Technical Drawing Essentials: Understanding Key Concepts
Elevation
The flat representation of a building facade, the side of a machine, or an object through geometric projection, regardless of perspective. It maintains all proportions and is obtained by parallel projection perpendicular to the plane of projection. Elevation, along with the plan, is one of the main representations in the dihedral system.
Dimensional
Something is two-dimensional if it has two dimensions, for example, width and length, but not depth. Planes are two-dimensional and can only contain one-dimensional or two-dimensional bodies.
Dimensions
An annotation is the measure of an object’s characteristic that must be specified in a technical drawing. Dimensioning, also known as sizing, must meet a set of rules to improve readability and facilitate the construction of a piece. Dimensioning is governed by ISO/R 129-1959.
Sketch
An outline of an image or idea, made by hand or copied from a model before creating the final drawing or any work of art. It can be considered an initial draft or an exercise in observation and technique. Generally, a sketch is not very accurate and sometimes only readable by the author.
Technical Drawing
A graphical representation of various types of objects to provide sufficient information for analysis, design development, and future construction and maintenance. It’s usually performed with computer-aided design (CAD) software or directly on paper or other supporting planes, which is called the field.
Angles
Used to draw horizontal, vertical, and inclined lines. When combined, they are used to draw parallel, perpendicular, and oblique lines. The two main types of set squares used in technical drawing are:
- The 45° set square, shaped like an isosceles triangle with a 90° angle and two 45° angles.
- The 60° set square, shaped like a scalene triangle with angles measuring 90°, 30°, and 60°.
Perspective
The way to represent an object on a flat surface, giving an idea of its position, height, width, depth, and volume.
Cavalier Perspective
An oblique parallel projection system where the dimensions of the projecting frontal plane and parallel elements are true to their actual size. In cavalier perspective, two dimensions (width and height, or x and y) are projected to their true scale, while the depth (z) is reduced by a 1:2 ratio. The X and Y axes form a 90-degree angle, and the Z axis is usually at 45 degrees (or 135 degrees) to both.
Dimetric Perspective
A dimetric perspective is a tool in technical drawing, part of axonometry, used to represent volumes. It involves drawing from two angles with the same amplitude and a different wide angle to form the three axes used for plotting the object. The most common angles for this view are 105° and 150°. This projection is typically used to represent parts that are longer than they are wide or high.
Military Perspective
An axonometric view, a system of representation using three Cartesian axes (X, Y, Z). In the drawing, the Z-axis is vertical, while the X and Y axes are horizontal, forming a 90° angle with each other. Typically, the X-axis is 120° from the Z-axis, while the Y-axis is 150° from the Z-axis. The main advantage is that distances in the horizontal plane retain their dimensions and proportions. Circles in the horizontal plane can be drawn with a compass without deformation. Circles in the vertical planes are represented as ellipses. To achieve the design, a reduction coefficient is applied to the Cartesian axes. In military perspective, the Z-axis is reduced by 2/3, while the X and Y axes are not reduced.
Plan
In architecture, a plan is a drawing that depicts, in orthogonal projection and to scale, the different levels of a building. It shows the figure of the ground or foundations, the horizontal section of walls and partitions at each level (usually at the height of the windows), and the configuration of the deck.
Three-Dimensional
A three-dimensional object or being has three dimensions. This means that each of its points can be located by specifying three numbers within a certain range.
Other Important Terms
Arc
Portion of a curve.
Bisection
The straight line passing through the apex of an angle, dividing it into two equal parts. Also defined as the locus of points equidistant from its sides.
Concentric
Describes geometric elements that share the same center.
Diameter
A line segment that joins two points of a circle passing through its center. Its length is equal to two radii.
Scale
The relationship between a drawn dimension and its corresponding real dimension.
Isometric
A case of the axonometric system where the three axes form equal angles of 120°.
Line
A succession of points resulting in a segment.
Freehand
Drawing without using drawing instruments like rulers, squares, or compasses.
Perpendicular
The condition of a line or plane forming a right angle with another line or plane.
Radius
The line segment that joins the center of a circle with a point on its circumference.
Straight
A sequence of points in one direction.
Reference
The “x” coordinate reflecting the distance along a land line (LT) of a point’s position in space.