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Fig. 46A. Dimensioning from a common datum. "A" is preferred but "B" is permitted where space is limited. Note the "block" arrow at the datum in case "B".

in inches.

Spotface all holes in castings. Local notes should be set as near to the relevant point as possible. Long 'leaders' (see below) should be avoided (see Fig. 42).

Dimensions, extension lines and leaders

(Note Extension lines are often known as 'projection' lines.)

(a) Dimension and extension fines are shown in Fig. 43. Wherever possible, dimension lines should lie outside the outline of the part.

(b) Extension lines are thin, and start just clear of the object lines. It is acceptable that there be a short dash at this end of the line, as at 'x'.

(c) Dimension lines should be thin, full lines, terminating in arrowheads not less than 3mm long. Note that on drawings made many years ago dimension lines were dotted - Fig. 44. This could lead to confusion with hidden detail and should not be used.

(d) Where an extension line refers to a point, there should be no gap, and it is acceptable that a small dot be set at the point of intersection for emphasis, Fig. 45.

(e) Centrelines should never be used as dimension lines. See Fig. 46.

(f) Dimension lines should never emerge as an extension of an outline but should be spaced away from it.

(g) Where several dimensions originate from a common reference plane, as in Fig. 46A, the dimensions should normally be as shown at (a). However, if space is short the method (b) may be used. Note the enlarged arrowhead at the reference plane, and that the dimensions are set close to the opposing arrowheads, (h) Leaders are lines that indicate where notes or isolated dimensions are intended to apply. Fig. 47 shows several examples. Leaders may terminate either with arrowheads or dots, arrows terminating ON

Fig. 47 Leaders and notes. Arrows finish ON the surface and dots are used within the body of the part.

a line, dots within the body of the object, (i) While extension and dimensions may cross others, leaders should never cross leaders; the notes should be so positioned that this is not necessary.

Dimensions

(a) If in feet and inches, either use 5ft 2in, or 5' 2"; not a mixture such as 5ft 2". If feet are included, inch dimensions should be fractional; 5'- 7-j, not 5' 7.5". If decimal fractions of the inch size are necessary, then the whole dimension must be in inches — 67.42". All dimensions of 24 inches and below are stated solely in inches.

(b) If all dimensions on the drawing are in inches, or all in millimetres, then the symbol" or mm may be omitted, but a note must be set on the drawing in a clear position stating the units used.

(c) Metric dimensions should be millimetres. Centimetres are forbidden. For on-site drawings or drawings of structures, metres and decimals may be preferable e.g. 140.5, not 140m 500mm.

(2) Fractions

(a) As has already been explained, fractions should have the bar horizontal, not sloping, -^not 1/2. An exception maybe made when space is restricted, but this should be avoided wherever possible.

as £ + ^ is discouraged by BSI, but it does help those who are not using 'sixty-fourths' all day long. It should be used only for the larger fractions, however, and only for sixty-fourths.

(c) It should be noted that a fractional dimension implies a 'rule' dimension unless it is qualified by a tolerance or limit. The words 'ream' and 'bore' applied to a fractionally dimensioned hole do imply that it should be dead size — or as near as your worn reamer will allow. The note 'Std' (Standard) to a hole has the same implication, though the 'standard' will be that particular works' standard tolerance (tolerances are dealt with later). (3) Decimal fractions (a) We have already noticed a few rules on page 45. You will recall that figures less than one should always be preceded by zero - 0.625. On dimensions especially the decimal

point should always be very bold and positive, and should be allotted a space as wide as the figure 6. The . is an unobtrusive little fellow and we must give it prominence. Even though the drawing may be decimally dimensioned, it is still in order - and desirable - to give inch drill sizes in fractional form e.g. 1/2" drill rather than 0.5" drill. On the other hand, when using sheet or wire gauges, or letter and number drills, it is preferable to indicate the size in decimals (or mm on metric

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