While delving into the physics of the plasma cutter may not be necessary for the person who wishes to answer the question – how to maintain a plasma cutter by taking it to a costly maintenance shop, the person who truly wants to have full control over the functioning and maintenance of his cutter needs to have some idea of how does a plasma cutter works.
In the following paragraphs, we have tried to simplify complex phenomena such as spark arcs and plasma to levels that you can quickly go through before starting out on your first cutting session.
Phase I: The Pressurized Air/Gas and the Electrode
While most of the plasma cutter reviews we’ve dealt with focus on the nozzle and the electrode, what these represent are mere enclosures for the real scene of developments. This scene comprises of a narrow shaft that runs through the body of the torch and through the nozzle (and the electrode that surrounds the mouth of the nozzle).
Connected to the source of pressurized air (or gas), this shaft is narrow enough to ensure that the air that passes through it during operation does not spread out and thus lose its pressure.
Surrounding the nozzle of course, is the electrode. Made of a base metal and endowed with a hafnium tip for improved conductivity, it sits around the nozzle’s mouth. By default, this electrode has a negative potential (similar to the negative end of a battery holder) and thus is primed to transfer electricity from the torch to the metal.
Phase II: Circuit and Plasma
When one of the best plasma cutters is turned on, it begins to force pressurized air through the nozzle. Pressurized air by itself, however, does not produce either the heat required for the cutter to work. As we’d mentioned elsewhere, this heat is produced instead by one of two methods:
- Contact method: The metal and the nozzle physically touch, creating a circuit that allows the electricity from the electrode to discharge itself onto the metal. This creates a spark/arc that sets off the cutting procedure.
- High Frequency Method (using pilot arc): Another method of setting off a spark is to use a high frequency unit that creates a pilot arc without requiring the unit to be in actual contact with the metal. The HF pilot arc reaches towards the metal and once it comes into contact with it, the pilot arc transforms into the cutting arc.
In either case, the arc that is created produces a region of extreme heat at the mouth of the nozzle. This heat transforms the air or gas passing through the nozzle into plasma. To make understanding how does a plasma cutter works easier, let us mention at this point that the name itself derive from this plasma , which is the fourth state of matter where superheated matter (atoms, ions, etc) loses its earlier characteristics (eg. the features of a gas) and becomes a mass of particles. Plasma is a good conductor of electricity and ensures that the circuit is maintained.
In case of the plasma cutter, this plasma reaches a temperature of about 35,000 degrees Fahrenheit. Further, since the air/gas is pressurized, the plasma retains the directional nature of the gas as well as the speed, thereby creating a fast moving, accurately aimed mass of plasma.
Phase III: Cutting and The “shielding gas”
Once the plasma hits the metal, it passes the heat onto the metal itself. If you’re wondering how does a plasma cutter works in such a situation since metals are good conductors of heat, let us add that the extreme temperature and fast plasma speed ensure that the transfer of heat is not fast enough. This leads to the metal in the immediate vicinity of the oncoming plasma to melt into liquid slag.
Such slag would remain in the molten state if the cutter is held in position, but quite inevitably, the cutter is moved to new areas. In doing so, the cutter “cuts” through the liquid slag, thus producing a “cut” around which the slag solidifies into metal once more once the cutter has moved on. All of this occurs in milliseconds, giving the impression that the machine is cutting through the metal like a saw.
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The process of cutting can be completed in two ways – either the cutter moves right through the metal and thus causes the nozzle to lose contact with the metal, or the electricity is switched off. In most cases, the former method is followed as it allows the user to make precise cuts.
Notably, once the cutter has lost contact with metal, it will lose the spark/arc and cannot recreate a pilot arc (if it is an HF pilot arc unit) as this requires a special starter mechanism. It would hence have to be started many times to handle complex cuts. On the plus side, it ensures that there are no injuries associated with excess running time.
All of this, however, does not account for one of the benefits of using a plasma cutter – the presence of extremely narrow slag belts/kerf areas along the cut. Slag belts/kerf areas are parts of the metal surrounding the cut that contains oxidized deposits of molten slag.
Such deposits can arise if the arc is erratic or too diffused, thereby heating up nearby impurities and allowing for the reaction of such impurities with oxygen (from the air) to produce unsightly marks on the metal.
To solve this, many machines come with a shielding gas mechanism. A shielding gas blows around the cutting gas and is often non-reactive. Such a non-reactive shield allows the plasma to stay focused and prevents the oxygen in the surroundings from interacting with the molten slag.
While some of the cutters are small and simple enough for people to observe how does a plasma cutter works in detail, others will provide only limited insights.
Whatever the case, one should rest assured that most cutters work along the lines given above and should any part malfunction, knowledge of the above can help locate the truant parts and replace them in the shortest possible time.
Further, knowledge of the procedure can help avoid injuries arising out of the improper handling of parts. This is especially so since the melting and solidifying of the metal is too fast and most humans consider the process as akin to metal-on-metal “cutting”.
In light of these benefits of knowing the procedure, we sincerely hope that this article will make cutting a safer and easier operation.