Agriculture Reference
In-Depth Information
through the brick. This, in turn, means that cutting is less stressful on the wire
which because it thins through repeated abrasion at the centre of the blade
length, breaks less often. The hardness of the rubber, occasional inclusions
within it and even the manner of how the cutter uses the bow saw, all play a
role in how long a blade will last. To gain a well-twisted wire it is important to
keep the brace pulled tightly to strain the wire during twisting. One must always
conclude this twisting action by turning the brace backwards several revolu-
tions so that the tension created in the wire is released, otherwise the blade
will spring dangerously; and this would be even more acute on a long length
of wire.
Hammond (1889, 91-3) provides variations on making blades and types of
bow saws used (see Fig. 130 on page 280):
…Some prefer to have the wire twisted before it is fixed to the frame; others to
fix it in the frame and twist it there.
If the latter way is adopted there must be a small winch attached to the bottom
of the frame at d., and a small shaft running through the latter, with a hook to
receive the wire. This wire is then fastened to a nail or screw at e, brought under
that end, and fastened to the hook of the shaft running through the end d. Then
if a plain piece of
3
⁄
16
wire is made to run through the top f g, and fastened with a
nut or thumbscrew, the winch can be turned and the wire twisted and tightened
at pleasure.
If the wire is twisted before it is fixed in the frame, the tightening is done by a
piece of strong string and windlass (such as carpenters tighten their bow-saws
with), or by means of two small rods of iron, each running half-way across from
f to g, with a thread worked on them in the centre where they meet and so
loosened or tightened by means of a 'union;' and this method, if properly con-
structed, is not to be despised…
Tensioning the Twisted Wire Blade in the Bow Saw and Cutting Bricks
With the wire twisted to suit the cutting required it was then only left to attach
it to the bow saw and to tension it sufficiently to enable it to perform as a blade.
This basically involved drawing the tops of the two handles inwards so their ends,
to which the blade is attached, move further apart thus tensioning the blade.
There were three main methods employed do this. The first involved a simple
twisted cord that is looped around the tops of the opposing handles, which is
then tensioned with a wooden 'toggle' that is placed and turned between the
two lengths of cord at the centre of the saw. Once sufficiently tensioned the tog-
gle is slid downwards so that it is prevented from un-winding by the centre bar
of the saw. The final two methods involve threaded steel rod placed across the
saw and through the tops opposing handles. The first and most simple method
