Types of Technical & Engineering Drawing Lines and Their Uses
The types and uses of various types of lines in technical & engineering drawing are as follows: (Note: Two diagrams at the end of this page illustrate how the various types of lines appear in drawings.)
Break lines are used to create breakouts on sections in order to shorten distances between parts of a drawing and give more clarity. Three types of lines are normally used as break lines; they have different line weights: long break lines, short break lines, cylindrical break lines.
Center Lines (or, long/short-dashed thin lines)
Center lines are used to locate or represent the centers of tools, circles, cylindrical surfaces or volumes, and symmetrical areas/objects, etc. Center lines are drawn as thin broken lines that have long and short dashes. In many instances, the long and short dashes vary in length; however, this depends on the scale or size of the drawing. Center lines could be extended and used as extension lines during dimensioning of objects or shapes.
Chain lines are broken or spaced parallel lines used to indicate either pitch lines (lines that show the pitch of gear teeth or sprocket teeth), center lines, developed views, or the features in front of a cutting plane. Usually, chain lines are applied at the beginning and end of long dashes, at center points as center lines, in dimensioning, or for other purposes.
Construction lines (light thin lines) are used to develop shapes and locations of features in technical & engineering drawings. After using construction lines to develop thick visible outlines of objects, they are either left on the sketches of many drawings, or cleaned off with an eraser.
Continuous thick lines
Continuous thick lines are used to represent visible edges and outlines of objects, shapes, and structures on paper or computer. They are usually dark and heavy solid lines which are very prominent in many drawings.
Continuous thin lines
Continuous thin lines are used to represent dimension lines, extension lines, projection lines, hatching lines for cross sections, leader lines, reference lines, imaginary lines of intersections, and short center lines.
Cutting plane lines (viewing plane lines)
Cutting plane lines are used to indicate the positions of cutting planes in sections, or during sectioning. Two types of cutting plane lines can be used.
The first type is a dark line that consists of one long dash and two short dashes spaced alternately. Long dashes are usually drawn at any length between 20 and 40mm, or a little bit more; it depends on the scale and size of the drawing. On the other hand, short dashes are usually drawn approximately 3mm long, and spaced at 1.5mm (between dashes).
The second type of cutting plane line consists of short dashes of equal lengths, approximately 6mm long, with a space (of length) of 1.5mm between each short dash.
Dimension lines are thin lines that have arrowheads at their opposite ends; they are used to show the precise length, breadth, width, and height of objects.
Extension lines are thin solid lines that are used to show the extent (beginning and end) of a dimension in a drawing. Extension lines are usually drawn at approximately 1.5mm away from the outlines of objects and extended 3mm longer than the outermost arrowheads located at the ends of dimension lines.
Freehand Break lines (or continuous narrow irregular lines)
Freehand break lines are lines drawn with freehand, and used to indicate short-breaks or irregular boundaries; they can be used to set the limits of partial views or sections.
Hatching lines (or section lines)
Hatching or section lines are used to indicate the sectional view or outlook of surfaces produced as a result of making arbitrary cuts on an object. Hatching lines are usually thin lines that are drawn at an angle of 45° and equally spaced.
Hidden lines are used to describe features that cannot be seen when objects are viewed from a particular direction; they consist of short and equally spaced thin dash lines and spaces. The dashes are usually three to four times longer than the space between them.
It is recommended that the dashes used in hidden lines should be approximately 3 mm long, and have a space of 1.0mm between each dash. On the other hand, the length of the dashes, and the space between them can be slightly altered, depending on the scale and size of the drawing.
Leader lines are used to show the dimensions of an object, feature, or structure whenever such dimensions are not clear enough after being placed beside objects, features, or drawn structures.
Long Break line (or continuous thin straight lines with zigzags)
Long break lines (or continuous straight lines with zigzags) show continuity of partially interrupted views; they are very suitable for computer-aided design (CAD) drawings.
Symmetry lines are imaginary lines that pass through the centers of areas, shapes, objects, and drawn structures; in most cases, symmetry lines divide objects into equal and similar-looking parts.
Visible lines are thick and continuous bold lines that are used to indicate the visible edges of objects. They usually stand out when compared with other lines.
The figures below are pictorial views of various types of lines used in technical & engineering drawing:
Figure 1: A drawing that shows various types of lines
Figure 2: A drawing that shows various types of lines