In a rectangular frame, represented partly finished in fig. 689, the tenons are commonly made on the shorter pieces, called the rails, and the mortises on the longer or the styles, which are always left somewhat longer than ultimately required, to prevent them from breaking out, either in making the mortises or in wedging up the frame. In carpentry, the panel is fitted in a groove, as at a, and is inserted or planted before the frame is glued up; hut in cabinet-work the panel is fitted in a rebate, as at b, and is fixed by slips of wood after the frame is finished.
When the styles and rails have been planed up to their widths and thicknesses, (see pp. 498 to 508), the internal length of the frame is marked on the styles at / /, and the width on the rails at w w; these lines are scribed on the four sides of each piece, with the square and scriber. The additional lines l', l' indicating the ultimate length of the style, are also marked.
The width of the enlarged tenon t t', is from one-half to two-thirds that of the entire rail; the inner haunch t, is required to be lower than the groove or rebate, and the outer haunch /', is generally about three times as wide as the inner, to V room for the wedges, and the end wood of the style exterior to them. The thickness of the tenon is commonly about one-third that of the style, but from the mode of work, its actual thickness, if not exceeding about 3/4-inch, becomes exactly the same as the width of the mortise-chisel employed.
The appropriate chisel having been selected, the gage-lines g g, corresponding with its width, are gaged on each edge of the styles and rails. Frequently the mortise-chisel is slightly stuck into the work to imprint its own width, by which to adjust the gages; and every piece is gaged from the face side, so that when the whole are put together they may be flush with one another.
The several styles to be mortised, if small, as in cabinet work, are placed side by side with their inner edges upwards, and are fixed upon the bench with the holdfast; the mortises are then commenced near the outer end, m', 690. The styles, if large as in carpentry, are placed upon the stout mortising stool; the workman sits upon them, and begins near the inner end m.
The mortises are made half-way through from the inner side of the rails, and are completed from the outer; and the operation is by no means difficult, provided the mortise-chisel, which although narrow is very thick and strong, is kept exactly perpendicular to the side of the wood, and truly to the gage-lines. The chisel is mostly held with its face towards the operator, and the first cut is perpendicular and about one-sixth from the end of the mortise, as at a, fig. 690; the chisel is driven with two or three blows of a mallet of proportionate size; the second cut is inclined, as at b, and between each of the inclined blows, the chisel is moved to loosen the chips. By the two cuts a triangular portion of wood or a core is loosened, and which is prized up by thrusting the chisel backwards through the dotted arc, the bevil or bulge of the chisel then resting upon the angle of the wood as a fulcrum.
The neighbouring lines in fig. 690 show the successive cuts employed in making the mortise; some workmen prefer taking the cuts a and b alternately, always prizing up the chips by thrusting the chisel from them, after each cut b; others prefer taking most of the cut a, at an earlier stage of the work. When the triangular incision reaches half way through the wood, it is extended in length cither by sloping cuts with the chisel, as at b, or with perpendicular cuts, as at c.
At the completion of the inner half of the mortise, the face of the chisel must be applied exactly perpendicular at each end, as a and c, and in releasing the shavings, the handle is moved towards the center of the mortise, using the cutting edge as the fulcrum, and not the angle of the wood, which would be thereby bruised. The style is now turned over, and the remaining half of the mortise is completed; but the outer ends are bevilled for the reception of the wedges, as marked in the diagram. The mortise is mostly left from the mortise-chisel, although when the two incisions do not exactly meet, it is needful to pare down the inequalities with an ordinary chisel.
The cutting of the tenon is less difficult of explanation than the mortise. The shoulders, or the transverse cuts of the tenon, generally made with the dovetail or carcase-saw, whilst the rail lies on the bench against the sawing-stop, or a peg near the corner of the bench; the rail may be held with the holdfast if preferred. The side or longitudinal cuts are usually made with the tenon or sash-saw, the rail being then fixed perpendicularly in the bench-screws.
These cuts, which remove two thin rectangular pieces called cheeks, should be made with great accuracy, and so as just to avoid encroaching on the gage lines; as the tenon is left from the saw, or at most the angle is cleared out with the corner of a chisel applied almost as a knife.
The haunches are marked by laying the end of the rail in contact with the gage lines on the inner side of the style, and marking the tenon from its corresponding mortise.
Tenons and mortises do not in all cases extend through the wood, and as they cannot be then wedged up, they have to depend exclusively on good fitting or surface contact, and the glue; in many cases also, screw-bolts, straps, and wooden pins arc used to draw the tenon into the mortise in various ways, subjects that are too varied to be here particularized.
In mortises that are wider or deeper than usual, it is a common practice to remove a portion of the wood with center-bits, or nose-bits, and to complete the mortises with firmer chisels.