2 2 Development of an Offset
Page 32
There are many joints in sheet
metal. Some joints are called
self-secured joints. This is
where we use the job’s own
metal. Examples are Groove
joint, Slip joint and Peined
joint. They all have different
amounts of metal involved in
their assembly. It is important
to allow for these joints when
marking out a pattern.
Figure I, shows the elevation.
Figure II, development of the
lower socket, seam on the
inside.
Figure III, development of the
lower socket, seam on the
outside.
After determining the outline
of the elevation of figure I as
mentioned in section 1 above,
draw the semi-circle at point d,
divide into six equal parts and
through points 2 and 6 draw
the so called generating lines
parallel to the middle line. The
joint line of the two pipe
sockets is a straight line. The
development is obtained as
described in table 1 by laying
off the line segment 1 to 2 (or
the arc in figure1) 12 times on
a base line, drawing the
perpendiculars at points a to g,
and on these points laying off
the respective line segments
aA, bB, etc. of figure I. As
mentioned above, it is very
important when stepping of
the 12 arc lengths or divisions
not to accumulate an error
which would make the
circumference longer or
shorter than required.
Note: The pipe can also be
developed by calculating it’s
circumference (diameter x π)
and then by dividing the
circumference in half,
quarters, and then each quarter
into three equal and accurate
divisions with no
accumulation or error.
2.2 Development of an
Offset
Figure I: Elevation.
Page 33
Figure II: Development of
socket A.
Figure III: Development of
socket B.
Figure IV: Development of
socket C.
First determine the outline of
the elevation of the offset in
figure I and take care to give
the three segments or pipes
the correct diameter. Then
draw the semicircle with the
centre at point 4, divide it into
six equal parts and through
points b, c, e, and f draw the
so-called generating lines
parallel to the middle lines of
the three segments. The
development of the two
sockets A and C is described
in the previous tables above.
For the development of the
middle socket B draw the
auxiliary line 1 to 7
perpendicular to the middle
line in figure 1. This auxiliary
line is constructed at a random
point on the centreline 11 to
18. The generating line yields
points 1 to 7.
For the development of figure
III determine the base line
with points 1 to 7 as usual.
Draw the perpendiculars at
these points and on these lines
lay off the various lengths of
the lines above and below the
base line. All these lines
should be the same length.