When you go to a museum, you often see artefacts that have been cut up. For example, to illustrate how a petrol engine works, the cylinder block can be cut in half and the cut faces are invariably painted red. In engineering drawing, cross-hatching is the equivalent of painting something red. It is used to show the internal details of parts which otherwise would become too complex to show or dimension.
The cross-hatch lines are usually equi-spaced and, for small parts, cover the whole of the 'red' cut area. They are normally positioned at 45° but if this is awkward because the part itself or a surface of it is at 45°, the hatching lines can be at another angle. Logical angles like 0°, 30°, 60° or 90° are to be preferred to peculiar ones like 18° (say). If sectioned parts are adjacent to each other, it is normal to cross hatch in different orientations (+ and -45°) or if the same orientation is used, to use double lines or to stagger the lines. Examples of single and double + and -45° cross-hatching lines are shown in the vice assembly drawing in Figure 3.1. An example of staggered cross-hatching is shown in the inverted plan drawing of the movable jaw in Figure 3.2.
If large areas are to be sectioned, there is no particular need to have the cross-hatching lines covering the whole of the component but rather the outside regions and those regions which contain details.
When sections are taken of long parts such as ribs, webs, spokes of wheels and the like, it is normally the convention to leave them unsectioned and therefore no cross-hatch lines are used. The reason for this is that the section is usually of a long form such that if it were hatched it would give a false impression of rigidity and strength. In the same way it is not normal to cross hatch parts like nuts and bolts and washers when they are sectioned. These are normally shown in their full view form unless, for example, a bolt has some specially machined internal features such that it is not an off-the-shelf item. Example of threads that are not cross-hatched can be seen in the vice assembly drawing in Figure 3.1.
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