Technical Bulletin

Lowering Plasma Cutting Costs

When equipment is properly sized and used correctly, and depending on material type and thickness, PAC (plasma arc cutting) can offer cutting speeds that are many times faster than oxy/fuel or mechanical processes. Increased cutting speeds, which can reach 25 to 100 inches per minute in hand held cutting applications, are often limited by the operators ability to move smoothly down the cut path rather than the system’s capability. This increase in cutting speed translates directly into lower cost per foot of cut. The increase in cutting speed coupled with lower consumable cost (air vs. oxygen and fuel), reduced distortion on thin material, suitability on a wide variety of materials, and immediate starts with no preheating make PAC a high productivity cutting option. To achieve the highest levels of cutting speeds and cut quality, it is critical to avoid a few operational and maintenance errors that are common in many shops. So let's look at some of these issues.

Air vs. Dual Gas

Compressed Air is the plasma gas used most commonly on the new generation of hand held plasma units and does an excellent job on sheet metal and thin stainless steel. The torches are light with only three or four consumable parts and part life is very good with proper use. For high volume cutting applications, using a dual gas system can offer improved cut quality and finish on aluminum and stainless steel by selecting the proper combination of plasma and assist gases. The development of hafnium electrodes which are compatible with oxygen has made PAC of steel up to ½” very cost competitive.

Air Quality

The development of plasma systems that use compressed air has led to the rapid growth of PAC in a wide range of applications. At the same time, the air supply is the most common factor in the lack of performance of these systems. First the air supply unit must be able to supply the required pressure specified for the system, and equally important, the volume of airflow needed for the material being cut. While it is easy to check the pressure, flow should be tested at the torch with a calibrated flowmeter and is often ignored. The other major factor in the performance of air plasma systems is the quality of the air. If the air contains contaminants, particularly water and oil, cut quality and part life can be reduced to the point where PAC is no longer cost effective. Most of the newer plasma packages come with a basic filter but none are sufficient in supplying the quality air your plasma system needs by itself. A three-stage system consisting of a water/particle filter, an oil filter, and an oil vapor filter, is recommended by manufacturers such as Hypertherm for best results. While not many shops have this level of air filtration, it shows the importance that manufacturers place on a clean air supply. The filters should be located close to the plasma power supply and checked on a regular basis. It is common to cut corners in this area to save some initial expense, but it is far more costly in the long run in terms of part life and time spent c leaning up poor quality cuts. Compressed air in high pressure cylinders can be used but is too costly to be practical in any but special situations

When cutting metal, the plasma arc transfers from the plasma torch electrode to the workpiece. The heat of the arc melts the metal, which is blown away by the high-pressure gas flow. Depending on the system, the arc is started either by a pilot arc or touching the work with the torch. The pilot arc system uses high frequency to initiate the arc, which can effect electronic equipment nearby. The touch start method used to shorten part life, but new technology such as Lincoln’s patented Vortech consumables, has offered considerable improvement. Not requiring high frequency allows consistent arc starting at distances two or three times what is practical with high freq starting. It is important to transfer the arc immediately to the work piece so the heat of the arc doesn’t impinge on the nozzle for an extended period, which will quickly destroy the part, usually by making the nozzle orifice out of round. Once the nozzle orifice is out of round, cut quality and speed is reduced and the ability to cut thicker material is diminished. To optimize torch and part life, it is critical to avoid firing the torch in the air away from the work. The arc should always be started on the edge of the plate or over the plate where piercing is necessary.

Another common problem in all arc welding and cutting applications is poor electrical connection between the power source and work-piece. To establish a good connection, the cable and clamp must be in good condition with the connections properly attached. The clamp should be in direct contact with the metal surface and any paint or rust should be removed. Poor electrical contact will make igniting the arc more difficult and diminish machine capacity.

It is always preferable to start cuts on the edge of the plate, but sometimes it is necessary to pierce through the plate to begin the cut. In hand-held cutting applications, a simple, effective technique can be used help prevent molten metal from splashing back on the torch parts and arcing out the torch head. When starting the pierce lay the torch on the workpiece at a shallow angle to the metal. After initiating the arc, roll the torch to a 90-degree angle and start moving along the line of travel. This will blow the molten metal created at the start before a hole is made away from the torch parts rather than back into the torch.

Over the long term, the money spent on replacing consumable torch parts will represent the largest cost associated with PAC and should be treated as such. This is also an area jealously protected by manufacturers from an increasing number of companies that produce after-market parts. The fact is that with today’s CNC machining capabilities and material analysis, there are many quality sources of replacement parts that are no longer protected by the original patent. Some plasma equipment manufacturers try to insinuate that using parts produced by someone else in itself cancels their warranty obligation, but after-market parts are a fact of life in every industry. The best strategy is to find a quality supplier that you feel provides the best value of cost and performance.

PAC can help increase the productivity of many cutting operations but like other welding and cutting processes you can only realize the full benefits if it is properly set up, correctly used, and well maintained.

EASY WAYS TO REDUCE WELDING COSTS

While many companies do a great job of getting the lowest prices on welding equipment and consumables, they often overlook the even more significant savings that are attainable by focusing on increasing the productivity of the welding operation. Given that in a typical welding operation using a manual process the cost of welding breaks down to 80% labor and overhead and 20% material, productivity improvements offer the largest potential savings and ones that will continue to pay dividends from year to year. With this in mind, a 10% reduction in product cost will yield a 2% improvement to the bottom line whereas 10% rise in welding productivity generates an 8% bottom line gain. Without a doubt we must continue to purchase cost effectively, but let’s not overlook the easy ways we can lower welding costs and keep our operations competitive for the future.

ELIMINATE OVERWELDING- This is undoubtedly the most common “cost raiser” in any welding operation. As the graphic shows, just a small increase in the size of a fillet weld results in a huge increase in the weld metal used and the time to make the weld. Generally, over-welding is simply due to the fact that the welding operator is not really sure of the size of the weld he is producing and has no way to know for sure what the actual size of his weld is. The tendency is to err on the “safe side” not realizing what this costs. This situation can easily be rectified by providing every operator with a weld fillet gauge, training him on its use and explaining the costs associated with making a weld larger than specified, such as higher material and labor cost and the increased potential for part distortion. It should be recognized that poor fit-up of the weld joint is also a major factor in the case of oversize welds which must be addressed if we expect the operator to meet the weld size requirements.

OPTIMIZE CURRENT WELDING PROCESSES AND PROCEDURES- The key to optimizing weld quality and controlling weld costs is found with “weld process knowledge”. Often welding parameters vary significantly among operators even when they are making identical welds. By bringing all the welding variables like wire feed speed, voltage, arc length and gun angle, into a specified range you begin to control quality and productivity. Cost reductions might be seen by changing wire size or switching to a shielding gas that will allow use of the high deposition spray arc mode of metal transfer rather than using the short-arc method for all applications. When welding carbon and stainless steels, flux cored, gas shielded wires often can offer higher deposition rates than solid wires especially where out of position welding is required. The high deposition rates provided by this process are the result the high current density, which is the relationship of the welding current (amperes) to the cross-sectional area of an electrode. With the thin sheath of the flux-cored wire being the primary current path, the current density is much higher than the equivalent size solid wire at the same current. The result is increased resistance heating which increases the melting rate of the wire and results in higher deposition rates of weld metal. These wires also have the benefit of excellent bead appearance, low spatter, even penetration, wide operation ranges, and generally much less training than required with solid wires. The other main factor determining weld cost is the operator duty cycle. This refers to the percentage of his time that the operator is actually welding. It is logical to say that the more time spent welding as opposed to other non-productive activities such as changing electrodes, chipping slag, or repositioning, more productive work can be accomplished.

Moving from manual stick electrode welding with a 30% duty cycle, to semi-automatic welding with a solid or flux cored wire allowing 45% operator on time is a big difference. If you can take that one step further and mechanize the travel, you have radically changed your welding cost structure and competitive position. Don’t get caught in the mindset that you have to make large capital investment to achieve worthwhile gains because many of the best payoffs involve using existing equipment and a little ingenuity.

POSITION PARTS- Except for some unique applications, welding in the flat and horizontal positions allows the use of the highest deposition materials and the fastest welding speeds regardless of the welding process being used. With a 1/8” 7018 stick electrode, a ¼” fillet weld made in the vertical position would produce about 4” of weld per minute. If the part is positioned to allow the weld to be made in the horizontal position, a larger high deposition electrode can be used to produce the same size weld at 14” per minute. This is a threefold increase in productivity and almost certainly will produce a better-looking weld requiring much less operator skill. This kind of productivity improvement gained by welding in the flat or horizontal position will hold true for all welding processes.

A LITTLE ECONOMICS DO ADD UP- The cumulative savings of small improvements will go a long way to lowering costs. Improve operating efficiency and reduce wasted time by looking at the use of larger wire packages, having a reasonable stock of consumable parts at the welding station, evaluating the shielding gas supply for integrity and efficiency, and streamlining the number of times parts are handled are just a few ways to increase the efficiency of your welding operations. Another area for possible savings would be taking a look at your gas flow rates and making sure they are set properly for the type of welding you are currently doing. For the shop that makes many short welds, i.e. tack welds, a surge suppressor might be worth a look. Significant gas cost savings can result.

NEW TECHNOLOGY MIGHT BE WORTH A LOOK- While some of the new technologies are worth a look let's not overlook what has gotten the welding industry to this point. The standard equipment without all the bells and whistles is still some of the best equipment available for the majority of the welding done today. When welding aluminum, stainless steel, and high nickel alloys, the new generation of inverter power sources with modified waveform technology might be beneficial for these specific applications. The benefits of these machines should be fully understood and documented for your particular application before a sizable capital investment is made. Often, the real benefit of these inverter machines is not their welding performance, but the fact that they are very energy efficient, which is extremely important with today’s high energy costs and they have a small footprint where space is at a premium.

HYPE vs. PERFORMANCE- With many shops using tri-mixes for the GMAW welding of steel, you might want to see what you are really getting with the addition of oxygen to the Argon /C02 mix. The more costly tri-mixes actually can lower the weld fusion potential and increase the chance of weld porosity. What’s really ironic is the 3 part gas mix in cylinders contains approximately 10% less gas than the conventional Argon CO2 mixes which in a large shop can mount up. The same is also true for the tri-mix used for welding stainless. This costly tri-mix when used for welding most thin gauge stainless in the short circuit mode increases the chance for weld burn through, distortion and oxidation than a lower cost 2 component mix like 98% argon/ 2% C02. On the other hand, the addition of 2-3% hydrogen to argon has been used in automatic GTAW welding of stainless steel but can also benefit many manual applications where bead size and appearance are important.

These are just a few ideas on places to look for savings. Your operators probably have some of their own ideas and would love to share them with someone who asks.

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