Workshop geometry

Some knowledge of elementary geometric drawing is necessary for the preparation of workshop drawings or 'full sizings', which are, in effect, accurately drawn full-size outlines of the object to be made, prepared from the smaller scale drawings of the designer. No elaborate equipment is necessary, nor need the setter-out be a skilled draughtsman, but he should understand the principles involved.


Drawings can be made on narrow rods (see Setting out and cutting lists, Chapter 36), on plywood sheets, or on a drawing-board which can be a piece of good-quality ply or laminboard with an outer facing of close-grained timber, planed up truly square on all four edges. A 30 in (762 mm) T-square is necessary which the craftsman can make for himself (339:1), using 1/8 in (3 mm) finish straight-grained mahogany for the blade, which should taper from about 4 in (101 mm) at the heel to 2 in (50 mm) at the tip, and a 12 in (304 mm) by 2 in (50 mm) by 1/2 in (12.5 mm) stock secured to the blade with short brass screws and a small dab of glue in the centre only. The stock is usually edged with a darker wood, ebony or close-grained rosewood, bevelled off as shown at 339:1a and b, and the blade is similarly treated, with the edge tongued. A hole is bored at the tip for the square to be hung under its own weight, and the wood is sealed and polished with shellac varnish or cellulose. Set-squares are also required, both 60° and 45° and up to 12 in (304 mm) in length (the key-jointed mahogany square edged with ebony [339:2] is excellent for workshop use), also a long wooden straight-edge, adjustable set-square or protractor, scale rules, dividers, pencil-compass and a beam-compass or set of trammel-points for large sweeps. Pencils should be HB, H and 4H of good quality, sharpened to a long point with about 1/4 in (6 mm) of lead exposed. Paper when used need only be the cheaper detail paper obtainable in widths of up to 5 ft (150 cm), or rolls of good-quality paper - hanger's lining paper which can be taped together to form larger sheets. Drawing-pins are now rarely used, and the paper can be taped to the board with draughting or cellulose tape.


Drawings should be lightly done at first, using a medium hard pencil and boldly outlining when correct, while a 4H pencil should be used for critical measurements. Moulded sections should be heavily outlined to show the correct profile, and all end-grain sections cross-hatched for easy identification. Curved lines should be drawn first and straight lines carried into them, while tracing through to another sheet or to the bare wood can be done with carbon paper or rubbing the back of the drawing with a very soft pencil and then tracing through from the face. Both plans and elevations can be superimposed on one sheet, outlining them in different coloured crayons, but to avoid confusion it is better to keep them well apart wherever possible.

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339 Drawing instruments: T-square and set-square

A 1 2 3 4




340 Division of lines, scales, etc.


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=

2 m or 1 cm = 2 cm, etc.), 2/5 m, 1/5 m, 1/10 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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