It was said in a previous case that with a wide rail and the tangents
on the same pitch as the straight rail, the inside of rail has a
crippled appearance. This can be remedied by having the wreath in two
pieces.
Fig. 1 shows the plan laid down so as to make the inside falling line
of rail a straight line; and the risers landing and starting placed in
the springing. To find the radius of centre line of rail, Fig. 1. Make
S N, Fig. 7, equal one step. From N draw N R at 45° with S N, and from
S draw a line at 60° with S N to meet the line from N in R, from R draw
square to and cut S N in P; then R P will be the radius for the inside
of the rail on plan.
Fig. 2 shows the development. Make H R equal stretch-out of centre line
of rail on plan. Set up one tread and risers above and below, with the
risers in the springing. Draw the centre falling line, resting on the
corners and passing through the centre, at the height of half a riser
above the landing. To develop tangents, make C S R Fig. 2, equal C B A,
Fig. 1, and complete tangents as shown. To draw the horizontal trace
at Fig. 1, make A B F, Fig. 1, equal R S F, Fig. 2. Join F C for the
required trace. Draw N O square to it.
Fig. 3 shows the face mould. Make A B F equal A B F and B C equal B C,
Fig. 2, and F C equal F C, Fig. 1. Make C P equal C N, Fig. 1. Draw the
major axis from N square to N C F. Draw from A parallel to F C P to
cut major axis in 7. Now if the drawing is correct A 7 will equal A R,
Fig. 1. With P as centre and N R, Fig. 1, as radius, strike an arc at
S; again, with 7 as centre and A R, Fig. 2, as radius, strike an arc
to intersect the first one in S. Join 7 S and P S extended, and P N
equal O N, Fig. 1. Draw from N parallel to 7 S to cut the major axis in
O, then O will be the centre.
Fig. 4 shows the sections and bevels. Make N N equal radius of centre
line of rail, Fig. 1, with O as centre, and for radius just touching
each tangent, which distances transfer to Fig. 4. All the sections in
this case will be in the centre of the plank.
Fig. 5 shows the face mould for the other side of the plank.
Fig. 6 shows the application of the face moulds to the stuff.
Both joints are square to tangents and face of plank in this case.
[Illustration: PLATE 25.]