Profile cutting: Options outlined

Oxy-fuel cutting, plasma cutting and laser cutting processes are developing to offer better cut quality, higher cutting speeds, lower operating costs and integration into automated production methods. Ian Kirkpatrick, general manager, ESAB Cutting Systems, explains

The most commonly used cutting method for mild or low-alloyed steels (3mm to 500mm) is oxy-fuel. Plate is preheated to its ignition temperature with a mixture of fuel gases (eg oxygen and propane or oxygen and acetylene) and then oxygen is used as the cutting gas. By mounting one or more cutting torches on a cutting gantry, which moves across the workpiece, the process can be easily mechanised.With the comparatively low cost of cutting heads, multiple cutting torches are used to provide simultaneous cutting of identical parts economically.

Oxy/fuel gas cutting offers a number of advantages as the capital cost of the equipment is relatively low, power requirements and maintenance costs are low too. Oxygen, propane, natural gas and acetylene are cheap and easy to come by.

Oxy-fuel gas cutting, however, is dependent to some degree on the surface condition of the material to be cut while its high heat input can result in a wide heat affected zone on the cut edge of the material.

The plasma cutting process is suitable for conductive metals of all types and produces a higher cutting speed in material of up to about 40mm. Recent advances in precision plasma have resulted in better quality and hole definition to rival laser. Plasma is suitable for bevelling materials that are going to be joined by welding. Underwater plasma cutting produces environmental benefits.

The most commonly used plasma systems are based on low-amp air plasma (using air as the plasma gas). Plasma cutting has been universally accepted as a valuable tool in all sectors of the modern metalworking industry, satisfying a range of applications.

Generally, plasma systems have a low capital cost, a low running cost and can be used either manually or as part of an automated profiling system. Indeed, the ability of plasma systems to cut sheet metal at speeds in excess of 6m/min has resulted in the development of high-speed CNC profiling machines to take advantage of the process.

Although the use of air as the plasma gas is acceptable for general-purpose cutting applications, specific gases do offer improved finishes. For example, when cutting mild steel, oxygen is usually the plasma gas that is used. Cutting this material with oxygen rather than air also offers a better finish, less dross, better weldability and higher cutting speeds.

Another approach to higher quality production cutting uses water injection plasma systems as radial or vortex water constricts the arc more effectively than a gas injection type. The water also cools the electrode and nozzle, which results in longer life for consumables. It enables cuts to be carried out under water to reduce noise and glare from the plasma.

Precision plasma systems have extended the application of plasma cutting into the areas of precision light to medium gauge applications. This technology is more precise than conventional plasma while offering similar flexibility. It works by concentrating the cutting arc in a very small diameter, using a much smaller nozzle aperture than conventional plasma systems.

This results in a beam energy density that is three to five times greater than that achieved with conventional plasma, which in turn results in a shallower temperature gradient in the arc. As a consequence, both edges of the cut are square, both the cut width and heat-affected zone are narrower and the heat-related distortion of thin material is less.

The laser cutting process produces high precision, extremely narrow sections and high cutting speeds in thin material. It is suitable for most materials - metals, plastic, glass, wood, insulation material. However, its main application is for cutting steel up to 25mm thickness. Laser cutting also offers high accuracy, as the tightly focused beam produces a narrow cut with a high quality edge, virtually no kerf and a very narrow heat-affected zone.

The cutting capacity of laser profiling systems is up to about 25mm steel plate, though the high cost of laser sources tends to limit its application to high value added applications rather than general fabrication.

Laser beam delivery and guiding systems enable fabricators to obtain the benefits of accuracy and quality on larger and thicker plate. Plate sizes to 20m long and 5m wide can be processed.

It is accepted that on outside profiles there is invariably little to choose between high density plasma and laser, but the latter certainly offers advantages where internal profiles, intricate work and fine holes are required.