Plasma Cutting Basics

Using a Plasma Cutter is one of the most efficient ways to cut through metal. Hot plasma is used to cut through electrically conductive materials, such as steel, aluminum, brass and copper. But, before we get started on how to use a plasma cutter, let's review the basics of plasma cutting technology and how it works.

First, it is important to understand the technology being applied when using a particular plasma cutting machine. Each machine has several specific "options" that are involved in the cutting process and therefore, the purchasing process.



PLASMA CUTTING PROCESS

Plasma cutters work by sending an electric arc through a gas that is passing through a constricted opening. The gas can be shop air, nitrogen, argon, oxygen. etc. This elevates the temperature of the gas to the point that it enters a 4th state of matter (we're all familiar with the first three: solid, liquid, and gas). Scientists call this additional state plasma. As the metal being cut is part of the circuit, the electrical conductivity of the plasma causes the arc to transfer to the work.

The restricted opening (nozzle) the gas passes through causes it to squeeze by at a high speed, like air passing through a venturi in a carburetor. This high speed gas cuts through the molten metal. The gas is also directed around the perimeter of the cutting area to shield the cut.

In many of today's better plasma cutters, a pilot arc between the electrode and nozzle is used to ionize the gas and initially generate the plasma prior to the arc transfer.

Other methods that have been used are touching the torch tip to the work to create a spark, and the use of a high-frequency starting circuit (like a spark plug). Neither of these latter two methods is compatible with CNC (automated) cutting.



HIGH FREQUENCY

High frequency refers to the type of ignition mechanism for your plasma cutter. Some machines use high frequency starting mechanism to ignite the spark at the torch tip of your plasma arc. This is a quality starting mechanism, however the high frequency emitted by the machine itself may interfere with your electrical and computer equipment in or around the work area. With these machines, the plasma arc is ignited by electrical current produced at a high frequency. Also, the torch tip must be in contact with the metal in order to ignite a spark.


PILOT ARC

The Pilot Arc technology is a manner of ignition that allows a spark from at the torch tip with out ever touching the material. With Pilot Arc a spark can be ignited at the torch tip several inches away from the cutting surface. The benefits of this cutting-edge technology are the ability to cut materials quickly and accurately with out the negative affects of high frequency, but more importantly, the ability to ignite a spark with out the tip touching the material will help protect and prolong the life of your consumables.


DUAL VOLTAGE

Certain plasma cutters have dual voltage capability. These machines, like Longevity FORCECUT 40D, use a different manner of igniting a spark at the torch tip. This means that each machine, regardless of its method for igniting a spark, are capable of operating at either 110V AC or 220V AC.


PROS & CONS OF PLASMA CUTTERS

Advantages

  1. Versatility - unlike oxy-acetylene cutting, plasma cutting works on any electrically conductive metal. The oxy-fuel process will not work on aluminum or stainless steel. Plasma also produces cleaner cuts due to dry air being used in most applications
  2. Speed - because of the high temperature of the plasma cutting arc (~20,000 degrees F), it's a much faster method of cutting than oxy-acetylene (~6,000 degrees F). There is also no pre-heat cycle. Plasma cutting is also much more precise and there is a much lower chance of surrounding areas being negatively affected by warping or pain damage.

Disadvantages

  1. Cost - plasma cutting definitely requires a higher upfront investment but once you are setup, you only need to replace your tips and consumable electrodes. Getting started in oxy-acetylene cutting may be cheaper at the start (see oxy-acetylene kits we recommend), but you then have the added expense of gas tank refills.


PLASMA CUTTER CONSUMABLES

The shield cup protects the rest of the consumables. With the plasma cutting process you tend to get an awful lot of sparks and molten metal. The shield's purpose is to prevent that from reaching the inner consumables. In some cases, the shield is a drag shield, which means it allows the operator to put the torch right on the metal without the need of an additional standoff. That's nice, especially for inexperienced cutters.

The nozzle does one of two things. First, it focuses the plasma arc. The larger the orifice, the less defined or wider the arc is. Nozzles with a wide orifice are used in gouging applications, while some have very small orifices that constrict and focus the arc. These are used for fine-feature, detailed cutting. Second, the nozzle helps to funnel gas flow, which also helps to shape and constrict the arc.

The electrode carries the current from the torch to the plate. It's a slender piece made of copper or silver and contains hafnium or tungsten. These materials have been found to be excellent conductors of electricity. The electricity comes from the machine out to the torch and goes into a cathode block, which the electrode is in contact with, and then focuses that charge on the end of the electrode through the hafnium/tungsten insert which then arcs to the plate. The nozzle takes that arc and really focuses it in a way that cuts metal, but the electrode carries the current. The nozzle tends to get very hot because it's right in the midst of the superheated gas. So the back of the electrode has a cooling device attached that helps transfer the heat out of the electrode.

And last, the swirl ring channels the gas in a couple of different directions as it comes from the machine and down the torch. First, the gas goes forward in a spiral manner around the electrode, around the arc, down through the nozzle, and out through the torch. The reason the ring needs to swirl is that it allows the gas, as it's going out through the nozzle, to be at different temperatures. The cooler gas travels along the outside - this gas is in contact with the nozzle and prevents it from burning up. If you don't have the swirl in there, gas mixing is inadequate and you could burn up your nozzle. The other purpose of the swirl ring is to channel the gas backward and pull the heat off the electrode.



SAFETY COMES FIRST!

Safety is always the highest priority on a welding job site. A worker can always repair a piece of equipment or grind out a bad weld and re-weld a joint. However, a personal injury can be permanent. Hazards in welding can include hot metal burns, arc burns, hearing loss, dismemberment, and loss of eyesight.

All of these potential hazards should be taken very seriously. Which is why there are plenty of product offerings to choose from in the personal safety category. Before starting any welding operations, safety equipment should be purchased or borrowed then used.



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