Getting the most from your Plasma Cutter

Plasma arc cutting is so fast, simple and versatile that many people push their oxy-fuel setups off to the side, using them only when they need to preheat metal. During plasma arc cutting, an electrical arc combines with a gas, typically air, to form a high temperature plasma arc that flows from an electrode to the work piece (which is any electrically conductive material) and slices through the material.

Compared to mechanical or oxy-fuel cutting methods, plasma cutting offers several advantages. Plasma cuts faster than oxy-fuel ? up to 75 ipm on 3/8 in steel. For example, Patrick Brinckerhoff, director of ride engineering for the Stratosphere Casino in Las Vegas, conducted his own cutting speed test prior to demolishing the High Roller roller coaster atop the Stratosphere Tower. A cut that took between seven and eight minutes with an oxy-fuel torch took only three minutes ten seconds with the plasma cutter.

By choosing a plasma cutter, Brinkerhoff didn't have to worry about safely transporting and storing acetylene. Other advantages include a smaller, more precise kerf (width of the cut), no preheat time, a smaller heat-affected zone and the ability to cut any type of electrically-conductive metal, including aluminum, brass, copper, titanium and galvanized steel.

Getting the most from your plasma cutter starts by ensuring you have the right equipment for your needs. We rate our plasma cutters by their cutting capacity. A rated capacity is the thickness of mild steel which can be precisely cut based on a steady travel speed of 10 ipm. A sever cut is the maximum cut the unit can achieve in ideal conditions at slower travel speeds. Thus a piece of equipment can be rated at 3/8 in and have a maximum sever cut of 5/8 in on mild steel. These ratings may vary by manufacturer.

Take the time to read your owner's manual. It contains important information on safety and getting the most from your plasma cutter. Then keep the manual handy so you and others can reference it as needed. The following information does not replace a thorough reading of that manual.

Take a Tip

Tips focus the plasma stream to the work piece. To extend their life, tips should be matched to the power settings you intend to use. A low-amp tip has a smaller orifice to maintain a narrow plasma stream at lower settings for use on thinner material. Using a 40-amp tip at an 80-amp setting distorts the tip orifice and will require replacement. Conversely, using an 80-amp tip on the lower settings will not focus the plasma stream as well and leads to a wider kerf. Extended tips are available for cutting in corners or when using patterns. Gouging tips are designed to spread the plasma stream and remove lots of material at once, enabling the arc to remove a large amount of material in a single pass.

Expect to get approximately two hours of continuous cutting time when using clean dry air and making longer cuts. Touching the tip to the work piece leads to double arcing and decreases tip life because it provides another path for the current to follow. However, moving the tip too far away from the work piece will decrease its cutting capacity. Maintain a 1/8 in distance between the tip and the work piece. A drag shield that attaches to the torch and holds the tip off of the work piece will ensure you maintain a constant 1/8 in standoff, making it ideal for unsteady hands as well as pattern cutting.

Before making your first cut on the work piece, perform a sample cut on the same type of material to check your settings and travel speed. If you plan to make a long cut, rehearse it beforehand to ensure you have adequate freedom of movement and can keep the torch perpendicular to the work piece. It's difficult to stop and start and still maintain a clean cut.

For more precise cutting, try using your non-cutting hand as a guide (see photo). This rest hand position gives freedom of movement in all directions while helping to maintain a constant standoff and steady your hand. When piercing thin material to start a cut, the arc may be able to punch through with little or no backsplash. On thicker material, however, start with the torch on a 45-deg angle so the first blast of metal has somewhere to go. If not, the metal will splash back and quickly wear your consumables.

Travel Speed

Your owner's manual should contain a chart comparing material thickness to travel speed in inches per minute. Generally, the faster you move the cleaner your cut will be, especially on aluminum. For thicker material, set the machine to full output and adjust your travel speed. On thinner material, turn down the amperage and change to a lower-amperage tip to maintain a narrow kerf.

If you're traveling at the correct speed, the arc should exit the material at a 15-deg to 20-deg angle opposite the direction of travel. If it's going straight down, you're moving too slowly and you'll have an unnecessary build-up of dross or slag. If you go too fast, molten metal will start spraying back into the tip. Since the arc will trail at an angle at the end of a cut, slow your cutting speed and angle the torch into cut through the last bit of metal.

Guides

For additional precision, clamp a straight edge to the work piece to guide the torch. If you need to make repeatable cuts of the same shape, create a template. Make sure the guide isn't flammable. Straight and circle guides are also available to help ensure precise cutting.

Steve Hidden is the product manager of plasma products with Miller Electric Mfg. Co., 1635 W. Spencer Street, Appleton, WI 54912-1079, 920-740-2401, Fax: 920-735-4147, shidde@millerwelds.com, www.millerwelds.com.