A dimensioning 'instruction' must consist of at least four things. Considering the 50mm width of the jaw and the 32mm spacing of the holes of the movable jaw drawing in Figure 3.15, these are:
■ Two projection lines which extend from the part and show the beginning and end of the actual dimension. They are projected from the part drawing and show the dimension limits. In Figure
3.15, the width is 50mm and the projection lines for this dimension show the width of the part. They are type B lines (thin, continuous and straight). These lines touch the outline of the part. The projection lines for the hole-centre spacing dimension of 32mm are centre lines. They are type G lines (thin, discontinuous, chain) which pass through the drawing just past where the holes are located.
■ A dimension line which is a type B line (thin, continuous and straight). In Figure 3.15, these dimension lines are the length of the dimension itself, i.e. '50' or '32' mm long.
■ A numerical value which is a length or an angle. In the Figure 3.15 example the dimensions are the '50mm' and '32mm' values. If a part is not drawn full size because it is too small or too large with respect to the drawing sheet, the actual dimension will be the value which it is in real life whereas the dimension line is scaled to the length on the drawing.
■ Two terminators to indicate the beginning and end of the dimension line. The terminators of '50' and '32' dimensions in Figure 3.15 are solid, narrow arrowheads. Other arrowhead types may be used. There are four types of arrowhead allowed in ISO, as shown in Figure 3.16. These four are the narrow/open (15°), the wide/open (90°), the narrow/closed (15°) and the narrow/solid (15°). An alternative to an arrowhead is the oblique stroke. When several dimensions are to be projected from the same position, the 'origin' indication is used, consisting of a small circle. These drawings are shown in Figure 3.16. An example of an origin indicator is shown in the movable jaw detailed drawing.
Many dimensioning examples can be seen in the movable jaw and hardened insert detail drawings. The dimensions in these two drawings follow the following convention. All terminators are of the solid arrow type, all projection lines touch the outside of the part outline, all dimension numerical values are placed above the dimension lines and all dimension values can be read from the left-hand bottom corner of the drawings.
The dimensioning convention used in the movable jaw and hardened insert detail drawings is the one which is the most commonly used one. However, alternative dimensioning conventions are allowed in the ISO standards. These will be covered in Chapter 4.
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