Sectional views

There are some instances when parts have complex internal geometries and one needs to know information about the inside as well as the outside of the artefact. In such cases, it is possible to include a section as one of the orthographic views. A typical section is shown in Figure 2.16. This is a drawing of a cover that is secured to another part by five bolts. These five bolts pass through the five holes in the edge of the flange. There is an internal chamber and some form of pressurised system is connected to the cover by the central threaded hole. The engineering drawing in Figure 2.16 is in third angle projection. The top drawing is incomplete. It is only half the full flange. This is because the part is symmetrical on either side of the horizontal centre line, hence the 'equals' signs at either end. This means that, in the observer's eye, a mirror image of the part should be placed below the centre line. Note that the view projected (beneath) from this plan view is not a side view but a section through the centre. In museums, it is normal practice to cut or section complex parts like engines to show the internal workings. Parts that are sectioned are invariably painted red (or any other bright colour!). In engineering drawing terms, the equivalent of painting something red is to use cross-hatching lines which, in the case of Figure 2.16, are placed at 45 ~ The ISO rules concerning the form and layout of such section lines is given in Chapter 3. The method of indicating the fact that a section has been taken on the view, from which the section is projected, is shown in the plan view of the flange. Here, the centre line has two thicker lines at either end with arrows showing the direction of viewing. Against the arrows are the capital letters W, and it is along these lines and in the direction of arrows that the sectional view is taken. The third angle projection view beneath is a section along the line AA, hence it is given the title 'Section AN. This method of showing the section position with a thickened line and arrows is explained further in the following chapter on ISO rules. Other examples of sections are given in the assembly drawing of a small hand vice (see Figure 1.11) and the detailed drawing of the movable jaw of the vice (see Figure 1.12). In the case of the movable jaw detailed drawing in Figure 1.12, the front view is shown on the top-left and the right-hand side drawing view is a right-hand section through the centre line. In this instance there are no section lines or arrows to indicate that it is a section through the centre. However, in this case, it should be obvious that the section is through the centre and therefore it is not necessary to include the arrows. However, this is not the case for the inverted planned view, which is a complicated half-section with two section plane levels on the left-hand side and a conventional inverted plan (unsectioned) view on the right-hand side. Because this is a complicated inverted plan view, the section line and arrows are shown to guide the viewer. Note that the crosshatched lines on the two different left-hand planes are staggered slightly. A different type of section is shown in the assembly drawing in Figure 1.11. Here the movable jaw (part number 3), the hardened insert (part number 2), the bush (part number 4), the bush screw (part number 5) and part of the jaw clamp screw (part number 6) are shown in section. This is what is termed a 'local' section because the whole side view is not in section but a part of it. The various parts in the section are cross-hatched with lines at different slopes and different spacings. The section limits are shown by the zig-zag line on the movable jaw and a wavy line on the jaw clamp screw. Another type of section is shown on the tommy bar of the assembly drawing. This is a small circle with cross-hatching inside. This is called a 'revolved section' and it shows that, at this particular point along the tommy bar, the cross-sectional shape is circular. In this instance the cross-sectional shape would be the same at any point along the tommy so it doesn't really matter where the section appears. The ISO standards dealing with sectional views are ISO 128-40:2001 and ISO 128-44:2001. 

Engineering Drawing for Manufacture
by Brian Griffiths
Publisher: Elsevier Science & Technology Books