Some home improvement projects involve the joining of timber.
A professional carpenter will scorn to use anything that is not self-locking; that is to say, when one piece is fitted into another it stays put and needs only the addition of adhesive, with or without nails or screws, to prevent it from sliding apart.
The most frequently encountered of such joins are dovetail and mortise-and-tenon.
Neither of these is difficult to make, though the greatest care must be taken to cut them square and true. The intricacies of a boxcomb, which is similar in principle to a dovetail, really require a special tool.
With a mortise-and-tenon, the mortise should be cut first, by boring a succession of holes with a brace and bit of the right width — (1) — and chiselling out the jagged parts (2). Then, using tenon saw and chisel, cut the tenon very slightly larger than the slot you have made so that it can subsequently be whittled down to make a precise fit (3). It is easier to take a little off the tenon than it is to enlarge the mortise if the former will not go into the latter.
Now you will want to pull the join together tightly and keep it in that position. To do this, bore a hole in the mortise of size to take a dowel of the same diameter (4), insert the tenon in the mortise, mark where the hole comes with a pencil and bore a hole in it about 1 mm (47 in) nearer the shoulder (5). Slightly sharpen the end of the dowel, which should be a little longer than required, glue its sides, hammer into the assembled parts and saw off flush (6).
For those who are not sufficiently dedicated to undertake the above joins, there are more simple ones that are quite adequate for the usual run of projects.
The simplest of all butt joins is just two lengths of wood held together by a nail. Its weakness lies in the fact that the nail is driven in the direction of the grain of the second piece and will easily come out. A cross grain, which will hold better, can be provided by boring a hole in the second piece and inserting a dowel, then screwing through both (B).
A butt join secured by a piece of quadrant moulding will be less likely to wobble (C). Glue and nail the moulding on to the two pieces. An elaboration of this is a plastic or composition corner joint,such as that shown in (D). Because this fitting is machine-cut it ensures a perfect right angle, rendering the use of a try square unnecessary. Or you can screw on a triangular plate of hardboard or metal of thickness in accordance with the strength required (E); and if this triangle is accurately cut and the joined timbers lined up with its edges you will also achieve a perfect right angle.
Slant or skew nailing (F) is useful for butting one piece of timber on to another in an awkward position.
Scarf joins are mainly employed for adding to the length of flooring joists and roofing timbers. The lower illustration in (G) shows a side plate reinforcement.
Corner half (H), T-half (i) and through bridle (J) are used in the making of fitted furniture where you want the perpendicular lengths of timber to run uninterruptedly from top to bottom of the piece.
Cross halving (K) is employed for crossing two pieces in the same plane, such as for the framework of a table top or tie beams of a shed roof.
Dowelled joins are extremely strong, but the ends of the abutting timbers must be perfectly squarely cut. Bore corresponding holes in each timber to take glued dowels. As air and surplus glue will become entrapped in the holes make a groove in the side of each dowel (L).
Tongued-and-grooved (called T and G) is bought ready cut to fit (M). It is used largely for flooring in preference to laying plain-edged planks which tend to curl at the edges and shrink to show draught-producing gaps.
To get the boards tightly up to one another, nail the first plank on to the flooring joists at right angles. Then temporarily nail a stout batten over the joists at a little distance away and use wedges to drive the second plank close up to the first (N). Move the batten along as you proceed. When you approach a wall running in the same direction as the floorboards you can, of course, dispense with the batten and use the wedges between wall and plank.
If you are nailing T and G over another piece of flat timber you can employ secret nailing as is shown in the second illustration of (M). Drive a nail askew through the tongue of the first plank, and the groove of the second plank will slip over to hide it. Such a join will, of course, be difficult to take apart without splintering the wood, should you wish to move it at a future date.
If you want to join two pieces of timber by cutting the ends at an angle of 450, you will need a mitre cutting board.
Glue and Screw
You will frequently come across the expressions glue and screw, glue and nail, or glue and pin.
You can join a thin piece of wood, ply or fibreboard,to a thicker piece (thin should always be pinned to thick not the other way round) by using panel pins or nails only. Greater strength will be achieved by skew nailing and an even stronger join will be made if screws are used.
The join can also be made with glue only. But you will have a stronger join if you use screws, nails or panel pins in conjunction with the glue. The glue gives surface adhesion and the screws, nails or pins will give depth adhesion.
Stresses and Strains
Expansion and contraction of timber are twice as great across the grain as with the grain; and as wood is from twenty-five to forty times stronger along the grain than across it, there is a possibility that the latter will split when subjected to varying humidities.
This is not likely when dimensions are small because the area of contact is itself small. But, with furniture incorporating two or more pieces of wood of considerable width joined together with screws, there may be a large area of grain running in one direction and a large area in another.
To get over the danger of splitting, design the furniture with all grains in the same direction, or, if this is not feasible, make the screw holes slightly elliptical to allow for movement. To make the ellipsis, bore a normal round hole in the piece taking the head and shank of the screw and elongate it with a cylindrical rasp. Where practicable, another solution to the problem is to attach the second piece of wood to the first loosely around the edges with moulding, as is the case with a panelled door.
Use masonry nails, which are made of particularly hard metal, for securing battens to walls on to which you intend fixing any form of panelling. Be careful to hit the nail on the head squarely. A slant blow of the hammer could cause it to snap off instead of bending, as an ordinary wire nail would. The snapped portion might fly in your face and a nasty accident result. When doing a lot of work with masonry nails it is wise to protect the eyes with goggles.
If you wish to attach anything that will be visible, bore a hole in the wall slightly deeper and wider than the screw being used, plug the hole with wood or fibre plug and screw into that, countersinking the screw head and filling level. Asbestos compound is an alternative to the plugging method. It presents a wider area of filling which will enable you, in fixing the screw, to correct any misalignment made by the boring tool.
Only solid walls can be plugged by these methods. If you want to fix sheathing to a lath-and-plaster or dry wall, you will have to find the nearest supporting stud and screw into that, or use a gravity or spring toggle, or ‘spring anchor’ toggle.
The employment of battens is not possible for sheathing walls unless they are even. But they may be dispensed with altogether by using a gap-filling adhesive which is squeezed on to the wall and the sheathing pressed on direct. There is also available a foam strip which is stuck on to the wall and, through its elasticity, will take up irregularities. These materials are particularly useful for use in, say, an extremely small kitchen where every inch of space is valuable.