Fig. 1 shows the plan of a half-space landing with the centre line of
rail struck with a 9-inch radius, and the risers starting and landing
placed in the springing. As the steps are only 10 inches it is very
evident that if the tangents A B and D E were the same pitch as the
straight rail the tangent across the well would pitch the wrong way,
which of course would not do. To obviate this the tangent A B must only
rise half a riser over B, and the tangent D E only fall half a riser
over D, and the tangent across the well level and the joint on the end
of shanks must be bevelled so as to joint square to the straight rail.
Fig. 1 shows the plan with the risers in the springing.
Fig. 2 shows the elevation. Make S N equal stretch-out of centre line
of rail. Set up one step above and below and draw landing; also draw
the centre falling as shown, this line must pass through the centre at
the height of half a riser above the landing, and make a good easing
into the straight rail above and below the springing. For development
of tangents make F B equal A B, Fig. 1, and draw from B to the joint
below the springing; draw R P square to the straight rail, which will
be the joint, and R N square to the tangent A B.
Fig. 3 shows the face moulds. As the tangent B C is level draw A B C
at right angles; make R A B equal R A B, Fig. 2, and B C equal B C,
Fig. 1, and complete the mould as usual. It will be seen that the
sections will be in the centre of the plank at both joints. For
section P, make F H, Fig. 2, equal A P, Fig. 1, and H J, Fig. 2,
equal H J, Fig. 3. The difference between J and the falling line is
what the section is out of the centre of plank as shown at Fig. 4.
The difference between the tangent A B and the falling line at the
springing shows that the section A is out of the centre. The stuff must
be thick enough to get this section out as required. Make R Y, Fig. 2,
equal half thickness of plank. Add Y C to the end of shank of one face
mould and reduce the other to the same amount, as shown by the shaded
parts.
[Illustration: PLATE 21.]