Engineering drawings are always drawn in orthographic protection. For the presentation of detailed drawings, this system has been found to be far superior to all others. The system has. however, the disadvantage of being very difficult to understand by people not trained in its usage. It is always easential that an engineer be able to communicate his ideas to anybody, particularly people who are not engineers, end it is therefore an advantage to be able to draw using a system of projection that is more easily understood. There are many systems of pictorial projection and this book deals with two: isometric and oblique projections. Of these two. iaomeuic presents the more natural looking view of an object.
True isometric projection is an application of orthographic projection and is dealt with in greater detail later
in this chaptsr. Tht most common form of isomeric projection it celled 'conventional isometric'. This is the method that is set in G.CE. 0' level and C.S.E. examination papers, although a knowledge of true isometric is sometimes assumed.
Conventional isometric projection (laometric drawing)
If you were to make a freehand drawing of a row of houses, the house furthest away from you would be the smallest house on your drawing. This is called the 'perspective' of the drawing and. in a perspective drawing, none of the lines are parallel. Isometric drawing ignores perspective altogether, lines are drawn parallel to each other and drawings can be made using e tee square and a set square. This is much simpler than perspective draw-ing.
Fig. 3/1 shows a shaped block drawn in conventional isometric projection.
You will note that there are three isometric axes. These are inclined at 120* to each other. One axis is vertical and the other two axes are therefore at 30° to the horizontal. Dimensions measured along these axes, or parallel to them, are true lengths.
The faces of the shaped block shown in Fig. 3/1 are all at 90° to each other. The result of this is that all of the lines in the isometric drawing are parallel to the isometric axes. If the lines are not parallel to any of the isometric axes, they are no longer true lengths. An example of this is shown in Fig. 3/2 which shows an isometnc drawing of a regular hexagonal prism. The hexagon is first drawn as a plana figure and a simple shape, in this case a rectangle, is drawn around the hexagon. The rectangle is easily drawn in tsometnc and the positions of the corners of the hexagon can be transferred from the plane figure to the isometric drawing with a pair of dividers.
The dimensions of the hexagon should all be 25 mm and you can see from Fig. 3/2 that lines not parallel to the isometric axes do not have true lengths.
Fig. 3/3 shows another hexagonal prism. This prism has been cut at en incline and this means that two extra views must be drawn so that sufficient information to drew the prism in isometric can be trensferred from the plane views to the isometric drawing.
This figure shows that, when making an isometric drawing, all dimensions must be measured parallel to one of the isometric axes.
Fig. 3/3 shows another hexagonal prism. This prism has been cut at en incline and this means that two extra views must be drawn so that sufficient information to drew the prism in isometric can be trensferred from the plane views to the isometric drawing.
This figure shows that, when making an isometric drawing, all dimensions must be measured parallel to one of the isometric axes.
Circles end curves drawn in isometric projection All of the (aces of a cube are square. If a cube is drawn in isometric protection, each square side becomes a rhombus If a circle is drawn on the face of a cube, the circle wiM change shape when the cube is drawn in isometnc projection. Fig. 3/4 shows how to plot the new shape of the circle.
The circle it first drawn as a plane figure, and is then divided into an even number of equal strips. The face of the cube is then divided into the same number of equal strips. Centre lines are edded end the measurement from the centra line of the cirda to the point where strip 1 crosses the circle is transfened from the plane drewing to the isometric drawing with a pair of dividers. This measurement is epplied above and below the centre line. This process is repeated for strips 2,3. etc.
The points which have been plotted should then be carefully joined together with a neat freehand curve. Fig. 3/5 illustrates how this system is used in practice. Since e circle can be divided into four symmetric«! quadrants, it is reaHy necessary to draw only a quarter of a circle instead of a whole plene circle.
The dimensions which are transferred from the plane circle to the isometric view ere called ordinate« and the system of transferring ordinates from plane figures to isometric views is not confined to circles. It may be used for any regular or irregular shape. Fig. 3/6 shows a shaped plete.
There are several points worth noting from Fig. 3/6.
(a) Since the plate is symmetrical about its centre line, only haH has been divided into strips on the plane Figure.
(b) In proportion to the plate, the holes are small. They have, therefore, ordinates much closer together so thst they can be drawn accurately.
(c) The point where the vee cut-out meet» the elliptical outline has its own ordinate ao that this point can be transferred exactly to the isometric view.
(d) Since the plate has a constant thickness, the top and bottom profiles are the same. A quick way of plotting the bottom profile is to draw a number of verticel lines down from the top profile and. with dividers set at the required thickness of plate, follow the top curve with the dividers, marking the thickness of the plate on each vertical line.
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