Widening joints

Widening joints are used to produce a board that is wider than those generally available to purchase. Boards produced in this way are almost certainly more stable than a single, wide, timber board. Examples of ways in which these boards are used include deep book shelving, the sides of a storage chest or a tabletop.

The plain butt or glued and rubbed joint is simple and quite effective for thicker timbers but is weak with thin boards of 12 mm. This joint can be strengthened with dowels or biscuits, or dominos if extra strength is required

The other widening joints (i.e. rebated, tongue and-groove, and loose or slip tongue) require some machining.

When using widening joints, two aspects should be considered:

Try to organise the grain to match the next piece to be attached so that the overall effect looks like one piece of timber (assuming that the board will show and appearance, or 'aesthetics', matters).
Ensure that the finished board is stable (minimising 'cup' or warp).

The laying out of the separate boards will affect both of these aspects so care and thought is required. With respect to matching the grain, tangential cut boards are more prone to cupping and are harder to match up due to the general curved lines of the grain. Radial cut timber is more stable and has straighter, more parallel, grain lines, which are easier to match up. The problem is that most stable radial cut timber (pine mainly) goes to the furniture industry while retail outlets get the tangential cut wood. Therefore, it is wise to select your timber carefully.

Figure 1.

Figure 2.

When considering ways to minimise cupping, boards should be arranged so that the end grain of each adjacent board alternates between curving up, then down, and so on (Figure 1). This has the effect of cancelling out any cupping. If the boards are radial cut (quarter sawn) then cupping is virtually eliminated and this process is not really necessary, which gives twice as many grain matching options. If you are only joining two or three boards, the combinations for matching the grain are considerable - check them all. When you have them set out as you want, mark each touching edge 'l' and T, '2' and '2', and so on, so they will always go back the same way

Now comes the planing (Figure 2). First, clamp the two edges to be joined in the vice and ensure that they are level with each other and that the marked numbers are both facing out or inwards. Next, use a sharp trying plane, set finely, and gently skim the edges.

When the edges are straight, gently place the plane on its side on the bench. Look along the edges with your eye and note that they might both be angled slightly. This does not matter because when you take one piece out and re-clamp the other, resting one piece on the other (number T touching the other number T) the low side will be counteracted by the high side and, therefore, they should end up in line as you look down the two surfaces

Dowelled Butt Joint

Dowels provide a convenient means by which to strengthen a Butt Joint (or Rubbed Joint) but great care has to be taken to ensure accurate location of the Dowels in each piece to be joined. Choose a Dowel diameter approximately one third the thickness of the joining edges.

Prepare the joining edges, Gauge their centres, then hold them together in a Vice for example.
Use a Try Square to square lines across where the Dowels are to be positioned.
Drill holes for the Dowels slightly more than 1/2 the length of the dowel.
Insert Dowel Centres into the drilled holes and press the two halves of the joint together such that the Dowel Centres mark the hole positions on the second piece.

Alternatively, a doweling jig can be used.

Biscuit Joint

A dedicated tool and an oddly shaped tenon combine to create a biscuit joint. At the heart of the process is a power tool called a biscuit joiner or a plate joiner.

To make a joint, use the tool to cut a shallow slot in each of the mating parts. Then, after adding glue to each slot, insert a thin, football-shaped biscuit into one slot. A little more than half the biscuit’s width goes into the slot; the other half sticks out. To complete the joint, just slip the mating slot onto the “tenon” and clamp the parts together.

Tongue and Groove

A traditional floor-boarding joint (now largely superseded by Chipboard panels but still a useful and strong method of joining together boards). The groove should be approximately one third the thickness of the joining edges and cut with a Plough Plane using the nearest width blade. The Tongue can be cut to match in the form of two Rebates but shaped and matching blades are available for Combination Planes to ensure a close fitting joint. They can also be machined using a Router.

Rebate

Groove and Feather

Also known ad a Loose Tongue or Keyed. Very similar to a Tongued and Grooved Joint but requiring only grooves to be cut into each of the joining edges thus simplifying construction. The tongue is made from solid timber or Plywood and fitted into the Grooves which are cut to a suitable width (e.g. the nearest standard Ply thickness to approximately one third the timber thickness). If tongues are made from solid timber, the Grain should be either diagonal or go across the width for strength.

Describe a process that can be used for making a widening joint using a portable power tool.

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