DEVELOPMENTS AND INTERSECTIONS
When you have read and understood this chapter,
you should be able to answer the following learning
Describe sheet metal developments.
Explain the differences among parallel, radial,
and triangulation developments.
Sheet metal drawings are also known as sheet metal
developments and pattern drawings, and we may use all
three terms in this chapter. This is true because the
layout, when made on heavy cardboard thin metal, a
wood, is often used as a pattern to trace the developed
shape on flat material. These drawings are used to
construct various sheet metal items, such as ducts for
heating, ventilation, and air-conditioning systems;
flashing, valleys, and downspouts in buildings; and parts
on boats, ships, and aircraft.
A sheet metal development serves to open up an
object that has been rolled, folded, or a combination of
both, and makes that object appear to be spread out on
a plane or flat surface. Sheet metal layout drawings are
based on three types of development: parallel, radial,
and triangulation. We will discuss each of these, but first
we will look at the drawings of corrections used to join
sheet metal objects.
JOINTS, SEAMS, AND EDGES
A development of an object that will be made
of thin metal, such as a duct or part of an aircraft
skin, must include consideration of the developed
surfaces, the joining of the edges of these surfaces,
and exposed edges.
The drawing must allow for
the additional material needed for those joints,
seams, and edges.
Figure 8-1 shows various ways to illustrate seams,
and edges. Seams are used to join edges. The seams
may be fastened together by lock seams, solder, rivets,
adhesive, or welds. Exposed edges are folded or wired
to give the edges added strength and to eliminate sharp
The lap seam shown is the least difficult. The pieces
of stock are merely lapped one over the other, as shown
in view C, and secured either by riveting, soldering, spot
welding, or by all three methods, depending on the
nature of the job. A flat lock seam (view C) is used to
construct cylindrical objects, such as funnels, pipe
sections, and containers.
Note that most of the sheet metal developments
illustrated in this chapter do not make any allowances
for edges, joints, or seams. However, the draftsman who
lays out a development must add extra metal where
The drafter must also show where the material will
be bent, and figure 8-2 shows several methods used to
mark bend lines. If the finished part is not shown with
the development, then drawing instructions, such as
bend up 90 degrees, bend down 180 degrees, and bend
up 45 degrees, should be shown beside each bend line.
Anyone who bends metal to exact dimensions must
know the bend allowance, which is the amount of
material used to form the bend. Bending compresses the
metal on the inside of the bend and stretches the metal
on its outside. About halfway between these two
extremes lies a space that neither shrinks nor stretches;
it is known as the neutral line or neutral axis, as shown
in figure 8-3. Bend allowance is computed along this
You should understand the terms used to explain
bend allowance. These terms are illustrated in figure
8-4 and defined in the following paragraphs:
LEGThe longer part of a formed angle.
FLANGEThe shorter part of a formed angle. If
both parts are the same length, each is called a leg.
MOLD LINE (ML)The line formed by extending
the outside surfaces of the leg and flange so they
intersect. It is the imaginary point from which base
measurements are shown on drawings.
BEND TANGENT LINE (BL)The line at which
the metal starts to bend.
BEND ALLOWANCE (BA)The amount of metal
used to make the bend.