FOCUS ON PLASMA CUTTING cut edge . This is probably the most popular variation , as many different gas combinations can be used to produce the best possible cut quality on a given material .
■ Water shield plasma cutting : This is a variation of the dual gas process where water is substituted for the shield gas . It produces improved nozzle and workpiece cooling along with better cut quality on stainless steel . This process is for mechanised applications only .
■ Water injection plasma cutting : This process uses a single gas for plasma and utilises water either radially or swirl-injected directly into the arc to greatly improve arc constriction , therefore arc density and temperatures increase . This process is for mechanised applications only .
■ Precision plasma cutting : This process produces superior cut quality on thinner materials at slower speeds . This improved quality is the result of using the latest technology to super-constrict the arc , dramatically increasing energy density . The slower speeds are required to allow the motion device to contour more accurately . This process is for mechanised applications only . produce high-quality parts . As the saying goes ‘ a chain is only as strong as its weakest link ’ and that is very much the case with a plasma cutting machine installation ,” Walsh told ISMR .
There are three major components inside the torch body itself ; electrode , gas baffle ( swirl baffle ) and nozzle . These items are called consumables . They are consumed over time during the plasma process and must be replaced . The electrode is connected to the negative side of a DC plasma power supply . The nozzle is connected to the positive side but is electrically isolated using a normally open relay .
Plasma cutting process variations
There are five basic variations of the plasma cutting process :
■ Conventional plasma cutting : This process generally uses a single gas ( e . g . air or nitrogen ) that cools and produces the plasma . Most of these systems are rated at under 100 Amps , for cutting materials under 5 / 8 ” thick . It is primarily used in hand-held applications .
■ Dual gas plasma cutting : This process utilises two gases , one for the plasma and one as a shield gas . The shield gas is used to shield the cut area from atmosphere , producing a cleaner
Plasma process variables
The variables involved in plasma cutting must be well controlled to achieve maximum cut quality , maximum nozzle / electrode life and maximum production . A balance must be maintained between them .
The purity of gas is essential for good cut quality and long electrode life . As far as gas pressure / flows are concerned , each nozzle is designed to perform at an optimum current based on a given gas pressure / flow . Increasing this pressure can result in a decrease in electrode life .
Gases required for most applications are a start gas , shield gas and cut gas . A few situations require a second shield gas . Argon gas is used for plasma marking . Material type and thickness , cutting quality , speed and production cost are variables to consider when selecting gas combinations .
“ High-tolerance plasma arc cutting ( HTPAC ) is an important development in plasma arc technology . The process gives better precision on material of under 12mm thick and can be a low-cost alternative to laser cutting ,” explained cutting specialist TWI , which celebrates its 75th anniversary this year . n
Water purity is also essential . The water injection plasma process requires de-ionized and filtered water . Suspended solids , dissolved minerals and other factors affect the conductivity of water and nozzle life and increase the possibility of high-frequency interference .
Cut water flow rate should be set to the amount specified . Excessive water flow will result in short electrode life and an unstable arc . Low water flow will result in insufficient cooling , affecting nozzle life .
Kerf is the width of material ( perpendicular to the torch and cut axis ) removed during the plasma cutting process . Kerf is affected by three major variables ; cutting speed , cutting amperage and standoff ( the distance maintained between torch and workpiece after piercing , but while cutting ).
Arc voltage is dependent upon current ( amperage ); nozzle orifice size ; standoff ; cut gas flow rate ; cut water flow rate ( if applicable ) and cutting speed .
18 | sheetmetalplus . com | ISMR July / August 2021