Plasma Machining & Cutting

We use the PLASMA CUTTING and PLASMA MACHINING processes to cut and machine steel, aluminum, metals and other materials of different thicknesses using a plasma torch. In plasma-cutting (also sometimes called PLASMA-ARC CUTTING), an inert gas or compressed air is blown at high speed out of a nozzle and simultaneously an electrical arc is formed through that gas from the nozzle to the surface being cut, turning a portion of that gas to plasma. To simplify, plasma can be described as the fourth state of matter. The three states of matter are solid, liquid and gas. For a common example, water, these three states are ice, water and steam. The difference between these states relates to their energy levels. When we add energy in the form of heat to ice, it melts and forms water. When we add more energy, the water vaporizes in the form of steam. By adding more energy to steam these gases become ionized. This ionization process causes the gas to become electrically conductive. We call this electrically conductive, ionized gas a “plasma”. The plasma is very hot and melts the metal being cut and at the same time blowing the molten metal away from the cut. We use plasma for cutting thin and thick, ferrous and nonferrous materials alike. Our hand-held torches can usually cut up to 2 inches thick steel plate, and our stronger computer-controlled torches can cut steel up to 6 inches thick. Plasma cutters produce a very hot and localized cone to cut with, and are therefore very suitable for cutting metal sheets in curved and angled shapes. The temperatures generated in plasma-arc cutting are very high and around 9673 Kelvin in the oxygen plasma torch. This offers us a fast process, small kerf width, and good surface finish. In our systems using tungsten electrodes, the plasma is inert, formed using either argon, argon-H2 or nitrogen gases. However, we also use sometimes oxidizing gases, such as air or oxygen, and in those systems the electrode is copper with hafnium. The advantage of an air plasma torch is that it uses air instead of expensive gases, thus potentially reducing overall cost of machining .

 

 

 

Our HF-TYPE PLASMA CUTTING machines use a high-frequency, high-voltage spark to ionize the air through the torch head and initiate arcs. Our HF plasma cutters do not require the torch to be in contact with the workpiece material at the start, and are suitable for applications involving COMPUTER NUMERICAL CONTROL (CNC) cutting. Other manufacturers are using primitive machines that require tip contact with the parent metal to start and then the gap separation occurs. These more primitive plasma cutters are more susceptible to contact tip and shield damage at starting.

 

 

 

Our PILOT-ARC TYPE PLASMA machines use a two step process for producing plasma, without the need for initial contact. In the first step, a high-voltage, low current circuit is used to initialize a very small high-intensity spark within the torch body, generating a small pocket of plasma gas. This is called the pilot arc. The pilot arc has a return electrical path built into the torch head. The pilot arc is maintained and preserved until it is brought into proximity of the workpiece. There the pilot arc ignites the main plasma cutting arc. Plasma arcs are extremely hot and are in the range of 25,000 °C = 45,000 °F.

 

 

 

A more traditional method we also deploy is OXYFUEL-GAS CUTTING (OFC) where we use a torch as in welding. The operation is used in cutting of steel, cast iron and cast steel. The principle of cutting in oxyfuel-gas cutting is based on oxidation, burning and melting of the steel. Kerf widths in oxyfuel-gas cutting are in the neighborhood of 1.5 to 10mm. The plasma arc process has been seen as an alternative to the oxy-fuel process. The plasma-arc process differs from the oxy-fuel process in that it operates by using the arc to melt the metal whereas in the oxy-fuel process, the oxygen oxidizes the metal and the heat from the exothermic reaction melts the metal. Therefore, unlike the oxy-fuel process, the plasma-process can be applied for cutting metals which form refractory oxides such as stainless steel, aluminium, and non-ferrous alloys.

 

 

 

PLASMA GOUGING a similar process to plasma cutting, is typically performed with the same equipment as plasma cutting. Instead of cutting the material, plasma gouging uses a different torch configuration. The torch nozzle and gas diffuser is usually different, and a longer torch-to-workpiece distance is maintained for blowing away metal. Plasma gouging can be used in various applications, including removing a weld for rework.

 

 

 

Some of our plasma cutters are built in to the CNC table. CNC tables have a computer to control the torch head to produce clean sharp cuts. Our modern CNC plasma equipment is capable of multi-axis cutting of thick materials and allowing opportunities for complex welding seams that are not possible otherwise. Our plasma-arc cutters are highly automated through the use of programmable controls. For thinner materials, we prefer laser cutting to plasma cutting, mostly because of our laser cutter's superior hole-cutting abilities. We also deploy vertical CNC plasma cutting machines, offering us a smaller footprint, increased flexibility, better safety and faster operation. The quality of the plasma cut edge is similar to that achieved with the oxy-fuel cutting processes. However, because the plasma process cuts by melting, a characteristic feature is the greater degree of melting towards the top of the metal resulting in top edge rounding, poor edge squareness or a bevel on the cut edge. We use new models of plasma torches with a smaller nozzle and a thinner plasma arc to improve arc constriction to produce more uniform heating at the top and bottom of the cut. This allows us to obtain near-laser precision on plasma cut and machined edges. Our HIGH TOLERANCE PLASMA ARC CUTTING (HTPAC) systems operate with a highly constricted plasma. Focusing of the plasma is achieved by forcing the oxygen generated plasma to swirl as it enters the plasma orifice and a secondary flow of gas is injected downstream of the plasma nozzle. We have a separate magnetic field surrounding the arc. This stabilises the plasma jet by maintaining the rotation induced by the swirling gas. By combining precision CNC control with these smaller and thinner torches we are capable to produce parts that require little or no finishing. Material removal rates in plasma-machining are much higher than in the Electric-Discharge-Machining (EDM) and Laser-Beam-Machining (LBM) processes, and parts can be machined with good reproducibility.

 

 

 

PLASMA ARC WELDING (PAW) is a process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode generally made of sintered tungsten and the workpiece. The key difference from GTAW is that in PAW, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exiting the orifice at high velocities and temperatures approaching 20,000 °C. Plasma arc welding is an advancement over the GTAW process. The PAW welding process uses a non-consumable tungsten electrode and an arc constricted through a fine-bore copper nozzle. PAW can be used to join all metals and alloys that are weldable with GTAW. Several basic PAW process variations are possible by varying the current, plasma gas flow rate, and the orifice diameter, including:

 

Micro-plasma (< 15 Amperes)

 

Melt-in mode (15–400 Amperes)

 

Keyhole mode (>100 Amperes)

 

In plasma arc welding (PAW) we obtain a greater energy concentration as compared to GTAW. Deep and narrow penetration is achievable, with a maximum depth of 12 to 18 mm (0.47 to 0.71 in) depending on the material. Greater arc stability allows a much longer arc length (stand-off), and much greater tolerance to arc length changes.

 

As a disadvantage however, PAW requires relatively expensive and complex equipment as compared to GTAW. Also the torch maintenance is critical and more challenging. Other disadvantages of PAW are: Welding procedures tend to be more complex and less tolerant to variations in fit-up, etc. Operator skill required is a little more than for GTAW. Orifice replacement is necessary.