Metal cutting consists of two methods: mechanical, and thermal. Plasma cutting belongs to the thermal field, which uses ionized gas to process metal. It is one of the most common techniques for cutting thick metal sheets. However, before we delve into the advantages and possibilities of plasma cutting, we should answer one more question.
What is plasma, and how does plasma cutting work?
You’ve no doubt heard of the three primary states of matter – solid, liquid, and gas. However, there is a fourth state of matter. Yes, this is plasma. It occurs in nature, but mainly in the upper layers of the Earth’s atmosphere. The famous aurora borealis is the result of solar wind made of it. Light and high-temperature fires also contain it. The same is true of our bodies. Altogether, it makes up about 99 percent of the visible universe. We encounter it in televisions, fluorescent and neon lights, and plasma cutters in everyday life. Plasma is an electrically conductive, ionized substance, similar to a gas. The similarity is due to the fact that in this case, the atoms are not in constant contact with each other. At the same time, however, plasma behaves like a liquid in terms of its ability to flow under the influence of electric and magnetic fields. Gases can be transformed into plasma by intense heating. For this reason, it is often referred to as ionized gas.
How does a plasma cutter work?
The general mechanics of the system are always the same. Plasma cutters use compressed air or other gases, such as nitrogen. The compressed gas comes into contact with the electrodes and then ionizes, creating higher pressure. As it increases, the plasma jet is pushed toward the cutting head. The cutting tip restricts the flow, creating a plasma jet. Since plasma is an electrical conductor, the workpiece is grounded by the cutting table. When the plasma arc contacts the metal, its high temperature melts it. At the same time, a high-velocity gas blows away the molten metal fragments.
Plasma machine – how to set it up and choose the speed of plasma cutting?
Regarding the machine itself, the most critical issues are space and safety. Unlike many hobby CNC systems, a plasma cutter requires a lot of space in a well-ventilated room to be safely operated. This is not a machine that can be safely tucked away in a corner like a 3D printer. The space around the device is needed as a buffer to protect against any stray sparks. Similarly, floors should be made of concrete, or other fire-resistant materials. Gas or compressors and computer systems also require space. The plasma arc can interfere with electronic systems, hence proper grounding is required. The computer must be connected to an uninterruptible power supply (UPS), thus ensuring constant and reliable power while the cutter is operating. If you plan to set up your own CNC plasma cutter, make sure the workshop has adequate power (according to specifications).
Types of gas
Plasma cutting uses different gases depending on the quality of the cut and the type of material:
- Oxygen can be used for mild steel up to 1 1/4 inches thick and roughing stainless steel and aluminum.
- A mixture of argon and hydrogen is preferred for the smooth cutting of stainless steel and aluminum, as it provides a high-quality cut.
- In smaller workshops, compressed air is the most commonly used gas. It is ideal for low-current applications cutting metal up to an inch thick.
- Other gases used in CNC plasma cutting include nitrogen and methane, which are most commonly used when cutting thin stainless steel.
CNC handheld plasma torch
Manual plasma torches are less expensive but more challenging to install on a CNC table. Plasma cutter power supplies designed for manual operation are usually much cheaper than those sold for mechanized cutting. The best manual CNC plasma cutting torches are the IPT-40 and IPT-60 variants. They work well with low start-up frequency and inexpensive consumables, using a central Euro connector, and can be upgraded to pencil torches in the future.
Cutting process methodology
Not all systems work in the same way. First, there is a lower-budget version, commonly called High Touch. This is unsuitable for CNC plasma cutters, as high frequencies can interfere with modern equipment and cause problems. This method uses a spark with both high voltage and frequency. The spark is created when the plasma torch touches the metal, closing the circuit and thus creating plasma. Another option is the arc ignition method. A high voltage and low current circuit creates a spark inside the flashlight. This creates a pilot arc, which is a small amount of plasma. When it contacts the workpiece, a cutting arc is completed, and the Operator can begin the cutting process. The third method is to use a spring-loaded plasma torch head. Pressing it against the workpiece creates a short circuit, which causes the current to flow. Releasing the pressure creates a pilot arc. The rest of the sequence is the same as in the previous method. This means that the arc makes contact with the workpiece.
Most small plasma cutters, especially those designed for manual cutting, cannot operate continuously at maximum current. The duty cycle refers to the length of time a device can operate continuously or intermittently over ten minutes. For example, a plasma cutter with a duty cycle of 70% at 50 amps can run for 7 of 10 minutes but requires at least a 3-minute break to maintain a safe operating temperature. If the user tries to cut longer than the rated duty cycle, the plasma cutter may reduce power or shut down completely. For mechanized plasma cutting, it’s ideal to have a cutter with a 100% duty cycle at the desired amperage, so you don’t have to worry about delays when scheduling moves. However, most inexpensive plasma cutters have a duty cycle of about 40-60% at maximum rated power, and the user must be careful not to cut too long. Typically, higher duty cycles can be achieved with less power (lower current). If you want to cut at 40A (up to 3/8 inch material), it is recommended that you purchase a 55A plasma cutter for higher duty cycles at 40A.
Power and air requirements
The maximum thickness of the material you plan to cut will determine the amperage required by the plasma cutter. As a rule of thumb, a 20A cutter can cut through 1/8 inch (~3.1 mm) of material, with each additional 10A of current adding 1/8 inch of capacity. For example, 30A will go through 1/4″, 40A will go through 3/8″, etc. Thicker metal is heavier. You may want to be able to cut material that is 1″ thick (~25.4 mm), but can your table support the weight of that material? If so, do you have the right tools for loading and unloading heavy parts (such as jacks)? If you decide to go with a high-current plasma cutter, does your building have an outlet that can supply enough power without tripping a circuit breaker? If the answer is „no,” you will need to use electrical wiring with the appropriate cross-sectional area and even install an additional power panel. A plasma cutter not only consumes electricity, but also compressed air. The compressed air from the nozzle of the plasma cutter is used to blow out the molten material during the cutting process. It also keeps the torch cool and directs the plasma jet into a tight column for precise cutting. An inadequate air compressor will result in poor cutting results. A plasma cutter’s power and air requirements are some of the biggest hurdles for CNC hobbyists to overcome. They become less burdensome if you stick to cutting thin materials.
Advantages and Disadvantages of Plasma Cutting
- Ability to process all conductive materials. Oxy-fuel cutting is also suitable for thick metals but is limited to ferrous metals.
- Excellent quality up to thicknesses from 50mm to 150mm.
- Medium-thickness cuts are relatively inexpensive.
- The best way to cut medium to thick stainless steel and aluminum.
- CNC machines provide high precision and repeatability.
- The ability to cut in water reduces the heat affected zone and noise level.
- The cutting edge is smaller compared to flame cutting.
- The cutting speed is faster than that of oxygen cutting.
- The same machine can also be used for plasma welding by manipulating the parameters.
- HAZ* is larger compared to laser cutting.
- The cut sheets are not as high quality, and the tolerance could be more precise with laser cutting.
- Does not reach the thickness of flame or water jet cutting.
- Wider edges than with laser cutting.
*Heat-affected zone (HAZ) is an unmelted area of metal that has changed material properties due to exposure to high temperatures. These changes in material properties are usually the result of welding or cutting at high temperatures. The HAZ is the area between the weld or cut and the underlying (intact) parent metal.
Proper cutting parameters, optimal plasma gas selection and flow rate, are needed for proper plasma cutter operation. We hope you find the answers to these questions after reading the above guide.