148 Geometry for curved joinery
PRACTICAL COMPASSES
Figure 8.10: To set out full- size curved shapes for jigs
and templates, etc, a beam compass or a radius rod is
required. As illustrated, the beam compass consists of
a pair of trammel heads or beam- compass heads and a
length of timber to be used as the beam; say of 40 ×
20mm prepared hardwood section. To improvise, a
radius rod can be easily made, consisting of a timber
lath (of, say 40 × 10mm prepared section), with a
panel pin or a small round wire- nail through one
end and the other end drilled to hold a pencil firmly.
Of course, the main disadvantage of a radius rod is
the omission of a fine- adjusting facility (such as the
eccentric point on a trammel head) and having to alter
the pin or nail’s position for every different setting.it in reverse order to reproduce the original shape – as
explained below.
For example, if you were making a shaped- headed
frame from an imprecise hardboard template of an
obvious- looking semi- elliptical arch shape, you must
first assume it to be a three- centre arch and set about
finding the three centres. This will enable you to
check (and improve the curvature very slightly) of the
template- shape by pivoting over the extremities of the
template with a beam compass or radius rod from the
three located centres. Figure 8.9(a) shows the hardboard
template displaying centre- finding lines marked on it by
a purpose- made wooden square which I call a geometric
centre- finder. A more detailed illustration of this is
shown in Figure 8.9(b). In wood- turning, a purpose-
made centre- finder, made of wood or steel – identical in
principle to mine, but much smaller – is used to find the
centres of circular items to be ‘turned’ on the lathe. It is
simply called a centre- finder and is shown at 8.9(c).
Trammel heads
Radius rod Panel pin or nailEccentric point
(turn for fine adjustment
of radius)Beam
Beam compassPencilPencilFigure 8.10 Details of a beam compass and an impro-
vised radius rod.