The first step a manufacturer must take when he wishes to make en article is to produce e drawing. First e designer will make a preliminary sketch and then a draughtsman will make a detailed drewing of the design. Since neither the designer nor the draughtsman will actually make the article, the drawings must be capable of being interpreted by the men in their workshops. These workshops may be sited a long way from the drawing office, even overseas, and so the drawings produced must be standardized so that anyone familiar with these standards could make the article required. Engineering drawing is. therefore, a language. In this modern age of rapid communication and international buying, from pins to complete atomic power stations, it is essential for the language to be intemetienal. This is the reason why you will often see symbols used on a drawing instead of words or abbreviations.
British Standard 308 gives the rules for engineering drawing and should be carefully studied by every prospective draughtsman.
The rest of this book tries to explain, within the framework of BS 308. the language of engineering drawing.
The first rule of engineering drawing is to standardize the projection that ia used There ere many to choose from. This book has deelt with three—isometric, oblique end orthographic. These three ere probably the best known. Both isometric and oblique projections have two big disadvantages. Firstly, it ia possible to see only two sides and either the top or the bottom in any one view. It is. of course, possible to draw more then one view, but this brings us to the second disadvantage. On any object, except the simplest, there are curves or arcs or circles. We have seen in Chapters 3 and 6 that, although it is a fairly simple operation to draw these circlea. it takes a considerable amount of time and. in industry, time costs money. For these reasons, isometric, oblique and like projections are not used as the standard projection.
The standerd projection uaed ia orthographic projection. This is the obvious projection to use because of its many advantages. It presents e true picture of each face: circles remain as circles; ellipses remain as ellipses; horizontal lines remain horizontal; and vertical lines remain vertical. There is no limit to the number of views that you can draw: if the object that you wish to draw is complicated, it is possible to show haH a dozen views: if it is simple, two will suffice. Equally important is the fact that, however many views are drawn, they are all related to each other in position
We still need to decide whether to draw in first or third angle orthographic projection but unfortunately it ia impossible to give e definite ruling on this. It ia traditional for the British Isles and the Commonwealth to draw in 1 st angle projection, but the United States of America and. more recently, the Continental countries have adopted 3rd angle projection. There is no doubt that eventually 3rd angle will become the international standard, but it will take a considerable time. First angle projection ia still widely taught in this country, but an examination candidate will need to be familiar with both projections since he may have to answer questions in either.
For full details of both 1st and 3rd angle projections turn beck to Chapter 10.
Sections have already been discussed at some length in Chapter 10 where their main epplicetion wea in finding the true shape across a body. When sections are used in engineering drawing, although the true shape is still found, the section is really used to show what is inside the object.
A drawing must be absolutely dear when it ieevea the drawing board. The person or persons using the drawing to make the object must have all the information that they need presented clearly and concisely so that they are not confused—even over the smallest point.
Suppose that you had to draw the assembly of the three speed gearbox on the reer hub of a pedal cycle. You probably know nothing about the interior of that hub. The reason that you know nothing about it is that you cannot see inside it If you ere to produce e drewing that can be read and understood by anybody, you can
draw as many views of the outside as you wish, but your drawing will still tell nothing about the gear train inside What Is raally needed is a view of the inside of the hub and this is precisely whet e section allows you to show.
haiched. Hatching should only be done when the cutting plane passes through a solid material.
Fig. 18/1 shows two sections projected from a simple bracket. You will notice that the sections are both projected from the Front Elevation. Sections can be projected from any elevation—you are not limited to the front elevation only. Thus, you can project e sectioned F E. from either the Plan or the End Elevation. A sectioned E.E. is projected from the F E and a sectioned Plan is projected from the F.E. It is not usual to project a sectioned E.E. from the Plan nor vice-versa.
The lines X-X and Y-Y are called the sectional cutting planes and this is a good description because you are. in fact, pretending to cut the bracket right through along these lines. Both the sectioned End Elevations are what you would see rf you had physically cut the bracket! along X-X end Y-Y, removed the material behind the cuning planes (that is. the side away from the arrowheads), and projected a normal E E. with the material removed. To avoid any misinterpretation, and to show the section quite clearly, wherever the cutting plane has cut through material the drawing is hatched. The standard hatching for sectioning is at 45* although it will be seen later that in exceptional circumstances this rule may be broken. You should also note that the cutting plane passes through the hole in the bracket and this is not
haiched. Hatching should only be done when the cutting plane passes through a solid material.
The lines X-X and Y-Y are of a particular nature. They are chain dotted lines thickened at the end of the chain dotted line. The letters X-X end Y-Y are not a random choice either Sectional cutting planes are usually given letters from the end of the alphabet although you will sometimes see other letters used.
Sectioning is a process which should be ueed only to simplify or clarify a drewing. You should certainly not put a section on every drawing that you do. There are some engineering details that H sectioned, lose their identity or create a wrong impression end these items ere never shown sectioned. A list of these items is shown below
Nuts and bolts Ball beerings and bell races Studs Roller bean ngs and roller rsces
Webs Gear Teeth
Webs are not shown sectioned because section lines scross a web give an impression of solidarity.
The question of clarity arises again when considering an assembly. i.e. more than one part If any of the parts are in the above list they are not hatched, but a finished product may be composed of several different parts made with several different materiels. In the days when productivity was not quite so vital, the draughtsman was a man who turned out drawings that were almost works of art. Since there was no printing as there is to-day only one drawing was made. Each different material was coloured when sectioned and each colour represented a different and specific metal. Later, when drawings were duplicated, colours were no longer used to any great extent and each metal was given its own type of shading and it was still possible to identify materials from the sectioned views. There are now so many types of materials and their alloys in use that it has become impossible to give them all their own type of line, and you can please yourself when deciding which line you will use for a particular section.
There are occasions when hatching at other than 45* is allowable This is when the hatching lines would be parallel or nearly parallel to one of the sides. Two examples of these are shown in Fig. 18/2.
If a very large piece of material has to be shown in section, then, in order to save time, it is necessary to hatch only the edges of the piece. An example of this is shown in Fig. 18/3.
If the object that you are drawing is symmetrical and nothing is to be gained by showing it all in section, then it is necessary to show only as much section as the drawing requires. This usually means drawing a half-section
Remembering that section« are used only to clarify a drawing, it is quite likely that you will come across a case where only a very small part of the drawing needs to be sectioned to denfy a point. In this case a part or scrap section is permitted. Two examples of this are shown in Fig. 18/4.
Fig. 18/4 Part or scrap sections
It is often necessary to show a small section showing the true shape across an object. There are two ways of doing this and they are both shown in Fig. 18/5. The revolved section ia obtained by revolving the section in its position end breeking the outline to eccommodete the section. The removed section is self-explanatory and should be used in preference to the revolved section if there is room on the drawing. It is so very much neater.
When very thin material* have to be shown in section and there is no room for hatching, then they are shown solid. The most common occurrence, of course, is when drawing sheet metal. If two or more parts are shown adjacent, a small place should be left between them. Fig. 18/6.
Fig. 18/8 attempts to show, in a completely hypothetical arrangement. as many details as possible about sectioning on one drawing. The drawing shows two lengths of ahaft joined by keyed flanges which ere bolted together. The shafts are supported by two identical plummer blocks which are. in turn, bolted to the base via four studs for each block. Only one stud is shown on the drawing. This is common practice on a complicated drawing when, rather than simplifying the drawing, it is complicated by too much detail. Note how the section line is thickened where it changes direction, as shown in Fig. 18/7.
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