ABSTRACT
Figure 15.1 : A beam compass or a radius rod is required to set out the full-size shape of the centre, either directly onto the rib material (single and twin-rib) or onto a hardboard or similar setting-out board (four-rib and multi-rib centres), from which a template is made of the common rib shape. The beam compass consists of a pair of trammel heads and a length of timber, say of 38 19 mm section, known as a beam. To improvise,
Figure 15.2(a) : Solid turning pieces are used for segmental arches with small rises up to about 75 mm. If the rise is too slight (say 10 mm rise, 900 mm span), then a beam compass or radius rod would not be practical for drawing the curve and a triangular trammel frame or trammel rod should be used. As illustrated in Figure 15.2(b) , to make the trammel rod or frame, mark the required span AB and rise CD on the rib material, place a board against CB and mark and cut line A 1 C 1 . As shown separately, make a sawcut at C 1 to take a pencil. To mark semi-segment CA, position pencil at C 1 and push the trammel against the protruding nails at A and B, moving to the left. Reverse the trammel and move to the right to mark semi-segment CB.
