Page:Cassells' Carpentry and Joinery.djvu/430

398 as a rule, it [would be found only in buildings of rather ordinary class. It will be seen by the plan that the jambs of the] parts do not radiate; the inner arris of the soffit of the arch is a semi-circle, and thus, on account of the radius of the plan not being great and the curvatures being very flat, the outer arris of the head of the frame can also be taken as a semicircle. A simple method will now be shown and explained of setting out and constructing the head with one mould only.

Circle-on-Circle Door Frame constructed with One Mould. — Fig. 1290 shows the front elevation of a 4-in. by 3-in. semicircular-headed solid door frame, with single rebated parallel jambs, oak sill, double rebated and weathered transom, a 2-in. glazed fixed fanlight, and a 2-in. four-panel door, moulded outside, with the bottom panels bead flush inside. Fig. 1291 shows a central vertical section, and Fig. 1292 the plan. In beginning a job of this description, an elevation of the head down to the transom should be set out full size on a rod, and immediately beneath it, and projected from it, the plan should also be set out. To get the thickness of the stuff required out of which to get the head, assuming that the head will be made in two pieces jointed at the centre, draw lines from the centre to the outside edges of the jambs, on the inside of the plan (see ′, Fig. 1292); also draw two lines parallel with these, touching the curve on the outside, as 1′ , which gives the thickness of the piece of stuff required. To find its length, either go through the same process in the elevation, drawing the ends square with the tangent lines, or preferably, make the mould, squared out from the lines just drawn, using ordinates to obtain this, as in the previous example; or it can be struck out with a trammel, being a quarter of an ellipse. Cut out two pieces to this mould square from the face, and make the joints at the centre and springing the same as the end of the mould. For the horizontal cut, set a bevel as shown on the plan, and apply it on the edge of the stuff from the face. A handrail screw and a couple of cross-tongues may be used for the head joint, the nuts, of course, going in from the top. When the joint is made, try the head, which will now have the appearance of Fig. 1293, without the lines, over its plan; its back and front faces should stand perpendicularly over the lines 1′ and a′, and its ends completely cover the sections of the jambs (Fig. 1292). The head has now to be worked to the plan curve.

Ascertaining Plan Curves. — Divide the soffit of the head of the frame into any number of equal parts between the springing lines (as in Fig. 1290), numbering them on each side, from springing to centre; the greater the number of parts, the more accurate will be the curve. From these points drop perpendiculars into the plan, cutting the tangents or block lines of the head (see Fig. 1292), and numbering the lines to correspond with the elevation. The utility of projecting the plan from the elevation will now be apparent. Next, place the head over its plan, as shown in Fig. 1293, keeping its centre perpendicularly over the centre line in the plan; with the aid of a set square, transfer to its face the lines 1, 2, 3, 4, etc., from the like numbered points in the plan. Lines must now be drawn on the top and bottom edges from these, parallel with the joint; and to do this, take the joint bevel, and apply it to each line in succession, holding the stock level, and the inside edge of the blade to the point from which the line has to be drawn. The head now having the lines drawn as in plan and elevation must have the points marked where the curve intersects these lines. Set a pair of compasses or spring dividers to the widths 1′ and 1′ ′ (Fig. 1292), and transfer them to the head at the springing joint on each side. Do the same throughout the series ′, transferring each width to its proper position on the top and bottom edges of the head, until all the points have been pricked off, as shown in the enlarged sketch of one side of the head (Fig. 1294). Now draw the curve through the points thus obtained, either by freehand or by the aid of a thin strip bent round the head and kept to the points. The two pieces can be worked off to the lines, keeping them straight across the face in the direction of the ordinates. They should be tested by moving a set square, held perfectly upright, carefully around