Fig. 1 shows plan of centre line of rail enclosed with tangent A B C.
Draw risers, landing, and starting half a tread from B. This will cause
the tangents on face moulds to be the same pitch as the straight rail.
This arrangement answers very well if the rail is small, but if the
rail is a wide one, say 4 inches or more, the inside has a crippled and
awkward appearance.
Fig. 2. Make R S equal stretch-out of the quadrant, Fig. 1. Set up
one tread and riser above and below, as they occur on the centre line
of rail, Fig. 1. For development of tangents make C 2 1 equal A B C,
Fig. 1. Square out a level line from the springing at S to cut the line
1 at A. Join A C for tangents, which will be the same pitch as the
straight rail.
Fig. 3 shows face mould. Make C B S equal C B A and B C equal B C,
Fig. 2, and S C equal the diagonal of the square on plan. Draw C O
and B O parallel to A B and B C. Draw sine major axis and sine minor
through O. Notice the major axis is parallel to the diagonal from A to
C, and the minor axis is the other diagonal. Both pitches being the
same throws the horizontal trace parallel to the diagonal from the
centre to B. One bevel will do for both shanks, as the pitches are the
same. The falling line will be in the centre of plank at both ends and
at the minor axis.
Fig. 4 shows the section, which is the same for both ends, only of
course the reverse hand.