5.13.
GLASS KNIFE MANUFACTURE
151
Only two tools are needed to break knives. The hardened wheel-cutters
that glaziers use to score glass so that it can be broken properly, are
generally available in hardware stores. One does need, in addition, a
pair of glazier's pliers [Fig. 15(f)]. These are designed so that they fit over
fairly thick glass, and their straight broad lips make a long linear contact
with both sides of the glass. These are available from major glass companies.
I think it is fair to say that almost everyone breaking knives today
modifies the glazier's pliers, and uses them in a way different from the
intention of the manufacturer and the professional glazier. It was Weiner
(1959) who first proposed fastening a narrow strip of adhesive tape over
the middle of the lower lip of the pliers. This then serves as a fulcrum,
and to break glass this is oriented just below the score line, so that the
tape is parallel to the score line in the glass, and the lips of the pliers
are right angles to it. [Figs. 15(b) and 16(b)]. When the pliers are squeezed
the tape serves as a pressure point, and the crack starts and develops from
there.
Experience indicates that Weiner's modification of glazier's pliers can
be improved by also adding two other thin strips of adhesive tape at the
lateral edges of the upper lip, which then serve as pressure points [Fig.
15(f)]. Also, if the lips of the pliers are covered with a thin layer of tape,
this exerts a damping effect, and allows the break to be made more slowly
than otherwise would be possible. This is an advantage, for slow breaks
yield much flatter fracture surfaces than do fast ones [Figs. 14(A) and (B)].
In practice, we use vinyl plastic electrical tape for covering the tips of
the pliers, and build up the center fulcrum and lateral pressure points
with strips of the same sort of tape, approximately 1 mm wide. The fulcrum and the pressure points become flattened after a few uses and have
to be re-established. It is possible to accomplish much the same end result
by grinding the tips of the pliers so that the lower lip is slightly convex
and the upper one concave. Such modified pliers are available from Ivan
S. Sorvall Inc., Norwalk, Connecticut. Saikotos and Maiolatesi (1964)
indicate that somewhat similar pliers are available from the Sommers
and Maca Glass Machinery Co., 5501 W. Ogden Ave., Chicago, Illinois.
ducing a relatively flat surface. (C) Score limited to a short starting line (arrow, lower
right), then slow "free-break," producing an almost perfectly flat surface.
(D) When two free-breaks intersect (at 90°), nearly straight and smooth edges can be
produced.
GLASS KNIFE MANUFACTURE
151
Only two tools are needed to break knives. The hardened wheel-cutters
that glaziers use to score glass so that it can be broken properly, are
generally available in hardware stores. One does need, in addition, a
pair of glazier's pliers [Fig. 15(f)]. These are designed so that they fit over
fairly thick glass, and their straight broad lips make a long linear contact
with both sides of the glass. These are available from major glass companies.
I think it is fair to say that almost everyone breaking knives today
modifies the glazier's pliers, and uses them in a way different from the
intention of the manufacturer and the professional glazier. It was Weiner
(1959) who first proposed fastening a narrow strip of adhesive tape over
the middle of the lower lip of the pliers. This then serves as a fulcrum,
and to break glass this is oriented just below the score line, so that the
tape is parallel to the score line in the glass, and the lips of the pliers
are right angles to it. [Figs. 15(b) and 16(b)]. When the pliers are squeezed
the tape serves as a pressure point, and the crack starts and develops from
there.
Experience indicates that Weiner's modification of glazier's pliers can
be improved by also adding two other thin strips of adhesive tape at the
lateral edges of the upper lip, which then serve as pressure points [Fig.
15(f)]. Also, if the lips of the pliers are covered with a thin layer of tape,
this exerts a damping effect, and allows the break to be made more slowly
than otherwise would be possible. This is an advantage, for slow breaks
yield much flatter fracture surfaces than do fast ones [Figs. 14(A) and (B)].
In practice, we use vinyl plastic electrical tape for covering the tips of
the pliers, and build up the center fulcrum and lateral pressure points
with strips of the same sort of tape, approximately 1 mm wide. The fulcrum and the pressure points become flattened after a few uses and have
to be re-established. It is possible to accomplish much the same end result
by grinding the tips of the pliers so that the lower lip is slightly convex
and the upper one concave. Such modified pliers are available from Ivan
S. Sorvall Inc., Norwalk, Connecticut. Saikotos and Maiolatesi (1964)
indicate that somewhat similar pliers are available from the Sommers
and Maca Glass Machinery Co., 5501 W. Ogden Ave., Chicago, Illinois.
ducing a relatively flat surface. (C) Score limited to a short starting line (arrow, lower
right), then slow "free-break," producing an almost perfectly flat surface.
(D) When two free-breaks intersect (at 90°), nearly straight and smooth edges can be
produced.