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Nonconventional

cutting of plateglass using hot

air jet.

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IntroductionGlass has been one of the important engineering

materials used by man since ancient times. Withthe advances made in the glass industry inrecent times, it has become the most versatileengineering material for use in both domesticand modern, industrial and laboratory

applications. Glass is chemically stable in theenvironment. It has an excellent property toresist attack by water vapor, CO2, and bacterialand plant organisms. Plate glass is used for

windows in buildings, ships, automobile,locomotive cars, and microwave ovens. It isused in decorative toys to astronomical mirrorsand telescopes to microscope.

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Advantages of glassAttractive looks.

Chemical stability towards

environment.

Nonporosity.

Transparent nature.

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Conventionally, plate glass cutting is usually done

by using a diamond point tool. Researcher have

developed other glass cutting methods such as laser

and abrasive water jet.

Disadvantages of cutting glass using diamond tool

is marking and scribing on the glass surface should by

made as per desired profile with extreme skill and thecut surface obtained is always irregular, wavy and with

poor surface finish.

Even though laser cutting of glass has some

advantages it possess some disadvantages also.The initial capital investment is required for laser cutting

systems and it leave a heat affected zone in the cut

edge which produce local thermal stress.

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Water jet cutting does not produce heat affected

zone but cost of equipment ,cost of power,

abrasive material is high and nozzles should bereplaced frequently due to wear.

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Disadvantage of the available glass cutting methods.

Sl.no. Cutting method Disadvantages

1. Diamond point tool Time consuming

Difficult to maintain close tolerance,

More wastage of material,

Non straight profiles cannot be cut in a single

step

2. Laser cutting Ve

Microcracks at the cut surface,

Evaporation and resolidification at the cut

Edge, leaves heat affected zone.

3. Abrasive water jet Very high initial capital equipment,

cutting Surface finish varies with the cutting speed

Nozzle wear, requires de-ionized high

pressure water.

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Nonconventional cutting

In this method, the hot air jet (200-2800C)

issuing from a nozzle is impinged on the plate

glass to be cut. The hot air jet or the X-Y

coordinate table, on which the plate glass is

placed, can be moved at the required speed.

Any complex profile could be easily and

economically generated in a single step by thismethod.

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Principle of hot air jet cutting

The method of hot air cutting of plate consists of

(a) A crack initiation at the scribed glass edge,and

(b) Propagation of the crack in glass owing to

thermal stresses set up by hot air jet.

The scratch at the starting edge act as a region of

stress concentration, hence the glass will breakor fail at that region.

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Heating a moving plate by a stationary hot air

jet causes thermal stresses in glass due to

nonuniform heating. The localized regions on theglass surface below the jet expand due to

heating, the posterior region of the crack tip

experiences a fall in temperature resulting in

contraction of the expanded element and hence

the crack advances at the tip. The crack

propagation is quicker if the posterior crack tip

region is subjected to a wet environment. This isdue to acceleration of contraction.

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Schematic diagram of the crack

propagation

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The key parameters influencing the cutting process

The thermal energy input is required to induce thermal

stress . The thermal energy input depends on the air

flow rate and its temperature.

The thermal energy input required varies with the

thickness and the composition of the glass to be cut.

Increasing the air pressure and temperature favour thecutting process.

The nozzle diameter also plays a important role on the

pattern of thermal energy input, thereby influencing the

cutting process. The nozzle of smaller diameter gives afiner air jet and hence, the cutting process is effective

and quicker and vice versa.

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Disadvantage of hot air jet cutting

It is impossible to cut interior holes. The

cutting process has to start at the glassedge.

It requires scratch for quick crack initiation.

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Conclusion

The glass cutting making use of hot air jet is

simple and cheaper than other processes andcan produce nonstraight profile and can

successfully cut plate glass in the thickness range

of 2-20mm.

It is a noncontact process and hence the

process is clean and Safe. The surface finish of

the cut section is smoother than those obtained

by the diamond tool and water jet cutting. Nomicrocracks is developed at the cut edge. The

process can be computerized.