Plasma Cutting

In simplest terms, plasma cutting is a process that uses a high velocity jet of ionized gas that is delivered from a constricting orifice. The high velocity ionized gas, that is, the plasma, conducts electricity from the torch of the plasma cutter to the work piece. The plasma heats the work piece, melting the material. The high velocity stream of ionized gas mechanically blows the molten metal away, severing the material. Plasma cutting can improve productivity and lower cutting costs. It does not require a preheat cycle, cuts any metal that conducts electricity, permits portability around job sites, minimizes the heat-affected zone (HAZ), and yields a cut with a small kerf. Plasma units also can gouge, pierce, bevel, cut holes, and trace shapes.

Plasma Torch

Plasma Cutting Compare to Oxyfuel cutting

Plasma cutting can be performed on any type of conductive metal - mild steel, aluminium and stainless are some examples. With mild steel, operators will experience faster, thicker cuts than with alloys. Oxyfuel cuts by burning, or oxidizing, the metal it is severing. It is therefore limited to steel and other ferrous metals which support the oxidizing process. Metals like aluminium and stainless steel form an oxide that inhibits further oxidization, making conventional oxyfuel cutting impossible. Plasma cutting, however, does not rely on oxidation to work, and thus it can cut aluminium, stainless and any

other conductive material. While different gasses can be used for plasma cutting, most people today use compressed air for the plasma gas. In most shops, compressed air is readily available, and thus plasma does not require fuel gas and compressed oxygen for operation. Plasma cutting is typically easier for the novice to master, and on thinner materials, plasma cutting is much faster than oxyfuel cutting. However, for heavy sections of steel (1 inch and greater), oxyfuel is still preferred since oxyfuel is typically faster and, for heavier plate applications, very high capacity power supplies are required for plasma cutting applications.

Plasma Cutter Principles

Plasma cutting is ideal for cutting steel and non-ferrous material less than 1 inch thick. Oxyfuel cutting requires that the operator carefully control the cutting speed so as to maintain the oxidizing process. Plasma is more forgiving in this regard. Plasma cutting really shines in some niche applications, such as cutting expanded metal, something that is nearly impossible with oxyfuel. And, compared to mechanical mean of cutting, plasma cutting is typically much faster, and can easily make non-linear cuts.

Plasma Cutting limitations

The plasma cutting machines are typically more expensive than oxyacetylene, and also, oxyacetylene does not require access to electrical power or compressed air which may make it a more convenient method for some users. Oxyfuel can cut thicker sections (>1 inch) of steel more quickly than plasma.

Cutting Technique

Step 1: Place the drag shield on the edge of the base metal, or hold the correct standoff distance (typically 1/8 inch). Direct the arc straight down.

Step 2: Press the trigger. After two seconds of preflow air, the pilot arc starts.

Step 3: Once the cutting arc starts, move the torch across the metal.

Step 4: Adjust speed so that the cutting sparks go through the metal and out the bottom of the cut.

Step 5: At the end of a cut, angle the torch slightly toward the final edge, or pause briefly to sever the metal completely.

Step 6: To cool the torch, postflow air continues for 20 to 30 seconds after you release the trigger; pressing the trigger during postflow instantly restarts the arc.

If the sparks are not visible at the bottom of the plate, the arc is not penetrating the metal. This can be caused if we move the torch too quickly, have insufficient amperage, or fail to direct the plasma stream straight into the metal.

Travelling at the right speed produces a clean cut with less dross on the bottom of the cut and little or no distortion of the metal. Slow travel speeds can overheat the metal, causing dross to accumulate. To minimize dross, increase travel speed or reduce amperage.

For an indication of how fast to move the torch, refer to the machine's cutting speed graph or check the speed for a rated cut. Dross also accumulates when we push a machine to cut a material at its maximum thickness. The only cure for this is a bigger machine.