Design-drawings are made to a fixed scale or representative fraction, with special features— mouldings, free-hand curves in which the radius is not constant, etc.—drawn full size. Inch scales can be 1/12 (1 in representing each actual foot in length), 1/4 (3 in to each foot) and V2
(6 in to each foot). The latter set are more easily translatable in the workshop for each 1/8 in on the full-size workshop rule is then equivalent to
1 in on the 1/8 scale drawing, with 1/4 in or 1/2 in equivalent to 1 in on the larger scales. Metric scales are usually 1/1 m (full size),1/2 m (1 m=
V20 m, etc. The larger scales are divided into centimetres and millimetres and the smaller into centimetres only, except in engineers' precision steel rules which can show very fine millimetre divisions. If other scales have to be constructed then it is necessary to know how to divide a given line into any number of equal parts and 340:1 shows the procedure where AB is the given line, AC another line drawn at any convenient angle and marked off with the workshop rule or dividers into the exact number of parts required: if B and C are then joined and parallel lines drawn from the upper marks with two set-squares (340:2) then AB will be similarly divided. Any scale, therefore, can be constructed on this principle, and assuming that an 1/8th scale is required then the base-line AB (340:1) is drawn exactly 11/2 in long to represent 1 ft of actual measurement, with AC divided into 12 equal parts; and metric scales are constructed in the same manner with AC divided into tenths. In some scales showing very fine divisions only the first whole unit is subdivided and marked 0, as in 340:5, and the scale is then read from right to left. Finer scales can be constructed on the diagonal principle to mark any number of divisions, but they must be very accurately drawn and 340:6 shows the method of construction where each base-line division is 1/8 in and each upright division 1/8 of 1/8 equalling 1/64. To read the scale add the number of base divisions to the number of vertical divisions; thus in the illustration five base divisions at 1/8th+five vertical divisions at 1/64=45/64. Metric scales showing divisions up to lOOths can be similarly constructed.
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