Page 219 - CITS - Welder - Trade Theory

WELDER - CITS

Fig 4

Equipment is available for both manual and mechanical cutting. A basic plasma arc cutting circuit is shown in
Fig.1. It employs direct current straight polarity (DCEN). The nozzle surrounding the electrode is connected to the
work piece (positive) through a current limiting resistor and a pilot arc relay contact.
The pilot arc between the electrode and nozzle is initiated by a high frequency generator connected between the
electrode and nozzle. The orifice gas ionized by the pilot arc is blown through the constricting nozzle orifice and
forms a low resistance path to ignite the main transferred arc between the electrode and the work piece when the
ON/ OFF switch is closed. The pilot arc relay may be opened automatically when the main arc ignites, to avoid
unnecessary heating of the constricting nozzle. The constricting nozzle is of copper and normally water cooled to
withstand the high plasma flame temperature (about 20000°K) and to have longer life.

In conventional gas plasma cutting, discussed above, the cutting gas can be argon, nitrogen, (argon + hydrogen),
or compressed air. For all the cutting gases other than compressed air, the non-consumable electrode material is
2% throated tungsten. In air plasma cutting (Fig 2) where dry, clean compressed air is used as the cutting gas, the
electrode of hafnium or zirconium. In used because tungsten is rapidly eroded in air. Wet and dirty compressed
air reduces the useful life of consumable parts and produces poor quality. Several process variations are used to
improve the cut quality for particular applications. Auxiliary shielding in the form of gas or water is used (Fig 3) to
improve the cut quality and to improve the nozzle life. Water injection plasma cutting (Fig 4) uses a symmetrical
impinging water jet near the constricting nozzle orifice to further con strict the plasma flame and to increase the
nozzle life. Good quality cut with sharp and clear edges with little or no dross is possible in water injection plasma
cutting. Process variables (Fig 5 & 6)

Fig 5 Fig 6

i Torch design - constricting nozzle shape and size.
ii Process variation - dual gas flow, water injection, air plasma.

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CITS : C G & M - Welder - Lesson 61-76 CITS : C G & M - Welder - Lesson 61-76