A plain, glued joint properly managed is often so strong that soft wood will split more readily than it will come asunder at the joint.
It is not uncommon to find that this joint has been used for thin sideboard and other tops, and in order to strengthen it blocks have been glued on underneath. They stiffen up a thin top if long enough, and so far are right; but the mode of application is often wrong. If a piece is glued on with its grain in opposite directions to that of the top, this cannot contract without splitting. The piece is apparently put cross-grained with the idea of binding the top so that it will not split, and is often done by the lower kind of trade cabinet-makers in London. I wish to warn readers against copying a bad example. Let the grain of the wood underneath be in the same direction as that of the top, and the risk of splitting, or in some cases of bending hollow, will be greatly reduced, if not done away with entirely.
As this is a matter of importance, illustrations, Figs. 78 and 79, are given showing the right and wrong way of placing such blocks or linings when they are fastened with glue. Of course, if the pieces are merely used to strengthen a joint, they need not be so wide as shown, as a narrow piece will do equally well. The subject will be found more treated under the heading, 'Lining up.'
Fig. 78.- Correct.
Fig. 79 - Wrong.
The dowelled joint is stronger than the one just described, for the reason that in addition to glue, wooden pegs, the dowels, connect the two pieces. The only difference in preparation is in connexion with these dowels, so that all that has been said about edges applies here also. Dowels can hardly be used on anything less than 3/4 in. stuff, and even this is rather thin for them unless they are slighter than usual. It will easily be seen that it is of the greatest importance that the holes are bored perpendicularly and with the utmost accuracy, for if the pegs slant one way, or the corresponding holes are not exactly opposite them, the edges of the wood cannot be brought into contact. Other important and, perhaps, not such obvious points must also be observed. The dowels must fit tightly, and be thoroughly dry before using. They should also completely fill the holes bored for them, or the surface of the wood may shrink over the empty space.
The edges of the wood being ready, the exact position for boring holes must be marked. Put both pieces in the bench-screw with the edges to be joined level with each other, and remember that it is the outsides of both boards as they lie in this position which will be the one surface and the insides the other. All that is necessary, then, is to set the gauge; either the marking or cutting may be used, to mark along as nearly as may be to the centre of the edge of each board. These lines being gauged from the outside faces of both must tally. Now with the square set off lines across both pieces at intervals, say, of a foot - there is no special distance for the dowels to be apart - and so get the centres of the holes. These are thus bound to be opposite to each other if carefully bored.
A clean cutting bit should be used to bore the holes with, and none is better for the purpose than a twist bit. The depth of the holes should be uniform, and may be about | in., so that each dowel-pin will be rather less than 11 in. There is no fixed length. To get the holes equally deep the bit gauge illustrated on page 97 may be used, or a simpler one be made out of a piece of wood with a hole through it for the bit, at any part of which it can be fastened by an ordinary screw-nail. Generally, however, no bit gauge is used, as sufficient accuracy can be got without, and as this may seem difficult of attainment to the novice, the hint that he should turn the brace an equal number of times at each hole will afford him a sufficient clue.
The holes being bored, widen their mouths slightly, so slightly that it may be considered as little more than removing the sharp edges, with the bit (countersink) for the purpose. When this has been done - and it may be explained as being partly to facilitate easy entrance of the dowels that the mouths are widened - put a little glue in the hole, and then hammer a piece of dowel wood home, cut it off at the right height with the saw, and treat all the remaining holes in the same way. Remember what was said about providing for escape of air and glue from under the dowel, and when explaining the construction of these pins. It may also be well to caution the novices against allowing the exuded glue to harden on the edges outside the dowel-pins, because it would prevent the boards coming up close. All the pegs being in, give a rub with a rasp on their edges to round the ends off a little, so that they enter the other holes easily. Glue is applied, and the boards are then to be brought into close contact as before.
The tongued joint is formed by one piece of wood having a projection along its centre and a corresponding groove in the other, as shown in Fig. 80. Special tools (match planes) are made for this purpose. They have not been mentioned elsewhere, for though used they are not so well adapted for cabinet-makers 'as for joiners' use. For fine work they are not always convenient, as if the corners of the cutter forming the tongue get rounded it is difficult or impossible to get a close joint. In any case the joint can be prepared with plough and rabbet planes. Made as shown in the illustration, it has no advantages over, even if it is as good as, the dowelled joint
Fig. 80 - Tongued Joint.
A much better way of forming a tongued joint is to plough a groove in each of the edges to be joined, and then insert the tonguing into each, as shown in Fig. 81.