COMPLEMENTARY CUTTING: EVALUATING AUTOMATED PLASMA & OXY-FUEL PROCESSES
Each process has clear-cut advantages, but the real difference lies in the major cost influences of equipment, consumables, cutting speed, installation, maintenance and cut quality.
When evaluating an automated XY cutting table, should you have plasma cutting capabilities, oxy-fuel capabilities or both? Each process has clear-cut advantages. Stainless steel, aluminum and other non-ferrous materials demands using the plasma process. Stainless steel won’t oxidize and aluminum dissipates heat too rapidly to cut cleanly with oxy-fuel. Cutting steel 2 in or thicker dictates using oxy-fuel, especially when multiple torches can be used; cut costs are approximately $0.2952 per ft vs. $0.4384 per ft for plasma.
Beyond these two situations, the best process for cutting steel from ½ in to 1½ in is a little less clear. Setting aside common factors (such as the controller, table, gantry and labor), major cost influencers include equipment, consumables and cutting speed. Other factors include installation, maintenance and cut quality.
Roughly speaking, a plasma system costs 10 times more than an oxy-fuel system. Its torch consumables cost more, too. Plasma’s economic advantage comes from speed. On materials from ½ in to 1½ in, a 400 amp precision plasma system, such as the Ultra-Cutâ 400, cuts up to three times faster (see Figure 1). Faster cutting speeds reduce cut cost per ft (see Figure 2). Piercing applications also favor plasma because plasma does not require pre-heating. Plasma can pierce 1¼ in steel in about 1.5 seconds, where oxy-fuel takes about 20 seconds. On parts with multiple pierces, an automated plasma system produces the best ROI.
On materials under ½ in, plasma’s advantage become more pronounced. Cutting speeds continue increase, up to 150 ipm for ¼ in steel. Note that different material thicknesses require changing torch consumables to optimize cut speed and quality. If you plan to change thickness often, note that some consumables cartridges, such as the XTä torch’s SpeedLokä cartridge, offer a keyless/no-tool change function that reduces change time to less than 60 seconds.
High-precision plasma systems using argon or nitrogen for the plasma gas can produce a clean, clear, easily readable line. Called plasma marking, fabricators increasingly use this process to distinguish similar components (such as left and right sides) and to permanently indentify components as part of the QA/QC process.
Oxy-fuel’s primary advantage comes from low component and consumables cost. For example, an oxy-fuel machine torch, hoses and required accessories cost just few thousand dollars, and a new tip costs about $25. Natural gas is practically free in the U.S. ($0.0001 per cu ft). Oxygen accounts for the largest operating expense (automated plasma systems can use oxygen, argon or nitrogen for the plasma gas, but at lower volumes). Thus, even though a 400 amp plasma system cuts 2 in steel at 30 ipm and oxy-fuel at 15 ipm, oxy-fuel’s lower purchase and consumables cost give it a distinct advantage.
Oxy-fuel also provides an advantage when multiple torches can cut the same pattern in parallel (see Figure 3). Setting up four to eight torches on the same gantry is relatively common, which overcomes plasma’s speed advantage. If you need to cut eight flanges from a 10 ft wide, 1 in thick plate, oxy-fuel is the way to go. However, the advantage may tip back to plasma if the part requires multiple pierces or when a limited part run can’t justify the capital expense of adding more torches.
Once installed, an oxy-fuel system operates almost maintenance free. Other than changing consumables, there’s not much that can go wrong with a torch, gas distribution and manifold system. While automated plasma systems are extremely robust, they are inherently more complex and require more maintenance.
A precision cut surface has the following characteristics:
- Square face (< 3 deg bevel)
- Smooth, with nearly vertical drag lines
- Little to no oxides
- Has little to no dross; what dross is present should be easy to remove
- A minimal heat affected zone (HAZ) and recast layer (recast = remelted metal deposited on cut edges)
- Demonstrates good mechanical properties in welded components
There are two primary differences between the processes. Oxy-fuel cuts with a 0 deg bevel, but the swirl of the plasma gas inherently creates a bevel on one side of the cut. High precision plasma cuts with 0 deg to 2 deg bevel and standard plasma cuts with a bevel greater than 3 deg. Depending on tolerance requirements, oxy-fuel may offer better parts fit without grinding . . . but then again, plasma produces a much smaller HAZ. Depending on weld procedure requirements, any thermally cut part may require grinding to remove the HAZ.
If you’re not sure which process sends the part to the next production step with the least amount of grinding, ask for cut samples and discuss the situation with your automation partner.
A GOOD PARTNER
No matter which process you choose, look for an automation partner that will work with you to set up, install and test the system. Understanding the principles of setting up an oxy-fuel torch, balancing a torch and correctly adjusting the flame require knowledge and training. With plasma, optimizing torch height during arc start and setting height after piercing greatly extends consumables life and improves cut quality. That, too, requires experience.
The good news for fabricators is that modern CNC controllers can manage one or more plasma torches and one or more oxy-fuel torches. There’s no doubt that automated, high-precision plasma cutting has replaced oxy-fuel in many applications. Fortunately, adding oxy-fuel capabilities to an XY plasma cutting table is a low-cost proposition. With oxy-fuel offering precision and lower cut costs in certain situations, smart fabrications usually opt to have the best of both worlds.
About the Author: John Henderson is a senior brand manager and Dirk Ott is the vice president of global plasma automation for Victor Technologies International, Inc., 16052 Swingley Ridge Road, Suite 300, St. Louis, MO 63017, 636-728-3000, www.victortechnologies.com.