342 Illustrated Theatre Production Guide 2 ed
has approximately the same strength and rigidity as a
2×4. Some small units are better constructed from 1
or even
3
4
stock when that rigidity is not required.
Structural Design for the Stage, by Holden and Sammler
(Focal, 1999), is an excellent resource for anyone inter-
ested in studying the structural properties of specific
materials.
Structural steel tubing of the sort used to construct
scenery is composed of mild steel, a name given to steel
with a low carbon content that has excellent malleability
and a Rockwell C scale hardness number in the 20s. It
is easily drilled with ordinary tools.
WELDING EqUIPMENT
Two very common types of welders are used to join
steel. One is the generic arc welder and the other is
the metal inert gas type, which is abbreviated as MIG. A
MIG welder is by far the easiest kind of welder
to use for welding mild steel and is far and away the
most popular choice. A MIG welder is somewhat
more expensive to purchase than a standard arc welder,
because its mechanism is much more complicated.
An arc welder is essentially a very large rectifier unit
that changes AC power into DC power. Electronically,
some machines are constant current while others are
constant voltage. Two long cables are attached to the
output of the machine. The ground has a clamp on the
end that gets connected to the steel you are welding.
The other cable has a smaller clamp to hold an electrode,
which is a consumable wire stick coated with flux. The
flux is a catalyst that aids in the welding process by
producing CO
2
gas when heated by the welding process.
The CO
2
gas prevents the steel from oxidizing at high
temperatures. When the electrode is brought into close
contact with the steel, a circuit is completed between
the (usually) positive output of the welder, the elec-
trode, the steel being welded, the ground clamp, and
the negative terminal of the welder.
Because the electrode is only tangentially in contact
with the steel, the electricity must jump across that small
gap in order to complete the circuit. When that happens,
a small spark or arc is created. The arc is very hot and
melts the steel. The electrode gets hot also, and the tip
of it melts as well. Protected by the flux, molten steel
from the electrode mixes with the molten steel on the
edges of the joint being welded. Three pieces of steel are
being melted and joined: the two pieces being con-
nected and the electrode from the welder itself. When
the molten steel cools and hardens, all three should be
joined at the molecular level, so that theoretically they
are all one homogeneous crystalline structure. (In reality,
the heating and cooling make a slight difference in the
composition of the welded area.)
There are several fairly problematic consequences
of arc welding. Of course the light produced is extremely
bright and will cause serious injury to your eyes if you
look at it from a close distance. (The distance from the
arc while you are welding is very, very close.) Special

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