The jack bar in a skylight is the same as the " common " bar in respect to its profile, and the miter at its lower end with the curb. At its upper end, however, it is required to miter against the side of the hip bar. instead of against the upper finish of the skylight. As the hip bar occupies an oblique position with reference to the jack bar, it is evident that a perfect miter between the two could not be effected without a modification or raking of the profile of the hip bar, all of which has been demonstrated in the preceding problem.
It may be here remarked that the raking of the profile of the hip bar is done not so much to affect a perfect joint with the top finish as to make a perfect miter with the jack bar, or what is the same thing, that the surfaces indicated by 2 3 of the profile of the hip bar in Fig. 408 shall lie in the same plane with that portion of the profile of the jack bar. However, as the raked hip bar presents exactly the same appearance when viewed in plan as a bar of normal profile, it will not be really necessary, so far as the miter cut on the jack bar is concerned, to perform the raking operation.
In Fig. 409 is shown a sectional and a plan view of a portion of a skylight containing the miter above referred to. The normal profile of the jack bar shown at F and F' is not exactly the same in its proportions as that of the preceding problem, but possesses the same general features. The view of the bar given in the section from A to B represents an oblique elevation of that side of the hip bar which is toward the jack bar. From B to D the view shows the side of the jack bar, while beyond D is shown a continuation of the full hip bar with its profile correctly placed in position at F3.
The first step before the pattern can be laid out is to obtain a correct intersection of the points in the plan, as at B1, and afterward an elevation of the same, as shown at B. Draw a normal profile of the jack bar in correct position in the plan, as shown at F1. Also place a profile of the hip bar in the plan of the same, as shown at P. As only the lateral projection of the points are here made use of a normal profile will answer as well as the raked profile shown, as above intimated. Number all the points in both profiles correspondingly, and from the points in each carry lines respectively parallel to their plans, intersecting as shown at B1. From the points of intersection of like numbers erect lines vertically into the sectional view, cutting lines of corresponding number drawn from the points in the profile V parallel to the lines of she rake, as shown near B. It will be seen that both sides of the profile F intersect with one side of the profile F2. both sets of intersection being numbered alike, as 11, 21, 31, etc. This gives rise to two miter lines at B in the sectional view. The line corresponding to the intersections on the upper side of the jack bar are here numbered 12, 22, 32, etc., while those points belonging exclusively to the lower intersection are numbered 32, .53 and 63.
A stretchout of the normal profile F may now he laid off on any line, as G H. drawn at right angles to the elevation of the jack bar, through which the usual measuring lines are drawn. Now place the blade of the T-square parallel to G H, and. bringing it against the various points in the two miter lines above described, cut corresponding measuring lines, carrying the points from the upper miter line into one side of the pattern and those from the lower one into the other side; then lines connecting the points of intersection, as shown from K to L, will constitute the required miter cut.
As it is desirable to cut the miter on the jack bar so as to fit over the hip bar (that is, so as not to cut the hip bar at all) and in order to prevent the surface from 4 to 5 of the jack bar from lapping on to a like portion of the hip bar, as shown between the points 41, 51 and x in the plan, the line from point 4 of F1 is allowed to intersect with the line from 5 of F, as shown at x, which point is carried into the sectional view and thence into the pattern, where it intersects with lines 4, as shown by x, so that the cut in the pattern is from x to 5 instead of from 4 to 5. For the same reason, if it is desired to prevent the surfaces 2 3 from overlapping the line from 2 of F1 may be intersected with 3 from F2, as shown at y, and carried into the pattern, as shown, producing the cuts in the pattern shown by the dotted lines 3 y in the place of those shown from 3 to 2.
Fig. 409. - Section and Plan of Miter at the Top of the Jack Bar in a Skylight and Pattern of the Same.