This technique is a
simple method for
joining two pieces of
timber. Although this joint
has many applications, it's
particularly useful for a
bearer on the inside of a
cabinet to secure a solid
wood top in place. In this
case, screws would fit into
slots cut in the bearer to
secure the top down. The
slots allow the top to
expand with the varying
humidity in addition to
preventing it from cupping
or bowing.
It's most useful, however, for connecting
middle legs to continuous top rails on
tables, where breaking the rail to form a
pair of tenons would weaken the structure
unnecessarily. This form of the joint is known
as the T-bridle. It's a relatively simple joint to
make, with the slot being cut fi rst and then
the tenon made to fi t it. The proportions
used are the same as for a standard
mortise and tenon, so when joining two
timbers of equal thickness, the slot should
be set centrally and should be no more than
one third of the timber's thickness.
Another use for this joint is
to secure two relatively
narrow pieces together to
provide additional strength
and give the appearance of a
much thicker piece when
used as a corner leg.
Height strength
As this is a mechanical joint
reinforced with adhesive - as
opposed to relying on the
glue alone - it's very strong.
Another advantage is the
ability to join the timbers
together without fear of them
slipping out of alignment as
cramps are tightened. Even if
the timbers are long and
slightly bowed they can be
joined together and will dry
straight.
Making the joint is
straightforward and requires
only one straight fluted cutter
and a router table fitted with
an adjustable fence and a
fine height adjuster.
The diameter of the cutter
will depend on the timbers
being joined, but as a general
rule a 10mm dia cutter would
be suitable for all but the
narrowest of timbers.
Depth gauge
The cutter can be set by eye
to form both the rebate and
tongue, but a handy gadget I
use is the depth gauge made
by Trend; as well as using it
to check the depth of grooves
and mortises, it can also
measure the projection of
saw blades and cutters.
Although a home-made
version is easy to make using
scrap wood and a steel ruler,
it's hardly worth the effort as
this version is both easy to
use and relatively
inexpensive.
1] Before cutting, decide which piece of wood is to have the groove cut
along the length and which one is to have a rebate cut to form the
tongue. Take the piece that'll have the tongue formed and hold it against
the fence and use the fine adjuster to place the edge of the cutter against
the edge of the wood.
2] Adjust the depth of cut using the fine height adjuster so the cutter will
cut a groove about 10mm deep. The depth isn't critical at this stage as
it'll depend on the project in hand. Run the wood from right to left over the
cutter pushing firmly against the fence. Depending on the density of the
wood, it may be necessary to cut the groove in two or more passes.
3] Without altering the position of either the router or the fence, adjust the
depth of cut to the outside edge of the groove just cut. The timber should
be held on its edge, acting as a guide at this stage. This process sets the
thickness of the tongue. Aim for a marginally greater depth to form a slightly
looser joint, thus giving some room for the adhesive to swell the timber.
4] This stage can be set by eye quite satisfactorily, but some kind of
measuring gauge makes it much easier and more accurate. The aim
is to set the edge of the cutter away from the fence, equivalent to the
depth of cut of the groove.
5] Here the measuring gauge is used to set the cutter the correct distance
away from the fence using the fine adjuster on this particular table. On
other tables it may be necessary to move the fence, and though this may
be a little trickier on a home-made table, it just takes a little care. As for
most techniques, it's worth testing this on some scrap timber first.
6] Push the timber against the fence and remove the waste timber. If this
can't be done in one pass, adjust the cutter position and remove a little
timber first and then re-set the cutter. An alternative is to stick some thin
plywood onto the fence on either side of the cutter and make the first pass,
then remove the plywood and make the second pass.
