50 Joinery joints
up a cutting gauge to the tongue’s required width
and then by scoring deeply on the face side and the
back side. I find that this method (depending on the
density of the timber) needs additional, final cutting
by running a sharp Stanley (craft) knife along the cut
gauge lines.
Plywood tongues
Figures 3.50(c)(d): Another type of tongue which
evolved from the two described above, is made by
cutting off strips of plywood and removing the sharp
arrises. Again, a cutting gauge can be used. As illus-
trated, the grain- direction of the plies needs to be
considered, to allow the greater number of plies to be
at 90° across the tongue’s width – at right angles to the
joint line. This relatively modern tongue also provides
good resistance to any side pressure applied to the
made- up tongued joint and is less time- consuming to
make. The only downside is in making the groove to
suit the tongue, which can be more problematic than
the other way round.
There are usually only three thicknesses of plywood
used for tongues, which relate to the thickness of
board being jointed. These are 3.5mm (⅛”), 6mm
(¼”) and 9mm (⅜”). The most commonly used – to
my knowledge – is the 6mm. Although there are no
hard and fast rules regarding the size of the tongue
and whether one or two should be used, the thick-
ness of the tongue for a single- tongued joint should
approximately equal a quarter of the board’s thick-
ness. For example, a 16mm- thick board ÷ 4 = 4mm;
this would approximate to using a 3.5mm plywood
tongue; a 20mm- thick board ÷ 4 = 5mm; this would
approximate to using a 6mm tongue. Using the rule
the other way round, a 9mm tongue × 4 = 36mm.
Therefore, 36mm (or the nearest commercial finished
size of 34mm) is the approximate maximum thick-
ness for 9mm tongues. Beyond this – unless you use
thicker plywood- tongues – I believe that two tongues
are required. Thicknesses of board, therefore, in excess
of 36mm, should have two tongues equal to a quarter
of the board’s thickness, with the remaining thickness
divided by three into the mid- and outer- area shoul-
ders. For example, a 45mm- thick board ÷ 4 = 11.25mm
(for two tongues) divided by 2 = 5.62mm per tongue,
which approximates to 6mm plywood tongues. The
remaining thickness = 45 – 2 × 6 = 33 divided by 3 =
11mm for each outer area and the mid- area shoulders.
Although – again – there are no hard and fast rules
regarding the width (or depth) of tongues, they should
not be too deep – or too shallow. As a guide, once you
have worked out the thickness and position of the
tongue(s) in relation to the board’s edge- thickness, the
half- depth of the tongue should approximately equal
the width of an outer shoulder. In the example above,
this was 11mm. So, 11 × 2 = 22mm tongue.Tongue and grooved edge joints
Figure 3.51: The tongue of this traditional edge-
joint, as illustrated, is formed from the material itself.
Although such joints can be made with hand planes
or portable powered routers, etc, because of the work
involved in producing the tongued edge and the likeli-
hood of uneven shoulders, they are usually regarded as
a job for a four- side moulding/cutting machine – usually
referred to as a four- cutter machine.‘F’ Joints
Figure 3.52: This type of edge joint is a modern
version of a tongue- and- groove joint, designed to
suit a machining process. As illustrated, the joint
resembles the letter ‘F’, which is a good interlock-
ing configuration and also increases the glue- line for
greater strength. Furthermore, it is not unattractive
in its end- grain appearance. It would not be impos-
sible to make a joint like this by hand, but it would be
exacting, tedious and time- consuming. Realistically,Figure 3.50 (c) Single plywood tongue; (d) Double
plywood tongue.
(c) (d)
Figure 3.51 Traditional tongue- and- grooved edges.