The principal thing to consider in planning stairs of this kind is to
place the riser starting, so as to get a good falling line. Fig. 1
shows the plan with the centre line enclosed with tangents A B C D E.
Fig. 2 shows the elevation with the centre falling line and development
of tangents. Make R N S equal stretch-out of centre line of rail,
Fig. 1. Draw the landing and the level part of falling line 4 inches
above it to cut the centre line at H. Draw H E to pitch of pitch-board
and complete the falling line from E to J; set up the height of a riser
above the landing on the right, and draw the top of step to cut the
line H E for position of riser. Make E P, Fig. 1, equal S P and draw
face of riser starting on plan. This riser must be slightly curved, so
as to get it the same width on the end as the others. For development
of tangents, make 1 2 3 4 E equal A B C D E, Fig. 1. From 4 square down
a line to cut H E in D; from where the falling line cuts the centre
line, square out a level line to cut 3 in C; join D C extended to cut
the line 2 in B; draw A B level. Then A B and B C will be the tangents
for the bottom mould, and C D and D E for the top mould; make E F,
Fig. 1, equal W F, Fig. 2, and join F C for H trace.
Fig. 3 shows the face mould for the upper wreath. Make E D F equal
E D F, Fig. 2, and F C equal F C, Fig. 1, and D C equal D C, Fig. 2.
Draw the major axis through the centre square to F C. With S as centre
and O N, Fig. 1, as radius, strike an arc at N; draw V L through the
centre and tangent to the arc. For section marked 3, make N H equal
O H, Fig. 1, draw H J parallel to V L, make J 3 equal H 3, Fig. 1.
Draw J P parallel to S N, and P L square to major axis, join L 3 for
section line.
Fig. 4 shows the sections, width of mould, &c. The process of getting
the bevels being the same in every case, it would be useless repetition
to describe it every time. The shaded parts of sections show the
superfluous stuff to come off the top side of wreath at each section.
Fig. 5 shows the face mould for the other side of the wreath. In
applying these moulds care must be taken to get the twist the right way.
Fig. 6 shows the face mould for the lower wreath. As this one has only
one pitch, draw A B C at right angles and make A B and B C equal A B
and B C, Fig. 2. With A as centre and A O, Fig. 1, as radius, strike an
arc, draw V L through the centre and tangent to the arc; make H S equal
O S, Fig. 1; draw S 4 parallel to V L, make 4 2 equal S 2, Fig. 1. Draw
4 5 parallel to A H, and 5 6 square to major axis A O; join 6 2 for
section line.
Fig. 7 shows the sections for this wreath. The section at the joint C
will be in the centre of plank, but that at A and 2 will be below the
centre. The difference between O H, Fig. 6, and Y J, Fig. 2, is what
the section on shank will be below the centre; and that between O 5,
Fig. 6, and 5 6, Fig. 2, for section 2. Draw the sections below the
centre as shown at Fig. 7.
Fig. 8 shows a sketch of the bottom wreath with the inside and outside
worked. The shaded parts show the superfluous stuff to come off top and
bottom.
[Illustration: PLATE 19.]