Technical drawings provide clear and accurate information how an object is to be manufactured. It shows and describes clearly and accurately the information required to build or manufacture a product.
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Technical drawing is a form of design communication based on line symbols recognized and understood worldwide. Hence, technical drawing is often referred to as a universal language understood (readable) by anyone regardless of the language they speak. Contractors, estimators, tradesmen and builders rely totally on technical drawings for the information they need to build, construct and manufacture a product. To work in design and manufacturing industries, the study of technical drawing –ability to prepare, read and interpret technical drawing is essential.
Line symbols used in technical drawing are often referred to as ALPHABET OF LINES. The use of line symbols enables engineers/designers to express features of designed products clearly and accurately. Line features vary not only by width but also by how they are graphically represented in a drawing. Line significance is conveyed by line weight or thickness of the line. Every line is drawn at different thickness and darkness to express contrast as well as importance. Lines that are less important are thin and light. Key to successful drafting is to have a good technical knowledge of these various line characteristics – to understand where and when to apply them in technical drawing.
ALPHABET OF LINE
1. OBJECT OR VISIBLE LINES – Thick dark line use to show outline of object, visible edges and surfaces.
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2. CONSTRUCTION LINE – Very light and thin line use to construct layout work.
3. DIMENSION LINE – Thin and dark lines use to show the size (span) of an object with a numeric value. Usually terminates with arrowheads or tick markings.
4. HIDDEN LINE – Short dash lines use to show non visible surfaces. Usually shows as medium thickness.
5. CENTRE LINE – Long and short dash lines. Usually indicates centre of holes, circles and arcs. Line is thin and dark.
6. EXTENSION LINE – Thin and dark line use to show the starting and ending of dimension.
7. CUTTING PLANE LINE – Extra thick line use to show cutaway views or plane of projection where a section view is taken. Arrow indicates direction of view.
8. SHORT AND LONG BREAK LINES –Short and long medium line use to show cutaway view of a long section.
9. LEADER LINE – Medium line with arrowhead to show notes or label for size or special information about a feature.
10. PHANTOM LINE – Long line followed by two short dashes use to show alternate position of a moving part.
11. SECTION LINE – Medium lines drawn at 45 degrees use to show interior view of solid areas of cutting plane line.