Engineering drawing techniques

Engineering drawing techniques

Engineering drawings can be produced using a variety of different techniques. The choice of technique is dependent upon a number drawing of factors such as:

Speed                          How much time can be allowed for producing the drawing. How soon the drawing can be commenced.

Media                          The choice will depend upon the equipment available (e.g. CAD or conventional drawing board and instruments) and the skill of the person producing the drawing.

Complexity                  The amount of detail required and the anticipated amount and frequency of development modifications.

Cost                            Engineering drawings are not works of art and have no intrinsic value. They are only a means to an end and should be produced as cheaply as possible. Both initial and ongoing costs must be considered.

Presentation                This will depend upon who will see/use the drawings. Non-technical people can visualize pictorial representations better than orthographic drawings.

Nowadays technical drawings are increasingly produced using computer aided drawing (CAD) techniques. Developments in software and personal computers have reduced the cost of CAD and made it more powerful. At the same time, it has become more `user friendly'. Computer aided drawing does not require the high physical skill required for manual drawing, which takes years of practise to achieve. It also has a number of other advantages over manual drawing. Let us consider some of these advantages:

Accuracy                    Dimensional control does not depend upon the draftsperson's eyesight.

Radii                            Radii can be made to blend with straight lines automatically.

Repetitive features      For example, holes round a pitch circle do not have to be individually drawn, but can be easily produced automatically by `mirror imaging'. Again, some repeated, complex features need only be drawn once and saved as a matrix. They can then be called up from the computer memory at each point in the drawing where they appear at the touch of a key.

Editing                         Every time you erase and alter a manually produced drawing on tracing paper or plastic film the surface of the drawing is increasingly damaged. On a computer you can delete and redraw as often as you like with no ill effects.

Storage                       No matter how large and complex the drawing, it can be stored digitally on floppy disk. Copies can be taken and transmitted between factories without errors or deterioration.

Prints                           Hard copy can be produced accurately and easily on laser printers, flat bed or drum plotters and to any scale. Colour prints can also be made.

Pictorial techniques

Engineering drawings such as general arrangement drawings and detail drawings are produced by a technique called orthographic drawing using the conventions set out in BS 308. Since we will be asking you to make orthographic drawings from more easily recognized pictorial drawings, we will start by introducing you to the two pictorial techniques widely used by draftspersons (Figures 2.38 and 2.39).

Oblique drawing

Figure 2.38 shows a simple oblique drawing. The front view (elevation) is drawn true shape and size. Therefore, this view should be chosen so as to include any circles or arcs so that these can be drawn with compasses. The lines forming the side views appear to travel away from you, so these are called `receders'. They are drawn at 45° to the horizontal using a 45° set-square. They may be drawn full length as in cavalier oblique drawing or they may be drawn half-length as in cabinet oblique drawing. This latter method gives a more realistic representation, and is the one we will be using.

Activity 2.24

(a)     Obtain a sheet of quadrille ruled paper (maths paper with 5 mm squares) and draw the box shown in Figure 2.38 full size. Use cabinet oblique projection.

(b)     Now use your compasses to draw a 50 mm diameter hole in the centre of the front (elevation) of the box.

(c)     Can you think of a way to draw the same circle on the side (receding) face of the box? It will not be a true circle so you cannot use your compasses.

Present your results in the form of a hand-constructed drawing with hand­written notes.

Isometric drawing

Figure 2.39a shows an isometric drawing of our previous box. To be strictly accurate, the vertical lines should be drawn true length and the

receders should be drawn to a special isometric scale. However, this sort of accuracy is rarely required and, for all practical purposes, we draw all the lines full size. As you can see, the receders are drawn at 30^° to the horizontal for both the elevation and the end view.

Although an isometric drawing is more pleasing to the eye, it has the disadvantage that all circles and arcs have to be constructed. They cannot be drawn with compasses.           Figure 2.39b-d shows you how to construct an isometric curve. You could have used this technique in Activity 2.24 to draw the circle on the side of the box drawn in oblique projection.

First, we draw the required circle. Then we draw a grid over it as shown in Figure 2.39b. Next number or letter the points where the circle cuts the grid as shown. Now, draw the grid on the side elevation of the box and step off the points where the circle cuts the grid with your compasses as shown in Figure 2.39c. All that remains is to join up the dots and you have an isometric circle as shown in Figure 2.39d.

Activity 2.25

(a)  Draw, full size, an isometric view of the box shown in Figure 2.39. Isometric ruled paper will be of great assistance if you can obtain some.

(b)  Draw a 50 mm diameter isometric circle on the TOP face of the box (remember that Figure 2.39 shows it on the side of the box).

Present your results in the form of a hand-constructed drawing with hand­written notes.

Another way of drawing isometric circles and curves is the 'four-arcs' method. This does not produce true curves but they are near enough for all practical purposes and quicker and easier than the previous method for constructing true curves. The steps are shown in   Figure 2.40.

1.   Join points B and E as shown in Figure 2.40b. The line BE cuts the line GC at the point J. The point J is the centre of the first arc. With radius BJ set your compass to strike the first arc as shown.

2.   Join points A and F as shown in Figure 2.40c. The line AF cuts the line GC at the point K. The point K is the centre of the second arc. With radius KF set your compasses to strike the second arc as shown. If your drawing is accurate both arcs should have the same radius.

3.   With Centre A and radius AF or AD strike the third arc as shown in Figure 2.40d.

4.   With Centre E and radius EH or EB strike the fourth and final arc as shown in       Figure 2.40e.

5.   If your drawing is accurate, arcs 3 and 4 should have the same radius.

Activity 2.26

Use the technique just described to draw a 40 mm diameter circle on the side face of our box. Start off by drawing a 40 mm isometric square in the middle of the side face. Present your results in the form of a hand-constructed drawing with hand-written notes.

Activity 2.27

(a)  Figure 2.41a shows some further examples of isometric drawings. Redraw them as cabinet oblique drawings.

(b)  Figure 2.41b shows some further examples of cabinet oblique drawings. Redraw them as isometric drawings. Any circles and arcs on the vertical surfaces should be drawn using the grid construction method. Any arcs and circles on the horizontal (plan) surfaces should be drawn using the 'four-arcs' method.

Present your results in the form of a hand-constructed drawing with hand­written notes.