Deciding which of the four major metal sheet and plate cutting processes is best for a given task is not always simple. Jim Colt, Application Technology Manager at Hypertherm, looks at which system would best suit a particular application.

  • The attributes of a cutting process that are most often important from a metal fabricator or job shop perspective are as follows. Capital equipment cost. While each process requires sophisticated CNC motion control and fume control equipment, as well as CAD and CAM software to unlock the best potential, there are differences in these requirements for each process. As an example, motion control systems on a laser designed to cut thin steel at high speeds are required to have much greater accuracy than motion control for waterjet or oxy-fuel systems that operate at lower speeds. These requirements have a direct, often large effect on capital equipment cost differences.
  • Cutting cost per part or per metre. This type of cost calculation must include consumables and utilities. In some cases, amortised capital equipment cost may also be included, as well as labour costs. It is necessary to ensure that the same type of inputs is used for each system when comparing relative costs. The cut costs should be broken down per-metre or per-part as opposed to per-hour, as this levels out the speed advantages of some processes. The cost per metre calculations will always show large advantages to the processes with the cut speed advantage.
  • Ease of use. This attribute puts the heaviest weight on software (especially CAM) as well as CNC control capability. Today’s best systems have relatively short learning curves, with embedded expertise often minimising the need for expert operators. While difficult to put a value on these advantages, it is necessary to think about this when introducing sophisticated systems.
  • Productivity. Often called throughput, this is essentially the amount of parts that are cut to specification at the end of a shift. Productivity or cut speed is the biggest influence on the real cost per part. In some cases, accuracy can be reduced in favour of a process that provides dramatically lower cut costs and a bigger pile of parts in less time.
  • Cut part accuracy. There are many ways to measure the accuracy of the wide variety of parts produced in metal fabricating. Often the outside contours of parts have much looser tolerances compared to inside details such as holes. Also, often only the top of a part is measured, yet the bottom (based on edge taper) will be substantially different in dimension. For simplicity, plus or minus expectations for tolerance as measured from the top of the part will be used here, as well as a reference to edge angularity for each process.
  • Edge quality and metallurgical effects. All of these processes will produce different effects on the cut-edge metallurgy that can affect machinability, formability and weldability.
  • Service and maintenance. Some of the long-term operating cost calculations with each form of these systems will be affected by the need for maintenance, as well as the level of expertise required to perform that maintenance.

For the following comparison, a ‘complete system’ capital equipment cost will be calculated. This would be the landed cost of a turnkey system on the shop floor, with a 1.5m x 3m cutting area, an industrial quality CNC machine, and CAD and CAM software.

Oxy-fuel

Oxy-fuel cutting is by far the simplest of the cutting technologies being discussed. The process essentially uses a fuel gas to heat steel to its ‘kindling’ temperature of around 980 deg. Celsius. Once the steel is at this temperature (preheat), a pure oxygen jet is activated to create an exothermic reaction, rapidly eroding the steel. Oxy-fuel can cut only mild steel, and does a good job in the thickness range 6m-150mm. Cutting speeds are better than other processes on material thicknesses over about 50mm. It is easy and inexpensive to add multiple torches to a CNC machine to cut multiple parts simultaneously.

  • Capital equipment cost. US$40k-US$50k for a turnkey system (relatively simple machine due to low speed requirements).
  • Cutting costs per part or meter. Gas usage is fairly high and cut speeds are slow, but the cost of cut parts gets more competitive with plasma as the steel gets thicker. Typically oxy-fuel has a moderately high cost per metre and gets better on 50mm and above.
  • Ease of use. Oxy-fuel cutting on a CNC machine requires a high skill level to get the best cut and maximum speed. Constant monitoring is generally required.
  • This is a relatively slow process with its advantages being based on lowest capital cost and the ability to cut very thick steel.
  • Cut part accuracy. With a good operator, oxy-fuel can typically achieve tolerances in the +/-0.75mm range and with edge angularity of less than one degree of edge taper.
  • Edge quality and metallurgy. A large heat-affected zone is produced due to low speeds, with edges typically rough and some dross removal required.
  • Service and maintenance. Simple to maintain, typically in-house.

High-definition plasma

High-definition plasma uses a high-temperature ionised gas to produce a high energy density cutting arc capable of cutting any conductive material. The latest systems can be almost completely automated and do their best work on 0.5mm-50mm steel thickness with a maximum up to 80mm, and do well on steel and aluminium from 0.5mm through to 160mm.

Capital equipment cost. US$75k-US$90k. Higher speeds are possible, but torch height control systems and better fume control are necessary for HyDefinition-class plasma cutting (HyDefinition is a patented consumable technology of Hypertherm that uses vented nozzle technology to deliver sharper top-edge quality, smoother cut surfaces with minimal angle deviation, and long nozzle life).

Cutting costs per part or metre. Plasma has the lowest cost per part on 6mm-50mm thick steel.

Ease of use. With the latest technology CNC and software, plasma is extremely easy to learn and use as expertise is embedded in CAM software.

Productivity. This is definitely the fastest, most productive process, being faster than laser on >6mm and faster than oxy-fuel up to 50mm.

Cut part accuracy. Typical cut part accuracy on steel is in the range +/- 0.38m-0.5 mm. Cut-edge angularity is two to three degrees on steel under about 9.5mm, and about one degree on steel 12.5mm-38mm thick.

Edge quality and metallurgy. A relatively narrow heat-affected zone is produced, typically less than 0.25mm, with very little edge hardening and excellent weldability, and smooth edges with minimal dross on steel.

Service and maintenance. Simple to maintain, typically in house or with factory phone tech support.

3kW fibre laser

Fibre lasers are the latest technology in laser cutting. These systems, using a solid state laser generator or power source, are far more efficient than the CO2 laser systems they are quickly replacing. Fibre lasers operate at a light wavelength that allows them to deliver the beam to the cutting head using a flexible fibre-optic cable, rather than the mirror-and-tube system used by CO2 lasers. This provides a simpler layout that requires far less maintenance. The laser uses a properly focused, high-energy laser to melt a small spot and an assist gas (typically oxygen for cutting steel) to remove the molten metal. A 3kW fibre laser can cut with similar speeds and power to a 4kW-5kW CO2 laser. Cut capability on steel is from thin gauge to about 20mm.

  • Capital equipment cost. US$400-500k. Laser systems require more precise motion control as well as a light tight enclosure for safety.
  • Cutting costs per part or metre. The big advantage of laser is with materials thinner than 6mm. Above this the speed is considerably reduced, though excellent cut quality and accuracy are maintained. It provides the lowest cut costs on thin materials, but is higher than plasma on steels above 6mm.
  • Ease of use. With the best technology CAM software and CNC control the system is as easy to operate as the newest plasma systems. All settings can be automated.
  • By far the best productivity on gauge thicknesses upwards of 1mm, but speeds are similar to plasma at around 6mm.
  • Cut part accuracy. The best technology fibre lasers can produce cut part tolerances in the range +/-0.25mm. This is better than plasma and almost as good as abrasive waterjet. Edge angularity on most materials and thicknesses is less than one degree.
  • Edge quality and metallurgy. Slightly narrower heat-affected zone compared to plasma.
  • Service and maintenance. Maintenance requirements have reduced dramatically and can generally be handled in-house in conjunction with phone technical support.

Abrasive waterjet

Waterjet technology has been used for decades cutting materials from cake to granite. Softer materials can be cut with a pure high-pressure waterjet forced through an orifice to increase its velocity and energy density. Abrasive water jets inject an abrasive (usually garnet) downstream of the orifice. Today’s best systems have pumps that can boost water pressure to as much as 6,900 bar, enhancing higher cutting speeds, but this has historically increased downtime for maintenance when pump seals fail periodically. The latest systems have improved rebuildability, allowing minimal downtime. No heat-affected zone is produced and abrasive waterjet can cut almost anything, at the best tolerances. The biggest downside of waterjet is that cutting speeds are lower compared to the other processes.

  • Capital equipment cost. Relatively simple motion control with relatively low speeds keeps the cost lower than laser and slightly higher than plasma.
  • Cutting costs per part or metre. Because cost per part is affected by cut speed, this is the most expensive cutting process. The consumable garnet also adds to costs.
  • Ease of use. Probably the easiest process to use. It can be fully automated with the best CAM and CNC systems and operator expertise can be low.
  • Slow on steels, but better on aluminium.
  • Cut part accuracy. By far the best cut part accuracy, typically within +/-0.127mm, and less than one degree of cut-edge taper.
  • Edge quality and metallurgy. Cut-edge smoothness and quality can be controlled by cut speed and the grit size of the abrasive. There is virtually no effect to base material metallurgy.
  • Service and maintenance. Simple to maintain, typically in-house.

Practical comparison

Hypertherm recently displayed 12.5mm steel parts that had been CNC cut with five different processes; a Powermax105 air plasma; a MAXPRO200 LongLife oxygen-based industrial plasma; a HyPerformance HPR130XD high-definition class plasma; a HyIntensity 3kW fibre laser; and a HyPrecision abrasive waterjet.

On superficial inspection, all parts appeared identical. However up close, edge taper and hole taper on the air plasma cut could clearly be seen. Furthermore, on examining the more expensive processes the angle of the taper diminishing, with less taper on the oxygen plasma, was almost completely absent with the high-definition plasma, and virtually non-existent with the laser and waterjet. While the cuts were very nice with all of the systems, the systems with higher capital and operating costs clearly offer cut quality and metallurgy advantages. Only using 12.5mm steel to compare processes may not be completely fair for those that desired excellent cut quality at the lowest price, as that would likely force the choice of the high-definition plasma. If the sample had been 3mm aluminium, the waterjet and laser would definitely be the most productive, most accurate processes. If the criterion is low capital cost with looser tolerances, perhaps the air plasma would be adequate with its extremely low capital cost.

Selecting the best cutting process is difficult as it depends on the specific application, the business needs and what areas are most critical. Oxy-fuel is limited to mild steel and is not effective on stainless or aluminium; it is typically used for very thick plate. Plasma provides an optimal mix of cut quality, productivity and operating costs for mild steel, stainless and aluminium across a wide range of thicknesses. Laser provides excellent cut quality and productivity on material less than 6mm and can be used up to 20mm. Waterjet can be used to cut a wide variety of materials, provides the best tolerances, and there is no heat affected zone, though cut speeds are lower.

www.hypertherm.com/en