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A simple method of cutting glass

by Hans Schmidt

In many countries around the world sections of bouttles are to be found - especially at markets in the informal sector - which are used as cheap drinking glasses, vases, or containers for everyday items. But bottle sections can be used for many purposes, not only in the househoId. They can also be used in schools, for any and of experiment not involving heating: bottle bottoms as watch glasses, a short section as a crystallisation dish, a longer section as a beaker, and the top section.

There are many methods of cutting bottles. In seminars on DIY production of teaching materials for science classes two methods have become established which are easy 10 master and produce well-cut glass. Both of these methods are based on the principle that heating causes stresses in the glass which cause it to crack.

Making a glass cutter

Some years ago, on Pashtunistan Square in Kabul we saw a boy using a simple device to cut bottles. The method was by no means new, but we improved it for laboratory use and also developed another method which works without electricity. However, as regards cutting accuracy the electric method is superior 10 all others. It can even be used to cut thin neon tubes.

The cutter has three main components:

· the cutting device (Ba- Bg)

· a connecting cable (Bb and Bh)

· an electric hotplate as a dropping resistor (Bi).

The cutting device is easy 10 assemble. First hammer two de-rusted steel nails or rods, 25 cm apart, into a dry wooden board (Bd) measuring approx. 30 x 15 x 4 cm. Fasten tensioning spring, about 2 - 3 cm long, to one of the nails the spring must be free 10 move. The cutting wire (Be) heats up and expands when current flows through it; the spring keeps it taut. The spring is held at the required cutting height by a wooden clothes peg (Ba).


Figure

Fasten a length of perforated metal strip (BI) measuring approx. 15 x 1.5 cm to the second nail (Bg) so that its height and length can be adjusted. The strip makes it possible to alter the tension of the cutting wire almost infinitely to suit bottles of different diameters, and to fix the wire at the desired cutting height.

Ordinary steel wire is not suitable for the cutting wire (Be), but wire from a heating spiral is ideal. Draw a length of heating spiral filament over the edge of a table to flatten it and then fasten eyes to its ends. Slip the eyes onto small hooks on the ends of the spring and the metal strip. The individual elements must all be good conductors.

Pass the cutting wire around the bottle so that there is no contact at the crossover point, but the gap is as small as possible. Temperatures at the crossover point can be very high - the wire often gets red-hot, and thin neon-tube glass cracks after less than one minute of heating. If the glass is thick, it will crack faster if you let a drop of water fall onto the crossover point of the cutting wire after about 1 - 2 minutes (Bj). The cracking of the glass will be clearly audible. After switching off the power supply, gentle tapping with a blunt object will help to continue the crack all round the bottle, even if the glass is thick.

The cutting device is connected in series to an ordinary electric hotplate (Bi) rated for 1000/1500 watts, with a heating element in the form of a flat open spiral. For cutting very thick glass, the resistance of the heating element can easily be reduced with a bridge. The cutting device is connected to the hotplate by a cable (Bh) with alligator clips or other metal clamps. The cutter can be completely dismantled, so that the hotplate can still be used in the kitchen.

Safety precautions

The cutter will also work with a 12volt car battery. In this case a dropping resistor (the hotplate) is not needed.

Caution: the entire cutting device is live when the power supply is on. Pull out the plug before touching it.

We have cut over a thousand bottle sections in seminars, so far without a single accident. Switches can easily cause both teachers and students to become careless. The orders "plug in" and "plug out" have proved to be the most effective way of ensuring safety.

The sharp edges of thicker glass can be smoothed with fine round or flat files, glasspaper, or a grindstone. But thin glass often cracks during this procedure (C).


Figure

Thick-walled glass can even be drilled. For this you need a twist drill with a tip ground to a pointed conical shape. Suitable lubricants are petroleum, diesel or a lubricating oil. Electric drills should be set to a low speed (D).


Figure