Plasma & Automated sheet metal cutting

Q. What Is Plasma & Plasma Cutting?
Plasma is a state of matter like a solid, liquid or gas. Adding heat to material causes the molecules in it to vibrate or move more quickly. When a solid is heated, the molecules start to vibrate more vigorously. Eventually the solid turns to a liquid and the molecules actually move around and collide with each other. As more energy or heat is added the motion becomes faster and more vigorous still and eventually the molecules move so quickly and collide so energetically (violently) that they separate and form a gas. If still more energy is added to the gas the molecules travel faster and thus collide with each other more violently and the gas changes to a plasma.

An atom consists of a positively charged core and is surrounded by negatively charged electrons. The molecules in a gas can be either individual atoms or collections of such atoms that are very closely connected. When plasma is formed, the collisions between the molecules eventually get so violent that, at first, the molecules will break up into the individual atoms and eventually some of the atoms will separate from, or lose, some of their electrons from their outer shell. The gas changes to plasma when a critical number of atoms lose electrons. When kinetic energy in a gas is increased, some of the electrons are freed from the atoms outer shell and plasma is formed.

When an atom releases electrons it becomes an ion. When enough energy is added to a gas that there is a balance between the number of atoms releasing electrons and those naturally recombining with free electrons, the gas is said to be ionized. This ionized gas is plasma. In plasma, there is an important fraction of ionized atoms at any given time, so there are always ions and electrons that are separate and free. The energy input into the gas has to be ongoing otherwise the gas will eventually cool enough that the electrons will mate back with the ionized atoms. When this happens, the plasma returns to a gas state.

Plasma is a thermal, very hot, electrically conducting gas, from positive and negative ions, electrons and excited and neutral atoms and molecules. In physics, we often talk about the 4th state. As the plasma gas a monatomic argon or diatomic hydrogen, nitrogen, oxygen gases and air can be used. The plasma gas is ionized and dissociated by the energy of the plasma arc. By a recombination of atoms and molecules outside the plasma jet of the system, plasma rapidly releases the received energy and enhances the effect of thermal plasma beam on the machined object. Plasma jet is usually additionally cooled with water. This allows the energy density of the plasma beam reach up to 2106 W/cm2. Due to the high temperature, the plasma expands and flows with supersonic speed in the direction of the machined object (anode). In the plasma arc, temperatures rise to 30 000 ° C; in conjunction with high kinetic energy of the plasma beam, a very high cutting speed can be used, depending on a material and a thickness of all electrically conductive materials.ssss

For the cutting process, a pilot arc between the cathode and the nozzle is first ignited by means of high voltage. This weaker energy pilot arc prepares partially, by ionizing, the path between the plasma torch and the machined object. By touching of the pilot arc and the machined object (cursory incision and cursory piercing) the power of the main arc is automatically increased. All kinds of steels, brass, aluminium, copper, stainless steel and other metal alloys can be cut.

ROAD TO PLASMA-CUTTING AUTOMATION
Fabricators note several reasons for considering the move from manual plasma-arc cutting (PAC) to automated cutting. A basic reason to automate is simple: manufacturers need quality cut parts at a reasonable price and at high rates of productivity.

Purchasing the right system
Facilitating the transition to automation requires a number of decisions. First, purchasing the right type of automated PAC setup needs a complete understanding of how the part requirements would apply to automation in terms of money. High-precision dual-gas cutting systems cost many Lakhs of Rupees, often accompanied by an equally expensive controller. While these setups provide very high cutting speeds, it is difficult, if not impossible, for a small fabricator to justify such an investment.

Automated plasma table
For cutting the same shape multiple times, using an automated plasma table saves hours of post-cut grinding, which enables Cornfield Customs to maintain price competitiveness. The move to an automated system increases productivity, and the machine produces good, clean cuts. Further, a dramatic reduction takes place in the amount of cut-edge finishing.

Quick Switch from Automated to Manual
While most users largely dedicate their PAC system to automated cutting on its table, arrangement can be made to quickly switch to manual cutting, by a quick-disconnect torch feature. This allows the user to efficiently switch from the setup’s 180-deg. mechanized cutting torch to a 70- or 90-deg. manual torch. Software automatically detects the selected torch to switch between automatic and manual modes.

Technical Tips For Better Plasma Cutting
Over the years, the air plasma arc process has been greatly refined so that plasma provides good quality cutting, gouging and piercing at very high speeds for a much lower cost. This cutting process is truly perfect for many industries. Plasma arc cutting is a process where an open arc can be constricted by passing through a small nozzle, or orifice, from the electrode to the workpiece. Although the technology behind the plasma arc may seem complicated, the process itself is very easy to learn and perform.

PLASMA – A CUT ABOVE OXYFUEL
While oxy-fuel was the most common method of cutting carbon steel in the past, plasma cutting is gaining favour because it provides numerous advantages. Plasma cuts faster than oxy-fuel; a pre-heat cycle is not required; the kerf width (the width of the cut) produced is small; and, it has a smaller heat-affected zone, which prevents the surrounding area from warping or damaging paint. In addition, plasma cutting can be used on any type of electricity-conducting metal (the oxy-fuel process cannot cut stainless steel or aluminium). Moreover, plasma cutting is a cleaner, less expensive and more convenient method of metal cutting because clean, dry air is used for most plasma cutting applications.

A properly installed air plasma arc cutting setup is safer than oxy-fuel gas cutting. This is because there is a chance of flashbacks and the danger of flammable gases in exposed hoses with oxy-fuel torches. Plasma cutting is easier for hole piercing due to the plasma jet. The plasma jet is the swirling force of air around the arc, in combination with the arc attachment, which focuses the heat into the metal. In addition, plasma arc piercing does not require the metal to be preheated, which saves production time. The plasma jet also makes for a better choice when cutting stacked materials.

PLASMA CUTTING APPLICATIONS
The plasma process can be used to cut any material that is a good electrical conductor. Unlike chemical cutting, it can be used on any metal for applications such as stack cutting, beveling, shape cutting, gouging and piercing.
Plasma cutting can be successfully performed on a variety of material sizes as well. Plasma can be used to cut anything from thin gauge aluminium to stainless and carbon steel up to several inches, depending upon the power of the cutting machine.

SETTING-UP A PLASMA ARC CUTTER
To set-up a plasma arc cutter, simply hook-up the compressed air to the plasma cutting unit. Three choices of air are available: bottled, in-house air supply or a small air compressor. Most plasma units have a built in regulator to maintain the proper flow of air for the system.
To set the amperage, or heat, of the cutting unit to the proper level, make a few practice cuts with the amperage set high. You can then adjust the amperage down according to your travel speed. If the amperage is too high or your travel speed is too slow, the material you are cutting may become hot and accumulate dross. Travelling at the right speed and using the right amount of heat will produce a very clean cut with less dross on the bottom of the cut, as well as little or no distortion to the metal.

OPERATING A PLASMA ARC FOR CUTTING
Begin cutting by placing the torch as close as possible to the edge of the base metal. Press the trigger to initiate the pre-flow air; the pilot arc will then light, followed by the cutting arc. Once the cutting arc starts, move the torch slowly across the metal. Adjust your speed so that the cutting sparks emerge from the bottom of the metal. You should be able to see the bottom of the workpiece and the arc should be directed straight down. If the sparks are not visible at the bottom of the plate, you have not penetrated the metal. This is because your travel speed is too fast, you have insufficient amperage, or the plasma stream is directed at an angle. At the end of a cut, angle the torch slightly towards the end of the cut or pause briefly to completely finish the cut. The post flow air will continue for a short period of time after the trigger is released to cool the torch and consumable parts, however cutting can be resumed immediately.

OPERATING A PLASMA ARC FOR GOUGING
Plasma arc gouging can be accomplished by placing the torch at approximately a 40-degree angle to the base metal. Press the trigger for the pre-flow air and pilot arc. When the cutting arc ignites, form the arc length a short distance from the workpiece. Further adjust the arc length and travel speed as needed. Do not gouge too deep, as several passes may be needed to accomplish the necessary gouge. Again, after releasing the trigger, the post-flow air will continue for a short period of time, but gouging can be resumed immediately.

OPERATING A PLASMA ARC FOR PIERCING
Piercing – creating a hole – can be performed by placing the torch at a 40-degree angle to the workpiece. Press the trigger. When the cutting arc is initiated bring the torch tip to a 90-degree angle and the arc will pierce the base metal. A good rule to follow is that you can pierce up to 1/2 of the maximum cutting thickness provided by the machine.

[Contributed by Ador Welding Ltd.]