection is any drawing, draft or representation of an idea or object that is carried out after considering views from various imaginary planes. Projections, which are quite similar to the direct views that one can see on televisions, can be used to represent actual objects if the following are employed:
- the eye of the viewer looking at the object.
- an imaginary plane of projection as dictated by the direction of the eye(s) of the viewer.
- projectors or imaginary lines of sight.
The theories behind projection have been widely used to draft 3-dimensional objects on 2-dimensional media such as papers and computer screens. The theory of projection is based on two variables:
- line of sight.
- plane of projection: plane from which images can be projected—depending on the axis.
2.1 Parallel Projection
Parallel projection is the type of projection in which the lines of sight or projectors are parallel to each other, and also perpendicular to the planes of objects or images. Parallel projection can be categorized or divided into orthographic, oblique, and axonometric projections.
(1) Orthographic projection
Orthographic projection (or drawing) is the type of projection in which 3-dimensional objects are represented in 2 dimensions by projecting planes (consisting of 2 major axes) of objects so that they are parallel with the plane of the media they are projected on.
Orthographic projection can also be defined as the type of projection in which views are taken on different planes of objects and drawn (or represented) in 2 dimensions as illustrated by the principal views shown in the figures below:
There are two types of orthographic projection: first angle projection, and third angle projection:
In first angle projection (i.e., European/international system) the front view is placed at the top of a medium (paper, computer screen, etc.) along with the right side view which is placed at the left side of the front view, while the left side view is placed at the right side of the front view, and the plan (T) is placed alone beneath the front view.
In third angle projection (i.e., American system) the plan (T) is placed alone at the top, while the front view is placed beneath the plan, and the right side view is placed at the right side of the front view, while the left side view is placed at the left side of the front view. (Note that third angle projection is more popular than first angle projection.)
If you would like to read more details about orthographic projection or drawing, click here.
(2) Oblique projection
Oblique projection is the type of projection in which any object is drawn in 3 dimensions, with each of the 3 dimensions (or major planes) consisting of two lines (or major axes: either xy, or yz, or xz) perpendicular to each other (i.e. 90°), and one of the 3 planes parallel to the plane of paper, or computer screen, etc.
In addition, one of the 3 planes is projected at either 30°, 45° or 60° to the x axis. Oblique projection is of 2 types: cavalier, and cabinet projection.
In cavalier projection, one of the 3 planes is drafted to represent a plane of an object “according to a given scale”, while in cabinet projection, one of the 3 planes is drafted to represent half of a plane of an object “according to half of a given scale”. A scale is any ratio (examples: 1:10, 1:100, 1:1000, etc.) of the size of an object on paper to the actual size of the same object in real life.
(3) Axonometric Projection
Axonometric projection is the type of projection that consists of three-dimensional drawings in which each of the 3 major axes (x, y, and z) of an object is drawn perpendicular to each other by either 30°, 45°, or 60°, and no plane of the object is drawn parallel to the plane of media—paper, computer screen, etc. Axonometric projection/drawing can be categorized into three types: isometric, dimetric, and trimetric projections.
Isometric projection is a method of projection/drawing in which the edges of 3-dimensional objects are represented by 3 axes perpendicular to each other and inclined to each other by 120° on the plane of media—paper or computer; also, 2 of the 3 axes are inclined at either 30°, 45°, or 60° to any imaginary x axis on any media.
In dimetric projection, 2 angles between any 2 major axes are unequal, while in trimetric projection, the 3 angles between the 3 major axes are unequal. Two different angles are required to construct 2 planes of objects in dimetric projections, while 3 different angles are required to construct 3 planes of objects in trimetric projections.
2.2 Perspective Projection
Perspective projection is the type of projection in which objects appear smaller as their distances from an observer increases: objects dimensions along a line of sight appear shorter than they actually are.
There are 3 types of perspective projections: 1 point, 2 point, and 3 point projections. One point perspective projections consists of 1 vanishing point, while 2 point and 3 point perspective projections consist of 2 and 3 vanishing points, respectively.
A vanishing point is a point of convergence where all lines of sight meet.
3.0 Objectives of technical drawing
The general objectives of studying technical drawing include the following:
- to develop skills in using universally accepted tools, symbols, scales and conventions to draw any visible object or invisible idea on paper, and computer.
- to understand orthographic and isometric projections and employ it in drafting/drawing ideas and objects using both projections, respectively.
- to understand and interpret technical drawings, sketches, and working drawings.
- to develop the ability to use imagination to observe, visualize and draft objects, ideas or concepts.
- to develop the ability to produce clean, accurate, neat, and informative drawings in a moderate amount of time.
- to develop the ability to take on any projects and draw environmental health science, civil and environmental engineering objects/structures.
4.0 Purpose of technical drawing
To draft and design objects or structures, and assess how they would appear in real life after they are manufactured, fabricated, assembled, constructed or built. For example, houses, septic tanks, drainages, etc., must be designed and assessed before they are built.
5.0 Application of technical drawing
Technical drawings have wide applications in any field in which planning and designing are required, such as architecture, manufacturing, engineering, construction, environment, estate management, etc.
Sanitarians, surveyors, environmental scientists, and civil/environmental engineers use technical drawings to supervise construction of layouts, structures, objects, and boundaries for various types of properties (houses, etc.).
Technical drawings are also used in situations where ideas/designs for objects and structures need to be modified, and different 2-dimensional views need to be assembled into 3-dimensional views.
Generally, technical drawings are used by a variety of professions, including but not limited to:
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