Frequently asked questions about oxyfuel cutting

Heavy fab shops know how important oxyfuel cutting is for processing material 2 in. and thicker. Others not involved with this cutting process might be surprised to learn of the quality cut edge oxyfuel delivers. Getty Images

If your fabricating company doesn’t cut a lot of really thick metal, you might not know a lot about mechanized oxyfuel cutting. That also might lead you to believe that the technology doesn’t have a place in modern fabricating operations. Well, that couldn’t be further from the truth.

The origins of oxyfuel cutting date back to the very early 19th century when French engineers Edmond Fouché and Charles Picard developed oxygen-acetylene welding in 1903. The process called for the use of pure oxygen, instead of air, to produce a flame hot enough to melt the steel on which the torch was focused. With an oxygen-acetylene flame that could reach more than 6,000 degrees F, the process proved to be especially useful in joining metals of all types, including alloy steels and aluminum, at least until the middle of the 20th century when arc welding became more widely adopted. Even with this shift, oxyfuel welding is still taught in many welding programs to demonstrate the evolution of welding technology over the years.

Oxyfuel cutting, on the other hand, is still being used in places where plate and structural steel are the dominant materials being processed. To learn why it remains important, let’s answer some of the questions many people without knowledge of the process ask.

What type of cutting process is oxyfuel cutting?

Technically, it’s not really cutting. It’s more like rapid oxidation because the cut edge is the result of a chemical reaction.

The reaction between pure oxygen and steel creates iron oxide. (Think of it as rapid, controlled rusting.) Preheat flames are used to raise the surface or edge of the steel to approximately 1,800 degrees F, which is indicated by a bright-red glow in the steel. Pure oxygen, introduced in a fine, high-pressure stream, is then directed toward that heated section of steel. As the steel is oxidized and the oxygen stream helps to blow away the resulting slag, the “cut” occurs. In mechanized systems, the preheat and oxygen stream is then moved at a constant speed to finish the cutting process.

What material is a candidate to be cut with oxyfuel cutting?

It has to be a ferrous material. You can’t cut aluminum or stainless steel.

If oxyfuel were used to cut metals other than carbon steel, you couldn’t do much cutting. Because these nonferrous metals have oxides that have a melting point higher than the base metal itself, they oxidize quickly when the oxyfuel torch is used on them and a protective crust is formed. No material is removed. Carbon steel produces an oxide with a lower melting point than itself, allowing for the rapid oxidation process to occur.

What is the thickest material I can cut on a mechanized oxyfuel table?

Oxyfuel loses some of its efficiencies and benefits on the thinner metals. That’s where you start looking at laser and plasma cutting more closely. Both are going to do a much better job, especially with lasers that can cut material up to 1 in. to 1.25 in. and plasma that can cut material up to 2 in. thick. Some might argue that 2 in. might not make sense for oxyfuel cutting, but even there it’s worthwhile to take a look at it because it is cost-effective to operate and delivers a very nice edge.

For the really thick stuff, 4 in. and higher, shops look at oxyfuel technology on their mechanized tables because they know what they need and the quality of cut they are going to get.

What sort of cut quality can I expect from oxyfuel cutting technology?

For one thing, you are going to get a 0-degree bevel. The edge is perfectly perpendicular to the flat surface on which the metal rests. That is something that usually shocks people, particularly those who are familiar with the bevel left by plasma cutting heads.

In addition to cutting plate, oxyfuel cutting technology is used to deliver beveled edges on thick-walled pipes.

More specifically, a quality oxyfuel cut has the following characteristics:

• Square top corner

• Flat cut face from top to bottom

• Square cut face with respect to top surface

• Clean, smooth surface with near vertical drag lines

• Little to no slag on bottom edge

Does it make sense to have only one oxyfuel cutting head on a mechanized cutting system?

It depends on the applications. Take, for instance, a steel service center. You might walk into one of these places and see a 12-head oxyfuel cutting system. Why so many cutting heads? They’re doing a tremendous amount of mild steel cutting.

So you get into situations where if your company is a steel service center, you might be looking to add more value to what you offer the customer. Historically, these service centers just sold you full sheets of steel. Today they’re having to do more than that. They will offer to cut the steel sheet size you need.

What gas source is recommended for mechanized oxyfuel cutting tables?

From an automated perspective, most shops with access to a natural gas supply choose to go this route. It’s easy to set up, and it’s very affordable.

For those that don’t have access to natural gas, propane typically is the next gas of choice. A shop can bring in a bulk propane containment system.

Usually those two make up the lion’s share of gas sources for heavy fab shops.

Do I need an experienced operator to get the most out of a mechanized oxyfuel table?

There was a time when the productivity of an oxyfuel cutting table depended on the skill of the operator. These “cutters” knew how to get the preheat flame just right before piercing without looking at gauges. They knew how to fine-tune the flame to deliver the best-quality edge without looking at cutting charts. But that skill-set just doesn’t exist as readily as it once did. That’s why automation has helped to make less experienced operators get up to speed more quickly with these new modern oxyfuel cutting systems.

Nowadays a system operator can plug in the parameters of the job: material, thickness, and cutting tip size. The gas pressures are set automatically, and the cut is orchestrated by the CAM software.

That user control interface really makes the difference. It reduces the amount of time needed for an operator to become comfortable with the oxyfuel cutting system and start producing quality parts without a lot of supervision.

Can the oxyfuel cutting system alert the operator as to when something like cutting tips need to be replaced?

Right now an oxyfuel cutting table still requires a good technician or table operator to identify when consumable sets need to be changed.

In plasma cutting, a system operator can see that large fluctuations are occurring with the voltage, which is indicative of problems with the consumable set. That doesn’t exist on the oxyfuel side of things. An operator is still needed to notice things like an abnormal oxygen stream or cut edges that don’t look right.

Luckily, you don’t have to buy an entire torch tip if it needs to be replaced. Because the consumables come as a set now, usually two to three pieces, you might need to replace only part of the cutting tip. For instance, an outer shield can be replaced if too much spatter is on it.

Final Recommendations

These questions and answers can help a fabricator understand oxyfuel cutting better, but if a shop plans to add this cutting capability, it really needs to answer its own set of questions. What exactly will you be using the cutting table for? What’s the mix of work now and in the near future? What results do you expect to achieve? Is there another process that should be considered?

There’s always variability in terms of making a decision. You just need to make sure you get all of those variables answered before you make your final determination.

John Henderson is senior director, industrial and specialty gas products, ESAB Welding & Cutting Products.