In fig. 699, sometimes called the secret dovetail, the pins and dovetails are both concealed, as neither of them extend through the work; the saw can be only used at the angle of 45 degrees, either for the pins or dovetails, and most of the work is done with the chisel. The angle is filled in with a corner line.
The lap dovetail, fig. 700, is often used for writing-desks, and similar works with rounded edges, and not having corner lines: the front of the desk, or p, is first rebated out to leave the lap, the pins are then made in this piece, and the dovetails are afterwards scribed on d, and made as in the last case; only a small portion of the end wood is then seen at the ends of the desk and this is in great measure removed from observation when the angle is rounded.
The mitre dovetail, fig. 701, requires each piece to be rebated out square, as in p, fig. 700; and after the pins and dovetails have been respectively made, the square rebates are converted into a mitre joint with a rebate plane. When finished, neither the pins, nor the modes of their concealment, are distinguishable and the work appears to have a plain mitre joint.
When the lid of a box has a dovetailed rim, or that the box and lid only differ in respect to depth, the box is technically said to have a tea-chest top, and four pieces of wood, sufficiently deep to make both the box and its cover, are then dovetailed together in either of the ways before mentioned. When the top and bottom of the box are also added, the six pieces present the appearance of a rectangular block, and which is known as a carcase, a term also applied to other entire framings. The saw used in cutting open the carcase, or in separating the top of the box from the bottom, is thence called a carcase saw.
This mode of work, besides saving much of the labour of dovetailing, ensures the exact agreement in size, and the general correspondence of the two parts; which it would be more difficult to obtain if they were separately made, especially in sloping works, such as portable writing-desks and others of similar character.
In every case where the box and the lid are made together, the line of division is gaged on the four sides exteriorly, and one of the dovetail pins is placed upon that line; but it is made fully as wide again as the others, to admit of division, and ye be of the ordinary size. If the joint-pin were made as usual, or left square, the carcase, on being cut open, would exhibit the rectangular lines of the pin and dovetail; to avoid which the joint-pin and dovetail should be pared away to the mitre, and then the cover and the box will also exhibit a mitre joint.
The top and bottom are fitted in various ways: sometimes they are glued on the square edges of the sides, but generally the sides and the top are both rebated, just as represented in fig. 700, on the supposition that p is the top, and d the side of the box; or they are rebated and mitred as in fig. 701.
A box made as above described, with mitred dovetails, with mitred joint-pins, and with the top and bottom rebated and mitred, would not show any joint, either within or without the box, except those constituting the margins of the twelve superficies of the work: in fact the joints would alone occur at the several angles, and escape observation, as will be apparent from the inspection of figure 701.
Such a box if neatly made, would be a finished specimen of work, but so much care is seldom taken, and it is more usual to employ corner lines and lippings to conceal the joints, or else to cover the box with veneers, and all of which are sometimes mitred. In these cases the interior frame or the carcase of the box is of common mahogany, and dovetailed in the manner of fig. 697; or in very inferior works, the fabric is of deal attached by glue and brads, the principal reliance being then placed on the veneer for uniting the parts and concealing the defects.
Having concluded this long but important digression, respecting the formation of tenons and dovetails, the consideration will be now resumed of the saws enumerated in the table on page 699.
The smith's screw head-saw, fig. 702, which, in the table, follows the back saws last noticed, differs from them in proportions, and also in the handle, which resembles that of a file; the blade is generally also thicker and harder, to accommodate it to its work. Some of the screw head-saws are made considerably smaller than those noticed in the table, the blade being a piece of watch-spring fixed in a brass back; but these little tools are generally made by the watch-maker, or other artizan requiring them.
In all screws that are made in the turning lathe, it is desirable, in separating them from the neighbouring metal, to use the turning tool, and to nick them in rather small behind the head. The little neck that is left, is broken through, just flattened with a file, and then slightly notched with a triangular file, as an entry for the screw-head saw; by these means the risk of notching the head otherwise than truly diametrical is avoided.
The comb-cutter's double saw shown in profile in fig. 704. and in section on a larger scale in fig. 703, is called a "stadda," and has two blades so contrived as to give, with great facility and exactness, the intervals between the teeth of combs, from the coarsest, to those having from 40 to 45 teeth in the inch.
The blades of the saw, or its plates, are made of thick steel, and are ground away on the edge as thin as the notches in the comb, either in the manner of a or b, and they have about 10 to 20 points in the inch, of slight pitch, fig. 644. The plates are fixed in the two grooves in the wooden handle or stock, by means of the stuffing, either two long wooden wedges, or folds of brown paper; the plates would rest in contact but for the introduction of the thin slip or tongue of metal l, called a languid, which is of the thickness of the teeth required in the comb, the one blade is in advance of the other from 1/16th to