Saturday, November 26, 2022

What to consider when choosing a plasma cutting table? Part I

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What to consider when choosing a plasma cutting table? I. Plasma cutting system selection

The multiple advantages currently offered by plasma arc cutting (or PAC) have made it possible to offer a wide variety of equipment that meets the various requirements of users.

Within that range, plasma cutting tables or pantographs are the next step up from manual PAC systems. These true mechanized plasma cutting workstations are generally beyond the budget and needs of a hobbyist or home user, but are a worthwhile purchase for larger shops and industries. Furthermore, tables that feature CNCs in particular provide precise control of the torch, creating straighter and faster cuts. They also allow us to save the settings and/or cutting pattern for reuse on future jobs, much like we save a graph or drawing to our computer or mobile device.

If our concern is, therefore, to acquire a plasma cutting table, we are going to develop a series of articles to address certain considerations that will help us in our decision. In this first installment we will specifically review the aspects involved in the plasma cutting system selection, whether or not provided with CNC functionality, although this is a highly desirable addition.

To better understand the essential details that we must take into account for a good choice, let us first examine how a plasma cutting table is constituted.

Plasma Cutting Table

In the figure above we can see a basic scheme of a CNC system with its main parts. In this scheme, the components on which we must focus our attention are fundamentally three:

  • The equipment that provides cutting technology.
  • The cutting table, including the design of the table itself, the Gantry/THC system, and the dust/fume containment system, as well as the type of motors that control the torch.
  • The software that runs the computer and designs the cutting patterns.

Now we are going to study in more detail the first fundamental component, that is, the equipment that provides the cutting system.

In a previous article we pointed out the main technologies currently offered by the plasma cutting market. However, each manufacturer constantly creates patented variants that combine these technologies and introduce improvements aimed at achieving better performance and cutting qualities. Many of these improvements also tend to adapt and make compatible the equipment intended for manual plasma cutting to cutting tables capable of offering greater performance.

When it comes to the systems themselves, one of the common features across manufacturers is that mechanized PAC systems are offered in single gas or dual gas (dual systems) configurations. Most single or dual gas torches incorporate a tip shield and a secondary flow of gas around the tip, called a shielding gas. The difference between one-gas and two-gas designs is simple: if the plasma gas and shielding gas are the same, it is a one-gas system. If they differ, they are two gas systems or dual systems.

Shielding gas selection depends on the specific cutting application and serves three purposes:

  • Cool the nozzle.
  • Prevent heat from radiating back into the nozzle.
  • Help remove molten material.

In this article we detail which gases and gas combinations are most used in plasma cutting and for which materials each one is applied.

Below we will review the most used systems that can be incorporated into PAC tables or that are directly designed for them.

Systems compatible with plasma cutting tables

conventional cap

In this process it is only the nozzle that produces the constriction of the arc and no shielding gas is used. The plasma gas is generally nitrogen or air, which is introduced into the torch around the electrode, and the electric arc is established between the nozzle and the substrate (transferred arc). Conventional PAC rigs that can be fitted to a table feature cutting capacities from 12mm to 38mm thick and are ideal for pipe and duct cutting, beveling, and 3-D robotic cutting. Let’s briefly look at the characteristics of each system.

air plasma: this type of cut is generic and cheap, since it only uses air, so the quality obtained is usually medium or low. Air is primarily used on carbon steel materials, although in certain applications it can also cut stainless steel and aluminum. The cutting capacity with air is high; however, the system has the drawback of carefully checking the quality of the air introduced, since it must not only be free of oil or dust, but mainly of humidity. Otherwise, the cost of consumables increases exponentially, so that what is saved in gases is spent on consumables.

plasma for nitrogen: in carbon steel with thicknesses greater than 30 mm it is necessary to use nitrogen cutting, although this is also applicable to aluminum and stainless steel. However, the case of carbon steel presents a disadvantage due to the possibility of future welding or another subsequent process, since it can cause nitriding problems. This means that before using nitrogen plasma in carbon steels, it is necessary to carefully analyze the combination of gases to be used. On the other hand, the use of nitrogen reduces the cutting capacity by up to 25% compared to air, but the life of the torch is extended by up to 300%.

PAC for oxygen

This process uses oxygen as the plasma gas (orifice) instead of nitrogen or air and leads to better cut quality. This translates into higher speed, lower bevel angles (perpendicularity), less roughness on the cut faces, a larger slag-free window, a narrower BEAM, and a more weld-friendly cut surface, which can reduce welding. number of deleterious defects in a structure.

In addition to drastically reducing nitrites on the cut surface, oxygen also reduces slag that sticks to the material during cutting. When working on carbon steel, oxygen reacts exothermically with the iron in the liquid metal, creating iron oxide. This reaction releases energy that further heats the liquid metal and reduces its viscosity, making it easier for the liquid metal to be removed by the plasma jet, leaving a clean, slag-free cutting edge at the bottom.

The problem with oxygen is that the cutting field is limited to 200 amps, which means a quality cut only in thicknesses of 25 or 30 mm maximum.

high definition PAC

also called precision cap, employs a nozzle design that increases arc constriction and energy density. Depending on the model and manufacturer, they work with maximum amperages between 130 and 800 A. Due to the higher energy of the arc, the quality of the cut edges and perpendicularity can be improved, especially in materials less than 3/8 inches thick. .

In this article we mention the main details of high definition plasma cutting equipment. The technological advances that this system incorporates, whose quality is immediately below laser cutting, makes it the most expensive to date. However, in addition to those mentioned on that occasion, the high-definition PAC units equipped with CNC have these other advantages:

  • Superior quality cutting in carbon steel, aluminum and stainless steel sheets, being able to cut, bevel and mark thicknesses of up to 160 mm.
  • Even tighter tolerances, of the order of +/- 0.0625.
  • Higher cutting speeds reduce response time.
  • It produces less dross, so the time spent on cleaning, if any, is minimal.
  • Table dimensions up to 3 meters wide x 12 meters long.
  • CAD/CAM nesting software that offers more than 250 pre-programmed shapes and provides efficient use of materials, minimizing dross.

This video gives us a series of examples of cuts made by the HyPerformance HPR800XD model from the leading firm Hypertherm, depending on different materials, thicknesses, amperages and combination of gases.

https://www.youtube.com/watch?v=ZSlH9TOay7A

summarizing

In addition to the above, when making the choice of plasma cutting equipment, we must understand that it does not make sense to acquire one that is unable to cut the thickness of the specific materials that we use, at least those that make up our largest volume of work. On the other hand, it would be a big inconvenience not to be able to get a job done just because our team can’t cut the sheet of material required.

Therefore, if we buy equipment that can cut much thicker material, we will not only benefit every time a new job comes along, but we will come out on top with every cut, because a higher amperage machine will always offer higher speed and better performance. cutting quality. In other words:

  • Higher cutting speed means less cutting time.
  • Higher cut quality and accuracy affects the quality of the workpiece and minimizes or eliminates rework, resulting in less manufacturing time.

If time is money, then we have gained enough time to offset the cost of more powerful equipment.

Continue with the second part of the technical article: How to choose a plasma cutting table?

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