A guide to mechanized plasma cutting

Mar 28, 2023

For many self-employed fabricators, the beginning of their entrepreneurial journey began with a welding power source. Welding skills travel, and an enterprising person can put them to work in pursuit of starting a business.

That trend doesn't seem to be changing anytime soon. According to the U.S. Bureau of Labor Statistics, jobs in the "welders, cutters, solderers, and brazers" sector are expected to grow to 427,300 in 2026, compared to 404,800 in 2016. Of that number, 24,000 will be self-employed, up 2,000 from 2016. The entrepreneurial spirit in the metal fabricating community is expected to be alive and well for the foreseeable future.

Of course, with any new business endeavor comes the challenges of growing the business. Most of these shops will look to do that by taking on larger weldments or more production work. With that in mind, many will consider making an investment in mechanized plasma cutting.

To help small-business owners or those with entrepreneurial aspirations, these frequently asked questions and accompanying answers can help provide some detail into what is possible with modern plasma cutting capabilities.

The fabricator's applications and budget drive the selection of the power source size. However, the main factors in the selection process include the type and thickness of the material to be cut.

Additional contributors are cut quality requirements, hole quality requirements, intricacies of the finished product, secondary processes, desired production rate (the number of parts needed and the speed at which they are produced), and any beveling requirements. (Beveling requires cutting through material at an angle other than 90 degrees, which increases the material thickness being cut through because of the angle).

A significant advancement in plasma cutting technology has been made in recent years that redefines plasma's cut quality on mild steel, stainless steel, and aluminum. Plasma cutting now offers extensive stainless steel and aluminum cutting options for many applications. For example, one of the newer plasma power sources now offers a three-gas mixing capability—argon, hydrogen, and nitrogen—which produces an enhanced result when cutting stainless steel and aluminum.

Table sizes can vary greatly from small (4 by 4 feet) to large (30 by 200 ft.) based on the fabricator's need.

Factors to consider when choosing table size include the material plate/sheet size, throughput requirements (dual table or another configuration to load multiple sheets/plates), and loading/unloading methods. Also, a very important consideration is the amount of space that the shop or manufacturing plant has available.

For most purposes, fabricators have their choice of two types of tables—downdraft tables and water tables. Downdraft tables typically are separated into zones that open and close as required to allow a fume extraction system to pull smoke from the cutting area and filter it or exhaust it from the room. Water tables often have an internal bladder that allows the water level to be raised and lowered depending on the cutting requirements at the time. Additionally, water tables do not have the additional requirement for fume extraction equipment that downdraft tables have. It is not recommended to cut aluminum (especially aluminum-lithium alloys) using a water table.

Fume ventilation capture should be addressed at the same time as the cutting table.

Water tables work by capturing smoke, dust, debris, particles, and slag in the water. This cools the slag and restricts smoke and other particulates from entering the workspace. During the cutting process, kinetic energy forces the fumes and particles into the water.

Downdraft tables remove the smoke from the workplace by pulling the smoke down into ductwork in the table and then delivering the smoke to a fume/smoke/dust collector. This collector filters the smoke and expels the filtered air.

Downdraft tables and fume/smoke/dust collector systems are designed with minimum coverage requirements so they must be sized accordingly. The size of a collector depends on the required airflow to contain the fume and particulate. The key factors in collector sizing are the size of the table, the amount of fume/smoke/dust that needs to be collected, and the material being cut.

For example, a wider table requires more airflow to remove the particulate. Larger-amperage power units and machines cutting with multiple torch heads produce more smoke. Different materials produce different types of particulate, such as hexavalent chromium from stainless steel.

Because of these variables, it is important for the fabricator to calculate the right collector size and the best filter media for the application when it is shopping for the cutting table.

A person familiar with the cutting process can learn the fundamentals of CNC programming in a short time with proper instruction (such as webinars, tutorials, and face-to-face instruction). These fundamentals can be picked up in a couple of days. As with any process, the programmer and the operator will pick up more detail over time based on the interest, curiosity, and practice.

Cutting machines require periodic cleaning, lubrication, and fluid checks per the manufacturer's recommendations, and their power sources and controls require periodic attention per the manufacturer as well. It is highly recommended that an annual preventive maintenance service program be performed on the cutting machine to ensure its longevity. These subjects should be addressed in depth at the time of any machine installation.

Suitable Material. The oxyfuel cutting process heats the metal to the temperature where it spontaneously ignites and a high-pressure stream of pure oxygen oxidizes the metal and blows it away. Because iron oxide melts at a lower temperature, this works well with carbon steel.

However, oxyfuel does not work with stainless steel because it doesn't oxidize. Aluminum melts at a higher temperature, so oxyfuel is not a good choice to cut this material.

Plasma will cut steel, stainless steel, and aluminum.

Operating Costs. Oxyfuel cutting uses fuel gases and oxygen to cut metals. The most common fuel gases used are natural gas (LPG) and acetylene, but propane, hydrogen, and even a combination of these also can be used. Generally, per-cubic-foot costs for natural gas and oxygen are significantly less than those for the gases used in the plasma cutting process.

Plus, the initial setup costs of the oxyfuel torch, hoses, and lifter are typically less than for the plasma cutting system. Once installed, oxyfuel torch consumables are generally cheaper to replace than the plasma consumables.

Speed. As a general rule, the oxyfuel cutting system is used when the cutting capacity requirements exceed the capacity of a plasma power source. Oxyfuel cutting is the choice with materials over 2 to 3 in. thick. With more intricate parts in thinner steel (and with stainless steel and aluminum), the plasma cutting system would be the best choice.

Comparing cutting speed and productivity, the plasma cutting system is much faster than a single oxyfuel torch up to 2.5 to 2.75 in. Oxyfuel has relatively slow cutting speeds.

Piercing. The big differentiator is piercing up to 2 in. Plasma cannot perform production piercing over 2 in. However, when piercing under 2 in., oxyfuel is extremely slow. For example, piercing into 1.25-in. steel plate with oxyfuel will take about 20 to 25 seconds. Plasma will take only about 1 to 2 seconds.

Oxyfuel's main disadvantage is slow piercing. If the fabricator's part is under 2 in. thick and requires many pierces for holemaking, then plasma cutting is the best choice.

The main takeaway is that oxyfuel can be a less expensive option using multiple heads to cut thicker carbon steel when the same pattern can be cut in parallel. This is unless a lot of piercing is needed in material up to 2 in. thick.

High-definition (HD) plasma is an advanced cutting process that delivers higher cut quality and angularity and faster cut speeds than conventional plasma cutting technology on materials up to 2 in. thick. This is possible because of a nozzle design that results in a narrower cutting arc.

An HD plasma cutting system allows for more automation when paired with a CNC machine and software. This automation allows machine operators with differing levels of experience and expertise to operate the machine and achieve superior cuts.

Yes, high-definition plasma can help eliminate secondary operations before the welding process. Air plasma systems leave a nitride edge, whereas HD systems don't. As a result, a fabricator doesn't need to grind the cut edge afterward. Edge cuts can be virtually dross-free, and holes will have little to no taper.

When HD is used with CNC automation, the cut quality and consistency from part to part lead to an increase in productivity.

Mike Best is executive vice president of sales, Hornet Cutting Systems, 430 W. Clay St., Valley Center, KS 67147, 316-755-3683, www.hornetcs.com.