Theatre Design & Technology - Dec 1973 - 9

knife length is limited to not much over 6". This is not to
deny the value of nichrome wire for small work where
the detail possible and a very noticeable improvement in
2

cut finish makes it obviously the preferable technique.
Nonetheless, the fact remains that moldings much over 6"
in breadth are unattainable with the usually available
gauges of nichrome while if deep convolutions are required by the design the maximum usable knife span is
reduced still further. Problems also occur when the usual
gauges of other types of wire are used, i.e., at working
temperatures the material becomes so soft that it is unable to maintain it geometry against cutting pressure. The
rod type heating elements used in many electric ovens
might be usable except that most types are essentially un-

careful to phase both primaries and secondaries correctly)
to give a continuous duty capacity of 75 amps.3 The
paralleled primaries of these transformers were connected
to the output of a 10 amp. autotransformer which
through the stepdown transformers gave the ability to

vary the open circuit output voltage from
to 6.3 volts
4
thus providing temperature control. Though the average
position of the autotransformer tended to remain fairly
constant the other variables involved (such as varying
knife length) was sufficiently uncontrollable as to make
adjustable output voltage a useful feature. The autotransformer was in turn fed by a standard 2 pole fusable
switch.

OUTPUT CIRCUITRY

formable after original manufacture.
The basic premise of our solution to the problem
(Figures 1 - 3) was that any conductor can be made a
heating element provided sufficient current is made to
flow. Though this solution is not difficult-as in the case
of small scale moulding with nichrome wire-where no
point of the knife is very far from support, in large scale
work where styrofoam is most useful, the length of the
knife requires a material of relatively large cross section.

KNI FE DESIGN

As can be seen in the schematic diagram each of the
transformer secondaries were brought out separately to a
terminal of one of the three-circuit 50 A range receptacles
with #4 AW.G. wire. Th is rather heavy wire was necessary
at these low voltages and relatively high currents since
under these conditions even small resistances could have
resulted in appreciable loss. Bringing the transformer leads
out separately to the range plug terminals is a recently
added feature. Its function is to allow the secondaries to
be connected in series rather than only in parallel by using
the special jumper shown in the schematic. The purpose
of this is to make higher output voltages available; the

Several criteria had to be satisfied in the selection of a
knife material. First, inasmuch as the forms were likely to
be quite complex it was desirable that the knife material
be formable at room temperature. Second, since presumably an infinite variety of moulding contours could
be encou ntered the knife material should be inexpensive

The three prongs of each of t~e range plugs of the
standard rig are paralleled together in the 2/0 welding
cable leads that connect the power supply to the knife

enough to be expendable. Another important considera-

clamps.

reason for which will be discussed later.

tion was the effect of the electrical resistance of the knife
material on the power supply. Assuming more or less
constant wattage to achieve cutting temperature, the
severity of current demands on the power supply would
be a function of heating element resistance and thus
a material of relatively high resistance would be desirable.
Happily, ordinary low carbon steel seemed to be a fairly
satisfactory compromise among the above criteria. Along
with its obvious physical attributes its electrical resistance
is approximately 6 times that of copper. Preliminary
experimentation indicated that for most purposes 1/4"
round iron bar stock provided a suitable knife cross
section.

POWER SUPPLY
Based upon the use of 1/4" round iron bar stock as the
knife material it was experimentally determined that given
an average knife length of 2' to 4', a current of between
150 and 200 amperes at 1.5 to 2.0 V .A.C. would be required. Thus the major construction involved in this
project was the power supply. The principal components
of the unit we constructed were a group of 3 heavy duty
120 V to 6.3 V stepdown filament transformers each
with a secondary current rating of 25 amps (Fig. 4). The
secondaries of these were connected in parallel (being

THEATRE DESIGN AND TECHNOLOGY

Figure 4.

DECEMBER. 1973

The power supply with the cover removed.

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