Very rarely does steel come in the shape or size needed. The plate is cut using one of the various methods available, usually thermal or erosion cutting. When steel is cut using a thermal process, heat is used to melt the metal. This is also known as burning. Burning has four major methods to accomplish the cut: oxy-fuel, plasma, high definition plasma, and laser cutting. While there are four options for thermal, there is one popular option for the erosion process- water jet cutting. Today we’re going to explore how each of these processes work and some of the benefits associated, starting with the thermal category.
Oxy-fuel uses, as the name suggests, pure oxygen to react with steel that has been preheated. The reaction from the immense heat being met with pure oxygen creates iron oxide resulting in melting the steel. This process can be thought of as rapid, controlled rusting. Oxy-fuel needs a specific material to be successful, carbon steel. Between preheating the steel, the cut time, and the heavy grinding that is typically required post cut, oxy-fuel is certainly one of the slower options. You might be asking if it’s slow, why choose it? This is a great option if you’re looking to burn carbon steel greater than 2 inches thick while still keeping a tight tolerance. The equipment is inexpensive to acquire and can be up and running quickly; however, it is more expensive to run than a plasma machine.
Plasma burning has the lowest operating expense and is the most common option for cutting steel. While oxy-fuel needs carbon steel, plasma requires any type of conductive material. The most common include steel, stainless steel, aluminum, brass, and copper. Where oxy-fuel excels at cutting thick materials, a plasma burn can accommodate materials up to 1.25 inches. Now that we know the materials we can use, how does it work? A high temperature ionized gas is forced through a narrow nozzle while an electrical current adds energy to ionize the flow. This creates a plasma arc, which can reach 40,000° F. The continuous movement of the plasma arc blows away the melted metal creating the cut. Often times, there is no preheating required, which makes it a faster process than oxy-fuel. With these advantages, it’s clear to see why this is the most popular cut technique.
(Fig. 1: Plasma Cutting Diagram)
High Definition Plasma Cutting
If you’re looking for a more precise cut, you might want to consider the plasma high definition method. As indicated by the name, the plasma arc is still utilized. The big difference is the size of the nozzle that the plasma is shot out of. Here we use a narrower nozzle to achieve cuts that are cleaner and have more squared edges. For the best results, you want to use metals between 3mm and 25mm thick. More precision means a tighter tolerance and a higher cost.
To round out the burning techniques lets dive into laser cutting. Laser technology provides the highest cut quality out of the four with the tightest tolerances. This, of course, means it is also the highest cost between the thermal options. There are two main types of lasers: CO2 and fiber lasers. CO2 lasers apply high voltages to a mixture of CO2, N2, and HE to generate a laser beam, which is reflected through mirrors to the cutting head. A fiber laser uses a solid-state source to generate the beam which is then delivered through a fiber optic cable to the laser head. Ideally, mild steel, stainless steel, brass, copper, or aluminum are used when laser cutting. Each material has a different thickness recommended, all falling between .25 inch and 1 inch. This option is ideal for people looking for extremely precise cuts for fine features. If you’re looking to cut holes less than a 1:1 diameter-to-thickness ratio, this is a great cut option.
(Fig. 2: CO2 laser diagram)
Each cut that uses heat has what is known as a heat affected zone (HAZ). The HAZ is the area of the material that has been affected by the heat used to cut. This can lead to the metal being a little bit stronger or weaker in that area, or possibly less ductile. Each process has a different HAZ associated with it, which is something you will want to take into consideration when deciding what cut process to use.
You don’t have to worry about the HAZ with waterjet cutting. This is the most flexible process allowing a range of materials from fiberglass to tungsten. Here we use an ultra-high water pressure ranging between 60,000 PSI and 90,000 PSI. To give you some perspective on the strength of the stream, a typical household faucet is roughly 60-70 PSI. Water is run through either an intensifier pump or a crankshaft pump. An intensifier pump uses a hydraulic oil to move a piston, which forces the water through a tiny hole. The crankshaft pump forces water through high-pressure tubing using crankshafts to move the plungers. Both pumps force the water through a nozzle, where abrasives can be added if you’re cutting stronger materials. The most common abrasive is garnet. The water is then formed into a thin, high-pressure beam ready to cut. This technique has the highest precision and tightest tolerances of all the processes we have talked about. It is also the most expensive option, but can certainly be worth it for the accuracy delivered.
(Fig. 3: Waterjet Cutting Diagram)
With the variety of options to choose from, deciding which way to cut your steel can seem daunting. Luckily, you don’t always have to choose. The company manufacturing your steel will usually choose the best option for your project. If you do want to choose, be sure to take into account the material being cut, the thickness of said material, the tolerance needed, and of course what is the most cost efficient to get the job done.