Theatre Design & Technology - Dec 1973 - 10

KNIFE CLAMPS
The construction of the knife clamps (Fig. 5) may
appear somewhat heavier than one might expect. The
reason for this is twofold: the first is to provide a low
resistance connection to the knife and adequate mechanical support. Second, however, is the fact that this large
chunk of metal tends to act as a heat sink and draw off
some of the knife heat in the immediate area of the clamp.
Thus at the base of the knife where mechanical stress is
greatest the knife ru ns somewhat cooler and is therefore
stronger where most needed. The clamps shown are made
of ordinary low carbon steel. Copper or brass wou Id have
been preferable both electrically and thermodynamically
but the cost was prohibitive. Nonetheless, the performance
of the steel clamps has been qu ite satisfactory. 5
COST & CAUTIONS
The economic feasibility of this project will depend for
the most part on the materials available, the anticipated
usage of the tool, and to some extent on the ingenuity of
the technician. For our part, we were extremely fortunate
in having all of the hardware described on hand with the
exception of the 59.00 cabinet for the power supply and a
few small fittings. The major cost items are obviously the
transformers. Our stepdown transformers were acqu ired
some years ago from surplus sources. The "off the shelf"
price of comparable units are approximately 520.00 to
S35.00. 6 Thus a cost of S60.00 to S105.00 plus about
S30.00 for the autotransformer is probable. If this strains
the budget excessively comprom ises are possible. For instance, a thinner (e.g. 3/16") knife would require only
about half as much current though admittedly the loss in
mechanical strength might require slower cuts or reduced
maximum unsupported knife length. A possible variation
on this approach would be to use a moderately large
gauge of nichrome-say B & S #8-and thus reduce the
current drawn by a factor of somewhere between 7 and 10
compared with steel stock of comparable diameter. This

would be particularly feasible if a source of smaller quantities of heavy gauge nichrome were located. If such a
source were not available then obviously the S 100.00
minimum purchase required by the manufacturer would
largely negate any savings in the cost of the power
supply. 1 A final alternate possibil ity is to deliberately use
too few or too small stepdown transformers and then
I imit the duty cycle and/or ventilate to keep them from
burning out. 7 There are limits to this, however, because of
losses due to internal resistance in the transformer secondaries themselves. In any event, transformer heating should
be monitored until some experience is gained with various
heats and knife dimensions.
Like many effective tools certain precautions should
be observed. The hot knife itself presents the obvious
danger of severe burns but if used with the slow burning
styrofoam, it is not a special fire hazard though flammables should obviously be kept well out of the area.
Shock hazard is reduced because the filament transformers
allow the secondaries to "float" above ground. However,
high potentially lethal output currents (albeit at low voltage) are available at the output terminals and should be
treated with respect. Another important precaution is to
provide adequate ventilation for the fumes given off by
the burning/melting process. Our usual procedure simply
is to wait for a pleasant day and move outside!
PERFORMANCE & APPLICATIONS
Aside from low material cost the use of a heated form
knife brings about a number of other benefits. Certainly
one of the most important of these is the creative flexibility it gives the designer. Ornate mouldings that would be
far too expensive or difficult or heavy to make by conventional means can now be easily fabricated. For the
technical director it means that large amounts of identically shaped mouldings can be produced without tying up
expensive power tools and without requiring the efforts of
skilled craftsmen. Indeed, once the knife is bent to shape
unskilled personnel can learn to operate this tool with
only a few minutes instruction.
The technique is not without its limitations. Generally
speaking, the surface produced by the knife is somewhat
more porous than one could wish and there is always the
intrinsic fragility of styrofoam. Surface porosity is somewhat controllable by varying knife temperature and feed
rates. The effect of these variables, however, seems to vary
somewhat with the specific type of styrofoam used and
thus optimum combinations have to be determined experimentally. One feature of operation, however, is always
present-namely that once a feed has been started it should
be kept as constant as possible and under no circumstances
should the feed be stopped for to do so for even a
moment would cause a noticeable trough to form at that
point.

Figure 5.

Both surface texture and strength are of course improved by using high density foam. Surface strength and

The knife clamps.

THEATRE DESIGN AND TECHNOLOGY

DECEMBER. 1973

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