BASIC INFORMATION

OXY-FUEL CUTTING PROCESS

The oxy-fuel cutting process is the oldest and most used, suited to cutting carbon steel, low alloy steels and titanium, the process is not suitable for cutting non-ferrous metals such as aluminium, stainless steels, nickel alloys, brass or copper. The thickness of material that can be cut ranges from 3mm to 300mm using standard equipment, using specialist equipment the range can be increased to 3000mm.

Fuel gases vary in performance verses cost, Acetylene produces the hottest flame at 3160°C, other gases include, MAPP - 2976°C, Propylene(LPG) - 2896°C, Propane - 2828°C, Natural gas - 2770°C. Lower cutting gas temperatures are reflected in longer pierce times, slower travel times and bigger heat affected zones (HAZ), before committing to use a fuel gas type be aware of the oxygen to fuel gas ratio used to achieve the desired cutting performance. Furthermore, gas delivery, storage or safety issues may influence the choice. The cutting process is performed by using a torch fitted with a suitable size nozzle, fuel gas and oxygen are fed under regulated pressure into the torch, pre-heating the material to a temperature between 700°C and 900°C the material should be bright red but not yellow in colour, this is known as the kindling temperature, introduction of the main oxygen jet causes an exothermic reaction, the steel becomes oxidised (dross) and is blown through the workpiece. Use of a CNC system is recommended to achieve the optimum cut profile finish and repeatability, the material should where possible be free of rust grease or other contaminants, correct nozzle size, gas pressures, flame shape, flame to workpiece height and torch travel speed should be checked and tested before production runs.

Mechanised cutting systems are suited for use in heavy fabrication, steel processing, shipyards. Manual oxy-fuel cutting and gouging is used in the above industries with the addition of cutting scrap metal, dismantling/decommissioning of process plant and ships.

PLASMA ARC CUTTING PROCESS

The plasma arc cutting process is the most versatile of the three processes in review, it is suitable for cutting all electrically conductive materials such as the most commonly used, carbon steels, low alloy steels, aluminium, stainless steels, nickel alloys and copper, in varying thicknesses from 0.5mm to in excess of 150mm.

Plasma arc cutting is speedier than oxy-fuel on a like for like material/thickness basis, plasma arc has the advantage of not recognising air gaps, so this enables materials to be stacked, also, laminated, hot dip coated, electro-plate, painted, rusty and heavily mill scaled material can be cut without major problems provided the material is well grounded.

Plasma arc cutting requires a plasma arc power supply, torch and gas supply. The most popular plasma arc power supplies are between 30A to 800A, they have a torch attached that is connected to the gas supply, systems are divided into two main categories, single gas or multi gas types. Single gas systems tend to be lower in cost to purchase but still deliver an acceptable cut finish to materials such as carbon steel and low alloy steels, the more advanced multi gas systems are suitable for cutting all conductive materials through the use of the appropriate consumables and gas combinations. Single gases used are generally clean dry compressed air or nitrogen. Multi gases used can be combinations of compressed air, oxygen, nitrogen, argon and hydrogen.

The plasma arc is generated from within the torch, high pressure gas is forced through a nozzle with a small diameter orifice, an electric arc that is generated by the plasma arc power supply is then passed through the high-pressure gas flow producing the plasma jet where the temperature is around 20,000°C, this temperature can be exceeded by the use of multi gas combinations, the plasma jet rapidly pierces through the material where the molten metal is blown away.

Similar to oxy-fuel CNC systems, plasma arc requires set parameters to function correctly, current (amperage), gas type/pressure, consumables – nozzle size/electrode, torch to workpiece height, torch travel speed all influence the end product results.

Manual plasma arc systems have additional versatility, their portability enables their use across a wider range of job sites, the systems can be used in combination with portable motorised/CNC carriages. Furthermore, manual plasma arc systems are being favoured for their gouging capability, quick cost-effective stock removal with a lower effect from heat input.

Mechanised cutting systems are suited for use in light to heavy fabrication, steel processing, shipyards. Manual plasma arc cutting and gouging is used in the above industries with the addition of cutting scrap metal, dismantling/decommissioning of process plant and ships.

LASER CAN CUT EVERYTHING

The laser cutting process is the newest of the three processes in review, laser cutting has seen remarkable development in laser beam generation and delivery. The laser beam used in the metal cutting industry has evolved from being a process where in the early 1970’s an oxygen laser jet was developed primarily for cutting titanium for the aero industries. Since then the CO2 gas lasers have become the most popular systems in the world, further development has given rise to the fibre laser cutting process, the fibre laser process is the most advanced form and is currently seen as the best.

Laser cutting power for metals has enormously increased over time, from 300W to cut 1.0mm carbon steel to 20,000W to cut 50mm carbon steel, 12,000W is accepted as being the high end norm to produce a 25mm thick cut in carbon steel. From the three thermal processes in review, laser is the most accurate process, it will cut material that is microns thick upwards, it will cut all metals including hot dip and electro plate galvanised steel, although there is a limiting thickness.

When all parameters are set correctly profiled parts require the least amount of 2nd ops, namely grinding or linishing. All laser systems perform best in clean workshop conditions, welding, grinding and other airborne contaminants can affect the quality of cut and longevity of the system, materials that are to be cut need to be clean and free from surface contaminants “cutting lotions” may be used to prevent the adhesion of micro spatter.

The laser beam is generated by one of three methods: Solid state, CO2 Gas or Fibre, fibre laser systems are the most advanced, the laser beam is transported to the head through a fibre, the advantage here is the length of the beam path remains constant thus eliminating costly downtime for resetting and adjusting beam transfer devices. The gases used in laser cutting are oxygen and nitrogen, when cutting carbon steel with oxygen as the assist gas, there is an exothermic reaction similar to the oxy-fuel process where the gas blows the dross through the workpiece. Nitrogen is used for cutting aluminium, stainless steels, nickel alloys, titanium and copper, nitrogen can also be used as the assist gas when cutting carbon and low alloy steels to produce a better finish and minimise 2nd ops.

Laser cutting systems require a substantial investment, to enable a quicker return on outlay, laser systems can be run on a “Lights out” basis, this is where highly automated production units operate totally unmanned overnight or with only a small number of people in attendance for breakdown cover. Whilst energy costs can be quite high, this is partly offset by the low cost of consumables.