If this diagram were folded, as [Fig. 94] was shown to be in [Fig. 93], the tangents of the elevation—namely, a″, b″, c″, d″—would stand over and above the plan tangents a, b, c, d of the well-hole. In practical work, this diagram must be drawn full size. It gives the correct length to each tangent as required on the face-mould, and furnishes also the data for the layout of the mould.
[Fig. 96] shows how to find the angle between the tangents of the face-mould for the bottom wreath, which, as shown in [Fig. 95], is to span over the first plan quadrant a b. The elevation tangents a″ and b″, as shown, will be the tangents of the mould. To find the angle between the tangents, draw the line a h in [Fig. 96]; and from a, measure to 2 the length of the bottom tangent a″ in [Fig. 95]; the length from 2 to h, [Fig. 96], will equal the length of the upper tangent b″, [Fig. 95].
From 2 to 1, measure a distance equal to 2-1 in [Fig. 95], the latter being found by dropping a perpendicular from w to meet the tangent b″ extended. Upon 1, erect a perpendicular line; and placing the dividers on 2, extend to a; turn over to the perpendicular at a″; connect this point with 2, and the line will be the bottom tangent as required on the face-mould. The upper tangent will be the line 2-h, and the angle between the two lines is shown at 2. Make the joint at h square to 2-h, and at a″ square to a″-2.
Fig. 100. Plan of Curved Steps and Stringer at Bottom of Stair.
The mould as it appears in [Fig. 96] is complete, except the curve, which is comparatively a small matter to put on, as will be shown further on. The main thing is to find the angle between the tangents, which is shown at 2, to give them the direction to square the joints.
In [Fig. 97] is shown how to find the angle between the tangents c″ and d″ shown in [Fig. 95], as required on the face-mould. On the line h-5, make h-4 equal to the length of the bottom tangent of the wreath, as shown at h″-4 in [Fig. 95]; and 4-5 equal to the length of the upper tangent d″. Measure from 4 the distance shown at 4-6 in [Fig. 95], and place it from 4 to 6 as shown in [Fig. 97]; upon 6 erect a perpendicular line. Now place the dividers on 4; extend to h; turn over to cut the perpendicular in h″; connect this point with 4, and the angle shown at 4 will be the angle required to square the joints of the wreath as shown at h″ and 5, where the joint at 5 is shown drawn square to the line 4-5, and the joint at h″ square to the line 4 h″.
[Fig. 98] is a diagram of tangents and face-mould for a stairway having a well-hole at the top landing. The tangents in this example will be two equally inclined tangents for the bottom wreath; and for the top wreath, one inclined and one level, the latter aligning with the level rail of the landing.