Article for Good Wood Working – February 2005
Housings, dados, carcasses, rebates
and grooves There are a lot of technical terms here. Terms not totally specific
to cabinet making, but certainly used in a specific way in carcass construction
and carcass design. Im going to be talking this month very largely about dados
and housings. These are joints primarily used for joining vertical carcass sides
to horizontal full width carcass components, shelves, dividers, things like that.
Quite often carcasses need these full width components simply to give the carcass
strength and integrity. A large bookcase for example will house quite a weight
of books. If its not attached to the wall,then the only components that are going
to give it strength are the carcass back and these fixed dividers. Carcasses will
bulge and sides deform alarmingly if these fixed components are not put in place.
Quite frequently clients will want shelves to be moveable to accommodate for example
books of different sizes so you would have to insist that one of your shelves
at least is fixed.
The simplest joint in this situation is a bare faced dado.
Now a dado is essentially just a shallow groove or trough cut across the grain.
It can be easily accomplished with a router, well have a look later on at exactly
how to do this, and a divider is then fitted directly in to the dado.
A refinement, and I should say a very common refinement, of this simple joint
is the stopped dado, or housing (dados and housings are terms that are used interchangeably,
Europeans tend to call them dados where Americans will call them housings). The
stopped dado will leave a clean butted joint at the front of the carcass where
the divider meets the upright and its good practice to take the dado as close
to the front of the shelf as is possible, certainly leaving it no more than 10mm
back from the front. The reason for this is that the joint at the front of this
is only held by glue and especially with a solid wood construction glue to the
end grain of the shelf component is not especially strong. The tendency in solid
wood especially is for the carcass side to slightly curl away at the front, breaking
this joint at this point. So try to take the dado or stopped housing as far forward
as you can.
Bear in mind when you are designing dados for a carcass that the back panel needs
to be fitted in to a rebate. If you fail to do this and just fit the back panelling
to a groove, then the dado or housing will be visible at the back of the carcass.
A further refinement of the dado is to put little shoulders down the side of the
divider or shelf component. This makes the dado slightly narrower, increases the
glue area, and makes the finished joint much neater and tidier. Its slightly
more complex to do, but it does give a cleaner job.
The next stage in complicating an otherwise simple joint is the the dovetailed
housing. Whilst straightforward dados and housings described above can be used
on either solid or veneered carcasses, I reserve the dovetailed housing for solid
wood carcasses. This is an extremely strong joint, but it has its complications
that make it an infrequently used joint. The challenge about the dovetailed housing,
stopped or otherwise, is that it is fitted from the back of the carcass. The dovetail
housing is not too big a problem given the fact that it is possible to buy router
bits that will do the job. Even milling the parts of the joint on the divider
is not an issue. Again a carefully set up router will do the job. The challenge
is gaining sufficient accuracy in the two mating parts to allow the safe assembly
of the two components. What tends to happen, and Ive seen this occur several
times, is that joints can be milled up and fitted together dry nice and tightly,
once glue is put on the joint however, the whole thing seizes up and will not
be persuaded to complete its travel down the housing. The sight of a young craftsman
beating the thing together with a larger then even larger hammer is terrifying.
So the divider slides in, jams up with some kind of hydraulic lock and wont go
the final few millimetres to come flush with the front of the cabinet. There are
two ways around this. One is to only put glue on the front of the carcass, this
allows the job to slide in and only start expanding with the glue as it is driven
home. This works but is still abit high risk for my liking. The other is to free
the joint sufficiently for the glue to move around inside it. This of course can
give an untidy glue line. Its not in my experience a clamp free joint and large
sash clamps are needed to pull the joint together. This joint is most successful
when one is machining up a whole lot of these components. You can spend a long
time setting up the joint to give just the right amount of slackness and then
mill the dovetailed tenon on all the dividers.
A solution to all this fiddling about that has been offered, though I admit to
having not tried it, is the tapered dovetailed housing. This has the benefit of
only fitting as the joint slides home and gets nearer to the end of its travel,
so overcoming the problem of glue jamming up the whole assembly.
Another method of dealing with solid wood carcass is the stopped dado with the
addition of stub or through tenons running through the thickness of the carcass
side. Stub tenons can be dropped into a carcass side and remain invisible within
the structure. Their purpose is just to increase the gluing area of the joint,
whereas through tenons give much greater strength as they are wedged from the
outside creating a mechanical joint. These joints can be quite decorative, but
they require careful joinery and immaculate wedging. One of the down sides is
that this kind of expressed joinery has a kind of Arts and Craft’s feel about
it that may not be appropriate for the piece of work you are doing. Working dados
or housings in carcass sides does require accuracy. Basically the modern router
makes the whole job relatively straightforward, but it isnt easy easy to mark
out and place your dados in exactly the right place. Dados that house drawer runners
for example have got to be dead parallel with one another otherwise the drawer
will just not fit. Carcass shelves or dividers can be twisted into out of parralel
housings but what happens is the carcass acquires a twist as a consequence which
has a knock on effect when you start to fit doors or carcass backs. Spend a little
time getting this joinery square and accurate and it will save you tons of time
later on dealing with the consequences of a twisted carcass.
When assembling joints like this, its a good idea to pre finish both components
and then have a small six inch rule to hand. Glue that squeezes out from the joint
can then be wiped off right into the corner with a six inch rule. You then wipe
over after squeegying all of the excess of glue of f with a rule, then wipe off
with a damp cloth if using pva, or cascamite. If the cloth is both damp and hot
it will remove excess glue without actually getting too much moisture on to the
job. A quick wipe with a dry cloth and the finish shouldnt be affected and all
of the glue will have been removed.