If you are in need of a cutting system for special materials and jobs, you need to consider the plasma cutting version. While it offers a huge number of benefits for most fabrication and other manufacturing operations.
It also has more than a few disadvantages that you will want to take into consideration before purchase. By knowing both sides of the equation, you will know enough to make an informative decision and purchase the most appropriate cutting system.
A plasma cutter is a thermal type of cutting device and is used to cut a variety of metals. A plasma arc (extremely hot, usually from 11000 degrees to 30000 degrees C) is used to melt metal along a cut line, and then the molten metal is removed from the workpiece.
The system consists of a plasma torch with an electrode and nozzle, a power supply that provides high amperage DC electricity, a cutting bed on which the metal is placed for cutting, a control system, and a cooling mechanism, preventing overheating of the equipment.
What is Plasma Arc Machining?
When a gas flow is heated to such a temperature that it becomes partly ionised it is known as ‘plasma’.
Plasma arc machining removes material by the eroding effect of a gas plasma, which is an ionized gas consisting of freely moving electrons and positively charged ions, together with neutral atoms.
Plasma arc machining is a metal removal process where the material is eroded from the workpiece by directing a high-velocity jet of high-temperature (in the region of 11,000°C to 30,000°C) ionised gas onto it.
Uses in the metalworking industry and aerospace. Gives high precision in the working of difficult materials such as titanium and nickel alloys, opening up many new applications.
Parts of Plasma Arc Machining
The following are the main parts of an arc plasma machining machine:
- Plasma Gun. Gas must be acted upon with a power supply (electricity) to become transformed into plasma; the more resistant the plasma, the higher the current.
- Gas Supply. The plasma is a source of compressed air, nitrogen, and other gases. In this case, the workpiece is shielded, and the melted metal is blown off by a second enclosed gas, which is inert.
- Cooling Water System. A cooling water system is used for the plasma torch, situated on the opposite side of the plasma cutter.
- Electrode and Nozzle. Specially built electrodes and nozzles squeeze and hold jet plasma, confining it to a beam of limited area appropriate for cutting.
- Workpiece. In PAM machining, the workpiece is placed underneath the plasma gun. Can cut carbon, some magnesium, stainless steel, aluminium, and steel alloys.
Working Principle of PAM
In a plasma torch, known as the gas (H2, N2, 02, etc.) is passed through a small chamber where a high-frequency spark (arc) is maintained between a tungsten electrode (cathode) and a copper nozzle (anode), both of which are water-cooled.
Working
In some torches, there is provided an extra flow of inert gas which surrounds the main flame to guard the gas from the atmosphere.
The high-velocity electrons generated by the arc collide with the gas molecules and, through dissociation of diatomic molecules of the gas, disrupt the cohesion of the gas atoms, which causes large numbers of the atoms to be freed, so liberating great quantities of thermal energy.
The plasma forming-gas is forced through a nozzle duct of the torch sufficiently to stabilise the arc.
The whole of the heating process takes place in the compressed zone of the nozzle duct. As a result, an almost high exit gas velocity is obtained, combined with a high core temperature up to 16,000 °C. The relative plasma jet thus melts the workpiece material, and the high-velocity gas stream blows the molten metal away.
The extent of the heat-affected zone depends on the work material, its thickness, and the cutting speed. For example, on a workpiece of 25 mm thickness, the extent of the zone is about 4 mm, and it is less at high cutting speeds.
A typical rate of flow for the gas is from 2 to 11 m/hr. A direct current, rated at about 400 V, 200 kW output is normally required. Arc current between 150 and 1000 A for a cutting rate of 250 to 1700 mm/min.
Accuracy
This is in the order of 1.5 mm with a similar surface finish. Tolerances on the width of slots and on the diameter of holes are generally ±0.8 mm on plates 6 to 30 mm thick and ± 3.0 mm on plates 100 to 150 mm thick.
Applications of Plasma Arc Machining
The applications of plasma arc machining are given below:
- This is used for cutting stainless steel and aluminium alloys predominantly.
- Profile cutting of metals, particularly of the above metals and alloys, has been one of the principal commercial applications of PAM.
- On the machining side, plasma has been used very satisfactorily in normal turning and milling of difficult materials.
Advantages of Plasma Arc Machining
Plasma cutting has grown increasingly popular for lots of applications and in several industries due to the many benefits it has over other types of cutting:
- Possibly the biggest benefit that a plasma cutter has is the accuracy of cutting that it provides, producing very clean and precise cuts. This is especially the case for a computer numerical controlled (CNC) system, where the machine is pre-programmed, and the processing is automated. This precision is taken to even greater heights by high definition (HD) models. Plasma cutting rarely produces any irregularities or jagged edges, provided it is used properly on the right type and thickness of materials. This is important for any detailed work or intricate patterns. It also means that little or no finishing is required, cutting processing time and cost.
- Speed of cutting is another significant advantage, particularly with thinner metals, where plasma will cut 25 mm thick steel at double the speed of oxy-fuel equipment. The quicker throughput dramatically improves productivity levels, especially with high-volume production.
- Plasma cutters are electric-powered and use gas sources, often compressed air or combinations of other gases. These are usually cheaper than those used for oxy-fuel cutting, keeping overall operating costs lower.
- The systems are very versatile, cutting through a wide range of conductive metals, including steel, copper, brass, and aluminum. They can also handle very intricate cuts to produce complex shapes, making them effectively ideal for medium-thickness metals as compared to oxy-fuel.
- Lower wastage levels follow from cleaner cutting, particularly in automated CNC systems, where human error is less likely. This reduces the need for re-work and improves productivity and flow.
- Minimal heat exposure leads to less distortion and warping due to a smaller heat-affected zone. Dross is also avoided or is easy to lift clear. This makes firmer cuts and improves the accuracy of items in the finished and cut states.
- With several consumables that can be replaced, plasma cutters tend to be cheaper and easier to keep running than laser and water jet models.
- And although plasma cutters use high electricity and extreme heat, they are actually safer to operate than laser systems, requiring no protective covering enclosures for the lasers that, in turn, require such arrangements. Inert gases are also safer than stored oxy-fuel gases. And once again, more dangerous operations are made even safer by automation, needing very little operator input.
- A plasma cutter costs about a third of the price of an equivalent laser or water jet system. For cutting thicker materials, the price gap is even larger, and the running costs remain lower as well, making plasma ideal for small to medium-sized companies.
Disadvantages of Plasma Arc Machining
When compared to just about any other cutting method, plasma machines have a lot of advantages. But there are still some drawbacks:
- Plasma cutting can only be done with conductive metals. Nonconductive materials need to use other means.
- Cutting depth of about 64 mm is the maximum for obtaining clean edges. Anything thicker needs to be treated with oxy-fuel or water jet.
- A plasma machine is still an expensive proposition. Generally, a power source and other features are included in a full turnkey installation. You match the machine you buy to the types of workloads you will have to handle. Financing can help ease the load on your budget.
- The noise level is always higher than 90 decibels, often more than 120. Ear protectors must be equally effective.
- Fumes and gases are produced. Ventilation and exhaust systems are desirable. Wear a mask.
- Bright light and sparks can injure eyes and burn skin. Operators need protective clothing and filtered eyewear.
- Quality is usually quite good, although lasers and waterjets will be better in some cases. The thinnest metals, less than about 3 mm, may warp somewhat, although proper control of torch height is helpful.
- Low-end, budget plasma cutters are usually made with substandard parts and software. You’ll get inconsistent results with frequent breakdowns and a short life. Pay for quality!