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Plasma and oxyfuel: A productive combination

How one plate cutting workhorse complements the other

Figure 1
For the right application, using both oxyfuel and plasma on one table shows how complementary these two plate cutting processes can be.

When it comes to CNC shape cutting in plate, plasma and oxyfuel have remained industry workhorses. Advancements in both have opened the door to more choices, and which choice to make depends, as always, on the application. To optimize plate fabrication, fabricators and steel service centers should start by asking three fundamental questions:

  1. What is the application?
  2. What are the cutting needs for the application, such as edge quality, production rate, hole quality, type of weld edge preparation, as well as material types and thicknesses?
  3. What are the restrictions, like budget and floor space?

Both plasma and oxyfuel machines have become simpler to use and require less operator intervention. Improvements in CNC, CAD/CAM, and nesting software have helped further both oxyfuel and plasma. All this helps operations having difficulty finding and retaining skilled talent. But even if companies didn’t have trouble with high operator turnover rates and inexperienced operators at the CNC, such advancements are the natural evolution of productivity. Part of this evolution involves taking advantage of each process’s best attributes (see Figures 1 and 2), and analyzing both processes shows just how complementary plasma arc and oxyfuel cutting really are.

Basic Process Considerations

Oxyfuel is simple and relatively inexpensive. Equipment investment is minimal, and the consumables used are common, low-cost fuel gases such as acetylene, propane, and natural gas. It’s generally used to cut mild steel plate 2 inches or greater (see Figure 3).

As an oxygen-based process, it requires carbon to cut effectively, restricting its use to mild steel applications. Only low-carbon and some low-alloy steels have oxides with a lower melting point than the base metal, so this means they can be cut effectively with oxyfuel.

When adjusted properly, oxyfuel cutting produces smooth, square cut surfaces. There is little slag on the bottom edge, and the top edge is only slightly rounded from the preheat flames. This makes the edge suitable for many applications without further treatment.

Fundamentally, oxyfuel is a slow cutting process. But when cutting thicker mild steel and applying multiple cutting torches simultaneously, oxyfuel has the productivity advantage over plasma cutting. This arrangement is cost-effective because all those torches share one gas delivery system. However, they usually work only for nests of similar or identical parts, because all torches ride on the same horizontal gantry.

System configurations commonly include four to six cutting torches, but high-production applications may use up to eight. Oxyfuel torches also can be configured to cut specialized edges like K bevels simultaneously in a single pass, with three torches configured to cut the bottom bevel, land, and top bevel (see Figures 4 and 5).

Plasma can cut any conductive material, including carbon/mild steel, stainless steel, aluminum, copper, and brass. It can gouge and mark, neither of which is possible with oxyfuel. Plasma also can cut metal with scale, rust, paint, or primer.

Though not necessarily reaching the precision cut quality levels of waterjet or lasers, plasma offers extremely good cut quality across a broad range of materials using different current ratings, usually between 30 and 450 amps, with higher current used for thicker stainless steel and aluminum.

And while it’s feasible to use multiple torches when plasma cutting, the additional cost usually limits systems to no more than two torches. That’s because each plasma torch usually needs a dedicated power source and gas management system.

Slag and dross, surface finish, cut angle, top edge rounding, and heat-affected zone (HAZ) are key when judging cut quality. Plasma cutting has progressed to the point of producing a virtually dross-free edge. Compared to oxyfuel, a plasma cut produces a narrower HAZ, which reduces effects such as plate warping and discoloration (see Figure 6).

While using a multitorch oxyfuel configuration provides high productivity, the cut quality produced by plasma can, in many applications, eliminate secondary processes like weld prep that may be necessary after an oxyfuel cut. By eliminating weld prep, a fabricator may be able to shorten overall manufacturing time, justifying the slower single-torch plasma cutting operation.

Both oxyfuel and plasma have advanced in the past decade, but plasma has done so at a much faster pace. Traditionally, parts thicker than 1 in. were cut with oxyfuel. Today, however, a plasma arc can cut mild steel up to 3 in. thick, with piercing capability up to 2 in. with the ability to cut thicker stainless and aluminum.

It takes a skilled worker to consistently produce a good oxyfuel cut. An experienced operator can do this by making the necessary process adjustments to gas flow, standoff, and speed.

Meanwhile, plasma is becoming more automated, so much so that it’s now less of an art and more of a configured science—and less dependent on operator skill. Plasma cutting systems offer different process levels; part programs are predefined; and built-in features and parameters such as pierce cuts, cut speeds, cut heights, standoffs, and automatic arc ignition simplify machine operation greatly. All this, combined with rapid cut-to-cut cycle times, arc voltage sensing for consistent cut quality, and longer consumables life, has fueled plasma cutting’s popularity.

At the same time, modern oxyfuel cutting systems have features such as internal ignition torches with flame sensing and electronics integrated into the torch for a more intelligent cutting process. The electronics control such features as integrated height sensing, which eliminates the need for a separate height sensor to keep the correct distance between the cutting nozzle and workpiece; integrated ignition that eliminates the need for and maintenance of external ignition devices; and quick nozzle change that allows technicians to rapidly change nozzles without tools, minimizing setup.

In terms of market adoption, plasma technology has gained the most ground globally, especially in markets such as China, South America, Southeast Asia, and India. In these regions oxyfuel cutting historically has been the thermal cutting method of choice simply because it’s simple, low-tech, and inexpensive. But now more of these operations are migrating to plasma cutting.

The Value of Both

In some cases, having both plasma and oxyfuel on the same machine allows the shop to take advantage of the strengths of each process. This is why fabricators and steel service centers that need to cut many materials often will consider machines equipped with multiple processes. If a shop does not have the capacity or the floor space to warrant dedicated oxyfuel and plasma systems, but needs the flexibility both technologies provide, a combination machine is an alternative (see Figure 7).

The shop can use the more accurate single-torch plasma for certain contours, and then switch to the oxyfuel process for the remaining contours, because oxyfuel consumes a less expensive fuel gas and is less costly to run. The result: The shop achieves the required part accuracy at a far lower cost than if it used the high-accuracy plasma process to cut the entire part.

Strengths and Weaknesses

Using both processes in one work envelope gives the fabricator free reign to think outside the box for ways to obtain the best results. Ultimately, shops can optimize cut quality, operating cost, or productivity, depending on what’s most important for the application.

Five years ago economic conditions forced many fabricators to reduce production shifts. As business improved, some chose multiprocess cutting systems that added capacity without increasing labor. After all, a single operator can run a machine with two processes, and one cutting table is less expensive than two. Combining plasma and oxyfuel also makes better use of floor space, which is a particular concern for fabricators in emerging markets where floor space is at a premium.

But there are downsides to any multiprocess system. For instance, it’s not practical to cut with both oxyfuel and plasma simultaneously. Combination machines cut with only one process at a time: oxyfuel followed by plasma, or vice versa.

Moreover, combining technologies brings up new requirements for the machine size, machine accuracy, and cutting precision. A precision plasma cutting system used to cut thin materials quickly does not require a large or robust machine to hold, say, 4-in. mild steel plate, but does require precision motion control (acceleration and deceleration) to achieve a narrow kerf width and, thus, good cutting results.

Because oxyfuel cutting equipment handles thicker plates, it requires a larger, more robust table. The larger kerfs and slower cut speeds make the equipment less sensitive to machine motion and acceleration aspects, so components need not be highly precise. Ball screws and rack-and-pinion drives are more than sufficient.

In combining the two technologies, though, a machine has to accommodate the needs of both the oxyfuel and plasma, with a robust table and precision motion control, which can include linear drives.

An Individual Decision

Using both mechanized oxyfuel and plasma cutting can make plate fabrication far more efficient and flexible. As these technologies advance, fabricators can produce their parts smarter, faster, with the highest quality, and at lower cost. Regardless, the choice to use these technologies on separate machines or on one system is as individual as the application requirements.

About the Author

Douglas Shuda

Director of Marketing - Global Cutting Technologies

411 S. Ebenezer Road

Florence, SC 29501

843-669-4411