In [Fig. 91], a view of [Fig. 90] is given, showing clearly the inclination of the tangents c″ and d″ over and above the plan tangents c and d. The central line of the wreath is shown extending along the sectional plane, over and above its plan lines, from one joint to the other, and, at the joints, made square to the inclined tangents c″ and d″. It is evident from the view here given, that the condition necessary to square the joint at each end would be to find the true angle between the tangents c″ and d″, which would give the correct direction to each tangent.
In [Fig. 92] is shown how to find this angle correctly as required on the face-mould to square the joints. In this figure is shown the same plan as in Figs. [90] and [91], and the same inclination to the tangents as in [Fig. 90], so that, except for the portion marked “Section,” it would be similar to [Fig. 90].
To find the correct angle for the tangents of the face-mould, draw the line m from d, square to the inclined line of the tangents c′ d″; revolve the bottom inclined tangent c′ to cut line m in n, where the joint is shown fixed; and from this point draw the line c″ to w. The intersection of this line with the upper tangent d″ forms the correct angle as required on the face-mould. By drawing the joints square to these two lines, they will butt square with the rail that is to connect with them, or to the joint of another wreath that may belong to the cylinder or well-hole.
Fig. 90. Two Tangents Equally Inclined.
Fig. 91. Plan Lines Projected into Oblique Plane
Inclined to Two Sides.