Fig. 14 Steps in deriving views by "pure" projection methods. Note the alternative methods of taking projection lines "round a corner".

front it would tend to move to the right.

To get a view of the bunker end (3), we do exactly the same thing, turning the object through 90 degrees; this time the loco has shunted forwards round the bend, so the view appears on the left of the main elevation. Again, to obtain the plan view, looking down on the boiler top (4) we turn the loco towards us through 90 degrees, and to obtain the underside view (5) we turn it the opposite way, so that the view appears above the initial position. If we now assemble all these views onto one sheet of paper, each in the same relative position to the main view (1), we get the relationship seen at the bottom of Fig. 13. It is the relative position of these views which is important, as the position of the view defines the face we are looking at. Not, perhaps, very important when making a drawing of a familiar object like this one, but if the drawing were of the full-size loco's inside cylinders it would be vital.

This method of projection is known as Natural, First Angle or English projection. Natural because it is the way children instinctively draw things; First angle, because the object is turned through 90 degrees, one right angle; and English because this distinguishes it from a different system originating in the USA about 1870 and which became the American standard in 1935. (I deal with the theoretical derivation of both systems in the appendix at the end of the book.). The use of projection not only assists the interpretation of a drawing, it makes it much easier to prepare the drawing in the first place. Look at Fig. 14. At (a) we have a simple solid - say a matchbox. We first draw the Front Elevation, A, by measurement. To draw the end elevation of face B we project the two lines marked

(i) to determine the height, and measure the width only. To obtain the plan view C, we need not measure anything, as the position of the lines can be obtained by projecting from the other two views. To turn these projection lines round the corner I have used a diagonal reference line, set at 45 degrees, as you see. Try this for yourself - but do it now Try every example in the book as you read it, you will find it much easier to follow that way.

At Fig. 14b is a slightly more complicated object. I have shown dimension lines to indicate the dimensions we know and can measure. In this case the Plan view is drawn first, and I have shown an alternative method of taking the projection lines round the corner. Try it for yourself, and draw the front elevation. Remember Fig. 13; keep visualising the turning of the object through 90 degrees to get a view of the face to be drawn. Plans - a digression

The habit seems to have grown of calling any drawing a Plan; indeed, there is an official Plans Service However, in the grammar of engineering drawing a Plan is specifically the view from above. The Plans Service provides Drawings or Prints, and I very strongly advise you to refer to them as such. If you write to any chief draughtsman for the plans of a machine you may not get what you want.

American or third angle projection

In this case the object is imagined to be set within a glass box and the outline drawn on the outside of the glass. If this box is then opened out the views appear as in Fig. 15. Compare this with Fig. 13. You will observe that the plan is now above the front elevation instead of below, and that the two end elevations are reversed.

Fig. 16 is a simpler comparison between the two systems. The method of deriving one view from another by projection lines is exactly the same in both cases; the difference lies in the relative position of the views.

Which is correct? The answer is that either will do, provided that the type of projection used is clearly indicated on the drawing. Although you can use either, you most certainly should not use both on the same drawing or, worse still, draw one view of an object in one style and another in the other; the quickest way of getting the sack in any drawing office.

Since writing this part I have come across several published drawings which commit just this crime. One is of a rather complex cylinder block. Five views are given, one in section. This section and the end view are in third angle. The plan, set directly below the elevation, is in first angle and the rear elevation is set out of projection but with no direction arrows to show the face it represents. I can read it - but

Was this article helpful?

0 0

Post a comment