SCARFING, the mode of joining two pieces of timber end to end, in such a manner that they may appear but one, and cannot be pulled asunder by a force applied in the direction of their length, without breaking otr part of the wood at the joint.
Other modes of uniting two timbers into one continuous length are sometimes practised; as for instance, the simple plan called fishing a beam, in which the ends abut against each other, and pieces of wood are added on each side, as shown in fig. I ; the whole being held together by iron bolts. The strain on the bolts may be reduced by indenting the pieces added at the joint into the beam, as represented in the lower part of the figure ; or by transverse keys of hard wood driven into grooves, of which one-half is eat in the beam, and the other in the supplementary pieces, in n similar manner to those shown in fig. 6.
Where neatness is more essential than strength, scarfed joints are preferred to any arrangement of this kind, because a beam united by them is of the same breadth and depth at the joints as at other parts. Figs. 2 and 3 represent two of the simplest forma of scarfing, in both of which the strain is borne wholly by the bolts. It is advisable to add a plate of iron ou the faces of the beam where the heads and nuts of the bolts pass through, and the ends of these plates may be turned into the wood, as shown in the cuts. Of these two plans the first appears rather preferable, because the screwing up of the bolts has no tendency to niter the position of the parte; while in the second it has a tendency to make the inclined faces slide upon each other, and thereby to open the joint.
It is desirable to avoid depending'sulely upon bolts for the strength of a scarf, owing to the effect of the shrinking of the timber, and the liability of the bolts to be, in consequence of their small dimensions, pressed into the wood. Fig. 4 is a scarf that may be used without bolts, although the addition of them adds much to the security of the joint. In this plan a key or wedge is driven gently into the square space at a, to bring the parts into their places. Two other illustra tions will suffice to explain other varieties of scarfing. Fig. 5 is a diagonal scarf, in which the parts are said to be tablnl together ; they being so cut and fitted to each other that no force can separate them longitudinally, without breaking, so long as the bolts hold them together sideways. Fig. 6 shows a very simple and good plan of
scarfing, which is easily executed with accuracy, owing to the absence of oblique faces. In this arrangement keys are used to resist any force tending to separate the beam in the direction of its fibres, instead of the parts being tabled together. The ends of the keys, which should be of hard wood, and let into both pieces of the beam to an equal depth, are shown by the dark tint in the cut.
Varieties may be almost infinitely multiplied by increasing the number of the faces, whether oblique or square, and uniting the parts either by tabling, keying, or a combination of the two; but in most cases the greatest simplicity should be aimed at, in order that the parts may the more readily be made to fit each other with accuracy. Very complicated scarfs have been used by some old carpenters, respecting which Robison observes that "many seem to aim at making the beam stronger than if it were of one piece ; " an absurdity too manifest to need refutation. Where a scarfed beam is exposed to transverse strains, the joint should be varied from the ordinary form ; but for these and some other contrivances to meet peculiar circum stances the reader is referred to the practical works of Tredgold, Nicholson, lee. When a piece of timber subject to compression in the direction of its length has to be scarfed, oblique faces should be avoided, because of their tendency to elide upon each other. Though bolts are commonly used to secure scarfed joints, iron hoops or straps, driven on tightly, have been recommended in their stead, and possess the advantage of not weakening the timber. In joints that depend wholly on bolts, Tredgold recommends that the sum of their areas should never be less than two-tenths of the area of the section of the beam. From the same authority we give the following rules for the length of scarfs :— In oak, ash, or elm, the whole length of the scarf should be six times the depth or thickness of the beam, where there are no bolts. In fir, without bolts, twelve times the depth.
The whole length of a scarf dependent wholly upon bolts should be in oak, ash, or elm, about three, and in fir, six times the depth of the beam.
When bolts and indents arc used together, the length of the scarf may be, in hard woods, twice, and in soft woods four times the depth. SCARIFIER. [Asantr. LAND.]