Metal cutting consists of two methods: mechanical and thermal. Plasma cutting belongs to the thermal domain, which uses ionized gas for metal processing. It is one of the most common techniques for cutting thick 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?
Undoubtedly, you've heard of the three basic states of matter - solid, liquid, and gas. However, there is a fourth state of matter. Yes, it's plasma. It occurs in nature, but mainly in the upper layers of the Earth's atmosphere. The famous aurora borealis is the result of the action of the solar wind made up of it. Light and high-temperature fires also contain it. The same applies to our bodies. Altogether, it makes up about 99 percent of the visible universe. In everyday life, we encounter it in televisions, fluorescent lights and neons, as well as in plasma cutters. Plasma is an electrically conductive, ionized substance, similar to gas. This similarity results from the fact that in this case, 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 converted into plasma through 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 utilize compressed air or other gases, such as nitrogen for example. The compressed gas comes into contact with electrodes, and then ionizes, creating higher pressure. As it increases, the plasma stream is pushed towards the cutting head. The cutting tip restricts the flow, creating a plasma stream. Since plasma is an electrical conductor, the workpiece is grounded through the cutting table. When the plasma arc touches the metal, its high temperature melts it. At the same time, the high-speed gas blows out the melted metal fragments.
Plasma Machine - How to Set It Up and How to Choose the Plasma Cutting Speed?
When it comes to the machine itself, the most important issues are related to space and safety. Unlike many hobbyist CNC systems, a plasma cutter requires a lot of space in a well-ventilated room to be operated safely. This is not a machine that can be safely tucked away in a corner like a 3D printer, for example. The space around the machine 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 the operation of electronic systems, hence proper grounding is required. The computer must be connected to an uninterruptible power supply (UPS), thus ensuring constant and reliable power during the operation of the cutter. If you plan to set up your own CNC plasma cutter, make sure that the workshop has the appropriate power (in accordance with the specification).
Types of gas
Plasma cutting utilizes various gases depending on the cutting quality and type of material:
- Oxygen can be used for soft steel up to 1 1/4 inches thick, but also for rough machining of stainless steel and aluminum.
- An argon and hydrogen mixture is preferred for smooth cutting of stainless steel and aluminum, as it provides high cutting quality.
- In smaller workshops, the most commonly used gas is compressed air. It is ideal for applications related to low current metal cutting up to an inch thick.
- Other gases used in CNC plasma cutting are nitrogen and methane, which are most commonly used when cutting thin stainless steel.
Handheld CNC Plasma Torch
Handheld plasma torches are cheaper, but more difficult to install on a CNC table. Power supplies for plasma cutters intended for manual operation are usually significantly cheaper than those sold for mechanized cutting. The best handheld torches for CNC plasma cutting are the IPT-40 and IPT-60 variants. They work well at low start-up frequencies and inexpensive consumables, using a central euro connector, and in the future they can be expanded to pencil torches.
Methodology of the cutting process
Not all systems operate in the same way. Firstly, there is a lower-budget version, commonly referred to as High Touch. This is not suitable for CNC plasma cutters, as high frequencies can disrupt the operation of modern devices and cause problems. This method uses a spark of both high voltage and frequency. Sparking occurs when the plasma torch touches the metal, closing the circuit and thus creating plasma. Another option is the arc ignition method. A high-voltage, low-current circuit generates a spark inside the torch. This creates a pilot arc, which is a small amount of plasma. When it comes into contact with the workpiece, a cutting arc is created and the Operator can now start the cutting process. The third method is the use of a spring plasma torch head. Pressing it against the workpiece causes a short circuit, which causes 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 comes into contact with the workpiece.
Work Cycle
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 the device can operate continuously or with breaks over ten minutes. For example, a plasma cutter with a 70% duty cycle at 50 amps can operate for 7 out of 10 minutes, but requires at least a 3-minute break to maintain a safe working 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, the ideal solution is to have a cutter with a 100% duty cycle at the desired current intensity, so you don't have to worry about delays when planning movements. 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 for too long. Typically, higher duty cycles can be achieved using lower power (lower current). If you want to cut at 40A (up to 3/8 inch material), it is recommended to 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 current intensity required by the plasma cutter. As a rule, a 20 A cutter can cut 1/8 inch (~ 3.1 mm) of material, with each additional 10 A of current adding 1/8 inch of capacity. For example, 30A will go through 1/4″, 40A will go through 3/8″, and so on. This may be obvious, but thicker metal is heavier. You may want to be able to cut material that is 1 inch thick (~25.4 mm), but can your table hold the weight of this material? If so, do you have the appropriate tools for loading and unloading heavy items (e.g., lifts)? If you decide on a high-current plasma cutter, does your building have an outlet that can provide the appropriate amount of current without tripping the breaker? If the answer is "no", you will need to use electrical wiring of the appropriate cross-section, and even install an additional power panel. A plasma cutter not only consumes electrical power but also compressed air. Compressed air from the nozzle of the plasma cutter is used to blow out molten material during the cutting process. It also keeps the burner cool and directs the plasma stream into a tight column for precise cutting. An improper air compressor will result in poor cutting results. The power and air requirements of the plasma cutter are some of the biggest hurdles for CNC hobbyists to overcome. They become less troublesome if you stick to cutting thin materials.
Advantages and Disadvantages of Plasma Cutting
Advantages
- Possibility of processing all conductive materials. Oxygen-fuel cutting is also suitable for thick metals, but is limited to ferrous metals.
- Excellent quality for thicknesses from 50mm to 150mm.
- Medium thickness cuts are relatively inexpensive.
- The best way to cut medium and thick stainless steel and aluminum.
- CNC Machines provide high precision and repeatability.
- Possibility of water cutting, thereby reducing the heat affected zone and noise level.
- The cutting edge is smaller compared to flame cutting.
- The cutting speed is higher than in the case of oxygen cutting.
- By manipulating parameters, the same machine can also be used for plasma welding.
Disadvantages
- HAZ* is larger compared to laser cutting.
- The cut sheets are not of such high quality, and the tolerance is not as precise as with laser cutting.
- It does not achieve the thickness of flame or water jet cutting.
- Wider edges than with laser cutting.
*The Heat Affected Zone (HAZ) is the non-melted area of metal that has undergone changes in material properties as a result of exposure to high temperature. These changes in material properties are usually the result of welding or high-temperature cutting. The Heat Affected Zone is the area between the weld or cut and the base (undisturbed) native metal.
Proper operation of a plasma cutter requires the application of appropriate cutting parameters, optimal selection of plasma gases, and the correct intensity of plasma gas flow. We hope that after reading the above guide, you will find answers to these questions.