Manual and machine drawing

Drawings can be produced by man or by machine. In the former, it is the scratching of a pencil or pen across a piece of paper whereas in the latter, it is the generation of drawing mechanically via a printer of some type.

In manual drawing, the various lines required to define an artefact are drawn on paper, using draughting equipment. The draughting equipment would typically consist of a surface to draw on, pens or pencils to draw with and aids like set-squares and curves to draw around. A typical drawing surface is a drawing board like the one shown in Figure 1.13 (courtesy of Staedtler). This is a student or lap drawing board with a horizontal ruler that can be moved vertically up and down the board. A small tongue on the left-hand side of the ruler runs in a channel on the side of the board. This allows horizontal lines to be drawn. Rotating the ruler through 90° can allow vertical lines to be drawn. In this position, the ruler tongue runs in the channel running along the bottom of the board (as shown). Alternatively, the arm can be kept in the horizontal position and a draughting head containing an integral set square can be used, which runs in a channel along the centre of the ruler. Often such a drawing head is rotatable and lines can be drawn at any angle. Such drawing boards are typically supplied in A3 and A4 sizes.

The drawing medium can either be a pencil or a pen. Black ink drawing pens are available in a variety of sizes corresponding to the ISO line thicknesses. If pencils are used, clutch pencils are recommended corresponding to the different ISO line thicknesses. A drawing board ruler, like the one described above, enables straight

figure 1.13 A drawing board (courtesy of Staedtler)

lines to be drawn but it cannot be used to draw circles. A pair of compasses are used to draw circles. A typical pair of compasses are shown in Figure 1.14 (courtesy of Staedtler). These are fairly expensive 'spring-bow' compasses, so named because the spring ring at the top provides tensioning and allows easy adjustment. Adjustment is achieved by rotating the central thumb wheel. This moves the legs further apart and allows larger diameter circles to be drawn. The compasses shown are pencil compasses that have a stylus point on the left and a pencil lead on the right. In the one shown, the right-hand side pencil leg can be removed and replaced with an ink cartridge pen. Alternative cheaper compasses are available with a simple hinged joint at the top. These are not as convenient to adjust but are more that adequate for everyday needs. Other draughting equipment which is useful but not necessarily mandatory are 'French' curves, flexi-curves, protractors, scaled rulers, lettering stencils and of course the obligatory eraser!

Machine-generated drawings are usually produced on a CAD system. The term 'CAD' is generally assumed to stand for 'computer aided design' but this is not necessarily the case in engineering drawing. The cheapest CAD systems are really two-dimensional 'computer aided draughting' packages used on standard PCs. Such a

Engineering Drawing Computer

Figure 1.14 Spring-bow compasses (courtesy of Staedtler)

two-dimensional draughting package was used to produce the drawings in this book. In this case, the lines are generated on a computer screen using a mouse or equivalent. When the drawing is complete, a printer produces a hard copy on paper. This can be simply plain paper or pre-printed sheets. Systems such as this are limited to two-dimensional drawing in which the computer screen is the equivalent of a piece of paper. True CAD packages are ones in which the computer assists the design process. Such packages can be used to predict stress, strain, deflections, magnetic fields, electrical fields, electrical flow, fluid flow, etc. In integrated CAD packages, artefacts and components are represented in three-dimensions on the two-dimensional screen (called three-dimensional modelling). Parts can be assembled together and modelling can be done to assist the design process. From these three-dimensional models, two-dimensional orthographic engineering drawings can be produced for manufacture.

Irrespective of whether an engineering drawing is produced by manual or machined means, the output for manufacturing purposes is a two-dimensional drawing that conforms to ISO standards. This provides a specification which has a legal status, thus allowing unambiguous manufacture.

References and further reading

BS 308:Part 1:1984, Engineering Drawing Practice, Part 1, Recommendations for General Principles, 1984. BS 308:Part 2:1985, Engineering Drawing Practice, Part 2, Recommendations for Dimensioning and Tolerance of Size, 1985. BS 308:Part 3:1972, Engineering Drawing Practice, Part 3, Geometric Tolerancing, 1972.

BS 8888:2000, Technical Product Documentation - Specification for Defining,

Specifying and Graphically Representing Products, 2000. Gillam B, 'Geometrical Illusions', Scientific American, January 1980. Article in the book The Perceptual World - Readings from Scientific America Magazine, edited by Irvin Rock, W H Freeman & Co., 1990. ISO 5457:1999, Technical Product Documentation - Sizes and Layouts of

Drawing Sheets, 1999. ISO Standards Handbook, 'Technical Drawings, Volume 1 - Technical Drawings in General, Mechanical Engineering Drawings & Construction Drawings', third edition, 1977.

ISO Standards Handbook, 'Technical Drawings, Volume 2 - Graphical Symbols, Technical Product Documentation and Drawing Equipment', third edition, 1977.

Parker M (editor), Manual of British Standards in Engineering Drawing and Design, second edition, British Standards Institute in association with Stanley Thornes Ltd, 1991.

PD 7308:1986, Engineering Drawing Practice for Schools and Colleges, 1986.

Ramachandran V S, 'Perceiving Shape from Shading', Scientific America, August 1988. Taken from the book The Perceptual World - Readings from Scientific America Magazine, edited by Irvin Rock, W H Freeman & Co., pp 127-138, 1990.

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