Carcase joints

Rebate

To create a neater finish at a corner, and produce a little more strength due to the increased glued surface area and a shoulder to butt against, rebate one piece and butt the other into it. Note that the amount of rebate cut out is normally from one half to three-quarters of the thickness (Fig. 1). A tongue and trench joint (sometimes known as a barefaced housing) can be used (Fig. 2). Avoid too tight a fit for this joint or the small end piece will break due to short end grain. This joint is time-consuming to produce and is not used so much now. The rebate is normally used for making boxes or drawer fronts designed for lightweight use.

Figure 1. Rebate joint

Figure 2. Tongue and trench joint

Scribed

Dovetail

This is probably the strongest of corner joints. It is used in box and carcase construction and especially for the fronts and backs of drawers, where the strain is directly on the joints, tending to pull the front and back from the sides. Figure 1. shows drawer parts and assembly.

Figure 1. Details of drawer joints

The three main types of dovetail joint are common, lapped and secret (or mitred). Note how the various parts are named in Figure 1.

In setting out, the slope, size and spacing of the pins are critical. A slope of 1 in 8 is used for harder woods (any higher angle could see the joint pull apart) and 1 in 6 for softer wood, although carpenters and joiners tend to use more slope: 1 in 4 despite the fact that this produces sharper corners that may break easily.

There is a 'rule of three', which is a simple standard for setting out these joints.

Figure 2. Common dovetail joint

Divide the thickness of the timber by the chosen slope (i.e. 4, 6 or 8).
Take three of these parts as the width of the pins at their wider side.
Allow spacing between pins as not more than three times the widest part of the pin (see Fig. 3). (Note that for a lapped dovetail (Fig. 4) the lap is one-third of the thickness of the timber being lapped.)

Figure 3. Sugested standard proportions for the common dovetail joint

Figure 4. Lapped dovetail joint

Steps in making a common dovetail joint

Mark carefully all face sides and face edges. It is important that these be correctly positioned in the whole assembly. Face sides must all face the one way, either inwards or outwards, and face edges all the one way, upwards or downwards.
Since all marking is done from and on the ends, square the timber to length before setting out.
Set out the pins. Mark the waste parts clearly and then cut accurately to the lines on the waste side using a light, fine-toothed saw (special dovetail saws are available) with gentle pressure. The waste is chiselled out from each side (to prevent splintering) and carefully pared to the marking line. The bulk of waste can be cut out with a coping saw or bandsaw. Note: An alternative method is to construct dovetails first.
Mark the pin sockets, using the pins as a template (Fig. 5). Your pencil needs a long taper and a sharp point for accurate marking.
Saw down the sides of the sockets to the gauged depth (Fig. 6) then remove waste with a bevel-edged chisel, working from each side. A coping saw may again be used to remove most waste before using a bevel-edged chisel.
Assemble the joint dry to test for fit and squareness. Test also for wind, which is usually caused by inaccurate marking or cutting. Adjust as necessary, then glue together and test again with a try square. Leave to set before cleaning off.

Lapped dovetail joints

See Figure 4. Though similar to the common dovetail, these joints were designed for use on the front of drawers so that the joint is not seen. These joints are a little more difficult to make because the saw cuts for pins cannot be carried right through the timber and most of the waste must be removed with chisels. Figure 8 illustrates the marking of the pins and lap. Figure 9 illustrates the cutting of the dovetail sockets with a bevelled edge chisel.

Hint: When marking the pins (Fig. 8) use a try square on one side of the two pieces to improve accuracy and reduce wind

Secret dovetail joints

See Figure 10. These joints are lapped over both pins and dovetails so that neither show on the outside. Laps are best mitred for a neat finish.

For both pieces, sawing and removal of waste are done as for the front piece of a lapped dovetail. In determining the proportions of the pins, only the part actually dovetailed is considered (i.e. two thirds of the total thickness).

General notes on dovetail joints

Dovetails should not be used under veneers. Shrinkage in the thickness of the timber will cause the ends of dovetails and pins to show through.
As it is difficult to cut all joints in an assembly exactly the same, the parts are not interchangeable: each part must be carefully marked.
Clean, accurate saw cuts make a better fitting joint than one that has to be pared to fit. Light pressure on the saw is essential; heavy-handed sawing makes a rough cut with burrs on the back.
Joint recesses must be cut accurately to the thickness of the timber or subsequent cleaning off may alter the finished size of the job.
Dovetail joints are secured with glue only A four-part assembly may require a light brace to keep it square while the glue sets.
Don't chisel, pare or clean up a joint that is clamped in a vice because, with no backing, it is likely that the joint will split. It must be clamped on a solid backing board.
When cleaning up a joint, move a wider chisel along a few millimetres at a time; less pressure is required.

Figure 5. Marking dovetail sockets from pins

Figure 6. Sawing pin sockets - note how waste is marked

Figure 7. Chiselling waste from sockets; block under cramp shoe protects work and bench hook protects the bench

Figure 8. Marking pins on lap from dovetails with pencil

Figure 9. Cutting back edge of sockets with bevelled-edge chisel

Figure 10. Secret or mitred dovetail joint

Housing

A housing or trench can be made, as in shelving, box dividers and drawer backs (Fig. 1). The depth of the trench should be no more than one third of the thickness of the timber or the joint becomes weak. A stopped housing (Fig. 2) is used when the housing is not meant to show on the front edge, giving a neater appearance.

Other, less used, housing joints are the sliding dovetail, the single-sided dovetail and the tapering dovetail. All of these housing joints are time consuming and uncommon now.

Figure 1. Through housing joint

Figure 2. Stopped housing joint

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