In the last paragraph of Problem 215 it is stated that the demonstration there given is applicable to the blank for a curved molding in a circular wall. There are many forms of arches and different methods of adapting them to the requirements of a curved wall. An arch may be semicircular or elliptical, having either the long or the short diameter of the ellipse as its width; and in either case it may be rampant, though rampant arches are seldom used. Any form of arch may be so constructed that its moldings project either from the exterior or from the interior surface of a curved wall. In adapting any form of arch to a circular wall the soffits and roof strip, or portions which appear level at the top of the arch, may, as they are carried around the curve of the arch, arrive at the springing line or top of the impost parallel to the center line in plan; or they may at this point be drawn radially to the curve of the wall.
In Fig. 742 is shown one-half of the elevation and plan of a semicircular window cap and a section on the center line of the same. M K L N is one-half the elevation of all the members constituting the molding, the lines of which are projected from the profile W as shown to the right of the center line. In the plan, those lines which in profile W were drawn horizontally are here drawn parallel to the center line B G. They might with equal propriety have been drawn radially toward the center of the curve E C. Should they be drawn radially the profile of the molding would remain nearly normal throughout its course, but when drawn as in the plan, Fig. 742, it will be seen that the profile is continually changing, that at the foot of the arch or top of the corbel being shown at Y in the plan. This profile is obtained by the usual operation of raking, as shown by the dotted lines.
As the blank for any curved molding is, to the pattern cutter, a flaring strip of metal, it simply becomes necessary to determine its width and the amount of flare which it may assume in the different parts of its course, after which its pattern may be arrived at by methods described in Problem 214 and those following. The direction of the line determining the amount of flare necessary for the strip to have is determined by the judgment of the pattern cutter and the requirements of the machinery used in "raising" the mold. Therefore in making the application of the demonstration given in Problem 215 to the window cap shown in Fig. 742 it is necessary to Bret draw the lines k g of the profile in elevation, and p r of the plan, establishing the flare and necessary width or stretchout at those points, after which the points p and r may be dropped upon the line M N of the elevation, and the points k and g carried horizontally to the center line K X. as shown. The points thus obtained in M N and K L must then be connected by the necessary arcs struck from X as center, thus completing an elevation of the Raring piece. Likewise the projection of the points g and k from K L must be Bel off from C on C G of the plan, as shown at g" and k". and the points thus obtained connected with r and p by arcs struck from the center used in describing the plan of the wall.
Fig. 743. - Elevation and Plan of an Arched Window Cap in a Circular Wall.
Should the width and flare of the blank, as deter-mined by the raked profile Y at the foot of the arch, so much from those of the normal profile at the top that parallel curved lines could not be drawn to connect the points at the foot with those at the top in either or both views, then centers must be found upon the center lines of the plan and of the elevation from which ares can be drawn connecting the required points. Having thus completed a plan and elevation of the flaring piece or blank, it will be seen by reference to Problem 215 that the lines p' r' and k' g' of the elevation, Fig. 742, correspond with A F and B E of Fig. 730; and that p r of the plan. Fig. 742. and the arcs drawn from it correspond with B' A' F' E' of Fig.
Fig. 743. - Method of Obtaining a Section through Molding.
780; after which the demonstration given in that problem may be followed to obtain the required pattern.
It will thus be seen from the foregoing that no matter what form of arch be used or in what manner it be placed upon the wall, the method of obtaining the pattern of its flaring surfaces remains the same.
It might under some circumstances be desirable to construct stays at several intervals between the top and the foot of an arch, similar to that shown at Y in Fig. 742, for the purpose of more accurately determining the flare in all parts of its sweep,or for the purpose of constructing a form of templet to assist in the operation of raising the mold. The profiles of such stays can be obtained by the usual operation of raking, which is fully described in numerous problems in the first section of this chapter. In Fig. 743 the operation of obtaining a profile upon the line X' X" is carried out in all its detail, resulting in the form shown at Q, and needs no further comment.
Fig. 744. - Perspective View of Templet for Use in liaising the Curved Mold.
In constructing a form or templet, as shown in Fig. 744, the outlines of the arch shown in the engraving must be obtained by developing extended sections of those lines on E C of the plan, as described in Problem 214 and those following, after which the stays can be placed upon lines drawn to correspond with those from which the respective sections were taken.