In designing joints and fastenings the carpenter should bear in mind not only the present position and form of the parts he places in contact, but also the changes that will certainly occur from the shrinking and settlement of the timbers, otherwise pressures will come upon parts not intended to receive them, and the pieces will frequently be crushed or split at the points of contact.
The principles which should be adhered to in designing joints and fastenings are laid down by Professor Rankine as follows: -
1. To cut the joints and arrange the fastenings so as to weaken the pieces of timber that they connect as little as possible.
2. To place each abutting surface in a joint as nearly as possible perpendicular to the pressure which it has to transmit.
3. To proportion the area of each surface to the pressure which it has to bear, so that the timber may be safe against injury under the heaviest load which occurs in practice, and to form and fit every pair of such surfaces accurately, in order to distribute the stress uniformly.
4. To proportion the fastenings so that they may be of equal strength with the pieces which they connect.
5. To place the fastenings in each piece of timber so that there shall be sufficient resistance to the giving way of the joint by the fastenings shearing or crushing their way through the timber.
The simplest forms of joints are the best, so that the parts may be fitted with the least possible inconvenience. Double abutments, such as that in Fig. 182, should be avoided, as they are difficult to fit; moreover, when the timber shrinks the whole strain may be thrown upon one of them.
The various forms of joints used in carpentry may be arranged as follows: - 1
Nature of Joint.
Form of Joint used, and page in which it is described.
Lapping, p. 58. Fishing, p. 59.
Scarfing, p. 60.
Tabling, p. 59.
Joints for lengthening " struts " or beams in compression ...
Joints for lengthening beams under cross strain ....
Joints for beams bearing on beams .
Halving, p. 64. Dovetailing, p. 64.
Tusk tenon, p. 68.
Chase mortises, p. 73.
Joints for beams on posts
Tenon, p. 67. Joggle, p. 70.
Bridle, p. 73.
Joints for posts on beams
Joints connecting struts with ties .
Oblique tenon, p. 70. Circular, p. 73.
Bridle, p. 73.
Joints connecting struts with posts .
Strut and beam joints ..
Mitre, p. 75.
Tie and brace joints ..
Dovetailing, p. 75. Notching, p. 66.
Suspending pieces ...
Fastenings are used for making joints more secure, and may be classified thus: -
Wedges, p. 76.
Trenails, p. 78
Keys, p. 77.
Screws, p. 78.
Wood pins, p. 77.
Bolts, p. 79.
Nails, p. 78.
Straps, p. 80.
Spikes, p. 78.
Sockets, p. 82.