Manual of Purpose-Made Woodworking Joinery

Templates and geometric- normal centre- finders 147

shaped window frame- and doorframe- heads usually

have to be fitted to the underside of existing arches,

where the original setting out method is unknown, the

tradesperson usually has to resort to making a hard-

board template on site. By trial- and- error marking,

cutting and ‘easing’ (planing), the template is fitted as

close as possible to the underside of the arch, but back

in the workshop, it will be necessary to improve the

curved shape to enable more precise working tem-

plates or jigs to be made. The improving is best done

by taxing your knowledge of arch geometry and using

at 78°. However, the straight- top crown lines must
always be tangential (at 90°) to the normal lines.

SERPENTINE OR EYEBROW

ARCHED SHAPE

Figure 8.8: This shape is sometimes encountered in
joinery and cabinetmaking design, usually as the
lower edge of top door- rails and – of course – the top
edge of the door- panels. First draw span AB, equal
to the reduced door- width between the top rail’s
shoulder lines. Establish centre line DG and mark
the chosen or given rise at C from E. Mark F from
A on the springing line, equal to about one- sixth of
the span. Draw diagonal line CF (if you need actual
lines when bisecting) and bisect it to cut the centre
line at G. Draw horizontal line HI, equal to the rail’s
depth at AH and BI. Draw lines GH and GI (shown
here as broken lines for clarity). These important lines
are geometric normal lines to the curves required in
reverse positions (cyma reversa) and each curve must
stop at these lines to reverse the radiating direction.
HIG are the three centres. To form the serpentine
shape, strike sector- curves AJ from H, JC from G and
BK from I. Note that this setting out is flexible and –
providing the cyma reversa principle is not violated – a
variety of low- or high- rise serpentine/eyebrow shapes
can be achieved.

H

AF

J

G

E

K

B

I

C

D

Figure 8.8 Serpentine or Eyebrow arched shape.

TEMPLATES AND GEOMETRIC-

NORMAL CENTRE- FINDERS

Figures 8.9(a)(b)(c): Mindful of the fact that in
maintenance- and refurbishment- work, replacement,

Figure 8.9 (a) Semi- elliptical shaped hardboard template

showing the site- marked springing line; the purpose- made

geometric centre- finder in position for further marking; and

a number of centre- finding lines marked from the ‘finder’

on the right of the centre line, showing two centres found.

Figure 8.9 (b) Front- and end- elevational views of a

purpose- made geometric centre- finder (of, say, 900mm

length), joined by a glued- and- screwed half- lap joint and

with tangentially positioned, projecting wooden dowels

of 9mm diameter; and (c) Similar views of the small,

plywood or steel centre- finder.

A = Tangent

B = Geometric normal

B

A

Equal

(b) (c)